Bipedalism is a form of
terrestrial locomotion
Terrestrial locomotion has evolved as animals adapted from aquatic to terrestrial environments. Locomotion on land raises different problems than that in water, with reduced friction being replaced by the increased effects of gravity.
As viewe ...
where an organism
moves by means of its two rear
limbs or
leg
A leg is a weight-bearing and locomotive anatomical structure, usually having a columnar shape. During locomotion, legs function as "extensible struts". The combination of movements at all joints can be modeled as a single, linear element ca ...
s. An
animal
Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...
or
machine
A machine is a physical system using Power (physics), power to apply Force, forces and control Motion, movement to perform an action. The term is commonly applied to artificial devices, such as those employing engines or motors, but also to na ...
that usually moves in a bipedal manner is known as a biped , meaning 'two feet' (from
Latin
Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
''bis'' 'double' and ''pes'' 'foot'). Types of bipedal movement include
walking
Walking (also known as ambulation) is one of the main gaits of terrestrial locomotion among legged animals. Walking is typically slower than running and other gaits. Walking is defined by an ' inverted pendulum' gait in which the body vaults ...
,
running
Running is a method of terrestrial locomotion allowing humans and other animals to move rapidly on foot. Running is a type of gait characterized by an aerial phase in which all feet are above the ground (though there are exceptions). This is ...
, and
hopping.
Several groups of modern species are habitual bipeds whose normal method of locomotion is two-legged. In the
Triassic
The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...
period some groups of
archosaurs (a group that includes
crocodile
Crocodiles (family (biology), family Crocodylidae) or true crocodiles are large semiaquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. The term crocodile is sometimes used even more loosely to inclu ...
s and
dinosaur
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
s) developed bipedalism; among the
dinosaur
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
s, all the early forms and many later groups were habitual or exclusive bipeds; the
bird
Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...
s are members of a clade of exclusively bipedal dinosaurs, the
theropod
Theropoda (; ), whose members are known as theropods, is a dinosaur clade that is characterized by hollow bones and three toes and claws on each limb. Theropods are generally classed as a group of saurischian dinosaurs. They were ancestrally c ...
s. Within
mammal
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
s, habitual bipedalism has evolved multiple times, with the
macropods,
kangaroo rats and mice,
springhare
''Pedetes'' is a genus of rodent, the springhares, in the family Pedetidae. Members of the genus are distributed across southern and Eastern Africa.
Species
A number of species both extant and extinct are classified in the genus ''Pedetes''. ...
,
hopping mice
A hopping mouse is any of about ten different Australian native mice in the genus ''Notomys''. They are rodents, not marsupials, and their ancestors are thought to have arrived from Asia about 5 million years ago.
All are brown or fawn, fading ...
,
pangolin
Pangolins, sometimes known as scaly anteaters, are mammals of the order Pholidota (, from Ancient Greek ϕολιδωτός – "clad in scales"). The one extant family, the Manidae, has three genera: '' Manis'', '' Phataginus'', and '' Smut ...
s and
hominin
The Hominini form a taxonomic tribe of the subfamily Homininae ("hominines"). Hominini includes the extant genera ''Homo'' (humans) and '' Pan'' (chimpanzees and bonobos) and in standard usage excludes the genus ''Gorilla'' (gorillas).
The ...
apes (
australopithecines, including
humans
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
) as well as various other extinct groups evolving the trait independently.
A larger number of modern species intermittently or briefly use a bipedal gait. Several
lizard
Lizards are a widespread group of squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbaenia alt ...
species move bipedally when running, usually to escape from threats. Many primate and bear species will adopt a bipedal gait in order to reach food or explore their environment, though there are a few cases where they walk on their hind limbs only. Several arboreal primate species, such as
gibbon
Gibbons () are apes in the family Hylobatidae (). The family historically contained one genus, but now is split into four extant genera and 20 species. Gibbons live in subtropical and tropical rainforest from eastern Bangladesh to Northeast India ...
s and
indriids
The Indriidae (sometimes incorrectly spelled Indridae) are a family of strepsirrhine primates. They are medium- to large-sized lemurs, with only four teeth in the toothcomb instead of the usual six. Indriids, like all lemurs, live exclusively on t ...
, exclusively walk on two legs during the brief periods they spend on the ground. Many animals rear up on their hind legs while fighting or copulating. Some animals commonly stand on their hind legs to reach food, keep watch, threaten a competitor or predator, or pose in courtship, but do not move bipedally.
Etymology
The word is derived from the
Latin
Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
words ''bi(s)'' 'two' and ''ped-'' 'foot', as contrasted with
quadruped
Quadrupedalism is a form of locomotion where four limbs are used to bear weight and move around. An animal or machine that usually maintains a four-legged posture and moves using all four limbs is said to be a quadruped (from Latin ''quattuor' ...
'four feet'.
Advantages
Limited and exclusive bipedalism can offer a species several advantages. Bipedalism raises the head; this allows a greater field of vision with improved detection of distant dangers or resources, access to deeper water for wading animals and allows the animals to reach higher food sources with their mouths. While upright, non-locomotory
limb
Limb may refer to:
Science and technology
*Limb (anatomy), an appendage of a human or animal
*Limb, a large or main branch of a tree
*Limb, in astronomy, the curved edge of the apparent disk of a celestial body, e.g. lunar limb
*Limb, in botany, ...
s become free for other uses, including manipulation (in primates and rodents), flight (in birds), digging (in the
giant pangolin
The giant pangolin (''Smutsia gigantea'') is the largest species in the family of pangolins. Members of the species inhabit Africa with a range stretching along the equator from West Africa to Uganda. It subsists almost entirely on ants and t ...
), combat (in bears, great
ape
Apes (collectively Hominoidea ) are a clade of Old World simians native to sub-Saharan Africa and Southeast Asia (though they were more widespread in Africa, most of Asia, and as well as Europe in prehistory), which together with its sister g ...
s and the large
monitor lizard) or camouflage.
The maximum bipedal speed appears slower than the maximum speed of quadrupedal movement with a flexible backbone – both the
ostrich
Ostriches are large flightless birds of the genus ''Struthio'' in the order Struthioniformes, part of the infra-class Palaeognathae, a diverse group of flightless birds also known as ratites that includes the emus, rheas, and kiwis. There are ...
and the
red kangaroo
The red kangaroo (''Osphranter rufus'') is the largest of all kangaroos, the largest terrestrial mammal native to Australia, and the largest extant marsupial. It is found across mainland Australia, except for the more fertile areas, such as sou ...
can reach speeds of , while the
cheetah
The cheetah (''Acinonyx jubatus'') is a large cat native to Africa and central Iran. It is the fastest land animal, estimated to be capable of running at with the fastest reliably recorded speeds being , and as such has evolved specialized ...
can exceed .
Even though bipedalism is slower at first, over long distances, it has allowed humans to outrun most other animals according to the
endurance running hypothesis
The endurance running hypothesis is a series of conjectures which presume humans evolved anatomical and physiological adaptations to run long distances
.
and, more strongly, that "running is the only known behavior that would account for the di ...
. Bipedality in
kangaroo rat
Kangaroo rats, small mostly nocturnal rodents of genus ''Dipodomys'', are native to arid areas of western North America. The common name derives from their bipedal form. They hop in a manner similar to the much larger kangaroo, but developed t ...
s has been hypothesized to improve locomotor performance, which could aid in escaping from predators.
Facultative and obligate bipedalism
Zoologists often label behaviors, including bipedalism, as "facultative" (i.e. optional) or "obligate" (the animal has no reasonable alternative). Even this distinction is not completely clear-cut — for example,
human
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
s other than infants normally walk and run in biped fashion, but almost all can crawl on hands and knees when necessary. There are even reports of humans who normally walk on all fours with their feet but not their knees on the ground, but these cases are a result of conditions such as
Uner Tan syndrome — very rare genetic neurological disorders rather than normal behavior.
Even if one ignores exceptions caused by some kind of injury or illness, there are many unclear cases, including the fact that "normal" humans can crawl on hands and knees. This article therefore avoids the terms "facultative" and "obligate", and focuses on the range of styles of locomotion ''normally'' used by various groups of animals. Normal humans may be considered "obligate" bipeds because the alternatives are very uncomfortable and usually only resorted to when walking is impossible.
Movement
There are a number of states of movement commonly associated with bipedalism.
