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''Australopithecus afarensis'' is an extinct
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...
of australopithecine which lived from about 3.9–2.9 million years ago (mya) in the Pliocene of East Africa. The first
fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
s were discovered in the 1930s, but major fossil finds would not take place until the 1970s. From 1972 to 1977, the International Afar Research Expedition—led by anthropologists
Maurice Taieb Maurice Taieb (22 July 1935 – 23 July 2021) was a French geologist and paleoanthropologist. He discovered the Hadar formation, recognized its potential importance to paleoanthropology and founded the International Afar Research Expedition (IAR ...
,
Donald Johanson Donald Carl Johanson (born June 28, 1943) is an American paleoanthropologist. He is known for discovering, with Yves Coppens and Maurice Taieb, the fossil of a female hominin australopithecine known as "Lucy" in the Afar Triangle region of Hada ...
and Yves Coppens—unearthed several hundreds of hominin specimens in Hadar,
Ethiopia Ethiopia, , om, Itiyoophiyaa, so, Itoobiya, ti, ኢትዮጵያ, Ítiyop'iya, aa, Itiyoppiya officially the Federal Democratic Republic of Ethiopia, is a landlocked country in the Horn of Africa. It shares borders with Eritrea to the ...
, the most significant being the exceedingly well-preserved skeleton AL 288-1 (" Lucy") and the site AL 333 ("the First Family"). Beginning in 1974, Mary Leakey led an expedition into Laetoli,
Tanzania Tanzania (; ), officially the United Republic of Tanzania ( sw, Jamhuri ya Muungano wa Tanzania), is a country in East Africa within the African Great Lakes region. It borders Uganda to the north; Kenya to the northeast; Comoro Islands ...
, and notably recovered fossil trackways. In 1978, the species was first described, but this was followed by arguments for splitting the wealth of specimens into different species given the wide range of variation which had been attributed to sexual dimorphism (normal differences between males and females). ''A. afarensis'' probably descended from '' A. anamensis'' and is hypothesised to have given rise to ''
Homo ''Homo'' () is the genus that emerged in the (otherwise extinct) genus '' Australopithecus'' that encompasses the extant species ''Homo sapiens'' ( modern humans), plus several extinct species classified as either ancestral to or closely rela ...
'', though the latter is debated. ''A. afarensis'' had a tall face, a delicate brow ridge, and prognathism (the jaw jutted outwards). The jawbone was quite robust, similar to that of
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 ...
s. The living size of ''A. afarensis'' is debated, with arguments for and against marked size differences between males and females. Lucy measured perhaps in height and , but she was rather small for her species. In contrast, a presumed male was estimated at and . A perceived difference in male and female size may simply be sampling bias. The leg bones as well as the Laetoli fossil trackways suggest ''A. afarensis'' was a competent biped, though somewhat less efficient at walking than humans. The arm and shoulder bones have some similar aspects to those of orangutans and gorillas, which has variously been interpreted as either evidence of partial tree-dwelling ( arboreality), or basal traits inherited from the chimpanzee–human last common ancestor with no adaptive functionality. ''A. afarensis'' was probably a generalist omnivore of both C3 forest plants and C4 CAM savanna plants—and perhaps creatures which ate such plants—and was able to exploit a variety of different food sources. Similarly, ''A. afarensis'' appears to have inhabited a wide range of habitats with no real preference, inhabiting open grasslands or woodlands, shrublands, and lake- or riverside forests. Potential evidence of
stone tool A stone tool is, in the most general sense, any tool made either partially or entirely out of stone. Although stone tool-dependent societies and cultures still exist today, most stone tools are associated with prehistoric (particularly Stone A ...
use would indicate meat was also a dietary component. Marked sexual dimorphism in primates typically corresponds to a polygynous society and low dimorphism to
monogamy Monogamy ( ) is a form of dyadic relationship in which an individual has only one partner during their lifetime. Alternately, only one partner at any one time ( serial monogamy) — as compared to the various forms of non-monogamy (e.g., pol ...
, but the group dynamics of early hominins is difficult to predict with accuracy. Early hominins may have fallen prey to the large carnivores of the time, such as big cats and hyenas.


