Human Evolution

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Middle Homo

By about 1.9 million years ago, the period of middle Homo had begun in Africa. Until recently, paleoanthropologists recognized one species in this period, Homo erectus. Many now recognize three species of middle Homo: H. ergaster, H. erectus, and H. heidelbergensis. However, some still think H. ergaster is an early African form of H. erectus, or that H. heidelbergensis is a late form of H. erectus.

The skulls and teeth of early African populations of middle Homo differed subtly from those of later H. erectus populations from China and the island of Java in Indonesia. H. ergaster makes a better candidate for an ancestor of the modern human line because Asian H. erectus has some specialized features not seen in some later humans, including our own species. H. heidelbergensis has similarities to both H. erectus and the later species H. neanderthalensis, although it may have been a transitional species between middle Homo and the line to which modern humans belong.

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Homo ergaster

Homo ergaster probably first evolved in Africa around 2 million years ago. This species had a rounded cranium with a brain size of between 700 and 850 cu cm (49 to 52 cu in), a prominent brow ridge, small teeth, and many other features that it shared with the later H. erectus. Many paleoanthropologists consider H. ergaster a good candidate for an ancestor of modern humans because it had several modern skull features, including relatively thin cranial bones. Most H. ergaster fossils come from the time range of 1.8 million to 1.5 million years ago.

The most important fossil of this species yet found is a nearly complete skeleton of a young male from West Turkana, Kenya, which dates from about 1.55 million years ago. Scientists determined the sex of the skeleton from the shape of its pelvis. They also determined from patterns of tooth eruption and bone growth that the boy had died when he was between 9 and 12 years old.

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The Turkana boy, as the skeleton is known, had elongated leg bones and arm, leg, and trunk proportions that essentially match those of a modern human, in sharp contrast with the apelike proportions of H. habilis and Australopithecus afarensis. He appears to have been quite tall and slender. Scientists estimate that, had he grown into adulthood, the boy would have reached a height of 1.8 m (6 ft) and a weight of 68 kg (150 lb). The anatomy of the Turkana boy indicates that H. ergaster was particularly well adapted for walking and perhaps for running long distances in a hot environment (a tall and slender body dissipates heat well) but not for any significant amount of tree climbing.

The oldest humanlike fossils outside of Africa have also been classified as H. ergaster, dated around 1.75 million years old. These finds, from the Dmanisi site in the southern Caucasus Mountains of Georgia, consist of several crania, jaws, and other fossilized bones. Some of these are strikingly like East African H. ergaster, but others are smaller or larger than H. ergaster, suggesting a high degree of variation within a single population.

H. ergaster, H. rudolfensis, and H. habilis, in addition to possibly two robust australopiths, all might have coexisted in Africa around 1.9 million years ago. This finding goes against a traditional paleoanthropological view that human evolution consisted of a single line that evolved progressively over time—an australopith species followed by early Homo, then middle Homo, and finally H. sapiens. It appears that periods of species diversity and extinction have been common during human evolution, and that modern H. sapiens has the rare distinction of being the only living human species today.

Although H. ergaster appears to have coexisted with several other human species, they probably did not interbreed. Mating rarely succeeds between two species with significant skeletal differences, such as H. ergaster and H. habilis. Many paleoanthropologists now believe that H. ergaster descended from an earlier population of Homo—perhaps one of the two known species of early Homo—and that the modern human line descended from H. ergaster.

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Homo erectus

Paleoanthropologists now know that humans first evolved in Africa and lived only on that continent for a few million years. The earliest human species known to have spread in large numbers beyond the African continent was first discovered in Southeast Asia. In 1891 Dutch physician Eugène Dubois found the cranium of an early human on the Indonesian island of Java. He named this early human Pithecanthropus erectus, or “erect ape-man.” Today paleoanthropologists refer to this species as Homo erectus.

H. erectus appears to have evolved in Africa from earlier populations of H. ergaster, and then spread to Asia sometime between 1.8 million and 1.5 million years ago. The youngest known fossils of this species, from the Solo River in Java, may date from as recently as 53,000 to 27,000 years ago (although that dating is controversial). So H. erectus was a very successful species—both widespread, having lived in Africa and much of Asia, and long-lived, having survived for possibly more than 1.5 million years.

H. erectus had a low and rounded braincase that was elongated from front to back, a prominent brow ridge, and an adult cranial capacity of 800 to 1,250 cu cm (50 to 80 cu in), an average twice that of the australopiths. Its bones, including the cranium, were thicker than those of earlier species. Prominent muscle markings and thick, reinforced areas on the bones of H. erectus indicate that its body could withstand powerful movements and stresses. Although it had much smaller teeth than did the australopiths, it had a heavy and strong jaw.

In the 1920s and 1930s German anatomist and physical anthropologist Franz Weidenreich excavated the most famous collections of H. erectus fossils from a cave at the site of Zhoukoudian (Chou-k’ou-tien), China, near Beijing (Peking). Scientists dubbed these fossil humans Sinanthropus pekinensis, or Peking Man, but others later reclassified them as H. erectus. The Zhoukoudian cave yielded the fragmentary remains of over 30 individuals, ranging from about 500,000 to 250,000 years old. These fossils were lost near the outbreak of World War II, but Weidenreich had made excellent casts of his finds. Further studies at the cave site have yielded more H. erectus remains.

