New Findings and New Questions The study of human ancestors is exciting and constantly changing. Since the 1990s, new discoveries in Africa have doubled the number of known hominine species. Those discoveries also doubled the length of the known hominine fossil record—from 3.5 million years to 7 million years, a time that corresponds closely to the time at which DNA studies suggest that the lineage that led to humans split from the lineage that led to chimpanzees. These new data have enhanced the picture of our species' past. Questions still remain as to how fossil hominines are related to one another—and to humans. In fact, the field is changing so rapidly that all we can present here is a sampling of current hypotheses.

Relatives Versus Ancestors Most paleontologists agree that the hominine fossil record includes seven genera—Sahelanthropus, Orrorin, Ardipithecus, Australopithecus, Paranthropus, Kenyanthropus, and Homo—and at least 20 species. These diverse hominine fossils stretch back in time roughly 7 million years. All these species are relatives of modern humans, but not all of them are human ancestors. To understand that distinction, think of your family. Your relatives may include aunts, uncles, cousins, parents, grandparents, and great-grandparents. All of these folks are your relatives, but only your parents, grandparents, and great-grandparents are your ancestors. Distinguishing relatives from ancestors in the hominine family is an ongoing challenge.

The Oldest Hominine? In 2002, paleontologists working in north-central Africa discovered a fossil skull roughly 7 million years old. This fossil, called Sahelanthropus, is a million years older than any known hominine. Sahelanthropus had a brain about the size of that of a modern chimp, but its short, broad face was more like that of a human. Scientists are still debating whether this fossil represents a hominine.

Australopithecus Some early hominine fossil species seem to belong to the lineage that led to modern humans, while others formed separate branches off the main hominine line. One early group of hominines, of the genus Australopithecus, lived from about 4 million to about 1.5 million years ago. These hominines were bipedal apes, but their skeletons suggest that they probably spent at least some time in trees. The structure of their teeth suggests a diet rich in fruit.

The best-known of these species is Australopithecus afarensis, which lived from roughly 4 million to 2.5 million years ago. The humanlike footprints in Figure 26–17, about 3.6 million years old, were probably made by members of this species. A. afarensis fossils indicate the species had small brains, so the footprints show that hominines walked bipedally long before large brains evolved. Other fossils of this genus indicate that males were much larger than females. You can see artists' conceptions of young female and adult female A. afarensis in Figure 26–18.

FIGURE 26–17 Laetoli Footprints Between 3.8 and 3.6 million years ago, members of a species of Australopithecus made these footprints at Laetoli in Tanzania. The footprints show that hominines walked upright millions of years ago.

In Your Notebook How long ago does DNA evidence suggest that the human lineage split from the chimpanzee lineage?

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