Paleoanthropology is the study of ancient humans. The closest living relatives of humans are gorillas and chimpanzees. We share a common ancestor; however, humans diverged about 4 to 8 million years ago, showing clear evidence of evolving distinctive hominid characteristics, such as upright bipedalism. As recently as about 30,000 years ago, there were many different species of early humans on Earth. However, almost all species of humans became extinct; only one—modern humans—survived.

Because it is impossible to observe organisms that only existed in the distant past, paleoanthropologists rely on fossil evidence to investigate the history of human evolution. Fossils preserve the remains (such as bones and teeth) and traces (such as footprints) of ancient life. By comparing the physical structures of fossils with those of other fossils or modern organisms, scientists can deduce information about prehistoric organisms and the evolution of species.

Comparative anatomy—the study of the structural similarities and differences of different organisms—helps scientists determine evolutionary relationships among organisms. For example, the forelimbs of humans, bats, cats, and whales have similar bone structure; although the forelimbs function in very different ways for each species, the similar structure indicates a common ancestor.

In 2000, an expedition led by anthropologist Zeresenay (Zeray) Alemseged discovered a remarkable fossil skeleton of an early human child in Dikika, Ethiopia. It is unusual for a child's skeleton to fossilize and remain intact, because juvenile bones are thin and weak. The skeletal remains, nicknamed Selam, are in exceptional condition and offer a rare opportunity for scientists to study a young early human ancestor. Selam was a young Australopithecus afarensis (about 3 years old at the time of her death) who lived about 3.3 million years ago. Careful analysis of Selam's anatomy indicates that A. afarensis was bipedal and likely an active climber, providing clues about human behavior during a pivotal period in human evolution.

Alemseged compared Selam to other fossils of early human relatives (adult A. afarensis as well as other species), modern human juveniles and adults, and other nonhuman great apes (such as chimpanzees and gorillas). He was able to determine Selam's age by looking at detailed X-rays of her skull, which showed adult teeth forming in the jaw; he identified her as female by comparing her canine teeth with others from the same species. Analysis of Selam's shoulder blades showed that she was more apelike than human. The shoulder socket in both juvenile and adult A. afarensis points up like the shoulder blades of modern nonhuman apes, suggesting that A. afarensis was adapted to climbing in trees. The pattern of growth is also apelike: the shoulder anatomy of juvenile and adult apes is similar, while the modern human shoulder blade changes shape as an individual grows from a baby to an adult. However, the anatomy of Selam's lower body is very humanlike and adapted to upright walking. Moreover, analysis of her brain size indicates evolution toward a humanlike growth rate of the brain.

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