Homo

• 1. Introduction

  ›

  Homo is the genus of great apes to which every person alive today belongs. Carl Linnaeus created it in the 18th century and coined the name Homo sapiens in 1758. He never wrote down what the genus included. At the time it held only humans, and he saw no reason to expect more members. The Latin word homo means human being, and that was definition enough. Then fossils began to surface, and the question stopped being simple. Today Homo holds one living species and a long list of extinct ones, and scientists still dispute many of them. Where does the genus really begin? Which fossils earn a place in it? And how separate were its members from one another?

  141513

• 2. Naming the Genus

  ›

  The Neanderthal became the first addition to Homo after Linnaeus. Homo neanderthalensis received its name in 1864, and Homo erectus followed in 1892. Between the late 19th and mid 20th centuries, researchers also proposed brand new genera for early human fossils. Pithecanthropus, Sinanthropus, Africanthropus and Atlanthropus were among them, alongside Protanthropus, Cyphanthropus, Telanthropus and Tchadanthropus. Most have since been merged into Homo, because Homo erectus is now understood as a single species that spread across a very wide area.

  In 2000, Wood and Richmond suggested a way to organize the wider family tree. They proposed that Hominini be a tribe containing all early humans and pre-humans since the split from chimpanzees. Within it, a subtribe called Hominina would hold the genus Homo alone. That would leave out earlier upright walkers such as Australopithecus, Orrorin tugenensis, Ardipithecus and Sahelanthropus. Rival labels existed, including Australopithecinae from Gregory and Hellman in 1939 and Preanthropinae from Cela-Conde and Altaba in 2002.

  Scientific papers today often use the plain names Neanderthal and Denisovan. The habit avoids three-part names such as H. sapiens neanderthalensis. It also avoids the Latin label incertae sedis, which means uncertain placement. Some recently identified groups, including the Denisova hominin, still lack a name that researchers agree on. Much of that uncertainty traces back to a harder question about how far back the genus actually reaches.

  1833212264

• 3. Where Homo Begins

  ›

  Chimpanzees and bonobos, which form the genus Pan, are the closest living relatives of Homo. Their ancestors and ours are estimated to have diverged between 5.7 and 11 million years ago, during the Late Miocene. Homo itself grew out of Australopithecus. Together with the genus Paranthropus, it is probably most closely related to Australopithecus africanus.

  Australopithecus garhi, sediba, africanus and afarensis have each been proposed as an ancestor or sister of the Homo lineage. All of them show features that resemble Homo, yet researchers have not agreed on which one gave rise to it. Two older markers help anchor the debate. The Laetoli footprints show that competent upright walking was inherited from Australopithecus more than four million years ago. Stone tool culture had begun by 2.5 to 3 million years ago.

  Tools were once treated as the sign that Homo had arrived, marking the start of the Lower Palaeolithic. In 2010, evidence was presented that Australopithecus afarensis used stone tools around 3.3 million years ago. That is close to a million years before the first Homo fossils. A mandible fragment known as LD 350-1, found in Afar, Ethiopia in 2013, is dated to 2.8 million years ago. It blends primitive traits of early Australopithecus with features seen in later Homo.

  Some authors push the origin of Homo close to or beyond 3 million years ago. A phylogenetic study combining anatomical, molecular and radiometric data puts it at 3.3 million years ago, within a range of 4.30 to 2.56 million. Others go the opposite way and doubt that Homo habilis belongs at all. They would start the genus with Homo erectus at roughly 1.9 million years ago.

  Brain size looks like a tidier yardstick, but it fails as one. The volume inside the skull rises from about 460 cubic centimeters in Australopithecus garhi to 660 in Homo habilis and 760 in Homo erectus. It reaches 1250 in Homo heidelbergensis and up to 1760 in Homo neanderthalensis. The climb had already begun among the australopithecines and kept going afterward, so no single number marks where the genus starts. The species long treated as the oldest member, Homo habilis, has itself come under suspicion.

  3738413536

• 4. Habilis and Erectus

  ›

  Homo habilis appeared about 2.1 million years ago, and its claim to the genus has weakened steadily. Even before 2010, some researchers argued it belonged in Australopithecus. Its strongest credential was undisputed tool use. That evidence lost force once Australopithecus tool use was traced back at least a million years earlier.

  Fossils from the Koobi Fora Formation, east of Lake Turkana in Kenya, upset the old family tree in 2007. A study of them argued that Homo habilis survived after Homo erectus had appeared. The two may have lived side by side for about half a million years, during the early Calabrian. If so, erectus did not descend directly from habilis. Both would instead share a common ancestor.

  Earlier finds in the Caucasus had pointed the other way. Specimens called Homo erectus georgicus seemed to show traits halfway between habilis and erectus. Their skull capacity rose from around 575 cubic centimeters in habilis to around 850 in erectus. Because the oldest erectus evidence sat outside Africa, some even imagined the species arising in Eurasia and returning to Africa.

  The publication of Dmanisi skull 5 in 2013 shook that picture again. It made it less certain that Asian erectus descended from an African form called Homo ergaster. Ergaster and erectus now look like variants of one species. It may have originated in either Africa or Asia, and by 0.5 million years ago it had spread widely across Eurasia, including Europe, Indonesia and China. The species proved adaptive and successful and lasted more than a million years. Around 500,000 years ago it gradually diverged into new species. A separate South African species, Homo gautengensis, was postulated in 2010 as a contemporary of erectus.