#
Standing
Standing, also referred to as orthostasis, is a position in which the body is held in an ''erect'' ("orthostatic") position and supported only by the feet. Although seemingly static, the body rocks slightly back and forth from the ankle in the s ...
. Staying still on both legs. In most bipeds this is an active process, requiring constant adjustment of balance.
#
Walking
Walking (also known as ambulation) is one of the main gaits of terrestrial locomotion among legged animals. Walking is typically slower than running and other gaits. Walking is defined by an ' inverted pendulum' gait in which the body vaults ...
. One foot in front of another, with at least one foot on the ground at any time.
#
Running
Running is a method of terrestrial locomotion allowing humans and other animals to move rapidly on foot. Running is a type of gait characterized by an aerial phase in which all feet are above the ground (though there are exceptions). This is ...
. One foot in front of another, with periods where both feet are off the ground.
#
Jumping
Jumping or leaping is a form of locomotion or movement in which an organism or non-living (e.g., robotics, robotic) mechanical system propels itself through the air along a ballistic trajectory. Jumping can be distinguished from running, gallo ...
/hopping. Moving by a series of jumps with both feet moving together.
Bipedal animals
The great majority of living terrestrial vertebrates are quadrupeds, with bipedalism exhibited by only a handful of living groups. Humans, gibbons and large birds
walk
Walking (also known as ambulation) is one of the main gaits of terrestrial locomotion among legged animals. Walking is typically slower than running and other gaits. Walking is defined by an ' inverted pendulum' gait in which the body vaults ...
by raising one foot at a time. On the other hand, most macropods, smaller birds, lemurs and bipedal rodents move by hopping on both legs simultaneously.
Tree kangaroo
Tree-kangaroos are marsupials of the genus ''Dendrolagus'', adapted for arboreal locomotion. They inhabit the tropical rainforests of New Guinea and far northeastern Queensland, along with some of the islands in the region. All tree-kangaroos ...
s are able to walk or hop, most commonly alternating feet when moving arboreally and hopping on both feet simultaneously when on the ground.
Extant reptiles
Many species of
lizards become bipedal during high-speed, sprint locomotion, including the world's fastest lizard, the spiny-tailed iguana (genus ''
Ctenosaura
''Ctenosaura'' is a lizard genus commonly known as spinytail iguanas or ctenosaurs. The genus is part of the large lizard family, Iguanidae and is native to Mexico and Central America. The name is derived from two Greek words: ctenos (κτεν ...
'').
Early reptiles and lizards
The first known biped is the
bolosaurid
Bolosauridae is an extinct family of ankyramorph parareptiles known from the latest Carboniferous (Gzhelian) or earliest Permian (Asselian) to the early Guadalupian epoch (latest Roadian stage) of North America, China, Germany, Russia and France. ...
''
Eudibamus
''Eudibamus'' is an extinct genus of biped bolosaurid ankyramorph parareptile known from the Free State of Thuringia of central Germany. It had a very small size reaching only 25-26 cm in length.
Discovery
''Eudibamus'' is known only from ...
'' whose fossils date from 290 million years ago. Its long hind-legs, short forelegs, and distinctive joints all suggest bipedalism. The species became extinct in the
early Permian 01 or '01 may refer to:
* The year 2001, or any year ending with 01
* The month of January
* 1 (number)
Music
* '01 (Richard Müller album), 01'' (Richard Müller album), 2001
* 01 (Son of Dave album), ''01'' (Son of Dave album), 2000
* 01 (Urban ...
.
Archosaurs (includes crocodilians and dinosaurs)
Birds
All
birds
Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...
are bipeds when on the ground, a feature inherited from their
dinosaur
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
ancestors. However,
hoatzin
The hoatzin ( ) or hoactzin ( ), (''Opisthocomus hoazin''), is the only species in the order Opisthocomiformes. It is a species of tropical bird found in swamps, riparian forests, and mangroves of the Amazon and the Orinoco basins in South Ameri ...
chicks have claws on their wings which they use for climbing.
Other archosaurs
Bipedalism evolved more than once in
archosaurs
Archosauria () is a clade of diapsids, with birds and crocodilians as the only living representatives. Archosaurs are broadly classified as reptiles, in the cladistic sense of the term which includes birds. Extinct archosaurs include non-avia ...
, the group that includes both dinosaurs and
crocodilia
Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period ( Cenomanian stage) and are the closest living ...
ns.
All dinosaurs are thought to be descended from a fully bipedal ancestor, perhaps similar to ''
Eoraptor
''Eoraptor'' () is a genus of small, lightly built, basal sauropodomorph. One of the earliest-known dinosaurs, it lived approximately 231 to 228 million years ago, during the Late Triassic in Western Gondwana, in the region that is now northwes ...
''.
Dinosaurs diverged from their
archosaur ancestors approximately 230 million years ago during the Middle to Late
Triassic
The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...
period, roughly 20 million years after the
Permian-Triassic extinction event wiped out an estimated 95 percent of all life on Earth.
Radiometric dating
Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares ...
of fossils from the early dinosaur
genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
''
Eoraptor
''Eoraptor'' () is a genus of small, lightly built, basal sauropodomorph. One of the earliest-known dinosaurs, it lived approximately 231 to 228 million years ago, during the Late Triassic in Western Gondwana, in the region that is now northwes ...
'' establishes its presence in the fossil record at this time. Paleontologists suspect ''Eoraptor'' resembles the
common ancestor
Common descent is a concept in evolutionary biology applicable when one species is the ancestor of two or more species later in time. All living beings are in fact descendants of a unique ancestor commonly referred to as the last universal comm ...
of all dinosaurs; if this is true, its traits suggest that the first dinosaurs were small, bipedal predators. The discovery of primitive, dinosaur-like ornithodirans such as ''
Marasuchus
''Marasuchus'' (meaning "Mara crocodile") is a genus of basal dinosauriform archosaur which is possibly synonymous with '' Lagosuchus''. Both genera lived during the Late Triassic in what is now La Rioja Province, Argentina. ''Marasuchus'' conta ...
'' and ''
Lagerpeton
''Lagerpeton'' is a genus of lagerpetid avemetatarsalian, comprising a single species, ''L. chanarensis''. First described from the Chañares Formation of Argentina by A. S. Romer in 1971, ''Lagerpeton'' anatomy is somewhat incompletely kn ...
'' in
Argentinian
Argentines (mistakenly translated Argentineans in the past; in Spanish (masculine) or ( feminine)) are people identified with the country of Argentina. This connection may be residential, legal, historical or cultural. For most Argentines, ...
Middle Triassic
In the geologic timescale, the Middle Triassic is the second of three epochs of the Triassic period or the middle of three series in which the Triassic system is divided in chronostratigraphy. The Middle Triassic spans the time between Ma and ...
strata supports this view; analysis of recovered fossils suggests that these animals were indeed small, bipedal predators.
Bipedal movement also re-evolved in a number of other
dinosaur
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
lineages such as the
iguanodonts
Iguanodontia (the iguanodonts) is a clade of herbivorous dinosaurs that lived from the Middle Jurassic to Late Cretaceous. Some members include '' Camptosaurus'', ''Dryosaurus'', ''Iguanodon'', '' Tenontosaurus'', and the hadrosaurids or "duck-bi ...
. Some extinct members of
Pseudosuchia
Pseudosuchia is one of two major divisions of Archosauria, including living crocodilians and all archosaurs more closely related to crocodilians than to birds. Pseudosuchians are also informally known as "crocodilian-line archosaurs". Prior to ...
, a sister group to the avemetatarsalians (the group including dinosaurs and relatives), also evolved bipedal forms – a poposauroid from the
Triassic
The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...
, ''
Effigia okeeffeae'', is thought to have been bipedal.
Pterosaur
Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 ...
s were previously thought to have been bipedal, but recent trackways have all shown quadrupedal locomotion.
Mammals
A number of groups of extant mammals have independently evolved bipedalism as their main form of locomotion - for example humans,
giant pangolin
The giant pangolin (''Smutsia gigantea'') is the largest species in the family of pangolins. Members of the species inhabit Africa with a range stretching along the equator from West Africa to Uganda. It subsists almost entirely on ants and t ...
s, the extinct
giant ground sloths
''Megatherium'' ( ; from Greek () 'great' + () 'beast') is an extinct genus of ground sloths endemic to South America that lived from the Early Pliocene through the end of the Pleistocene. It is best known for the elephant-sized type species ' ...
