Pre-human

''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 related to modern humans (depending on the species), most notably ''Homo erectus'' and ''Homo neanderthalensis''. The genus emerged with the appearance of ''Homo habilis'' just over 2 million years ago. ''Homo'', together with the genus ''Paranthropus'', is probably sister to ''Australopithecus africanus'', which itself had previously split from the lineage of '' Pan'', the chimpanzees.

''Homo erectus'' appeared about 2 million years ago and, in several early migrations, spread throughout Africa (where it is dubbed ''Homo ergaster'') and Eurasia. It was likely that the first human species lived in a hunter-gatherer society and was able to control fire. An adaptive and successful species, ''Homo erectus'' persisted for more than a million years and gradually diverged into new species by around 500,000 years ago.

Anatomically modern humans (''Homo sapiens'') emerged close to 300,000 to 200,000 years ago, in Africa, and ''Homo neanderthalensis'' emerged around the same time in Europe and Western Asia. ''Homo sapiens'' dispersed from Africa in several waves, from possibly as early as 250,000 years ago, and certainly by 130,000 years ago, the so-called Southern Dispersal beginning about 70–50,000 years ago leading to the lasting colonisation of Eurasia and Oceania by 50,000 years ago. Both in Africa and Eurasia, ''Homo sapiens'' met with and interbred with archaic humans. Separate archaic (non-''sapiens'') human species are thought to have survived until around 40,000 years ago (Neanderthal extinction).

Names and taxonomy

The Latin noun ''homō'' (genitive ''hominis'') means "human being" or "man" in the generic sense of "human being, mankind". The binomial name ''Homo sapiens'' was coined by Carl Linnaeus (1758). Names for other species of the genus were introduced beginning in the second half of the 19th century (''H. neanderthalensis'' 1864, ''H. erectus'' 1892).

Even today, the genus ''Homo'' has not been strictly defined. Since the early human fossil record began to slowly emerge from the earth, the boundaries and definitions of the genus ''Homo'' have been poorly defined and constantly in flux. Because there was no reason to think it would ever have any additional members, Carl Linnaeus did not even bother to define ''Homo'' when he first created it for humans in the 18th century. The discovery of Neanderthal brought the first addition.

The genus ''Homo'' was given its taxonomic name to suggest that its member species can be classified as human. And, over the decades of the 20th century, fossil finds of pre-human and early human species from late Miocene and early Pliocene''—or, indeed, delineating ''Homo'' from '' Pan.'' Even so, classifying the fossils of ''Homo'' coincides with evidence of: (1) competent human bipedalism in ''Homo habilis'' inherited from the earlier ''Australopithecus'' of more than four million years ago, as demonstrated by the Laetoli footprints; and (2)  human tool culture having begun by 2.5 million years ago.

From the late-19th to mid-20th centuries, a number of new taxonomic names including new generic names were proposed for early human fossils; most have since been merged with ''Homo'' in recognition that ''Homo erectus'' was a single species with a large geographic spread of early migrations. Many such names are now dubbed as "synonyms" with ''Homo'', including ''Pithecanthropus'', ''Protanthropus'', ''Sinanthropus'', ''Cyphanthropus'', ''Africanthropus'', ''Telanthropus'', ''Atlanthropus'', and ''Tchadanthropus''.

Classifying the genus ''Homo'' into species and subspecies is subject to incomplete information and remains poorly done. This has led to using common names ("Neanderthal" and "Denisovan"), even in scientific papers, to avoid trinomial names or the ambiguity of classifying groups as ''incertae sedis'' (uncertain placement)—for example, ''H. neanderthalensis'' vs. ''H. sapiens neanderthalensis'', or ''H. georgicus'' vs. ''H. erectus georgicus''. Some recently extinct species in the genus ''Homo'' have only recently been discovered and do not as yet have consensus binomial names (see Denisova hominin). Since the beginning of the Holocene, it is likely that ''Homo sapiens'' (anatomically modern humans) has been the only extant species of ''Homo''.

John Edward Gray (1825) was an early advocate of classifying taxa by designating tribes and families. Wood and Richmond (2000) proposed that Hominini ("hominins") be designated as a tribe that comprised all species of early humans and pre-humans ancestral to humans back to ''after'' the chimpanzee-human last common ancestor; and that Hominina be designated a subtribe of Hominini to include ''only'' the genus ''Homo'' — that is, ''not'' including the earlier upright walking hominins of the Pliocene'', ''Orrorin tugenensis'', ''Ardipithecus'', or ''Sahelanthropus''. Designations alternative to Hominina existed, or were offered: ''Australopithecinae'' (Gregory & Hellman 1939) and ''Preanthropinae'' (Cela-Conde & Altaba 2002); and later, Cela-Conde and Ayala (2003) proposed that the four genera ''Australopithecus'', ''Ardipithecus'', ''Praeanthropus'', and ''Sahelanthropus'' be grouped with ''Homo'' within Hominini (sans ''Pan'').

