Homo

Homo is a genus of great ape that emerged from the early homininian genus Australopithecus, encompassing a single extant species, Homo sapiens, along with a number of extinct species classified as either ancestral or closely related to modern humans, collectively called archaic humans. Homo, together with the genus Paranthropus, is probably most closely related to the species Australopithecus africanus within Australopithecus.'''' The closest living relatives of Homo are of the hominin genus Pan, with the ancestors of Pan and Homo estimated to have diverged around 5.7–11 million years ago during the Late Miocene.
The oldest member of the genus is Homo habilis, with fossil records of just over 2 million years ago. H. erectus appeared about 2 million years ago and spread throughout Africa and Eurasia in several migrations. The species was adaptive and successful, and persisted for more than a million years before gradually diverging into new species around 500,000 years ago.
Anatomically modern humans emerged close to 300,000 to 200,000 years ago in Africa, and H. neanderthalensis emerged around the same time in Europe and Western Asia. H. sapiens dispersed from Africa in several waves, from possibly as early as 250,000 years ago, and certainly by 130,000 years ago, with the so-called Southern Dispersal, beginning about 70,000–50,000 years ago, leading to the lasting colonisation of Eurasia and Oceania by 50,000 years ago. H. sapiens met and interbred with archaic humans in Africa and in Eurasia. Separate archaic human species including Neanderthals are thought to have survived until around 40,000 years ago.

Names and taxonomy

The Latin noun homō means "human being" or "man" in the generic sense of "human being, mankind". The binomial name Homo sapiens was coined by Carl Linnaeus. Names for other species of the genus were introduced from the second half of the 19th century.
The genus Homo has not been strictly defined, even today. Since the early human fossil record began to slowly emerge from the earth, the boundaries and definitions of the genus 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 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 times produced a rich mix for debating classifications. There is continuing debate on delineating Homo from Australopithecus—or, indeed, delineating Homo from Pan. Even so, classifying the fossils of Homo coincides with evidence of: 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 human tool culture having begun by 2.5 million years ago to 3 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 regarded 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, even in scientific papers, to avoid trinomial names or the ambiguity of classifying groups as incertae sedis —for example, H. neanderthalensis vs. H. sapiens neanderthalensis, or H. georgicus vs. H. erectus georgicus. Some recently extinct species in the genus have been discovered only lately and do not as yet have consensus binomial names. Since the beginning of the Holocene, it is likely that Homo sapiens has been the only extant species of Homo.
John Edward Gray was an early advocate of classifying taxa by designating tribes and families. Wood and Richmond proposed that Hominini 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 such as Australopithecus, Orrorin tugenensis, Ardipithecus, or Sahelanthropus. Designations alternative to Hominina existed, or were offered: Australopithecinae and Preanthropinae ; and later, Cela-Conde and Ayala proposed that the four genera Australopithecus, Ardipithecus, Praeanthropus, and Sahelanthropus be grouped with Homo within Hominini.

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, 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. This finds support in a recent phylogenetic study in hominins that by using morphological, molecular and radiometric information, dates the emergence of Homo at 3.3 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, 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 Australopithecina 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 the 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. 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 by 0.5 Mya.

''Homo erectus''

Homo erectus has often been assumed to have developed anagenetically from H. 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. For example they showed increased cranial capacity from around 575 cm³ in H. habilis to around 850 cm³ in H. erectus. 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. argued that H. habilis may have survived beyond the emergence of H. erectus, so that the evolution of H. erectus would not have been anagenetical, and H. erectus would have existed alongside H. habilis for about half a million years, during the early Calabrian. On 31 August 2023, researchers reported, based on genetic studies, that a human ancestor population bottleneck occurred "around 930,000 and 813,000 years ago... lasted for about 117,000 years and brought human ancestors close to extinction."
Weiss estimated that there have been about 44 billion members of the genus Homo from its origins to the evolution of H. erectus, about 56 billion individuals from H. erectus to the Neolithic, and another 51 billion individuals since the Neolithic. This provides the opportunity for an immense amount of new mutational variation to have arisen during human evolution.
A separate South African species Homo gautengensis has been postulated as contemporary with H. erectus in 2010.