, numerous species of jumping
rodent
Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are na ...
s and
macropods. Humans, as their bipedalism has been extensively studied, are documented in the next section. Macropods are believed to have evolved bipedal hopping only once in their evolution, at some time no later than 45 million years ago.
Bipedal movement is less common among
mammal
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
s, most of which are
quadruped
Quadrupedalism is a form of locomotion where four limbs are used to bear weight and move around. An animal or machine that usually maintains a four-legged posture and moves using all four limbs is said to be a quadruped (from Latin ''quattuor' ...
al. All primates possess some bipedal ability, though most species primarily use quadrupedal locomotion on land. Primates aside, the
macropods (kangaroos, wallabies and their relatives),
kangaroo rats and mice,
hopping mice
A hopping mouse is any of about ten different Australian native mice in the genus ''Notomys''. They are rodents, not marsupials, and their ancestors are thought to have arrived from Asia about 5 million years ago.
All are brown or fawn, fading ...
and
springhare
''Pedetes'' is a genus of rodent, the springhares, in the family Pedetidae. Members of the genus are distributed across southern and Eastern Africa.
Species
A number of species both extant and extinct are classified in the genus ''Pedetes''. ...
move bipedally by hopping. Very few non-primate mammals commonly move bipedally with an alternating leg gait. Exceptions are the
ground pangolin
The ground pangolin (''Smutsia temminckii''), also known as Temminck's pangolin, Cape pangolin or steppe pangolin, is one of four species of pangolins which can be found in Africa, and the only one in southern and eastern Africa. The animal was ...
and in some circumstances the
tree kangaroo
Tree-kangaroos are marsupials of the genus ''Dendrolagus'', adapted for arboreal locomotion. They inhabit the tropical rainforests of New Guinea and far northeastern Queensland, along with some of the islands in the region. All tree-kangaroos ...
. One black bear,
Pedals
A pedal (from the Latin '' pes'' ''pedis'', "foot") is a lever designed to be operated by foot and may refer to:
Computers and other equipment
* Footmouse, a foot-operated computer mouse
* In medical transcription, a pedal is used to control ...
, became famous locally and on the internet for having a frequent bipedal gait, although this is attributed to injuries on the bear's front paws.
Primates
![Muybridge runner](https://upload.wikimedia.org/wikipedia/commons/6/6b/Muybridge_runner.jpg)
Most bipedal animals move with their backs close to horizontal, using a long tail to balance the weight of their bodies. The primate version of bipedalism is unusual because the back is close to upright (completely upright in humans), and the tail may be absent entirely. Many primates can stand upright on their hind legs without any support.
Chimpanzees,
bonobos,
gorilla
Gorillas are herbivorous, predominantly ground-dwelling great apes that inhabit the tropical forests of equatorial Africa. The genus ''Gorilla'' is divided into two species: the eastern gorilla and the western gorilla, and either four or fi ...
s, gibbons and
baboon
Baboons are primates comprising the genus ''Papio'', one of the 23 genera of Old World monkeys. There are six species of baboon: the hamadryas baboon, the Guinea baboon, the olive baboon, the yellow baboon, the Kinda baboon and the chacma ba ...
s exhibit forms of bipedalism. On the ground
sifaka
A sifaka (; ) is a lemur of the genus ''Propithecus'' from the family Indriidae within the order Primates. The name of their family is an onomatopoeia of their characteristic "shi-fak" alarm call. Like all lemurs, they are found only on the isl ...
s move like all
indrids with bipedal sideways hopping movements of the hind legs, holding their forelimbs up for balance.
Gelada
The gelada (''Theropithecus gelada'', am, ጭላዳ, translit=č̣əlada), sometimes called the bleeding-heart monkey or the gelada baboon, is a species of Old World monkey found only in the Ethiopian Highlands, living at elevations of above se ...
s, although usually quadrupedal, will sometimes move between adjacent feeding patches with a squatting, shuffling bipedal form of locomotion. However, they can only do so for brief amounts, as their bodies are not adapted for constant bipedal locomotion.
Humans are the only primates who are normally biped, due to an extra curve in the spine which stabilizes the upright position, as well as shorter arms relative to the legs than is the case for the nonhuman great apes. The evolution of
human
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
bipedalism
Bipedalism is a form of terrestrial locomotion where an organism moves by means of its two rear limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped , meaning 'two feet' (from Latin ''bis'' 'double' ...
began in primates about four million years ago,
or as early as seven million years ago with ''
Sahelanthropus
''Sahelanthropus tchadensis'' is an extinct species of the Homininae (African apes) dated to about , during the Miocene epoch. The species, and its genus ''Sahelanthropus'', was announced in 2002, based mainly on a partial cranium, nicknamed '' ...
''
or about 12 million years ago with ''
Danuvius guggenmosi
''Danuvius guggenmosi'' is an extinct species of great ape that lived 11.6 million years ago during the Middle–Late Miocene in southern Germany. It is the sole member of the genus ''Danuvius''. The area at this time was probably a woodland ...
''. One hypothesis for human bipedalism is that it evolved as a result of differentially successful survival from carrying food to share with group members, although there are alternative hypotheses.
; Injured individuals
Injured chimpanzees and bonobos have been capable of sustained bipedalism.
Three captive primates, one macaque
Natasha
Natasha (russian: Наташа) is a name of Slavic origin. The Slavic name is the diminutive form of Natalia.
Notable people
* Natasha, the subject of ''Natasha's Story'', a 1994 nonfiction book
* Natasha Aguilar (1970–2016), Costa Rican sw ...
and two chimps,
Oliver and Poko (chimpanzee), were found to move bipedally . Natasha switched to exclusive bipedalism after an illness, while Poko was discovered in captivity in a tall, narrow cage. Oliver reverted to
knuckle-walking
Knuckle-walking is a form of quadrupedal walking in which the forelimbs hold the fingers in a partially flexed posture that allows body weight to press down on the ground through the knuckles. Gorillas, bonobos, and chimpanzees use this style o ...
after developing arthritis. Non-human primates often use bipedal locomotion when carrying food.
Limited bipedalism
Limited bipedalism in mammals
Other mammals engage in limited, non-locomotory, bipedalism. A number of other animals, such as
rats,
raccoon
The raccoon ( or , ''Procyon lotor''), sometimes called the common raccoon to distinguish it from other species, is a mammal native to North America. It is the largest of the procyonid family, having a body length of , and a body weight of ...
s, and
beavers will squat on their hindlegs to manipulate some objects but revert to four limbs when moving (the beaver will move bipedally if transporting wood for their
dams
A dam is a barrier that stops or restricts the flow of surface water or underground streams. Reservoirs created by dams not only suppress floods but also provide water for activities such as irrigation, human consumption, industrial use ...
, as will the raccoon when holding food).
Bear
Bears are carnivoran mammals of the family Ursidae. They are classified as caniforms, or doglike carnivorans. Although only eight species of bears are extant, they are widespread, appearing in a wide variety of habitats throughout the Nor ...
s will fight in a bipedal stance to use their forelegs as weapons. A number of mammals will adopt a bipedal stance in specific situations such as for feeding or fighting.
Ground squirrel
Ground squirrels are members of the squirrel family of rodents ( Sciuridae), which generally live on or in the ground, rather than trees. The term is most often used for the medium-sized ground squirrels, as the larger ones are more commonly known ...
s and
meerkats will stand on hind legs to survey their surroundings, but will not walk bipedally.
Dog
The dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated descendant of the wolf. Also called the domestic dog, it is derived from the extinct Pleistocene wolf, and the modern wolf is the dog's nearest living relative. Do ...
s (e.g.
Faith
Faith, derived from Latin ''fides'' and Old French ''feid'', is confidence or trust in a person, thing, or In the context of religion, one can define faith as "belief in God or in the doctrines or teachings of religion".
Religious people often ...