Evolution

''Australopithecus'' and the appearance of ''Homo''

Several species, including ''Australopithecus garhi'', ''Australopithecus sediba'', ''Australopithecus africanus'', and ''Australopithecus afarensis'', have been proposed as the ancestor or sister of the ''Homo'' lineage. These species have morphological features that align them with ''Homo'', but there is no consensus as to which gave rise to ''Homo''.

Especially since the 2010s, the delineation of ''Homo'' in ''Australopithecus'' has become more contentious. Traditionally, the advent of ''Homo'' has been taken to coincide with the first use of stone tools (the Oldowan industry), and thus by definition with the beginning of the Lower Palaeolithic. But in 2010, evidence was presented that seems to attribute the use of stone tools to ''Australopithecus afarensis'' around 3.3 million years ago, close to a million years before the first appearance of ''Homo''. LD 350-1, a fossil mandible fragment dated to 2.8 Mya, discovered in 2013 in Afar, Ethiopia, was described as combining "primitive traits seen in early ''Australopithecus'' with derived morphology observed in later ''Homo''. Some authors would push the development of ''Homo'' close to or even past 3 Mya. Others have voiced doubt as to whether ''Homo habilis'' should be included in ''Homo'', proposing an origin of ''Homo'' with ''Homo erectus'' at roughly 1.9 Mya instead.

The most salient physiological development between the earlier australopithecine species and ''Homo'' is the increase in endocranial volume (ECV), from about in ''A. garhi'' to in ''H. habilis'' and further to in ''H. erectus'', in ''H. heidelbergensis'' and up to in ''H. neanderthalensis''. However, a steady rise in cranial capacity is observed already in ''Autralopithecina'' and does not terminate after the emergence of ''Homo'', so that it does not serve as an objective criterion to define the emergence of the genus.

''Homo habilis''

''Homo habilis'' emerged about 2.1 Mya. Already before 2010, there were suggestions that ''H. habilis'' should not be placed in genus ''Homo'' but rather in ''Australopithecus''. The main reason to include ''H. habilis'' in ''Homo'', its undisputed tool use, has become obsolete with the discovery of ''Australopithecus'' tool use at least a million years before ''H. habilis''. Furthermore, ''H. habilis'' was long thought to be the ancestor of the more gracile ''Homo ergaster'' (''Homo erectus''). In 2007, it was discovered that ''H. habilis'' and ''H. erectus'' coexisted for a considerable time, suggesting that ''H. erectus'' is not immediately derived from ''H. habilis'' but instead from a common ancestor. With the publication of Dmanisi skull 5 in 2013, it has become less certain that Asian ''H. erectus'' is a descendant of African ''H. ergaster'' which was in turn derived from ''H. habilis''. Instead, ''H. ergaster'' and ''H. erectus'' appear to be variants of the same species, which may have originated in either Africa or Asia and widely dispersed throughout Eurasia (including Europe, Indonesia, China) by 0.5 Mya.

''Homo erectus''

''Homo erectus'' has often been assumed to have developed anagenetically from ''Homo habilis'' from about 2 million years ago. This scenario was strengthened with the discovery of ''Homo erectus georgicus'', early specimens of ''H. erectus'' found in the Caucasus, which seemed to exhibit transitional traits with ''H. habilis''. As the earliest evidence for ''H. erectus'' was found outside of Africa, it was considered plausible that ''H. erectus'' developed in Eurasia and then migrated back to Africa. Based on fossils from the Koobi Fora Formation, east of Lake Turkana in Kenya, Spoor et al. (2007) argued that ''H. habilis'' may have survived beyond the emergence of ''H. erectus'', so that the evolution of ''H. erectus'' would not have been anagenetically, and ''H. erectus'' would have existed alongside ''H. habilis'' for about half a million years (), during the early Calabrian.

A separate South African species ''Homo gautengensis'' has been postulated as contemporary with ''Homo erectus'' in 2010.

Phylogeny

A taxonomy of ''Homo'' within the great apes is assessed as follows, with ''Paranthropus'' and ''Homo'' emerging within ''Australopithecus'' (shown here cladistically granting ''Paranthropus'', ''Kenyanthropus'', and ''Homo''). The exact phylogeny within ''Australopithecus'' is still highly controversial. Approximate radiation dates of daughter clades are shown in millions of years ago (Mya). ''Graecopithecus,'' ''Sahelanthropus'', ''Orrorin'', possibly sisters to ''Australopithecus'', are not shown here. Note that the naming of groupings is sometimes muddled as often certain groupings are presumed before any cladistic analysis is performed.

