Humans, Early

In 1871 Charles Darwin, the man who gained fame with his theory of evolution, discussed the origins of humans in a book called Descent of Man. Darwin noted that chimpanzees and gorillas—humans' closest animal relatives—are found only in Africa and suggested that Africa was also the birthplace of the human species. Modern paleoanthropologists—the scientists who study early humans and their relatives—agree with Darwin. Fossils and other evidence indicate that human ancestors, and probably modern humans as well, appeared first in Africa.

The story of human origins in Africa and of how humans populated the rest of the world is not complete. It will probably change as archaeologists uncover more fossils. Even the evidence that now exists has given rise to different interpretations and theories, and some ideas concerning early humans are hotly debated. Still, most scientists today agree on a broad outline of human evolution that begins in Africa millions of years ago.

HUMAN ANCESTORS

Darwin did not believe that humans were descended from apes and monkeys. Instead he believed that both humans and apes descended from a common ancestor. Fossil evidence suggests that a variety of apelike creatures, now extinct, lived in Africa at least 25 million years ago. In time, the first human ancestors evolved from one of these species.

The First Hominids

Human beings are bipedal, meaning that they normally walk on two feet. Bipedal walking is one of the main physical characteristics of hominids, members of the Hominidae family to which humans belong.

Probably sometime between 8 million and 6 million years ago, one of the ancient African ape species became bipedal. Scientists do not know what caused the shift from four-footed to two-footed walking, which marked the beginning of hominids and the first step in human evolution. Some believe that widespread climate change transformed large areas of African forest into savanna plains. This environment suited creatures that could stride upright and look far ahead. But this theory has not been proved.

Over several million years, a variety of different hominids evolved in Africa, the only continent where traces of such ancient human ancestors have been found. The earliest known hominid is called Ardipithecus ramidus. Fossils found in ETHIOPIA, and possibly some from KENYA, belong to this species; they are about 4.4 million years old. The head and teeth of A. ramidus were very similar to those of apes, but it seems to have walked upright. It had already separated from the evolutionary line leading to the modern African gorilla and chimpanzee.

A better-known hominid species, Australopithecus afarensis, lived from at least 3.6 million to about 2.8 million years ago. At a site called Hadar in Ethiopia, fossil diggers discovered a nearly complete female A. afarensis skeleton. Nicknamed Lucy, this hominid became one of the most famous fossils in the world. Another important find came from Laetoli in TANZANIA, where the paleoanthropologist Mary Leakey discovered A. afarensis footprints in volcanic ash. The tracks confirm that this hominid walked upright, though scientists who have studied its hand and foot bones believe that it also spent a lot of time in trees. A. afarensis was small by modern standards, probably about 4 feet tall and weighing between 64 and 99 pounds, with males much larger than females. The same is true of a similar species, Australopithecus africanus, that lived around the same time in what is now SOUTH AFRICA. Kenyanthropus platyops, a fossil found in 1999 in northern Kenya, suggests that another species with a flatter face and smaller teeth lived at about the same time as A. afarensis.

Other australopithecines also evolved in Africa, but scientists are not certain which species was the ancestor of later hominids, including today's humans. Some fossil evidence suggests that one or more kinds of australopithecines survived until about 1.5 million years ago in eastern and southern Africa. By that time, however, more advanced hominids had developed and were living alongside them.

Early, Extinct Humans

The new type of advanced hominid appeared around 2.5 million years ago. With a larger brain and a flatter, less apelike face than the australopithecines, it belonged to the genus Homo, a subcategory of hominids that includes all species considered to be true humans.

The earliest of these humans that scientists know about is called Homo habilis, which means “handy man.” The name was given in 1964 by the paleoanthropologist Louis B. Leakey, who discovered some of the first Homo fossils in Tanzania's Olduvai Gorge along with the oldest recognizable stone tools. Although some scholars think that the later australopithecines may have made and used simple tools, the evidence is unclear. There is no doubt, however, that soon after 2 million years ago, Homo habilis turned flakes of stone into tools for cutting and scraping.

By this time the genus Homo had evolved into several kinds of humans in East Africa. Other types may have lived in other parts of the continent. Paleoanthropologists do not always agree on the Homo species of a fossil find—often mere fragments, a jawbone, a few teeth, a hip bone. Discovering a complete skull is rare, and even more rare is finding different parts of the skeleton of a single individual. When comparing two skull fragments, one archaeologist might see them as members of different species, while another could regard them as differences within a single species.

A remarkably complete skeleton found in 1984 at Nariokotome, near Lake Turkana in Kenya, shows how hominid fossils can be interpreted in different ways. The skeleton belongs to a boy who was about 11 years old when he died, some 1.6 million years ago. Researchers estimate that if he had reached adulthood, he would have been taller than 6 feet and weighed around 150 pounds. Some experts regard him as an example of a human species they call Homo ergaster, which later evolved into Homo erectus. But most scientists think that the Turkana boy already belonged to H. erectus.