) can stand or move on two legs if trained, or if birth defect or injury precludes
quadruped
Quadrupedalism is a form of locomotion where four limbs are used to bear weight and move around. An animal or machine that usually maintains a four-legged posture and moves using all four limbs is said to be a quadruped (from Latin ''quattuor' ...
alism. The
gerenuk
The gerenuk (; so, garanuug; ''Litocranius walleri''), also known as the giraffe gazelle, is a long-necked antelope found in parts of East Africa. The sole member of the genus ''Litocranius'', the gerenuk was first described by the naturalist ...
antelope stands on its hind legs while eating from trees, as did the extinct
giant ground sloth
''Megatherium'' ( ; from Greek () 'great' + () 'beast') is an extinct genus of ground sloths endemic to South America that lived from the Early Pliocene through the end of the Pleistocene. It is best known for the elephant-sized type species ' ...
and
chalicotheres. The
spotted skunk
The genus ''Spilogale'' includes all skunks commonly known as spotted skunks. Currently, there are four accepted extant species: ''S. gracilis'', ''S. putorius'', ''S. pygmaea'', and ''S. angustifrons''. New research, however, proposes that ther ...
will
walk on its front legs when threatened, rearing up on its front legs while facing the attacker so that its
anal glands, capable of spraying an offensive oil, face its attacker.
Limited bipedalism in non-mammals (and non-birds)
Bipedalism is unknown among the
amphibian
Amphibians are tetrapod, four-limbed and ectothermic vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terres ...
s. Among the non-
archosaur reptile
Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...
s bipedalism is rare, but it is found in the "reared-up" running of
lizard
Lizards are a widespread group of squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbaenia alt ...
s such as
agamids
Agamidae is a family of over 300 species of iguanian lizards indigenous to Africa, Asia, Australia, and a few in Southern Europe. Many species are commonly called dragons or dragon lizards.
Overview
Phylogenetically, they may be sister to th ...
and
monitor lizards. Many reptile species will also temporarily adopt bipedalism while fighting. One genus of
basilisk lizard can run bipedally across the surface of water for some distance. Among
arthropod
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...
s,
cockroach
Cockroaches (or roaches) are a paraphyletic group of insects belonging to Blattodea, containing all members of the group except termites. About 30 cockroach species out of 4,600 are associated with human habitats. Some species are well-known as ...
es are known to move bipedally at high speeds. Bipedalism is rarely found outside
terrestrial animal
Terrestrial animals are animals that live predominantly or entirely on land (e.g. cats, dogs, ants, spiders), as compared with aquatic animals, which live predominantly or entirely in the water (e.g. fish, lobsters, octopuses), and amphibians, ...
s, though at least two types of
octopus
An octopus ( : octopuses or octopodes, see below for variants) is a soft-bodied, eight- limbed mollusc of the order Octopoda (, ). The order consists of some 300 species and is grouped within the class Cephalopoda with squids, cuttle ...
walk bipedally on the sea floor using two of their arms, allowing the remaining arms to be used to camouflage the octopus as a mat of algae or a floating coconut.
Evolution of human bipedalism
There are at least twelve distinct hypotheses as to how and why bipedalism evolved in humans, and also some debate as to when. Bipedalism evolved well before the large human brain or the development of stone tools. Bipedal specializations are found in ''
Australopithecus
''Australopithecus'' (, ; ) is a genus of early hominins that existed in Africa during the Late Pliocene and Early Pleistocene. The genus ''Homo'' (which includes modern humans) emerged within ''Australopithecus'', as sister to e.g. ''Austral ...
'' fossils from 4.2 to 3.9 million years ago and recent studies have suggested that obligate bipedal hominid species were present as early as 7 million years ago.
Nonetheless, the evolution of bipedalism was accompanied by significant evolutions in the spine including the forward movement in position of the
foramen magnum, where the spinal cord leaves the cranium.
Recent evidence regarding modern human
sexual dimorphism
Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most ani ...
(physical differences between male and female) in the
lumbar spine
The lumbar vertebrae are, in human anatomy, the five vertebrae between the rib cage and the pelvis. They are the largest segments of the vertebral column and are characterized by the absence of the foramen transversarium within the transverse p ...
has been seen in pre-modern primates such as ''
Australopithecus africanus
''Australopithecus africanus'' is an extinct species of australopithecine which lived between about 3.3 and 2.1 million years ago in the Late Pliocene to Early Pleistocene of South Africa. The species has been recovered from Taung, Sterkfonte ...
''. This dimorphism has been seen as an evolutionary adaptation of females to bear lumbar load better during
pregnancy
Pregnancy is the time during which one or more offspring develops ( gestates) inside a woman's uterus (womb). A multiple pregnancy involves more than one offspring, such as with twins.
Pregnancy usually occurs by sexual intercourse, but ca ...
, an adaptation that non-bipedal primates would not need to make. Adapting bipedalism would have required less shoulder stability, which allowed the shoulder and other limbs to become more independent of each other and adapt for specific suspensory behaviors. In addition to the change in shoulder stability, changing locomotion would have increased the demand for shoulder mobility, which would have propelled the evolution of bipedalism forward. The different hypotheses are not necessarily mutually exclusive and a number of selective forces may have acted together to lead to human bipedalism. It is important to distinguish between adaptations for bipedalism and adaptations for running, which came later still.
The form and function of modern-day humans' upper bodies appear to have evolved from living in a more forested setting. Living in this kind of environment would have made it so that being able to travel arboreally would have been advantageous at the time. It has also been proposed that, like some modern-day apes, early hominins had undergone a
knuckle-walking
Knuckle-walking is a form of quadrupedal walking in which the forelimbs hold the fingers in a partially flexed posture that allows body weight to press down on the ground through the knuckles. Gorillas, bonobos, and chimpanzees use this style o ...
stage prior to adapting the back limbs for bipedality while retaining forearms capable of
grasp
A grasp is an act of taking, holding or seizing firmly with (or as if with) the hand. An example of a grasp is the handshake, wherein two people grasp one of each other's like hands.
In zoology particularly, prehensility is the quality of an ap ...
ing. Numerous causes for the evolution of human bipedalism involve freeing the hands for carrying and using tools,
sexual dimorphism
Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most ani ...
in provisioning, changes in climate and environment (from
jungle
A jungle is land covered with dense forest and tangled vegetation, usually in tropical climates. Application of the term has varied greatly during the past recent century.
Etymology
The word ''jungle'' originates from the Sanskrit word ''ja ...
to
savanna
A savanna or savannah is a mixed woodland-grassland (i.e. grassy woodland) ecosystem characterised by the trees being sufficiently widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach the ground to ...
) that favored a more elevated eye-position, and to reduce the amount of skin exposed to the tropical sun. It is possible that bipedalism provided a variety of benefits to the hominin species, and scientists have suggested multiple reasons for evolution of human bipedalism. There is also not only the question of why the earliest hominins were partially bipedal but also why hominins became more bipedal over time. For example, the postural feeding hypothesis describes how the earliest hominins became bipedal for the benefit of reaching food in trees while the savanna-based theory describes how the late hominins that started to settle on the ground became increasingly bipedal.
Multiple factors
Napier (1963) argued that it is unlikely that a single factor drove the evolution of bipedalism. He stated "''It seems unlikely that any single factor was responsible for such a dramatic change in behaviour. In addition to the advantages of accruing from ability to carry objects – food or otherwise – the improvement of the visual range and the freeing of the hands for purposes of defence and offence may equally have played their part as catalysts."'' Sigmon (1971) demonstrated that chimpanzees exhibit bipedalism in different contexts, and one single factor should be used to explain bipedalism: preadaptation for human bipedalism. Day (1986) emphasized three major pressures that drove evolution of bipedalism: food acquisition, predator avoidance, and reproductive success. Ko (2015) stated that there are two questions main regarding bipedalism 1. Why were the earliest hominins partially bipedal? and 2. Why did hominins become more bipedal over time? He argued that these questions can be answered with combination of prominent theories such as Savanna-based, Postural feeding, and Provisioning.
Savannah-based theory
According to the Savanna-based theory,
hominines came down from the tree's branches and adapted to life on the savanna by walking erect on two feet. The theory suggests that early hominids were forced to adapt to bipedal locomotion on the open savanna after they left the trees. One of the proposed mechanisms was the knuckle-walking hypothesis, which states that human ancestors used quadrupedal locomotion on the savanna, as evidenced by morphological characteristics found in ''Australopithecus anamensis'' and ''Australopithecus afarensis'' forelimbs, and that it is less parsimonious to assume that knuckle walking developed twice in genera ''Pan'' and ''Gorilla'' instead of evolving it once as
synapomorphy
In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have ...
for ''Pan'' and ''Gorilla'' before losing it in Australopithecus. The evolution of an orthograde posture would have been very helpful on a savanna as it would allow the ability to look over tall grasses in order to watch out for predators, or terrestrially hunt and sneak up on prey.