Taxonomy


Research history

Beginning in the 1930s, some of the most ancient hominin remains of the time dating to 3.8–2.9 million years ago were recovered from East Africa. Because '' Australopithecus africanus'' fossils were commonly being discovered throughout the 1920s and '40s in South Africa, these remains were often provisionally classified as ''Australopithecus''
aff. ''Species affinis'' (commonly abbreviated to: ''sp.'', ''aff.'', or ''affin.'') is taxonomic terminology in zoology and botany. In open nomenclature it indicates that available material or evidence suggests that the proposed species is related to, h ...
''africanus''. The first to identify a human fossil was German explorer Ludwig Kohl-Larsen in 1939 by the headwaters of the Gerusi River (near Laetoli, Tanzania), who encountered a maxilla. In 1948, German palaeontologist
Edwin Hennig Edwin Hennig (27 April 1882 – 12 November 1977) was a German paleontologist. Career Edwin Hennig was one of five children of a merchant who died when Hennig was 10 years old. Starting in 1902, Hennig studied natural sciences, anthropolog ...
proposed classifying it into a new
genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...
, "''Praeanthropus''", but he failed to give a species name. In 1950, German anthropologist
Hans Weinert Hans may refer to: __NOTOC__ People * Hans (name), a masculine given name * Hans Raj Hans, Indian singer and politician ** Navraj Hans, Indian singer, actor, entrepreneur, cricket player and performer, son of Hans Raj Hans ** Yuvraj Hans, Punjabi a ...
proposed classifying it as ''Meganthropus africanus'', but this was largely ignored. In 1955, M.S. Şenyürek proposed the combination ''Praeanthropus africanus''. Major collections were made in Laetoli, Tanzania, on an expedition beginning in 1974 directed by British palaeoanthropologist Mary Leakey, and in Hadar, Ethiopia, from 1972 to 1977 by the International Afar Research Expedition (IARE) formed by French geologist
Maurice Taieb Maurice Taieb (22 July 1935 – 23 July 2021) was a French geologist and paleoanthropologist. He discovered the Hadar formation, recognized its potential importance to paleoanthropology and founded the International Afar Research Expedition (IAR ...
, American palaeoanthropologist
Donald Johanson Donald Carl Johanson (born June 28, 1943) is an American paleoanthropologist. He is known for discovering, with Yves Coppens and Maurice Taieb, the fossil of a female hominin australopithecine known as "Lucy" in the Afar Triangle region of Hada ...
and Breton anthropologist Yves Coppens. These fossils were remarkably well preserved and many had associated skeletal aspects. In 1973, the IARE team unearthed the first knee joint, AL 129-1, and showed the earliest example at the time of bipedalism. In 1974, Johanson and graduate student Tom Gray discovered the extremely well-preserved skeleton AL 288–1, commonly referred to as " Lucy" (named after the 1967 Beatles song '' Lucy in the Sky with Diamonds'' which was playing on their
tape recorder An audio tape recorder, also known as a tape deck, tape player or tape machine or simply a tape recorder, is a sound recording and reproduction device that records and plays back sounds usually using magnetic tape for storage. In its present ...
that evening). In 1975, the IARE recovered 216 specimens belonging to 13 individuals, AL 333 "the First Family" (though the individuals were not necessarily related). In 1976, Leakey and colleagues discovered fossil trackways, and preliminarily classified Laetoli remains into ''
Homo ''Homo'' () is the genus that emerged in the (otherwise extinct) genus '' Australopithecus'' that encompasses the extant species ''Homo sapiens'' ( modern humans), plus several extinct species classified as either ancestral to or closely rela ...
'' spp., attributing ''Australopithecus''-like traits as evidence of them being transitional fossils. In 1978, Johanson,
Tim D. White Tim D. White (born August 24, 1950) is an American paleoanthropologist and Professor of Integrative Biology at the University of California, Berkeley. He is best known for leading the team which discovered Ardi, the type specimen of ''Ardipithecu ...
and Coppens classified the hundreds of specimens collected thus far from both Hadar and Laetoli into a single new species, ''A. afarensis'', and considered the apparently wide range of variation a result of sexual dimorphism. The species name honours the Afar Region of Ethiopia where the majority of the specimens had been recovered from. They later selected the jawbone LH 4 as the
lectotype specimen In biology, a type is a particular specimen (or in some cases a group of specimens) of an organism to which the scientific name of that organism is formally attached. In other words, a type is an example that serves to anchor or centralizes the ...
because of its preservation quality and because White had already fully described and illustrated it the year before. ''A. afarensis'' is known only from East Africa. Beyond Laetoli and the Afar Region, the species has been recorded in Kenya at Koobi Fora and possibly
Lothagam Lothagam is a geological formation located in Kenya, near the southwestern shores of Lake Turkana, from Kanapoi. It is located between the Kerio and Lomunyenkuparet Rivers on an uplifted fault block. Lothagam has deposits dating to the Miocene-P ...
; and elsewhere in Ethiopia at Woranso-Mille, Maka, Belohdelie, Ledi-Geraru and Fejej. The
frontal bone The frontal bone is a bone in the human skull. The bone consists of two portions.'' Gray's Anatomy'' (1918) These are the vertically oriented squamous part, and the horizontally oriented orbital part, making up the bony part of the forehead, ...
fragment BEL-VP-1/1 from the Middle Awash, Afar Region, Ethiopia, dating to 3.9 million years ago has typically been assigned to ''A. anamensis'' based on age, but may be assignable to ''A. afarensis'' because it exhibits a
derived Derive may refer to: *Derive (computer algebra system), a commercial system made by Texas Instruments * ''Dérive'' (magazine), an Austrian science magazine on urbanism *Dérive, a psychogeographical concept See also * *Derivation (disambiguation ...
form of
postorbital constriction In physical anthropology, post-orbital constriction is the narrowing of the cranium (skull) just behind the eye sockets (the orbits, hence the name) found in most non-human primates and early hominins. This constriction is very noticeable in non-hu ...
. This would mean ''A. afarensis'' and ''A. anamensis'' coexisted for at least 100,000 years. In 2005, a second adult specimen preserving both skull and body elements, AL 438–1, was discovered in Hadar. In 2006, an infant partial skeleton, DIK-1-1, was unearthed at Dikika, Afar Region. In 2015, an adult partial skeleton, KSD-VP-1/1, was recovered from Woranso-Mille. For a long time, ''A. afarensis'' was the oldest known African great ape until the 1994 description of the 4.4-million-year-old '' Ardipithecus ramidus'', and a few earlier or contemporary taxa have been described since, including the 4-million-year-old ''A. anamensis'' in 1995, the 3.5-million-year-old '' Kenyanthropus platyops'' in 2001, the 6-million-year-old '' Orrorin tugenensis'' in 2001, and the 7- to 6-million-year-old ''Sahelanthropus tchadensis'' in 2002. Bipedalism was once thought to have evolved in australopithecines, but it is now thought to have begun evolving much earlier in habitually arboreal primates. The earliest claimed date for the beginnings of an upright spine and a primarily vertical body plan is 21.6 million years ago in the Early Miocene with '' Morotopithecus bishopi''.


