Homo is the genus that comprises the species Homo sapiens, which includes modern humans, as well as several extinct species classified as ancestral to or closely related to modern humans, most notably Homo erectus. The genus is between 2 and 3 million years old, taken to emerge with the appearance of Homo habilis and possibly that of Homo gautengensis. Homo is derived from the genus Australopithecus, which itself had previously split from the lineage of Pan, the chimpanzees. Taxonomically, Homo is the only genus assigned to the subtribe Hominina which, with the subtribes Australopithecina and Panina, comprise the tribe Hominini (see evolutionary tree below). All species of the genus Homo plus those species of the australopithecines that arose after the split from Pan are called hominins.
Homo erectus appeared about two million years ago in East Africa (where it is dubbed Homo ergaster) and, in several early migrations, it spread throughout Africa and Eurasia. It was likely the first hominin to live in a hunter-gatherer society and to control fire. An adaptive and successful species, Homo erectus persisted for almost 2 million years before suddenly becoming extinct about 70,000 years ago (0.07 Ma)â€"perhaps a casualty of the Toba supereruption catastrophe.
The subspecies Homo sapiens sapiens, anatomically modern humans, emerged about 200,000 years ago (0.2 Ma) in East Africa (see Omo remains). Modern humans migrated from Africa as recently as 60,000 years ago. During Upper Paleolithic times they spread throughout Africa, Eurasia, Oceania, and the Americas, and they encountered archaic humans along the way during these migrations. Homo sapiens sapiens is the only surviving species and subspecies of the genus Homo. Archaic humans survived until about 40,000 years ago (see H. neanderthalensis), and possibly until as late as the times of the Epipaleolithic culture (about 12,000 years ago). DNA analysis provides some evidence of interbreeding between archaic and modern humans.
Names and taxonomy
- See Homininae for an overview of taxonomy.
The Latin noun homÅ (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 properly 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 times produced a rich mix for debating classifications. There is continuing debate on delineating Homo from Australopithecusâ€"or, indeed, delineating Homo from Pan, as one body of scientists argue that the two species of chimpanzee should be classed with genus Homo rather than Pan. Even so, classifying the fossils of Homo coincides with evidences of: 1) competent human bipedalism in Homo habilis inherited from the earlier Australopithecus of more than four million years ago, (see Laetoli); and 2) human tool culture having begun by 2.5 million years ago.
From the late-19th to mid-20th century, 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 and singular 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") in even scientific papers to avoid trinomial names or the ambiguity of classifying groups as incertae sedis (uncertain placement)â€"for example, H. neanderthalensis vs. H. sapiens neanderthalensis, or H. georgicus vs. H. erectus georgicus. Some recently extinct species in the genus Homo are only recently discovered and do not as yet have consensus binomial names (see Denisova hominin and Red Deer Cave people).
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 Homoâ€"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 (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 Hominina.
Evolution
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 as to which gave rise to Homo. The advent of Homo was traditionally taken to coincide with the first use of stone tools (the Oldowan industry), and thus by definition with the beginning of the Lower Palaeolithic. The emergence of Homo also coincides roughly with the onset of Quaternary glaciation, the beginning of the current ice age.
A fossil mandible fragment dated to 2.8 million years ago which may represent an intermediate stage between Australopithecus and Homo was discovered in 2015 in Afar, Ethiopia (LD 350-1). Some authors would push the development of Homo past 3 Mya, by including Kenyanthropus (a fossil dated 3.2 to 3.5 Mya, usually classified as an australopithecine species) into the genus Homo.
The most salient physiological development between the earlier australopithecine 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 genus Homo, 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.
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 (1.9 to 1.4 million years ago), during the early Calabrian.
Migration
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.
List of species
The species status of H. rudolfensis, H. ergaster, H. georgicus, H. antecessor, H. cepranensis, H. rhodesiensis, H. neanderthalensis, Denisova hominin, Red Deer Cave people, and H. 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.
Homo naledi was discovered near Johannesburg, South Africa in 2013 and announced on 10 September 2015. Fossils indicate the hominin was 1.45â€"1.5 meters tall and had a small brain. The fossils have been dated to be between 335,000 and 236,000 years old.
See also
- List of human evolution fossils (with images)
- Nature timeline
References
Further reading
- Serre; Langaney, André; Chech, Mario; Teschler-Nicola, Maria; Paunovic, Maja; Mennecier, Philippe; Hofreiter, Michael; Possnert, Göran; Pääbo, Svante; et al. (2004). "No evidence of Neandertal mtDNA contribution to early modern humans". PLoS Biology. 2 (3): 313â€"7. PMC 368159 . PMID 15024415. doi:10.1371/journal.pbio.0020057.Â
External links
- Exploring the Hominid Fossil Record (Center for the Advanced Study of Hominid Paleobiology at George Washington University)
- Hominid species
- Prominent Hominid Fossils
- Mikko's Phylogeny archive
- Homo at the Encyclopedia of Life
- Human Timeline (Interactive) â€" Smithsonian, National Museum of Natural History (August 2016).