  Genetic studies add a darker footnote. On the 31st of August 2023, researchers reported a population bottleneck among human ancestors, from a possible 100,000 individuals down to about 1,000. They placed it between roughly 930,000 and 813,000 years ago. It lasted about 117,000 years and brought human ancestors close to extinction.

  4542764746

• 5. Out of Africa

  ›

  By about 1.8 million years ago, Homo erectus lived in two places far apart. In East Africa it is called Homo ergaster, and in Western Asia it is called Homo georgicus. The ancestors of the Indonesian Homo floresiensis may have left Africa even earlier. Over the following 1.5 million years, erectus and its relatives or descendants spread through Africa and Eurasia.

  Homo heidelbergensis reached Europe by about 0.5 million years ago. After about 300,000 years ago, Homo neanderthalensis and Homo sapiens developed. Homo naledi was already present in Southern Africa by that same time.

  Homo sapiens soon spread across Africa and then to Western Asia in several waves. The earliest may have come as far back as 250,000 years ago, and the movement is certain by 130,000 years ago. A discovery announced in July 2019 complicated the story further. In Apidima Cave, in the Peloponnese of Greece, anthropologists reported 210,000-year-old remains of Homo sapiens and 170,000-year-old remains of a Neanderthal. The sapiens bones were more than 150,000 years older than any earlier sapiens find in Europe.

  The best known exodus is the Southern Dispersal, dated to around 60,000 years ago. Other estimates place its start between 70,000 and 50,000 years ago. It led to the lasting settlement of Oceania and Eurasia by anatomically modern humans, who were established there by 50,000 years ago.

  Living populations carry the mark of these journeys. The San people of Southern Africa show the deepest split among extant groups, estimated at close to 130,000 years, or possibly more than 300,000. Australo-Melanesians separated from other non-Africans on the order of 60,000 years ago. Europeans and East Asians separated about 50,000 years ago, and repeated mixing between populations has continued throughout Eurasia since the Holocene began.

  6967727365

• 6. Shared Genes

  ›

  Around 45,000 years ago, according to the genomes of people outside sub-Saharan Africa, numerous separate mixing events took place with Neanderthals. In some cases they also involved Denisovans. Homo sapiens met and interbred with archaic humans in Africa as well as Eurasia, so the genus did not branch cleanly like a tree.

  Genetic evidence points to an archaic lineage that split from other human lines about 1.5 million years ago. It was perhaps Homo erectus, and it may have interbred with the Denisovans around 55,000 years ago. An erectus-like lineage of that age appears to have reached modern humans by way of the Denisovans. The signal shows up especially in Papuans and aboriginal Australians.

  The mixing was not limited to the distant past. The genetic structure of some sub-Saharan African groups suggests input from a west Eurasian population about 3,000 years ago.

  Some archaic lines lasted astonishingly late. Fossils show that Homo erectus in the strict sense survived until at least 117,000 years ago. Homo floresiensis lasted until 50,000 years ago. Neanderthals and other separate archaic human species are thought to have survived until around 40,000 years ago, though most were extinct or absorbed into expanding sapiens populations by then.

  A general pattern in mammals helps explain all this. Populations that are splitting into species can typically still interbreed for several million years after they begin to diverge genetically. That means every Homo species could in principle have interbred with the others. One proposal even suggests Homo naledi may be a hybrid with a late surviving australopith, though those lineages are generally considered long extinct.

  6061626370

• 7. Counting the Species

  ›

  A minimalist approach to human taxonomy recognizes at most three species. Homo habilis would stay, though its membership in Homo is questionable. Homo erectus would absorb most archaic varieties as subspecies, with Homo heidelbergensis as a late or transitional form. Homo sapiens would include the Neanderthals and other varieties as subspecies. Anthropology and palaeontology have no generally agreed method for drawing these lines.

  The opposite habit has a long record. Researchers have often postulated new human species from as little as a single fossil. Homo cepranensis rests on one skull cap found in Italy, dated to about 450,000 years ago and possibly belonging to Homo heidelbergensis. Penghu 1, a single individual from Taiwan around 100,000 years old, may be Homo erectus or a Denisovan. Homo luzonensis, from the Philippines, is known from three individuals who lived between 134,000 and 49,000 years ago.

  The bodies behind these names differ remarkably. Homo floresiensis, from Indonesia, stood about 100 centimeters tall, a little over 3 feet. It weighed about 25 kilograms and had a cranial capacity near 400 cubic centimeters, with seven individuals known. Homo naledi, from South Africa, weighed about 45 kilograms and is known from 15 individuals dated between 335,000 and 236,000 years ago. Homo antecessor of western Europe, living between 1.2 million and 800,000 years ago, stood about 175 centimeters and weighed around 90 kilograms.

  The status of Homo rudolfensis, ergaster, georgicus, antecessor, cepranensis, rhodesiensis, neanderthalensis, the Denisova hominin and floresiensis all remains under debate. Heidelbergensis and neanderthalensis are so closely related that they have been treated as subspecies of Homo sapiens. Meanwhile, a group called Nesher Ramla Homo, known from several individuals in Israel and dated between 140,000 and 120,000 years ago, was added to the roster in 2021 with its classification still uncertain.

  4899839798

• 8. Billions of People

  ›

  Kenneth M. Weiss estimated that about 44 billion individuals of the genus Homo lived from its origins to the evolution of Homo erectus. He put the next stretch, from Homo erectus to the Neolithic, at about 56 billion. Another 51 billion have lived since the Neolithic began. Those billions belong to a genus that, since the start of the Holocene, has likely had only one extant species, Homo sapiens.

  4847