Classification

''A. afarensis'' is now a widely accepted species, and it is now generally thought that ''Homo'' and ''Paranthropus'' are sister taxa deriving from ''Australopithecus'', but the classification of ''Australopithecus'' species is in disarray. ''Australopithecus'' is considered a grade taxon whose members are united by their similar physiology rather than close relations with each other over other hominin genera. It is unclear how any ''Australopithecus'' species relate to each other, but it is generally thought that a population of '' A. anamensis'' evolved into ''A. afarensis''. In 1979, Johanson and White proposed that ''A. afarensis'' was the last common ancestor between ''Homo'' and '' Paranthropus'', supplanting ''A. africanus'' in this role. Considerable debate of the validity of this species followed, with proposals for synonymising them with ''A. africanus'' or recognising multiple species from the Laetoli and Hadar remains. In 1980, South African palaeoanthropologist
Phillip V. Tobias Phillip Vallentine Tobias (14 October 1925 – 7 June 2012) was a South African palaeoanthropologist and Professor Emeritus at the University of the Witwatersrand in Johannesburg. He was best known for his work at South Africa's hominid fossil ...
proposed reclassifying the Laetoli specimens as ''A. africanus afarensis'' and the Hadar specimens as ''A. afr. aethiopicus''. The skull KNM-ER 1470 (now ''
H. rudolfensis ''Homo rudolfensis'' is an extinct species of archaic human from the Early Pleistocene of East Africa about 2 million years ago (mya). Because ''H. rudolfensis'' coexisted with several other hominins, it is debated what specimens can be confiden ...
'') was at first dated to 2.9 million years ago, which cast doubt on the ancestral position of both ''A. afarensis'' or ''A. africanus'', but it has been re-dated to about 2 million years ago. Several ''Australopithecus'' species have since been postulated to represent the ancestor to ''Homo'', but the 2013 discovery of the earliest ''Homo'' specimen, LD 350-1, 2.8 million years old (older than almost all other ''Australopithecus'' species) from the Afar Region could potentially affirm ''A. afarensis'' ancestral position. However, ''A. afarensis'' is also argued to have been too derived (too specialised), due to resemblance in jaw anatomy to the robust australopithecines, to have been a human ancestor. Palaeoartist Walter Ferguson has proposed splitting ''A. afarensis'' into "''H. antiquus''", a relict dryopithecine "''Ramapithecus''" (now '' Kenyapithecus'') and a subspecies of ''A. africanus''. His recommendations have largely been ignored. In 2003, Spanish writer Camilo José Cela Conde and evolutionary biologist Francisco J. Ayala proposed reinstating "''Praeanthropus''" including ''A. afarensis'' alongside '' Sahelanthropus'', '' A. anamensis'', '' A. bahrelghazali'' and '' A. garhi''. In 2004, Danish biologist Bjarne Westergaard and geologist Niels Bonde proposed splitting off "''Homo hadar''" with the 3.2-million-year-old partial skull AL 333–45 as the holotype, because a foot from the First Family was apparently more humanlike than that of Lucy. In 2011, Bonde agreed with Ferguson that Lucy should be split into a new species, though erected a new genus as "''Afaranthropus antiquus''". In 1996, a 3.6-million-year-old jaw from Koro Toro, Chad, originally classified as ''A. afarensis'' was split off into a new species as '' A. bahrelghazali''. In 2015, some 3.5- to 3.3-million-year-old jaw specimens from the Afar Region (the same time and place as ''A. afarensis'') were classified as a new species as '' A. deyiremeda'', and the recognition of this species would call into question the species designation of fossils currently assigned to ''A. afarensis''. However, the validity of ''A. bahrelghazali'' and ''A. deyiremeda'' is debated. Wood and Boyle (2016) stated there was "low confidence" that ''A. afarensis'', ''A. bahrelghazali'' and ''A. deyiremeda'' are distinct species, with '' Kenyanthropus platyops'' perhaps being indistinct from the latter two.