[Dean, F. 2000. Primate diversity. W.W. Norton & Company, Inc: New York. Print.] It was also suggested in P. E. Wheeler's "The evolution of bipedality and loss of functional body hair in hominids", that a possible advantage of bipedalism in the savanna was reducing the amount of surface area of the body exposed to the sun, helping regulate body temperature. In fact,
Elizabeth Vrba
Elisabeth S. Vrba (born 17 May 1942) is a paleontologist at Yale University who developed the turnover-pulse hypothesis.
Education
Vrba earned her Ph.D. in Zoology and Palaeontology at the University of Cape Town, in 1974. Vrba studied zoolog ...
's
turnover pulse hypothesis supports the savanna-based theory by explaining the shrinking of forested areas due to global warming and cooling, which forced animals out into the open grasslands and caused the need for hominids to acquire bipedality.
[Shreeve, James]
"Sunset on the savanna"
, ''Discover'', 1996.
Others state hominines had already achieved the bipedal adaptation that was used in the savanna. The fossil evidence reveals that early bipedal hominins were still adapted to climbing trees at the time they were also
walking
Walking (also known as ambulation) is one of the main gaits of terrestrial locomotion among legged animals. Walking is typically slower than running and other gaits. Walking is defined by an ' inverted pendulum' gait in which the body vaults ...
upright. It is possible that bipedalism evolved in the trees, and was later applied to the savanna as a vestigial trait. Humans and orangutans are both unique to a bipedal reactive adaptation when climbing on thin branches, in which they have increased hip and knee extension in relation to the diameter of the branch, which can increase an arboreal feeding range and can be attributed to a convergent evolution of bipedalism evolving in arboreal environments. Hominine fossils found in dry grassland environments led anthropologists to believe hominines lived, slept, walked upright, and died only in those environments because no hominine fossils were found in forested areas. However, fossilization is a rare occurrence—the conditions must be just right in order for an organism that dies to become fossilized for somebody to find later, which is also a rare occurrence. The fact that no hominine fossils were found in forests does not ultimately lead to the conclusion that no hominines ever died there. The convenience of the savanna-based theory caused this point to be overlooked for over a hundred years.
Some of the fossils found actually showed that there was still an adaptation to arboreal life. For example,
Lucy
Lucy is an English feminine given name derived from the Latin masculine given name Lucius with the meaning ''as of light'' (''born at dawn or daylight'', maybe also ''shiny'', or ''of light complexion''). Alternative spellings are Luci, Luce, Lu ...
, the famous ''
Australopithecus afarensis
''Australopithecus afarensis'' is an extinct species of australopithecine which lived from about 3.9–2.9 million years ago (mya) in the Pliocene of East Africa. The first fossils were discovered in the 1930s, but major fossil finds would not ...
'', found in Hadar in Ethiopia, which may have been forested at the time of Lucy's death, had curved fingers that would still give her the ability to grasp tree branches, but she walked bipedally. "
Little Foot
"Little Foot" (Stw 573) is the nickname given to a nearly complete Australopithecus fossil skeleton found in 1994–1998 in the cave system of Sterkfontein, South Africa.
Originally nicknamed "little foot" in 1995 when four ankle bones in a mus ...
," a nearly-complete specimen of ''
Australopithecus africanus
''Australopithecus africanus'' is an extinct species of australopithecine which lived between about 3.3 and 2.1 million years ago in the Late Pliocene to Early Pleistocene of South Africa. The species has been recovered from Taung, Sterkfonte ...
'', has a divergent big toe as well as the ankle strength to walk upright. "Little Foot" could grasp things using his feet like an ape, perhaps tree branches, and he was bipedal. Ancient pollen found in the soil in the locations in which these fossils were found suggest that the area used to be much more wet and covered in thick vegetation and has only recently become the arid desert it is now.
Traveling efficiency hypothesis
An alternative explanation is that the mixture of savanna and scattered forests increased terrestrial travel by proto-humans between clusters of trees, and bipedalism offered greater efficiency for long-distance travel between these clusters than quadrupedalism. In an experiment monitoring chimpanzee metabolic rate via oxygen consumption, it was found that the quadrupedal and bipedal energy costs were very similar, implying that this transition in early ape-like ancestors would not have been very difficult or energetically costing. This increased travel efficiency is likely to have been selected for as it assisted foraging across widely dispersed resources.
Postural feeding hypothesis
The postural feeding hypothesis has been recently supported by Dr. Kevin Hunt, a professor at
Indiana University
Indiana University (IU) is a system of public universities in the U.S. state of Indiana.
Campuses
Indiana University has two core campuses, five regional campuses, and two regional centers under the administration of IUPUI.
*Indiana Universi ...
. This hypothesis asserts that chimpanzees were only bipedal when they eat. While on the ground, they would reach up for fruit hanging from small trees and while in trees, bipedalism was used to reach up to grab for an overhead branch. These bipedal movements may have evolved into regular habits because they were so convenient in obtaining food. Also, Hunt's hypotheses states that these movements coevolved with chimpanzee arm-hanging, as this movement was very effective and efficient in harvesting food. When analyzing fossil anatomy, ''
Australopithecus afarensis
''Australopithecus afarensis'' is an extinct species of australopithecine which lived from about 3.9–2.9 million years ago (mya) in the Pliocene of East Africa. The first fossils were discovered in the 1930s, but major fossil finds would not ...
'' has very similar features of the hand and shoulder to the chimpanzee, which indicates hanging arms. Also, the ''
Australopithecus
''Australopithecus'' (, ; ) is a genus of early hominins that existed in Africa during the Late Pliocene and Early Pleistocene. The genus ''Homo'' (which includes modern humans) emerged within ''Australopithecus'', as sister to e.g. ''Austral ...
'' hip and hind limb very clearly indicate bipedalism, but these fossils also indicate very inefficient locomotive movement when compared to
human
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
s. For this reason, Hunt argues that bipedalism evolved more as a terrestrial feeding posture than as a walking posture.
A related study conducted by
University of Birmingham
, mottoeng = Through efforts to heights
, established = 1825 – Birmingham School of Medicine and Surgery1836 – Birmingham Royal School of Medicine and Surgery1843 – Queen's College1875 – Mason Science College1898 – Mason Univers ...
, Professor Susannah Thorpe examined the most arboreal great ape, the
orangutan
Orangutans are great apes native to the rainforests of Indonesia and Malaysia. They are now found only in parts of Borneo and Sumatra, but during the Pleistocene they ranged throughout Southeast Asia and South China. Classified in the genu ...
, holding onto supporting branches in order to navigate branches that were too flexible or unstable otherwise. In more than 75 percent of observations, the orangutans used their forelimbs to stabilize themselves while navigating thinner branches. Increased fragmentation of forests where A. afarensis as well as other ancestors of modern humans and other apes resided could have contributed to this increase of bipedalism in order to navigate the diminishing forests. Findings also could shed light on discrepancies observed in the anatomy of A. afarensis, such as the ankle joint, which allowed it to "wobble" and long, highly flexible forelimbs. If bipedalism started from upright navigation in trees, it could explain both increased flexibility in the ankle as well as long forelimbs which grab hold of branches.
Provisioning model
One theory on the origin of bipedalism is the behavioral model presented by
C. Owen Lovejoy, known as "male provisioning".
Lovejoy theorizes that the
evolution
Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
of bipedalism was linked to monogamy. In the face of long inter-birth intervals and low reproductive rates typical of the apes, early
hominids
The Hominidae (), whose members are known as the great apes or hominids (), are a taxonomic family of primates that includes eight extant species in four genera: '' Pongo'' (the Bornean, Sumatran and Tapanuli orangutan); ''Gorilla'' (the ...
engaged in pair-bonding that enabled greater parental effort directed towards rearing offspring. Lovejoy proposes that male provisioning of food would improve the offspring survivorship and increase the pair's reproductive rate. Thus the male would leave his mate and offspring to search for food and return carrying the food in his arms walking on his legs. This model is supported by the reduction ("feminization") of the male canine teeth in early hominids such as ''
Sahelanthropus tchadensis
''Sahelanthropus tchadensis'' is an extinct species of the Homininae (African apes) dated to about , during the Miocene epoch. The species, and its genus ''Sahelanthropus'', was announced in 2002, based mainly on a partial cranium, nicknamed ' ...
