Evolution

Researchers uncover the most complete picture yet of how Australopithecus sediba, a possible immediate human ancestor, walked and moved.

The team analysed two skeletons: a juvenile male (known as MH1), a female (MH2) and the isolated shinbone of an adult male (MH4). The two-million-year-old skeletons were found together (along with the fossils of several other animals, such as sabre-toothed cats, birds and mice) in 2008 and it has been speculated that they belong to a mother and son. It's only now, over 4 years later, that researchers have been able to fully analyse them.

The skeletons reveal that while Au. sediba was capable of bipedal walking, it did so in a very different manner than us. Traits such as a small heel (reminiscent of a chimp's) mean that it walked using a strange rotation of the knees and hip, with feet turned inwards. While it could walk bipedally, it would not have been a good runner - its narrow shoulders mean it could not swing its arms as we do (which saves energy). Our broad chest is one of the reasons humans are good long-distance runners, and the evidence suggests Au. sediba could not walk or run as well as we can.

But walking was not Au. sediba's only method of locomotion. Its long arms, shoulder blades and the narrow top of its ribcage are all very similar to those of orangutans and indicate a lifestyle that was at least partly arboreal. Brachiation (when apes swing from tree to tree with their arms) may have been a way for Au. sediba to get around.

The spine and teeth of the specimens demonstrate some human-like characteristics. Au. sediba likely had the same number of vertebrae as us (with a human-like hollow back) but it was more flexible than ours. The teeth bear similarities to Au. africanus, suggesting that these two australopiths were distinct from Au. afarensis (the species the famous "Lucy" skeleton belongs to).

As for whether Au. sediba is an immediate common ancestor to the Homo genus, that debate is a long way from being settled. Not all remains are as complete as these; many members of the Hominid line are known from fragmentary remains which can't be properly compared against Au. sediba, and some argue we have Homo fossils that predate these skeletons. More fossils may be the only way to definitely answer where Au. sediba lies in our family tree.

Photo: In the centre is a reconstructed Au. sediba based on the material of MH1, MH2 and MH4. Shown against the skeletons of a small-bodied modern woman (left) and a chimp (right). Credit to Lee Berger.

General articles:
http://news.nationalgeographic.com/news/2013/13/130411-homo-ancestor-hominin-skeleton-lucy-australopithecus-sediba-science/

http://www.bbc.co.uk/news/science-environment-22108784

http://www.livescience.com/28656-closest-human-ancestor-was-pigeon-toed.html

http://www.sciencedaily.com/releases/2013/04/130411142719.htm

On the ribcage, feet and running: http://www.sciencedaily.com/releases/2013/04/130411142942.htm

On different forms of bipedalism among human ancestors: http://www.sciencedaily.com/releases/2013/04/130411142710.htm

What Au. sediba's teeth tell us: http://www.sciencedaily.com/releases/2013/04/130411142935.htm