Anatomy


Skull

''A. afarensis'' had a tall face, a delicate brow ridge, and prognathism (the jaw jutted outwards). One of the biggest skulls, AL 444–2, is about the size of a female gorilla skull. The first relatively complete jawbone was discovered in 2002, AL 822–1. This specimen strongly resembles the deep and robust gorilla jawbone. However, unlike gorillas, the strength of the
sagittal The sagittal plane (; also known as the longitudinal plane) is an anatomical plane that divides the body into right and left sections. It is perpendicular to the transverse and coronal planes. The plane may be in the center of the body and divi ...
and
nuchal The nape is the back of the neck. In technical anatomical/medical terminology, the nape is also called the nucha (from the Medieval Latin rendering of the Arabic , "spinal marrow"). The corresponding adjective is ''nuchal'', as in the term ''n ...
crests (which support the temporalis muscle used in biting) do not vary between sexes. The crests are similar to those of chimpanzees and female gorillas. Compared to earlier hominins, the incisors of ''A. afarensis'' are reduced in breadth, the canines reduced in size and lost the honing mechanism which continually sharpens them, the premolars are molar-shaped, and the molars are taller. The molars of australopiths are generally large and flat with thick enamel, which is ideal for crushing hard and brittle foods. The brain volume of Lucy was estimated to have been 365–417 cc, specimen AL 822-1 about 374–392 cc, AL 333-45 about 486–492 cc, and AL 444-2 about 519–526 cc. This would make for an average of about 445 cc. The brain volumes of the infant (about 2.5 years of age) specimens DIK-1-1 and AL 333-105 are 273–277 and 310–315 cc, respectively. Using these measurements, the brain growth rate of ''A. afarensis'' was closer to the growth rate of modern humans than to the faster rate in chimpanzees. Though brain growth was prolonged, the duration was nonetheless much shorter than modern humans, which is why the adult ''A. afarensis'' brain was so much smaller. The ''A. afarensis'' brain was likely organised like non-human ape brains, with no evidence for humanlike brain configuration.


Size

''A. afarensis'' specimens apparently exhibit a wide range of variation, which is generally explained as marked sexual dimorphism with males much bigger than females. In 1991, American anthropologist
Henry McHenry Henry Malcolm McHenry (born May 19, 1944) is a professor of anthropology at the University of California, Davis, specializing in studies of human evolution, the origins of bipedality, and paleoanthropology. McHenry has published on the comparative ...
estimated body size by measuring the joint sizes of the leg bones and scaling down a human to meet that size. This yielded for a presumed male (AL 333–3), whereas Lucy was . In 1992, he estimated that males typically weighed about and females assuming body proportions were more humanlike than apelike. This gives a male to female body mass ratio of 1.52, compared to 1.22 in modern
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, cultu ...
s, 1.37 in chimpanzees, and about 2 for
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 ...
s and orangutans. However, this commonly cited weight figure used only three presumed-female specimens, of which two were among the smallest specimens recorded for the species. It is also contested if australopiths even exhibited heightened sexual dimorphism at all, which if correct would mean the range of variation is normal body size disparity between different individuals regardless of sex. It has also been argued that the femoral head could be used for more accurate size modeling, and the femoral head size variation was the same for both sexes. Lucy is one of the most complete Pliocene hominin skeletons, with over 40% preserved, but she was one of the smaller specimens of her species. Nonetheless, she has been the subject of several body mass estimates since her discovery, ranging from for absolute lower and upper bounds. Most studies report ranges within . For the five makers of the Laetoli fossil trackways (S1, S2, G1, G2 and G3), based on the relationship between footprint length and bodily dimensions in modern humans, S1 was estimated to have been considerably large at about tall and in weight, S2 and , G1 and , G2 and , and G3 and . Based on these, S1 is interpreted to have been a male, and the rest females (G1 and G3 possibly juveniles), with ''A. afarensis'' being a highly dimorphic species.


