Homo is the genus of hominids that includes modern humans and species closely related to them. The genus is estimated to be about 2.3 to 2.4 million years old, possibly having evolved from australopithecine ancestors, with the appearance of Homo habilis It is the only genus in the subtribe Hominina. Several species, including Australopithecus garhi, Australopithecus sediba, Australopithecus africanus, and Australopithecus afarensis, have been proposed as the direct ancestor of the Homo lineage. These species have morphological features that align them with Homo, but there is no consensus on which gave rise to Homo, assuming it was not an as of yet undiscovered species.
The most salient physiological development between the earlier australopith species and Homo is the increase in cranial capacity, from about 450 cm3 (27 cu in) in A. garhi to 600 cm3 (37 cu in) in H. habilis. Within the Homo genus, cranial capacity again doubled from H. habilis through Homo ergaster or H. erectus to Homo heidelbergensis by 0.6 million years ago. The cranial capacity of H. heidelbergensis overlaps with the range found in modern humans.
The advent of Homo was thought to coincide with the first evidence of stone tools (the Oldowan industry), and thus by definition with the beginning of the Lower Palaeolithic; however, recent evidence from Ethiopia now places the earliest evidence of stone tool usage at before 3.39 million years ago. The emergence of Homo coincides roughly with the onset of Quaternary glaciation, the beginning of the current ice age.
Homo sapiens (modern humans) is the only surviving species in the genus, all others having become extinct. Homo neanderthalensis, traditionally considered the last surviving relative, died out about 40,000 years ago, though recent discoveries suggest that another species, Homo floresiensis, may have lived much more recently. The other extant Homininaeâ€"the chimpanzees and gorillasâ€"have a limited geographic range. In contrast, the evolution of humans is a history of migrations and admixture. Humans repeatedly left Africa to populate Eurasia and finally the Americas, Oceania, and the rest of the world.
Naming
In the biological sciences, particularly anthropology and palaeontology, the common name for all members of the genus Homo is "human". The word homo is Latin meaning "human", and came to mean "man" in New Latin. The word "human" itself is from Latin humanus, an adjective cognate to homo, both thought to derive from a Proto-Indo-European word for "earth" reconstructed as *dhǵhem-.
The binomial name Homo sapiens was coined by Carl Linnaeus (1758).
Names for other species were introduced beginning in the second half of the 19th century (H. neanderthalensis 1864, H. erectus 1892). A couple of recently discovered, recently extinct, species in the genus Homo do not have accepted binomial names yet, Denisova hominin, and Red Deer Cave people. Classification of the genus Homo into species and subspecies is poorly defined and subject to incomplete information, leading to difficulties in binomial naming, and the use of common names, such as Neanderthal and Denisovan, even in scientific papers.
Species
The species status of Homo rudolfensis, Homo ergaster, H. georgicus, H. antecessor, H. cepranensis, H. rhodesiensis, Homo neanderthalensis, Denisova hominin, Red Deer Cave people and Homo floresiensis remains under debate. H. heidelbergensis and H. neanderthalensis are closely related to each other and have been considered to be subspecies of H. sapiens. Recently, nuclear DNA from a Neanderthal specimen from Vindija Cave has been sequenced using two different methods that yield similar results regarding Neanderthal and H. sapiens lineages, with both analyses suggesting a date for the split between 460,000 and 700,000 years ago, though a population split of around 370,000 years is inferred. The nuclear DNA results indicate about 30% of derived alleles in H. sapiens are also in the Neanderthal lineage. This high frequency may suggest some gene flow between ancestral human and Neanderthal populations due to mating between the two.
Migration and admixture
H. habilis, which is considered the first member of the genus Homo, might have given rise to H. ergaster (however this is questionable, as some finds suggest that the species were contemporaneous). Some of H. ergaster migrated to Asia, where they are named Homo erectus, and to Europe with Homo georgicus. H. ergaster in Africa and H. erectus in Eurasia evolved separately for almost two million years and presumably separated into two different species. Homo rhodesiensis, who were descended from H. ergaster, migrated from Africa to Europe and became Homo heidelbergensis and later (about 250,000 years ago) Homo neanderthalensis and the Denisova hominin in Asia. The first Homo sapiens, descendants of H. rhodesiensis, appeared in Africa about 250,000 years ago. About 100,000 years ago, some H. sapiens sapiens migrated from Africa to the Levant and met with resident Neanderthals, with some admixture. Later, about 70,000 years ago, perhaps after the Toba catastrophe, a small group left the Levant to populate Eurasia, Australia and later the Americas. A subgroup among them met the Denisovans and, after further admixture, migrated to populate Melanesia. In this scenario, non-African people living today are mostly of African origin ("Out of Africa model"). However, there was also some admixture with Neanderthals and Denisovans, who had evolved locally (the "multiregional hypothesis"). Recent genomic results from the group of Svante Pääbo also show that 30,000 years ago at least three major subspecies coexisted: Denisovans, Neanderthals and anatomically modern humans. Today, only H. sapiens remains, with no other extant species.
