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Australopithecus africanus

Australopithecus afarensis

Primate ancestor

Figure 8.16 The evolutionary relationships among hominin species. This tree represents the current consensus among scientists who are attempting to uncover human evolutionary history. Note that modern humans are the last remaining species of a group that was once highly diverse and consisted of several species that coexisted.

Australopithecus africanus

Australopithecus afarensis

Ardepithecus y Australopithecus anamensis ramidus

Primate ancestor

Figure 8.16 The evolutionary relationships among hominin species. This tree represents the current consensus among scientists who are attempting to uncover human evolutionary history. Note that modern humans are the last remaining species of a group that was once highly diverse and consisted of several species that coexisted.

both species since they diverged must be clear. Like humans, modern chimpanzees have been evolving over the five million years since they diverged from humans—in other words, a missing link would not look like a modern chimpanzee with some human features or a cross between the two species (Figure 8.17). While the history of hominins is becoming clearer as new fossils are being identified, much less work has been done on the evolutionary history of chimpanzees.

Furthermore, if we examine the theory of common descent more closely, we can see that accepting this theory does not require the identification of a missing link. The theory of evolution is also supported by evidence of intermediate forms between a modern organism and its ancestors. This evidence should be much easier to locate, and it is the type of evidence provided by the ho-minin fossil record (review Figures 8.14 and 8.16). Besides being bipedal, humans differ from other apes in having a relatively large brain, a flatter face, and a more extensive culture. The oldest hominins are bipedal but are otherwise similar to other apes in skull shape, brain size, and probable lifestyle. More

Figure 8.17 The common ancestor of humans and chimpanzees? Although in popular representations the missing link between humans and chimpanzees is pictured as in this satirical image of Charles Darwin, both humans and chimpanzees have been evolving separately for at least five million years, and their common ancestor is unlikely to resemble either modern species.

modern hominins show greater similarity to modern humans, with flattened faces and increased brain size (Figure 8.18). Even younger fossil finds indicate the existence of symbolic culture and extensive tool use, trademarks of modern humans.

The ancient hominin-modern human transition is not the only fossil record that supports the theory of common descent. Examples of well-described transitions include one between ancient reptiles and mammals, and another between ancient and modern horses (Figure 8.19); many others have been found as well.

Figure 8.18 The ape-to-human transition. Ancient hominins display numerous apelike characteristics, including a large jaw, small brain case, and receding forehead. More recent hominins have a reduced jaw, larger brain case, and smaller brow ridge, much like modern humans.
Figure 8.19 The fossil record of horses. Horse fossils provide a fairly complete sequence of evolutionary change from small, catlike animals with five toes to the modern horse, with one massive toe.
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