Torso

DIK-1-1 preserves an oval hyoid bone (which supports the tongue) more similar to those of chimpanzees and gorillas than the bar-shaped hyoid of humans and orangutans. This would suggest the presence of laryngeal air sacs characteristic of non-human African apes (and large gibbons). Air sacs may lower the risk of hyperventilating when producing faster extended call sequences by rebreathing exhaled air from the air sacs. The loss of these in humans could have been a result of speech and resulting low risk of hyperventilating from normal vocalisation patterns. It was previously thought that the australopithecines' spine was more like that of non-human apes than humans, with weak
neck vertebra In tetrapods, cervical vertebrae (singular: vertebra) are the vertebrae of the neck, immediately below the skull. Truncal vertebrae (divided into thoracic and lumbar vertebrae in mammals) lie caudal (toward the tail) of cervical vertebrae. In sau ...
e. However, the thickness of the neck vertebrae of KSD-VP-1/1 is similar to that of modern humans. Like humans, the series has a bulge and achieves maximum girth at C5 and 6, which in humans is associated with the brachial plexus, responsible for nerves and muscle innervation in the arms and hands. This could perhaps speak to advanced motor functions in the hands of ''A. afarensis'' and competency at precision tasks compared to non-human apes, possibly implicated in stone tool use or production. However, this could have been involved in head stability or posture rather than dexterity. A.L. 333-101 and A.L. 333-106 lack evidence of this feature. The neck vertebrae of KDS-VP-1/1 indicate that the nuchal ligament, which stabilises the head while distance running in humans and other cursorial creatures, was either not well developed or absent. KSD-VP-1/1, preserving (among other skeletal elements) 6 rib fragments, indicates that ''A. afarensis'' had a bell-shaped ribcage instead of the barrel shaped ribcage exhibited in modern humans. Nonetheless, the constriction at the upper ribcage was not so marked as exhibited in non-human great apes and was quite similar to humans. Originally, the vertebral centra preserved in Lucy were interpreted as being the T6, T8, T10, T11 and L3, but a 2015 study instead interpreted them as being T6, T7, T9, T10 and L3. DIK-1-1 shows that australopithecines had 12 thoracic vertebrae like modern humans instead of 13 like non-human apes. Like humans, australopiths likely had 5 lumbar vertebrae, and this series was likely long and flexible in contrast to the short and inflexible non-human great ape lumbar series.


Upper limbs

Like other australopiths, the ''A. afarensis'' skeleton exhibits a mosaic anatomy with some aspects similar to modern humans and others to non-human great apes. The pelvis and leg bones clearly indicate weight-bearing ability, equating to habitual bipedal, but the upper limbs are reminiscent of orangutans, which would indicate arboreal locomotion. However, this is much debated, as tree-climbing adaptations could simply be basal traits inherited from the great ape last common ancestor in the absence of major selective pressures at this stage to adopt a more humanlike arm anatomy. The shoulder joint is somewhat in a shrugging position, closer to the head, like in non-human apes. Juvenile modern humans have a somewhat similar configuration, but this changes to the normal human condition with age; such a change does not appear to have occurred in ''A. afarensis'' development. It was once argued that this was simply a byproduct of being a small-bodied species, but the discovery of the similarly sized ''
H. floresiensis ''Homo floresiensis'' also known as "Flores Man"; nicknamed "Hobbit") is an extinct species of small archaic human that inhabited the island of Flores, Indonesia, until the arrival of modern humans about 50,000 years ago. The remains of an in ...
'' with a more or less human shoulder configuration and larger ''A. afarensis'' specimens retaining the shrugging shoulders show this to not have been the case. The scapular spine (reflecting the strength of the back muscles) is closer to the range of gorillas. The forearm of ''A. afarensis'' is incompletely known, yielding various brachial indexes ( radial length divided by humeral length) comparable to non-human great apes at the upper estimate and to modern humans at the lower estimate. The most complete ulna specimen, AL 438–1, is within the range of modern humans and other African apes. However, the L40-19 ulna is much longer, though well below that exhibited in orangutans and gibbons. The AL 438-1 metacarpals are proportionally similar to those of modern humans and orangutans. The ''A. afarensis'' hand is quite humanlike, though there are some aspects similar to orangutan hands which would have allowed stronger flexion of the fingers, and it probably could not handle large spherical or cylindrical objects very efficiently. Nonetheless, the hand seems to have been able to have produced a precision grip necessary in using
stone tool A stone tool is, in the most general sense, any tool made either partially or entirely out of stone. Although stone tool-dependent societies and cultures still exist today, most stone tools are associated with prehistoric (particularly Stone A ...
s. However, it is unclear if the hand was capable of producing stone tools.