'' and ''
Ardipithecus ramidus
''Ardipithecus ramidus'' is a species of australopithecine from the Afar region of Early Pliocene Ethiopia 4.4 million years ago (mya). ''A. ramidus'', unlike modern hominids, has adaptations for both walking on two legs ( bipedality) and life i ...
'', which along with low body size dimorphism in ''Ardipithecus'' and ''Australopithecus'', suggests a reduction in inter-male antagonistic behavior in early hominids.
[Lovejoy CO. Science. 2009 326(5949):74e1-8.] In addition, this model is supported by a number of modern human traits associated with concealed ovulation (permanently enlarged breasts, lack of
sexual swelling
Sexual swellings are enlarged areas of genital and perineal skin occurring in some female primates that vary in size over the course of the menstrual cycle. Thought to be an honest signal of fertility, male primates are attracted to these swell ...
) and low sperm competition (moderate sized testes, low sperm mid-piece volume) that argues against recent adaptation to a polygynous reproductive system.
However, this model has been debated, as others have argued that early bipedal hominids were instead polygynous. Among most monogamous primates, males and females are about the same size. That is
sexual dimorphism
Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most ani ...
is minimal, and other studies have suggested that ''
Australopithecus afarensis
''Australopithecus afarensis'' is an extinct species of australopithecine which lived from about 3.9–2.9 million years ago (mya) in the Pliocene of East Africa. The first fossils were discovered in the 1930s, but major fossil finds would not ...
'' males were nearly twice the weight of females. However, Lovejoy's model posits that the larger range a provisioning male would have to cover (to avoid competing with the female for resources she could attain herself) would select for increased male body size to limit predation risk. Furthermore, as the species became more bipedal, specialized feet would prevent the infant from conveniently clinging to the mother - hampering the mother's freedom and thus make her and her offspring more dependent on resources collected by others. Modern monogamous primates such as gibbons tend to be also territorial, but fossil evidence indicates that ''Australopithecus afarensis'' lived in large groups. However, while both gibbons and hominids have reduced canine sexual dimorphism, female gibbons enlarge ('masculinize') their canines so they can actively share in the defense of their home territory. Instead, the reduction of the male hominid canine is consistent with reduced inter-male aggression in a pair-bonded though group living primate.
Early bipedalism in homininae model
Recent studies of 4.4 million years old ''
Ardipithecus ramidus
''Ardipithecus ramidus'' is a species of australopithecine from the Afar region of Early Pliocene Ethiopia 4.4 million years ago (mya). ''A. ramidus'', unlike modern hominids, has adaptations for both walking on two legs ( bipedality) and life i ...
'' suggest bipedalism. It is thus possible that bipedalism evolved very early in
homininae
Homininae (), also called "African hominids" or "African apes", is a subfamily of Hominidae. It includes two tribes, with their extant as well as extinct species: 1) the tribe Hominini (with the genus ''Homo'' including modern humans and numerou ...
and was reduced in chimpanzee and gorilla when they became more specialized. Other recent studies of the foot structure of ''
Ardipithecus ramidus
''Ardipithecus ramidus'' is a species of australopithecine from the Afar region of Early Pliocene Ethiopia 4.4 million years ago (mya). ''A. ramidus'', unlike modern hominids, has adaptations for both walking on two legs ( bipedality) and life i ...
'' suggest that the species was closely related to African-ape ancestors. This possibly provides a species close to the true connection between fully bipedal hominins and quadruped apes. According to Richard Dawkins in his book "
The Ancestor's Tale
''The Ancestor's Tale: A Pilgrimage to the Dawn of Life'' is a science book by Richard Dawkins and Yan Wong on the subject of evolution, which follows the path of humans backwards through evolutionary history, describing some of humanity's cou ...
", chimps and bonobos are descended from ''
Australopithecus
''Australopithecus'' (, ; ) is a genus of early hominins that existed in Africa during the Late Pliocene and Early Pleistocene. The genus ''Homo'' (which includes modern humans) emerged within ''Australopithecus'', as sister to e.g. ''Austral ...
'' gracile type species while gorillas are descended from ''
Paranthropus''. These apes may have once been bipedal, but then lost this ability when they were forced back into an arboreal habitat, presumably by those australopithecines from whom eventually evolved hominins. Early
hominine
Homininae (), also called "African hominids" or "African apes", is a subfamily of Hominidae. It includes two tribes, with their extant as well as extinct species: 1) the tribe Hominini (with the genus ''Homo'' including modern humans and numerou ...
s such as ''
Ardipithecus ramidus
''Ardipithecus ramidus'' is a species of australopithecine from the Afar region of Early Pliocene Ethiopia 4.4 million years ago (mya). ''A. ramidus'', unlike modern hominids, has adaptations for both walking on two legs ( bipedality) and life i ...
'' may have possessed an arboreal type of bipedalism that later independently evolved towards knuckle-walking in chimpanzees and gorillas and towards efficient walking and running in modern humans (see figure). It is also proposed that one cause of
Neanderthal extinction
Neanderthals became extinct around 40,000 years ago. This timing, based on research published in ''Nature'' in 2014, is much earlier than previous estimates, and derives from improved radiocarbon dating methods analyzing 40 sites from Spain to ...
was a less efficient running.
Warning display (aposematic) model
Joseph Jordania from the University of Melbourne recently (2011) suggested that bipedalism was one of the central elements of the general defense strategy of early hominids, based on
aposematism
Aposematism is the advertising by an animal to potential predators that it is not worth attacking or eating. This unprofitability may consist of any defences which make the prey difficult to kill and eat, such as toxicity, venom, foul taste or ...
, or
warning display and
intimidation
Intimidation is to "make timid or make fearful"; or to induce fear. This includes intentional behaviors of forcing another person to experience general discomfort such as humiliation, embarrassment, inferiority, limited freedom, etc and the victi ...
of potential predators and competitors with exaggerated visual and audio signals. According to this model, hominids were trying to stay as visible and as loud as possible all the time. Several morphological and behavioral developments were employed to achieve this goal: upright bipedal posture, longer legs, long tightly coiled hair on the top of the head,
body painting
Body painting is a form of body art where artwork is painted directly onto the human skin. Unlike tattoos and other forms of body art, body painting is temporary, lasting several hours or sometimes up to a few weeks (in the case of mehndi or " ...
, threatening synchronous body movements, loud voice and extremely loud rhythmic singing/stomping/drumming on external subjects. Slow locomotion and strong body odor (both characteristic for hominids and humans) are other features often employed by aposematic species to advertise their non-profitability for potential predators.
Other behavioural models
There are a variety of ideas which promote a specific change in behaviour as the key driver for the evolution of hominid bipedalism. For example, Wescott (1967) and later Jablonski & Chaplin (1993) suggest that bipedal threat displays could have been the transitional behaviour which led to some groups of apes beginning to adopt bipedal postures more often. Others (e.g. Dart 1925) have offered the idea that the need for more vigilance against predators could have provided the initial motivation. Dawkins (e.g. 2004) has argued that it could have begun as a kind of fashion that just caught on and then escalated through sexual selection. And it has even been suggested (e.g. Tanner 1981:165) that male phallic display could have been the initial incentive, as well as increased sexual signaling in upright female posture.
Thermoregulatory model
The thermoregulatory model explaining the origin of bipedalism is one of the simplest theories so far advanced, but it is a viable explanation. Dr. Peter Wheeler, a professor of evolutionary biology, proposes that bipedalism raises the amount of body surface area higher above the ground which results in a reduction in heat gain and helps heat dissipation. When a hominid is higher above the ground, the organism accesses more favorable wind speeds and temperatures. During heat seasons, greater wind flow results in a higher heat loss, which makes the organism more comfortable. Also, Wheeler explains that a vertical posture minimizes the direct exposure to the sun whereas quadrupedalism exposes more of the body to direct exposure. Analysis and interpretations of
Ardipithecus
''Ardipithecus'' is a genus of an extinct hominine that lived during the Late Miocene and Early Pliocene epochs in the Afar Depression, Ethiopia. Originally described as one of the earliest ancestors of humans after they diverged from the chimp ...
reveal that this hypothesis needs modification to consider that the
forest
A forest is an area of land dominated by trees. Hundreds of definitions of forest are used throughout the world, incorporating factors such as tree density, tree height, land use, legal standing, and ecological function. The United Nations' ...
and
woodland
A woodland () is, in the broad sense, land covered with trees, or in a narrow sense, synonymous with wood (or in the U.S., the ''plurale tantum'' woods), a low-density forest forming open habitats with plenty of sunlight and limited shade (see ...
environmental
preadaptation
Exaptation and the related term co-option describe a shift in the function of a trait during evolution. For example, a trait can evolve because it served one particular function, but subsequently it may come to serve another. Exaptations are common ...
of early-stage
hominid bipedalism preceded further refinement of bipedalism by the pressure of
natural selection
Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Charle ...
