Evolution Update

Evolution Update

Expanding the Diversity of Early Human Locomotion

Brandon Kieft November 15, 2015

Researchers working in the Rising Star Cave System in South Africa have recently published their discovery of an early human ancestor that is the first known hominin with a combination of modern feet and a small brain.

You probably don’t spend much time thinking about feet, and even less time pondering about their extremely adept ability to carry us around while walking upright. In fact, the Homo sapiens foot has several features that make it highly adapted to striding bipedalism – but it was not always so easy.

An international collaboration of scientists from three countries has published their investigation on the foot of Homo naledi, a hominid ancestor recently discovered in the Rising Star Cave System of South Africa.

The researchers analyzed a nearly complete adult right foot and over 100 various foot fragments found at the site. They found that the morphology and inferred function of the H. naledi foot was very human-like compared to other early human relatives that had been uncovered previously.

Surprisingly, however, the group cited recent work on full H. naledi skeletons from the same cave system whose findings conclude that the skeleton of this early hominin is quite primitive (though the fossils themselves have not been dated). This is interesting because the feet of other early hominids that have primitive body plans and small craniums, such as H. erectus and H. habilis, do not appear to have this human-like quality.

The team made their conclusions based on several anatomical measurements, one of which is called the power arm/load arm ratio. This value, which was calculated to average 40.8 for H. naledi, is quite similar to upright modern humans and distinct from our non-bipedal chimpanzee cousins.

Because the fossils have no associated date, the researchers came up with three possible conclusions. First, if this early hominid were alive during the early Pliocene (~4-5 million years ago), it would boast the most modern foot of any known early human during that era. Second, an earlier date such as the mid to late Pliocene (~2.5-3 million years ago) would indicate that its feet share features with known hominids of the time. Thirdly, if the remains were shown to be relatively recent, possibly an era overlapping with H. floresiensis and H. neanderthalensis, then this discovery would add to the diversity of foot morphology leading right up to H. sapiens.

All in all this investigation, like many others concerning human ancestors, has shown that H. naledi possessed a combination of primitive and derived features.
However, H. naledi is the first known hominin with this combination of such modern feet and legs with a small brain size.

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