Lower limbs

The australopith pelvis is platypelloid and maintains a relatively wider distance between the hip sockets and a more oval shape. Despite being much smaller, Lucy's
pelvic inlet The pelvic inlet or superior aperture of the pelvis is a planar surface which defines the boundary between the pelvic cavity and the abdominal cavity (or, according to some authors, between two parts of the pelvic cavity, called lesser pelvis an ...
is wide, about the same breadth as that of a modern human woman. These were likely adaptations to minimise how far the centre of mass drops while walking upright in order to compensate for the short legs (rotating the hips may have been more important for ''A. afarensis''). Likewise, later ''Homo'' could reduce relative pelvic inlet size probably due to the elongation of the legs. Pelvic inlet size may not have been due to fetal head size (which would have increased
birth canal In mammals, the vagina is the elastic, muscular part of the female genital tract. In humans, it extends from the Vulval vestibule, vestibule to the cervix. The outer vaginal opening is normally partly covered by a thin layer of mucous membrane ...
and thus pelvic inlet width) as an ''A. afarensis'' newborn would have had a similar or smaller head size compared to that of a newborn chimpanzee. It is debated if the platypelloid pelvis provided poorer leverage for the hamstrings or not. The heel bone of ''A. afarensis'' adults and modern humans have the same adaptations for bipedality, indicating a developed grade of walking. The big toe is not dextrous as is in non-human apes (it is adducted), which would make walking more energy efficient at the expense of arboreal locomotion, no longer able to grasp onto tree branches with the feet. However, the foot of the infantile specimen DIK-1-1 indicates some mobility of the big toe, though not to the degree in non-human primates. This would have reduced walking efficiency, but a partially dextrous foot in the juvenile stage may have been important in climbing activities for food or safety, or made it easier for the infant to cling onto and be carried by an adult.


Palaeobiology


Diet and technology

''A. afarensis'' was likely a generalist omnivore. Carbon isotope analysis on teeth from Hadar and Dikika 3.4–2.9 million years ago suggests a widely ranging diet between different specimens, with forest-dwelling specimens showing a preference for C3 forest plants, and bush- or grassland-dwelling specimens a preference for C4 CAM savanna plants. C4 CAM sources include grass, seeds, roots, underground
storage organ A storage organ is a part of a plant specifically modified for storage of energy (generally in the form of carbohydrates) or water. Storage organs often grow underground, where they are better protected from attack by herbivores. Plants that have ...
s, succulents and perhaps creatures which ate those, such as
termite Termites are small insects that live in colonies and have distinct castes ( eusocial) and feed on wood or other dead plant matter. Termites comprise the infraorder Isoptera, or alternatively the epifamily Termitoidae, within the order Blat ...
s. Thus, ''A. afarensis'' appears to have been capable of exploiting a variety of food resources in a wide range of habitats. In contrast, the earlier ''A. anamensis'' and ''Ar. ramidus'', as well as modern savanna chimpanzees, target the same types of food as forest-dwelling counterparts despite living in an environment where these plants are much less abundant. Few modern primate species consume C4 CAM plants. The dental anatomy of ''A. afarensis'' is ideal for consuming hard, brittle foods, but microwearing patterns on the molars suggest that such foods were infrequently consumed, probably as fallback items in leaner times. In 2009 at Dikika, Ethiopia, a rib fragment belonging to a cow-sized hoofed animal and a partial femur of a goat-sized juvenile
bovid The Bovidae comprise the biological family of cloven-hoofed, ruminant mammals that includes cattle, bison, buffalo, antelopes, and caprines. A member of this family is called a bovid. With 143 extant species and 300 known extinct species, t ...
was found to exhibit cut marks, and the former some crushing, which were initially interpreted as the oldest evidence of butchering with stone tools. If correct, this would make it the oldest evidence of sharp-edged stone tool use at 3.4 million years old, and would be attributable to ''A. afarensis'' as it is the only species known within the time and place. However, because the fossils were found in a
sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicat ...
unit (and were modified by abrasive sand and gravel particles during the fossilisation process), the attribution to hominin activity is weak.


Society

It is highly difficult to speculate with accuracy the group dynamics of early hominins. ''A. afarensis'' is typically reconstructed with high levels of sexual dimorphism, with males much larger than females. Using general trends in modern primates, high sexual dimorphism usually equates to a polygynous society due to intense male–male competition over females, like in the harem society of gorillas. However, it has also been argued that ''A. afarensis'' had much lower levels of dimorphism, and so had a multi-male kin-based society like chimpanzees. Low dimorphism could also be interpreted as having had a monogamous society with strong male–male competition. Contrarily, the canine teeth are much smaller in ''A. afarensis'' than in non-human primates, which should indicate lower aggression because canine size is generally positively correlated with male–male aggression.