. This then allowed for the more efficient exploitation of the hotter conditions
ecological niche
In ecology, a niche is the match of a species to a specific environmental condition.
Three variants of ecological niche are described by
It describes how an organism or population responds to the distribution of resources and competitors (for ...
, rather than the hotter conditions being hypothetically bipedalism's initial stimulus. A feedback mechanism from the advantages of bipedality in hot and open habitats would then in turn make a forest preadaptation solidify as a permanent state.
Carrying models
Charles Darwin wrote that "Man could not have attained his present dominant position in the world without the use of his hands, which are so admirably adapted to the act of obedience of his will". Darwin (1871:52) and many models on bipedal origins are based on this line of thought. Gordon Hewes (1961) suggested that the carrying of meat "over considerable distances" (Hewes 1961:689) was the key factor. Isaac (1978) and Sinclair et al. (1986) offered modifications of this idea, as indeed did Lovejoy (1981) with his "provisioning model" described above. Others, such as Nancy Tanner (1981), have suggested that infant carrying was key, while others again have suggested stone tools and weapons drove the change. This stone-tools theory is very unlikely, as though ancient humans were known to hunt, the discovery of tools was not discovered for thousands of years after the origin of bipedalism, chronologically precluding it from being a driving force of evolution. (Wooden tools and spears fossilize poorly and therefore it is difficult to make a judgment about their potential usage.)
Wading models
The observation that large primates, including especially the great apes, that predominantly move quadrupedally on dry land, tend to switch to bipedal locomotion in waist deep water, has led to the idea that the origin of human bipedalism may have been influenced by waterside environments. This idea, labelled "the wading hypothesis", was originally suggested by the
Oxford
Oxford () is a city in England. It is the county town and only city of Oxfordshire. In 2020, its population was estimated at 151,584. It is north-west of London, south-east of Birmingham and north-east of Bristol. The city is home to the ...
marine biologist
Marine biology is the scientific study of the biology of marine life, organisms in the sea. Given that in biology many scientific classification, phyla, family (biology), families and genera have some species that live in the sea and others th ...
Alister Hardy
Sir Alister Clavering Hardy (10 February 1896 – 22 May 1985) was an English marine biologist, an expert on marine ecosystems spanning organisms from zooplankton to whales. He had the artistic skill to illustrate his books with his own drawing ...
who said: "It seems to me likely that Man learnt to stand erect first in water and then, as his balance improved, he found he became better equipped for standing up on the shore when he came out, and indeed also for running." It was then promoted by
Elaine Morgan
Elaine Morgan OBE, FRSL (7 November 1920 – 12 July 2013), was a Welsh writer for television and the author of several books on evolutionary anthropology. She advocated the aquatic ape hypothesis, which she advocated as a corrective to what ...
, as part of the
aquatic ape hypothesis
The aquatic ape hypothesis (AAH), also referred to as aquatic ape theory (AAT) or the waterside hypothesis of human evolution, postulates that the ancestors of modern humans took a divergent evolutionary pathway from the other great apes by becom ...
, who cited bipedalism among a cluster of other human traits unique among primates, including voluntary control of breathing, hairlessness and subcutaneous fat.
The "
aquatic ape hypothesis
The aquatic ape hypothesis (AAH), also referred to as aquatic ape theory (AAT) or the waterside hypothesis of human evolution, postulates that the ancestors of modern humans took a divergent evolutionary pathway from the other great apes by becom ...
", as originally formulated, has not been accepted or considered a serious theory within the anthropological scholarly community.
Others, however, have sought to promote wading as a factor in the origin of human bipedalism without referring to further ("aquatic ape" related) factors. Since 2000
Carsten Niemitz
Carsten Niemitz (born 29 September 1945 in Dessau) is a German anatomist, ethologist, and human evolutionary biologist.
Life and work
Niemitz studied biology, mathematics, medicine and art history at the Universities of Giessen, Freiburg, Gött ...
has published a series of papers and a book
on a variant of the wading hypothesis, which he calls the "amphibian generalist theory" (german: Amphibische Generalistentheorie).
Other theories have been proposed that suggest wading and the exploitation of aquatic food sources (providing essential nutrients for human brain evolution or critical fallback foods) may have exerted evolutionary pressures on human ancestors promoting adaptations which later assisted full-time bipedalism. It has also been thought that consistent water-based food sources had developed early hominid dependency and facilitated dispersal along seas and rivers.
Consequences
Prehistoric fossil records show that early hominins first developed bipedalism before being followed by an increase in brain size. The consequences of these two changes in particular resulted in painful and difficult labor due to the increased favor of a narrow pelvis for bipedalism being countered by larger heads passing through the constricted birth canal. This phenomenon is commonly known as the
obstetrical dilemma.
Non-human primates habitually deliver their young on their own, but the same cannot be said for modern-day humans. Isolated
birth
Birth is the act or process of bearing or bringing forth offspring, also referred to in technical contexts as parturition. In mammals, the process is initiated by hormones which cause the muscular walls of the uterus to contract, expelling the f ...
appears to be rare and actively avoided cross-culturally, even if birthing methods may differ between said cultures. This is due to the fact that the narrowing of the hips and the change in the pelvic angle caused a discrepancy in the ratio of the size of the head to the
birth canal
In mammals, the vagina is the elastic, muscular part of the female genital tract. In humans, it extends from the vestibule to the cervix. The outer vaginal opening is normally partly covered by a thin layer of mucosal tissue called the hyme ...
. The result of this is that there is greater difficulty in birthing for hominins in general, let alone to be doing it by oneself.
Physiology
Bipedal movement occurs in a number of ways and requires many mechanical and neurological adaptations. Some of these are described below.
Biomechanics
Standing
Energy-efficient means of standing bipedally involve constant adjustment of balance, and of course these must avoid
overcorrection. The difficulties associated with simple standing in upright humans are highlighted by the greatly increased risk of falling present in the elderly, even with minimal reductions in control system effectiveness.
Shoulder stability
Shoulder stability would decrease with the evolution of bipedalism. Shoulder mobility would increase because the need for a stable shoulder is only present in arboreal habitats. Shoulder mobility would support suspensory locomotion behaviors which are present in human bipedalism. The forelimbs are freed from weight-bearing requirements, which makes the shoulder a place of evidence for the evolution of bipedalism.
Walking
![Illu vertebral column](https://upload.wikimedia.org/wikipedia/commons/5/53/Illu_vertebral_column.svg)
Unlike non-human apes that are able to practice bipedality such as
''Pan'' and ''
Gorilla
Gorillas are herbivorous, predominantly ground-dwelling great apes that inhabit the tropical forests of equatorial Africa. The genus ''Gorilla'' is divided into two species: the eastern gorilla and the western gorilla, and either four or fi ...
'', hominins have the ability to move bipedally without the utilization of a bent-hip-bent-knee (BHBK)
gait, which requires the engagement of both the hip and the knee joints. This human ability to walk is made possible by the
spinal curvature humans have that non-human apes do not. Rather, walking is characterized by an "inverted pendulum" movement in which the
center of gravity
In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force ma ...
vaults over a stiff leg with each step.
Force plate
Force platforms or force plates are measuring instruments that measure the ground reaction forces generated by a body standing on or moving across them, to quantify balance, gait and other parameters of biomechanics. Most common areas of appli ...
s can be used to quantify the whole-body kinetic & potential energy, with walking displaying an out-of-phase relationship indicating exchange between the two.
This model applies to all walking organisms regardless of the number of legs, and thus bipedal locomotion does not differ in terms of whole-body kinetics.
