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From Apes to Modern Humans
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The next big change in the hominin lineage happened a little over 2 million years ago. From the early australopithecines two different features developed. One was the appearance of large cheek teeth and powerful chewing muscles. These hominins had sagittal crests running from the front to the back of the skull where massive muscles were attached to move their large jaws. They probably did a lot of heavy chewing and ate coarse, fibrous plant foods. These hominins became known as robust australopithecines. The robust australopithecines were geographically diverse, with A. robustus in South Africa and A. boisei in East Africa. They lived until about 1.5 million years ago. At the same time, another group of hominins developed big brains rather than big teeth. Their faces were also smaller and less prognathous than the robust australopithecines. This group has been designated as belonging to our genus, Homo. There were two early forms in East Africa at about 1.5 to 2 million years ago: H. habilis and H. rudolfensis. There is a significant amount of variation between these two types, more than what is normally found between males and females (sexual dimorphism), which is why they are defined as two different species. Along with bigger brains came changes in behavior such as incorporating more meat into their diets and the manufacture and use of tools. Homo was also the first hominin to venture outside of Africa. The primarily Asian form of Homo is known as H. erectus and the African form is H. ergaster. H. ergaster is also known as "Early H. erectus." At first, they were found only in Eastern and Southern Africa, but after about a million years ago, similar forms were found showing the same characteristic brow ridges as far as China and Java. The spread of H. erectus from Africa to Asia probably occurred closer to 2 million year ago. The Asian line was probably divided to some extent geographically, with distinct populations in Southeast and Northeast Asia that persisted until around 400,000 years ago as other populations of archaic Homo evolved. However, recent studies have dated possible fossils of H. erectus in Java to only 50,000 years ago, meaning that at least one population of H. erectus lived at the same time as modern humans. The African form of Homo and its descendants gave rise to H. heidelbergensis and eventually H. sapiens. In Europe, H. heidelbergensis evolved into the European Neanderthals (H. neanderthalensis). In Africa, archaic forms of H. sapiens emerged. These forms of H. sapiens had large brains like ours, were still robustly built with relatively larges faces like H. erectus, but lacked the distinctive long, low head shape and unbroken brow ridges. The last major change to occur among the hominins was the appearance of anatomically modern humans, which are characterized by smaller faces, higher and more rounded crania, more lightly built skeletons and behavioral flexibility. Modern humans dispersed all over the world, showing geographical variation but no major evolutionary divergence.
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About 5 million years ago, some
apes started to habitually walk on two feet. This pattern of bipedalism
differentiated them from the other apes and marked the beginning of the
lineage of hominins. Fossils of early hominins have been found primarily
in eastern and southern Africa. The earliest major group of hominins
belong to the genus Australopithecus. They had relatively human-like bodies and an ape-like head: protruding jaws, big teeth, small cranium and low forehead. Not much changed over the next 2 million years, during which hominins were made up of a number of species of these ape-like bipeds.