Birth

The platypelloid pelvis may have caused a different birthing mechanism from modern humans, with the neonate entering the inlet facing laterally (the head was transversally orientated) until it exited through the
pelvic outlet The lower circumference of the lesser pelvis is very irregular; the space enclosed by it is named the inferior aperture or pelvic outlet. It is an important component of pelvimetry. Boundaries It has the following boundaries: * anteriorly: the pub ...
. This would be a non-rotational birth, as opposed to a fully rotational birth in humans. However, it has been suggested that the shoulders of the neonate may have been obstructed, and the neonate could have instead entered the inlet transversely and then rotated so that it exited through the outlet oblique to the main axis of the pelvis, which would be a semi-rotational birth. By this argument, there may not have been much space for the neonate to pass through the birth canal, causing a difficult childbirth for the mother.


Gait

The Laetoli fossil trackway, generally attributed to ''A. afarensis'', indicates a rather developed grade of bipedal locomotion, more efficient than the bent-hip–bent-knee (BHBK) gait used by non-human great apes (though earlier interpretations of the gait include a BHBK posture or a shuffling movement). Trail A consists of short, broad prints resembling those of a two-and-a-half-year-old child, though it has been suggested this trail was made by the extinct bear '' Agriotherium africanus''. G1 is a trail consisting of four cycles likely made by a child. G2 and G3 are thought to have been made by two adults. In 2014, two more trackways were discovered made by one individual, named S1, extending for a total of . In 2015, a single footprint from a different individual, S2, was discovered. The shallowness of the toe prints would indicate a more flexed limb posture when the foot hit the ground and perhaps a less arched foot, meaning ''A. afarensis'' was less efficient at bipedal locomotion than humans. Some tracks feature a long drag mark probably left by the heel, which may indicate the foot was lifted at a low angle to the ground. For push-off, it appears weight shifted from the heel to the side of the foot and then the toes. Some footprints of S1 either indicate asymmetrical walking where weight was sometimes placed on the anterolateral part (the side of the front half of the foot) before toe-off, or sometimes the upper body was rotated mid-step. The angle of gait (the angle between the direction the foot is pointing in on touchdown and median line drawn through the entire trackway) ranges from 2–11° for both right and left sides. G1 generally shows wide and asymmetrical angles, whereas the others typically show low angles. The speed of the track makers has been variously estimated depending on the method used, with G1 reported at 0.47, 0.56, 0.64, 0.7 and 1 m/s (1.69, 2, 2.3, 2.5 and 3.6 km/h; 1.1, 1.3, 1.4, 1.6 and 2.2 mph); G2/3 reported at 0.37, 0.84 and 1 m/s (1.3, 2.9 and 3.6 km/h; 0.8, 1.8 and 2.2 mph); and S1 at . For comparison, modern humans typically walk at . The average step distance is , and stride distance . S1 appears to have had the highest average step and stride length of, respectively, and whereas G1–G3 averaged, respectively, 416, 453 and 433 mm (1.4, 1.5 and 1.4 ft) for step and 829, 880 and 876 mm (2.7, 2.9 and 2.9 ft) for stride.


Pathology

Australopithecines, in general, seem to have had a high incidence rate of vertebral pathologies, possibly because their vertebrae were better adapted to withstand suspension loads in climbing than compressive loads while walking upright. Lucy presents marked thoracic kyphosis (hunchback) and was diagnosed with
Scheuermann's disease Scheuermann's disease is a self-limiting skeletal disorder of childhood. Scheuermann's disease describes a condition where the vertebrae grow unevenly with respect to the sagittal plane; that is, the posterior angle is often greater than the an ...
, probably caused by overstraining her back, which can lead to a hunched posture in modern humans due to irregular curving of the spine. Because her condition presented quite similarly to that seen in modern human patients, this would indicate a basically human range of locomotor function in walking for ''A. afarensis''. The original straining may have occurred while climbing or swinging in the trees, though, even if correct, this does not indicate that her species was maladapted for arboreal behaviour, much like how humans are not maladapted for bipedal posture despite developing arthritis. KSD-VP-1/1 seemingly exhibits compensatory action by the neck and lumbar vertebrae (gooseneck) consistent with thoracic kyphosis and Scheuermann's disease, but thoracic vertebrae are not preserved in this specimen. In 2010, KSD-VP-1/1 presented evidence of a valgus deformity of the left ankle involving the
fibula The fibula or calf bone is a leg bone on the lateral side of the tibia, to which it is connected above and below. It is the smaller of the two bones and, in proportion to its length, the most slender of all the long bones. Its upper extremity ...
, with a bony ring developing on the fibula's joint surface extending the bone an additional . This was probably caused by a fibular fracture during childhood which improperly healed in a
nonunion Nonunion is permanent failure of healing following a broken bone unless intervention (such as surgery) is performed. A fracture with nonunion generally forms a structural resemblance to a fibrous joint, and is therefore often called a "false j ...
. In 2016, palaeoanthropologist John Kappelman argued that the fracturing exhibited by Lucy was consistent with a proximal humerus fracture, which is most often caused by falling in humans. He then concluded she died from falling out of a tree, and that ''A. afarensis'' slept in trees or climbed trees to escape predators. However, similar fracturing is exhibited in many other creatures in the area, including the bones of antelope,
elephant Elephants are the largest existing land animals. Three living species are currently recognised: the African bush elephant, the African forest elephant, and the Asian elephant. They are the only surviving members of the family Elephantida ...
s, giraffes and rhinos, and may well simply be taphonomic bias (fracturing was caused by fossilisation). Lucy may also have been killed in an animal attack or a mudslide. The 13 AL 333 individuals are thought to have been deposited at about the same time as one another, bear little evidence of carnivore activity, and were buried on a stretch of a hill. In 1981, anthropologists James Louis Aronson and Taieb suggested they were killed in a flash flood. British archaeologist Paul Pettitt considered natural causes unlikely and, in 2013, speculated that these individuals were purposefully hidden in tall grass by other hominins (funerary caching). This behaviour has been documented in modern primates, and may be done so that the recently deceased do not attract predators to living grounds.