In humans, walking is composed of several separate processes:
*Vaulting over a stiff stance leg
*Passive ballistic movement of the swing leg
*A short 'push' from the ankle prior to toe-off, propelling the swing leg
*Rotation of the hips about the axis of the spine, to increase stride length
*Rotation of the hips about the horizontal axis to improve balance during stance
Running
![Running 3](https://upload.wikimedia.org/wikipedia/commons/3/36/Running_3.jpg)
Early hominins underwent post-cranial changes in order to better adapt to bipedality, especially running. One of these changes is having longer hindlimbs proportional to the forelimbs and their effects. As previously mentioned, longer hindlimbs assist in
thermoregulation
Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperature ...
by reducing the total surface area exposed to direct sunlight while simultaneously allowing for more space for cooling winds. Additionally, having longer limbs is more energy-efficient, since longer limbs mean that overall muscle strain is lessened. Better energy efficiency, in turn, means higher
endurance
Endurance (also related to sufferance, resilience, constitution, fortitude, and hardiness) is the ability of an organism to exert itself and remain active for a long period of time, as well as its ability to resist, withstand, recover from a ...
, particularly when running long distances.
Running is characterized by a spring-mass movement.
Kinetic and potential energy are in phase, and the energy is stored & released from a spring-like limb during foot contact,
achieved by the
plantar arch
The plantar arch is a circulatory anastomosis formed from:
* deep plantar artery, from the dorsalis pedis - a.k.a. dorsal artery of the foot
* lateral plantar artery
The plantar arch supplies the underside, or sole, of the foot.
The plantar ar ...
and the
Achilles tendon
The Achilles tendon or heel cord, also known as the calcaneal tendon, is a tendon at the back of the lower leg, and is the thickest in the human body. It serves to attach the plantaris, gastrocnemius (calf) and soleus muscles to the calcaneus ( ...
in the foot and leg, respectively.
Again, the whole-body kinetics are similar to animals with more limbs.
Musculature
Bipedalism requires strong leg muscles, particularly in the thighs. Contrast in
domesticated
Domestication is a sustained multi-generational relationship in which humans assume a significant degree of control over the reproduction and care of another group of organisms to secure a more predictable supply of resources from that group. A ...
poultry
Poultry () are domesticated birds kept by humans for their eggs, their meat or their feathers. These birds are most typically members of the superorder Galloanserae (fowl), especially the order Galliformes (which includes chickens, quails, a ...
the well muscled legs, against the small and bony wings. Likewise in humans, the
quadriceps
The quadriceps femoris muscle (, also called the quadriceps extensor, quadriceps or quads) is a large muscle group that includes the four prevailing muscles on the front of the thigh. It is the sole extensor muscle of the knee, forming a large ...
and
hamstring
In human anatomy, a hamstring () is any one of the three posterior thigh muscles in between the hip and the knee (from medial to lateral: semimembranosus, semitendinosus and biceps femoris). The hamstrings are susceptible to injury.
In quadrupe ...
muscles of the thigh are both so crucial to bipedal activities that each alone is much larger than the well-developed biceps of the arms. In addition to the leg muscles, the increased size of the
gluteus maximus
The gluteus maximus is the main extensor muscle of the hip. It is the largest and outermost of the three gluteal muscles and makes up a large part of the shape and appearance of each side of the hips. It is the single largest muscle in the human ...
in humans is an important adaptation as it provides support and stability to the trunk and lessens the amount of stress on the joints when running.
Respiration
Quadrupeds
Quadrupedalism is a form of locomotion where four limbs are used to bear weight and move around. An animal or machine that usually maintains a four-legged posture and moves using all four limbs is said to be a quadruped (from Latin ''quattuor' ...
, have more restrictive breathing
respire while moving than do bipedal humans.
"Quadrupedal species normally synchronize the locomotor and respiratory cycles at a constant ratio of 1:1 (strides per breath) in both the trot and gallop. Human runners differ from quadrupeds in that while running they employ several phase-locked patterns (4:1, 3:1, 2:1, 1:1, 5:2, and 3:2), although a 2:1 coupling ratio appears to be favored. Even though the evolution of bipedal gait has reduced the mechanical constraints on respiration in man, thereby permitting greater flexibility in breathing pattern, it has seemingly not eliminated the need for the synchronization of respiration and body motion during sustained running."
Respiration through bipedality means that there is better breath control in bipeds, which can be associated with brain growth. The modern brain utilizes approximately 20% of energy input gained through breathing and eating, as opposed to species like chimpanzees who use up twice as much energy as humans for the same amount of movement. This excess energy, leading to brain growth, also leads to the development of
verbal communication
Linguistics is the scientific study of human language. It is called a scientific study because it entails a comprehensive, systematic, objective, and precise analysis of all aspects of language, particularly its nature and structure. Linguis ...
. This is because breath control means that the muscles associated with breathing can be manipulated into creating sounds. This means that the onset of bipedality, leading to more efficient breathing, may be related to the origin of verbal language.
Bipedal robots
![Asimo](https://upload.wikimedia.org/wikipedia/commons/f/fd/Asimo.jpg)
For nearly the whole of the 20th century, bipedal
robot
A robot is a machine—especially one programmable by a computer—capable of carrying out a complex series of actions automatically. A robot can be guided by an external control device, or the control may be embedded within. Robots may be c ...
s were very difficult to construct and
robot locomotion
Robot locomotion is the collective name for the various methods that robots use to transport themselves from place to place.
Wheeled robots are typically quite energy efficient and simple to control. However, other forms of locomotion may be more ...
involved only wheels, treads, or multiple legs. Recent cheap and compact computing power has made two-legged robots more feasible. Some notable biped robots are
ASIMO,
HUBO
HUBO ( ko, 휴보; designated KHR-3) is a walking humanoid robot, head mounted on a life-size walking bipedal frame, developed by the Korea Advanced Institute of Science and Technology (KAIST) and released on January 6, 2005. According to ...
,
MABEL and
QRIO. Recently, spurred by the success of creating a fully passive, un-powered bipedal walking robot,
those working on such machines have begun using principles gleaned from the study of human and animal locomotion, which often relies on passive mechanisms to minimize power consumption.
See also
*
Allometry
Allometry is the study of the relationship of body size to shape, anatomy, physiology and finally behaviour, first outlined by Otto Snell in 1892, by D'Arcy Thompson in 1917 in '' On Growth and Form'' and by Julian Huxley in 1932.
Overview
Allom ...
*
Orthograde posture
*
Quadrupedalism
Quadrupedalism is a form of locomotion where four limbs are used to bear weight and move around. An animal or machine that usually maintains a four-legged posture and moves using all four limbs is said to be a quadruped (from Latin ''quattuor ...
Notes
References
Sources
*
Further reading
*
Darwin, C., "
The Descent of Man and Selection in Relation to Sex
''The Descent of Man, and Selection in Relation to Sex'' is a book by English naturalist Charles Darwin, first published in 1871, which applies evolutionary theory to human evolution, and details his theory of sexual selection, a form of biolo ...
", Murray (London), (1871).
*
Dart, R. A., "Australopithecus africanus: The Ape Man of South Africa" ''Nature'', 145, 195–199, (1925).
*
Dawkins, R., "
The Ancestor's Tale
''The Ancestor's Tale: A Pilgrimage to the Dawn of Life'' is a science book by Richard Dawkins and Yan Wong on the subject of evolution, which follows the path of humans backwards through evolutionary history, describing some of humanity's cou ...
", Weidenfeld and Nicolson (London), (2004).
*DeSilva, J., "First Steps: How Upright Walking Made Us Human" HarperCollins (New York), (2021)
*Hewes, G. W., "Food Transport and the Origin of Hominid Bipedalism" ''American Anthropologist'', 63, 687–710, (1961).
*Hunt, K. D., "The Evolution of Human Bipedality" ''Journal of Human Evolution'', 26, 183–202, (1994).
*
Isaac, G. I., "The Archeological Evidence for the Activities of Early African Hominids" In:Early Hominids of Africa (Jolly, C.J. (Ed.)), Duckworth (London), 219–254, (1978).
*
*
*Tanner, N. M., "On Becoming Human", Cambridge University Press (Cambridge), (1981)
*
*Wheeler, P. E. (1984) "The Evolution of Bipedality and Loss of Functional Body Hair in Hominoids." ''Journal of Human Evolution'', 13, 91–98,
*
External links
The Origin of BipedalismHuman Timeline (Interactive)–
Smithsonian,
National Museum of Natural History
The National Museum of Natural History is a natural history museum administered by the Smithsonian Institution, located on the National Mall in Washington, D.C., United States. It has free admission and is open 364 days a year. In 2021, with 7 ...
(August 2016)
{{portal bar, Evolutionary biology, Science
Terrestrial locomotion
Animal anatomy
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