Homo erectus—”upright man”—ranged farther than any of the other early human species. Its remains have been found in North Africa, in sites in MOROCCO and ALGERIA, as well as at several sites in eastern and southern Africa. In addition, H. erectus is also the first human species known to have left the African continent. Sites in China and on the Indonesian island of Java have yielded fossils of H. erectus, and some of those from Java may be 1.8 million years old, almost as old as the earliest known African examples. In 1999 scientists working in the southeastern European nation of Georgia discovered skulls and bones that were 1.7 million years old, but they are unsure whether the fossils belong to Homo ergaster or Homo erectus.

From evidence found in caves and other sites, paleoanthropologists know that the Homo erectus of Africa and Asia were nomadic individuals who gathered wild plants and hunted for meat. They made flaked stone tools, and around 1.4 million years ago, those in Africa began making larger, multipurpose stone tools such as cleavers and hand axes. Homo erectus survived for more than 1 million years but eventually gave way to new human species, including Homo sapiens—modern humans, the only hominid species that still exists.

MODERN HUMANS

Nearly all paleoanthropologists think that Homo erectus was the direct or indirect ancestor of Homo sapiens. How the change took place, however, is the subject of vigorous debates and disputes. Only additional fossil finds and further study, perhaps with research techniques not yet developed, will settle the question.

One clue may lie in a group of African and European fossils with features that fall somewhere between those of H. erectus and H. sapiens. Some scientists think that these represent a species called Homo heidelbergensis that lived between 500,000 and 200,000 years ago. H. heidelbergensis evolved into a type of human called H. neanderthalensis, or Neanderthal man, that lived in Europe and western Asia before becoming extinct around 30,000 years ago. Before they disappeared, the Neanderthal people shared their world with modern Homo sapiens for some time. But where, when, and how did those modern humans emerge? The most recent scientific discoveries and interpretations have led to two general theories: the multiregional theory and the “out of Africa” theory.

The Multiregional Theory

Some scholars believe that Homo erectus evolved into Homo sapiens in more than one region of the world. According to the multiregional theory, H. erectus left Africa before 1.8 million years ago, spread across Asia and possibly Europe, and then began to evolve into modern humans, who have some differences in appearance around the world.

Although no H. erectus fossils have yet been found in Europe, supporters of the multiregional theory believe that H. erectus existed in Europe, where it evolved into the Neanderthals. Some scientists who accept this interpretation believe that H. erectus and its various regional descendants are really just one species, Homo sapiens, reaching back 1.8 million years into the past.

The “Out of Africa” Theory

The alternate view is that although Homo erectus spread to Asia and Europe, its evolution there ended with extinction. Meanwhile in Africa, Homo sapiens evolved in a separate line which developed and spread around the world. If so, all humans alive today are descended from a relatively recent origin in Africa. Africa has produced the oldest known H. sapiens fossils. A skeleton from Omo in Ethiopia and a skull and leg bone from Guomde in Kenya could be as old as 130,000 years, while remains from South Africa are between 120,000 and 70,000 years old.

According to the “out of Africa” theory, H. heidelbergensis or some other descendant of African H. erectus evolved into H. sapiens between 200,000 and 120,000 years ago. By about 100,000 years ago, part of the H. sapiens population had spread from Africa into neighboring regions. They reached Australia by about 40,000 years ago, using boats or rafts. By walking across a land bridge between Siberia and Alaska, or by sailing to the southern Alaskan islands, they reached the Americas between 35,000 and 15,000 years ago.

The “out of Africa” theory says that all regional variations in the modern human population developed during and after the migration from Africa. If this theory is true, there is no genetic connection between the ancient H. erectus populations of China and Indonesia and the present-day inhabitants of those countries. All people alive today are descended from the Homo sapiens population that evolved in Africa and spread out from there 100,000 years ago. Outside Africa, H. erectus evolved into new human types, including the Neanderthals, but these became extinct. They probably did not contribute to the ancestry of modern humans, although some scientists have wondered whether H. sapiens might have interbred with early human species such as the Neanderthals.

The African Eve

In 1987 the technology of DNA testing gave rise to a new theory about human origins. Researchers focused on a particular kind of DNA called mitochondrial DNA (mtDNA), which mothers pass to their children. By comparing samples from around the world, the researchers were able to examine mtDNA differences among various populations for clues to the relationships among those groups. The results showed that African mtDNA reflected more genetic diversity than European or Asian mtDNA. This suggested that the African DNA was older, which was interpreted as evidence that humans originated in Africa.

One version of this theory became a popular myth, claiming that all present-day human mtDNA could be traced back to a female ancestor in Africa about 200,000 years ago. Some people called this mythical woman the “African Eve,” after the biblical story of the first woman. The “African Eve” theory attracted public attention, but it was not based on well-grounded conclusions. In addition, the term “Eve” gave the false impression that there had been a single ancestor, instead of an ancestral population.

Additional DNA testing has, however, given support to the general “out of Africa” theory. Homo erectus certainly came from Africa, and there is growing support for the theory that its modern descendants originated there as well. In any case, human evolution is an ongoing process that will not end with Homo sapiens, so long as the species survives. (See also Archaeology and Prehistory, Leakey Family.)