Human Evolution

V

The Genus Homo

People belong to the genus Homo, which first evolved at least 2.3 million to 2.5 million years ago. The earliest members of this genus differed from the australopiths in at least one important respect—they had larger brains than did their predecessors.

The evolution of the modern human genus can be divided roughly into three periods: early, middle, and late. Species of early Homo resembled gracile australopiths in many ways. Some early Homo species lived until possibly 1.6 million years ago. The period of middle Homo began perhaps between 2 million and 1.8 million years ago, overlapping with the end of early Homo. Species of middle Homo evolved an anatomy much more similar to that of modern humans but had comparatively small brains. The transition from middle to late Homo probably occurred sometime around 200,000 years ago. Species of late Homo evolved large and complex brains and eventually language. Culture also became an increasingly important part of human life during the most recent period of evolution.

A

Origins

The origin of the genus Homo has long intrigued paleoanthropologists and prompted much debate. One of several known species of australopiths, or one not yet discovered, could have given rise to the first species of Homo. Scientists also do not know exactly what factors favored the evolution of a larger and more complex brain—the defining physical trait of modern humans.

Louis Leakey originally argued that the origin of Homo related directly to the development of toolmaking—specifically, the making of stone tools. Toolmaking requires certain mental skills and fine hand manipulation that may exist only in members of our own genus. Indeed, the name Homo habilis (meaning “handy man”) refers directly to the making and use of tools.

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However, several species of australopiths lived at the same time as early Homo, making it unclear which species produced the earliest stone tools. Recent studies of australopith hand bones have suggested that at least one of the robust species, Australopithecus robustus, could have made tools. In addition, during the 1960s and 1970s researchers first observed that some nonhuman primates, such as chimpanzees, make and use tools, suggesting that australopiths and the apes that preceded them probably also made some kinds of tools.

According to some scientists, however, early Homo probably did make the first stone tools. The ability to cut and pound foods would have been most useful to these smaller-toothed humans, whereas the robust australopiths could chew even very tough foods. Furthermore, early humans continued to make stone tools similar to the oldest known kinds for a time long after the gracile australopiths died out.

Some scientists think that a period of environmental cooling and drying in Africa set the stage for the evolution of Homo. According to this idea, many types of animals suited to the challenges of a drier environment originated during the period between about 2.8 million and 2.4 million years ago, including the first species of Homo. A toolmaking human might have had an advantage in obtaining alternative food sources as vegetation became sparse in increasingly dry environments. The new foods might have included underground roots and tubers, as well as meat obtained through scavenging or hunting. However, some scientists disagree with this idea, arguing that the period during which Homo evolved fluctuated between drier and wetter conditions, rather than just becoming dry. In this case, the making and use of stone tools and an expansion of the diet in early Homo—as well as an increase in brain size—may all have been adaptations to unpredictable and fluctuating environments. In either case, more scientific documentation is necessary to strongly support or refute the idea that early Homo arose as part of a larger trend of rapid species extinction and the evolution of many new species during a period of environmental change.

B

Early Homo

Paleoanthropologists generally recognize two species of early Homo—Homo habilis and H. rudolfensis (although other species may also have existed). The record is unclear because most of the early fossils that scientists have identified as species of Homo—rather than robust australopiths who lived at the same time—occur as isolated fragments. In many places, only teeth, jawbones, and pieces of skull—without any other skeletal remains—indicate that new species of smaller-toothed humans had evolved as early as 2.5 million years ago. Scientists cannot always tell whether these fossils belong to late-surviving gracile australopiths or early representatives of Homo. The two groups resemble each other because Homo likely descended directly from a species of gracile australopith.

B

1

Homo habilis

Between 1960 and 1963, at Olduvai Gorge, Tanzania, a team led by Louis and Mary Leakey discovered the remains of an early human that seemed distinctly different from the australopiths. In 1964 Louis Leakey, South African paleoanthropologist Philip Tobias, and British primate researcher John Napier concluded that these remains represented a new species, which they named Homo habilis. The scientists placed the species in the genus Homo because its brain was estimated to be significantly larger than that of any known australopith. Other scientists questioned whether the amount of brain enlargement was sufficient for inclusion of the species in Homo, and even whether H. habilis was different from Australopithecus africanus, as the teeth of the two species look similar. However, scientists now widely accept both the genus and species names designated by the Olduvai team. According to recent estimates, H. habilis had a brain volume that ranged from 590 to 690 cu cm (36 to 42 cu in), well above the range for australopithecines.

H. habilis lived in eastern and possibly southern Africa between about 1.9 million and 1.6 million years ago, and maybe as early as 2.4 million years ago. Although the fossils of this species somewhat resemble those of australopiths, H. habilis had smaller and narrower molar teeth, premolar teeth, and jaws than did its predecessors and contemporary robust australopiths.

A fragmented skeleton of a female from Olduvai shows that she stood only about 1 m (3.3 ft) tall, and the ratio of the length of her arms to her legs was greater than that in the australopith Lucy. At least in the case of this individual, therefore, H. habilis had very apelike body proportions. However, H. habilis had more modern-looking feet and hands capable of producing tools. Some of the earliest stone tools from Olduvai have been found with H. habilis fossils, suggesting that this species made and used the tools at this site.

Scientists began to notice a high degree of variability in body size as they discovered more early Homo fossils. This could have indicated that H. habilis had a large amount of sexual dimorphism. For instance, the Olduvai female skeleton was dwarfed in comparison with some other fossils—exemplified by a sizable early Homo cranium from East Turkana in northern Kenya. However, the differences in size actually exceeded those expected between males and females of the same species, and this finding later helped convince scientists that another species of early Homo had lived in eastern Africa.

B

2

Homo rudolfensis

This second species of early Homo was given the name Homo rudolfensis, after Lake Rudolf (now Lake Turkana). The best-known fossils of H. rudolfensis come from the area surrounding this lake and date from about 1.9 million years ago. Paleoanthropologists have not determined the entire time range during which H. rudolfensis may have lived.

This species had a larger face and body than did H. habilis. The cranial capacity of H. rudolfensis averaged about 750 cu cm (46 cu in). Scientists need more evidence to know whether the brain of H. rudolfensis in relation to its body size was larger than that proportion in H. habilis. A larger brain-to-body-size ratio can indicate increased mental abilities. H. rudolfensis also had fairly large teeth, approaching the size of those in robust australopiths. The discovery of even a partial fossil skeleton would reveal whether this larger form of early Homo had apelike or more modern body proportions. Scientists have found several modern-looking thighbones that date from between 2 million and 1.8 million years ago and may belong to H. rudolfensis. These bones suggest a body size of 1.5 m (5 ft) and 52 kg (114 lb).

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