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Before the discovery of Australopithecus afarensis in the 1970s, most anthropologists believed that an increase in brain capacity had preceded bipedal locomotion. However, this reconstruction of the Lucy skeleton shows that the A. afarensis was bipedal even with a brain size of around 400 cc.

But the actual Lucy skeleton does not include feet, and only has two partial leg bones.

How do anthropologists determine, based on the bones preserved, that A. afarensis was bipedal?

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Mainly because of the characteristics of the pelvic joint with the femur and the foramen magnum (the hole the spinal cord uses to connect with the encephalon). Also, there are more specimens other than Lucy, and there also exist fossil footprints (for instance the Laetoli footprints). All this evidence points to a bipedal walking.

enter image description here

Source: http://www.talkorigins.org/faqs/homs/pelvis.html

As you can see, the Asutralopithecus pelvis resembles human pelvis more than chimpanzee's (wich is adapted to a primarily quadrupedal movement).

enter image description here

Source: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Primates.html

In this image you can see the position of the foramen magnum in a typical pure quadrupedal mammalian, such as a dog. The foramen inserts in the posterior part of the cranium. In the human, the foramen inserts in the base of the skull, due to de erected position. In chimpanzees, which have quadrupedal march, but can also display some bipedal movements, the foramen is slightly displaced towards the basis of the cranium, in an intermediate position between both.

enter image description here

Source: http://www.fcnym.unlp.edu.ar/museo/educativa/serypertenecer/principal/australopitecinos.html (Spanish)

In this image it shows the foramen in chimpanzees, Australopithecus and modern humans, and you can see that the position in the second is at the base of the skull, just like in Homo.

It is assumed that the bipedal locomotion is an adaptation to the savanna environment, where trees were rare. Bipedalism also allows a better visual field and it has more energetic efficiency.

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It would be great if you could add citations for where the information and figures come from. –  kmm Aug 16 '13 at 0:18
    
It'd be nice if you added how the foramen magnum, femur, and pelvis show how A. Afarensis was bipedal. –  user10283 Nov 20 at 3:07

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