Palaeoecology

''A. afarensis'' does not appear to have had a preferred environment, and inhabited a wide range of habitats such as open grasslands or woodlands, shrublands, and lake- or riverside forests. Likewise, the animal assemblage varied widely from site to site. The Pliocene of East Africa was warm and wet compared to the preceding
Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...
, with the
dry season The dry season is a yearly period of low rainfall, especially in the tropics. The weather in the tropics is dominated by the tropical rain belt, which moves from the northern to the southern tropics and back over the course of the year. The ...
lasting about four months based on floral, faunal and geological evidence. The extended rainy season would have made more desirable foods available to hominins for most of the year. Africa 4–3 million years ago featured a greater diversity of large carnivores than today, and australopithecines likely fell prey to these dangerous creatures, including hyenas, '' Panthera'', cheetahs, and the sabre-toothed '' Megantereon'', ''
Dinofelis ''Dinofelis'' is a genus of extinct sabre-toothed cats belonging to the tribe Metailurini or possibly Smilodontini. They were widespread in Europe, Asia, Africa and North America at least 5 million to about 1.2 million years ago (Early Pliocene ...
'', ''
Homotherium ''Homotherium'', also known as the scimitar-toothed cat or scimitar cat, is an extinct genus of machairodontine saber-toothed predator, often termed scimitar-toothed cats, that inhabited North America, South America, Eurasia, and Africa during ...
'' and '' Machairodus''. Australopithecines and early ''Homo'' likely preferred cooler conditions than later ''Homo'', as there are no australopithecine sites that were below in elevation at the time of deposition. This would mean that, like chimpanzees, they often inhabited areas with an average diurnal temperature of , dropping to at night. At Hadar, the average temperature from 3.4 to 2.95 million years ago was about .


See also

* '' Ardipithecus ramidus'' * '' Australopithecus anamensis'' * '' Australopithecus bahrelghazali'' * ''
Australopithecus deyiremeda ''Australopithecus deyiremeda'' is an extinct species of australopithecine from Woranso–Mille, Afar Region, Ethiopia, about 3.5 to 3.3 million years ago during the Pliocene. Because it is known only from three partial jawbones, it is unclear ...
'' * ''
Kenyanthropus ''Kenyanthropus'' is a hominin genus identified from the Lomekwi site by Lake Turkana, Kenya, dated to 3.3 to 3.2 million years ago during the Middle Pliocene. It contains one species, ''K. platyops'', but may also include the 2 million year ...
'' * LD 350-1 * List of fossil sites (with link directory) * List of human evolution fossils (with images) * Lomekwi


References

72. Bonnefille, R. ; Potts, R.; Chalie, F.; Jolly, D. (2004). "High-Resolution Vegetation and Climate Change Associated with Pliocene Australopithecus afarensis". Proceedings of the National Academy of Sciences. 101 (33): 12125–12129. Bibcode:2004PNAS..10112125B. doi:10.1073/pnas.0401709101. PMC 514445. PMID 15304655.


Further reading

* * * *


External links


Becoming Human: Paleoanthropology, Evolution and Human Origins



The Smithsonian's Human Origins Program

Human Timeline (Interactive)
Smithsonian {{Authority control afarensis Pliocene primates Mammals described in 1978 Fossil taxa described in 1978 Prehistoric Ethiopia Prehistoric Kenya Pliocene mammals of Africa Tool-using mammals Taxa named by Donald Johanson Taxa named by Tim D. White Archaeology of Eastern Africa