Other important fossil sites for this species in China include Lantian, Yuanmou, Yunxian, and Hexian. Researchers have also recovered many tools made by H. erectus in China at sites such as Nihewan and Bose, and other sites of similar age (at least 1 million to 250,000 years old).

Ever since the discovery of H. erectus, scientists have debated whether this species was a direct ancestor of later humans, including H. sapiens. The last populations of H. erectus—such as those from the Solo River in Java—may have lived as recently as 53,000 to 27,000 years ago, at the same time as did populations of H. sapiens. Modern humans could not have evolved from these late populations of H. erectus, a much more primitive type of human. However, earlier East Asian populations could have given rise to H. sapiens.

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Homo heidelbergensis

Many paleoanthropologists believe that early humans migrated into Europe by 800,000 years ago, and that these populations were not Homo erectus. A growing number of scientists refer to these early migrants into Europe—who predated both Neandertals and H. sapiens in the region—as H. heidelbergensis. The species name comes from a 500,000-year-old jaw found near Heidelberg, Germany.

Scientists have found few human fossils in Africa for the period between 1.2 million and 600,000 years ago, during which H. heidelbergensis or its ancestors first migrated into Europe. Populations of H. ergaster (or possibly H. erectus) appear to have lived until at least 800,000 years ago in Africa, and possibly until 500,000 years ago in northern Africa. When these populations disappeared, other massive-boned and larger-brained humans—possibly H. heidelbergensis—appear to have replaced them. Scientists have found fossils of these stockier humans at sites in Bodo, Ethiopia; Saldanha (also known as Elandsfontein), South Africa; Ndutu, Tanzania; and Kabwe, Zimbabwe.

Scientists have come up with at least three different interpretations of these African fossils. Some scientists place the fossils in the species H. heidelbergensis and think that this species gave rise to both the Neandertals (in Europe) and H. sapiens (in Africa). Others think that the European and African fossils belong to two distinct species, and that the African populations—which, in this view, were not H. heidelbergensis but a separate species—gave rise to H. sapiens. Yet other scientists advocate a long-held view that H. erectus and H. sapiens belong to a single evolving lineage, and that the African fossils belong in the category of archaic H. sapiens (archaic meaning not fully anatomically modern).

The fossil evidence does not clearly favor any of these three interpretations over another. A growing number of fossils from Asia, Africa, and Europe have features that are intermediate between early H. ergaster and H. sapiens. This kind of variation makes it hard to decide how to identify distinct species and to determine which group of fossils represents the most likely ancestor of later humans.

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Why Did Humans Spread Out of Africa?

Humans evolved in Africa and lived only there for as long as 4 million years or more, so scientists wonder what finally triggered the first human migration out of Africa (a movement that coincided with the spread of early human populations throughout the African continent). The answer to this question depends, in part, on knowing exactly when that first migration occurred. Some studies claim that sites in Asia and Europe contain crude stone tools and fossilized fragments of humanlike teeth that date from more than 1.8 million years ago. Although these claims remain unconfirmed, small populations of humans may have entered Asia prior to 1.8 million years ago, followed by a more substantial spread between 1.6 million and 1 million years ago. Early humans reached northeastern Asia by around 1.4 million years ago, inhabiting a region close to the perpetually dry deserts of northern China. The first major habitation of central and western Europe, on the other hand, does not appear to have occurred until between 1 million and 500,000 years ago.

Scientists once thought that advances in stone tools could have enabled early humans such as Homo erectus to move into Asia and Europe, perhaps by helping them to obtain new kinds of food, such as the meat of large mammals. If African human populations had developed tools that allowed them to hunt large game effectively, they would have had a reliable source of food wherever they went. In this view, humans first migrated into Eurasia based on a unique cultural adaptation.

By 1.5 million years ago, early humans had begun to make new kinds of tools, which scientists call Acheulean. Common Acheulean tools included large handaxes and cleavers. While these new tools might have helped early humans to hunt, the first known Acheulean tools in Africa date from later than the earliest known human presence in Asia. Also, most East Asian sites over 200,000 years old contain only simply shaped cobble and flake tools. In contrast, Acheulean tools were more finely crafted, larger, and more symmetrical. Thus, the earliest settlers of Eurasia did not have a true Acheulean technology, and advances in toolmaking alone cannot explain the spread out of Africa.

Another possibility is that the early spread of humans to Eurasia was not unique, but rather part of a wider migration of meat-eating animals, such as lions and hyenas. The human migration out of Africa occurred during the early part of the Pleistocene Epoch, between 1.8 million and 780,000 years ago. Many African carnivores spread to Eurasia during the early Pleistocene, and humans could have moved along with them. In this view, H. erectus was one of many meat-eating species to expand into Eurasia from Africa, rather than a uniquely adapted species. Relying on meat as a primary food source might have allowed many meat-eating species, including humans, to move through many different environments without having to quickly learn about unfamiliar and potentially poisonous plants.

However, the migration of humans to eastern Asia may have occurred gradually and through lower latitudes and environments similar to those of Africa. If East African populations of H. erectus moved at only 1.6 km (1 mi) every 20 years, they could have reached Southeast Asia in 150,000 years. Over this amount of time, humans could have learned about and begun relying on edible plant foods. Thus, eating meat may not have played a crucial role in the first human migrations to new continents. Careful comparison of animal fossils, stone tools, and early human fossils from Africa, Asia, and Europe will help scientists to better determine what factors motivated and allowed humans to venture out of Africa for the first time.

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