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Fossil Evidence of Bipedalism

  • Teacher Resource
  • Posted 03.10.10
  • NOVA

In this video segment adapted from NOVA, see how paleoanthropologists—including Don Johanson, with his famous discovery of the Australopithecus afarensis "Lucy"—have used the fossil record to identify a large number of fairly similar bipedal species that encompass ancestors of humans and related species. These species, which walked upright like humans, but resembled apes in small brain size and in facial structure, flourished as a group for millions of years. Scientists still debate which of these species was our direct ancestor. The video features a comparison of Lucy's fossilized pelvis bone with that of an ape, and shows how strikingly similar Lucy's is to a human one.

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NOVA Fossil Evidence of Bipedalism
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  • Media Type: Video
  • Running Time: 3m 06s
  • Size: 9.3 MB
  • Level: Grades 8-12

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Source: NOVA:"Becoming Human, Part 1: First Steps"

This media asset was adapted from NOVA:"Becoming Human, Part 1: First Steps".

Background

Our big brains set humans apart from other animals. Our ability to walk upright also distinguishes us from apes, with whom we share a common ancestor. However, in Darwin's day, and for many decades afterwards, too few fossils had been found to solve the mystery of which came first: intelligence or bipedalism.

Donald Johanson's 1973 discovery of "Lucy," a half-complete skeleton of a previously unknown species of hominid, proved critical to understanding this aspect of human evolution. In offering evidence for how humans diverged from apes, scientists cite specific features in Lucy's anatomy. Although she displayed subtle but important differences in skull structure, she largely retained an apelike upper body. However, it was her lower body, in particular her pelvis and knee joints, that helped scientists determine the order in which changes occurred.

Bipedalism, or walking on two feet, is what separated the earliest human ancestors from the earliest ape ancestors. The shape and position of the pelvis (hip), femur (leg), and tibia (shinbone) distinguish bipeds from quadrupeds, or animals that walk on four feet. As the video states, Lucy was like a human from the waist down. Her knee joints locked straight, a feature characteristic of upright walkers. And in bone structure and orientation, her pelvis more closely resembled a modern human's than a chimp's.

Lucy was clearly capable of walking bipedally. Her funnel-shaped ribcage and broad pelvis indicate that she probably had a large belly, like a modern ape, reflecting an adaptation to a relatively low-quality, high-bulk diet. However, the thick waist this gave her would have hindered her flexibility, and her high shoulders and the shape of her torso suggest it would have been difficult for her to swing her arms as humans do when running.

In this way, a three-million-year-old fossil showed that bipedalism was the first step toward becoming human—long before the earliest stone tools were made or any language was developed. Lucy's species, Australopithecus afarensis, was just one of many apelike human ancestors that walked upright on two legs. Subsequent fossil finds of much earlier bipedal hominids have confirmed this. As a group, small-brained bipedal apes inhabited Earth for millions of years before modern humans, Homo sapiens, evolved.

Interestingly, one compelling hypothesis, proposed by evolutionary biology professor Daniel Lieberman, suggests that bipedalism evolved to preserve energy. As seen by examining the fossil record, several species of upright walkers evolved with subtle anatomical differences between them. Minor increases in hip extension and leg length, for example, would have saved early transitional forms at least some energy. The compound effect of small changes ultimately yielded large energy savings.

To learn more about the discovery of "Lucy" and bipedalism, check out Finding Lucy, The Transforming Leap, from Four Legs to Two, and Walking Tall.

To learn more about human evolution, check out Laetoli Footprints, Evolving Ideas: Did Humans Evolve?, and Origins of Humankind.

To learn how scientists classify fossils of human ancestors and other species, check out Bones of Contention.

Questions for Discussion

    • Which of Lucy's characteristics were decidedly more apelike? Which were decidedly more human in appearance?
    • Discuss how the discovery of Lucy provided evidence to support the assertion that bipedalism evolved before large brains did.
    • Why do you think bipedalism evolved in hominids before large brains did?

Resource Produced by:


					WGBH Educational Foundation

Collection Developed by:


						WGBH Educational Foundation

Collection Funded by:


						National Science Foundation



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