Several of the ''Homo'' lineages appear to have surviving progeny through introgression into other lines. Genetic evidence indicates an archaic lineage separating from the other human lineages 1.5 million years ago, perhaps ''H. erectus'', may have interbred into the Denisovans about 55,000 years ago. Fossil evidence shows ''Homo erectus'' s.s. survived at least until 117,000 yrs ago, and the even more basal ''Homo floresiensis'' survived until 50,000 years ago. A 1.5 million years Homo erectus-like lineage appears to have made its way into modern humans through the Denisovans and specifically into the Papuans and aboriginal Australians. The genomes of non-sub-Saharan African humans show what appear to be numerous independent introgression events involving Neanderthal and in some cases also Denisovans around 45,000 years ago. The genetic structure of some sub-Saharan African groups seems to be indicative of introgression from a west Eurasian population some 3,000 years ago.

Some evidence suggests that ''Australopithecus sediba'' could be moved to the genus ''Homo'', or placed in its own genus, due to its position with respect to e.g. ''Homo habilis'' and ''Homo floresiensis''.

Dispersal

By about 1.8 million years ago, ''Homo erectus'' is present in both East Africa (''Homo ergaster'') and in Western Asia (''Homo georgicus''). The ancestors of Indonesian ''Homo floresiensis'' may have left Africa even earlier.

''Homo erectus'' and related or derived archaic human species over the next 1.5 million years spread throughout Africa and Eurasia (see: Recent African origin of modern humans). Europe is reached by about 0.5 Mya by ''Homo heidelbergensis''.

''Homo neanderthalensis'' and ''Homo sapiens'' develop after about 300 kya. ''Homo naledi'' is present in Southern Africa by 300 kya.

''H. sapiens'' soon after its first emergence spread throughout Africa, and to Western Asia in several waves, possibly as early as 250 kya, and certainly by 130 kya. In July 2019, anthropologists reported the discovery of 210,000 year old remains of a ''H. sapiens'' and 170,000 year old remains of a ''H. neanderthalensis'' in Apidima Cave, Peloponnese, Greece, more than 150,000 years older than previous ''H. sapiens'' finds in Europe.

Most notable is the Southern Dispersal of ''H. sapiens'' around 60 kya, which led to the lasting peopling of Oceania and Eurasia by anatomically modern humans. ''H. sapiens'' interbred with archaic humans both in Africa and in Eurasia, in Eurasia notably with Neanderthals and Denisovans.

Among extant populations of ''Homo sapiens'', the deepest temporal division is found in the San people of Southern Africa, estimated at close to 130,000 years, or possibly more than 300,000 years ago. Temporal division among non-Africans is of the order of 60,000 years in the case of Australo-Melanesians. Division of Europeans and East Asians is of the order of 50,000 years, with repeated and significant admixture events throughout Eurasia during the Holocene.

Archaic human species may have survived until the beginning of the Holocene, although they were mostly extinct or absorbed by the expanding ''H. sapiens'' populations by 40 kya (Neanderthal extinction).

List of lineages

The species status of ''H. rudolfensis'', '' H. ergaster'', ''H. georgicus'', ''H. antecessor'', '' H. cepranensis'', ''H. rhodesiensis'', '' H. neanderthalensis'', Denisova hominin, and ''H. floresiensis'' remain under debate. ''H. heidelbergensis'' and ''H. neanderthalensis'' are closely related to each other and have been considered to be subspecies of ''H. sapiens''.

There has historically been a trend to postulate new human species based on as little as an individual fossil. A "minimalist" approach to human taxonomy recognizes at most three species, ''Homo habilis'' (2.1–1.5 Mya, membership in ''Homo'' questionable), ''Homo erectus'' (1.8–0.1 Mya, including the majority of the age of the genus, and the majority of archaic varieties as subspecies, including ''H. heidelbergensis'' as a late or transitional variety) and ''Homo sapiens'' (300 kya to present, including ''H. neanderthalensis'' and other varieties as subspecies). "Species" does in this context not necessarily mean that hybridization and introgression were impossible at the time. However, it is often used as a convenient term, but it should be taken to mean to be a generic lineage at best, and clusters at worst. In general definitions and methodology of "species" delineation criteria are not generally agreed upon in anthropology or paleontology. Indeed, mammals can typically interbreed for 2 to 3 million years or longer, so all contemporary "species" in the genus ''Homo'' would potentially have been able to interbreed at the time, and introgression from beyond the genus ''Homo'' can not ''a priori'' be ruled out. It has been suggested that ''H. naledi'' may have been a hybrid with a late surviving ''Australipith'' (taken to mean beyond ''Homo'', ed.), despite the fact that these lineages generally are regarded as long extinct. As discussed above, many introgressions have occurred between lineages, with evidence of introgression after separation of 1.5 million years.

See also

* List of human evolution fossils ''(with images)''
* Multiregional origin of modern humans

Footnotes

References

Bibliography

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External links

Exploring the Hominid Fossil Record
( Center for the Advanced Study of Hominid Paleobiology at George Washington University)

Hominid species




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Human Timeline (Interactive)
– Smithsonian, National Museum of Natural History (August 2016).

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Hominina
Primate genera
Human evolution
Taxa named by Carl Linnaeus
Mammal genera with one living species