Feb 27 2009

Two Stories of Human Evolution

Two new bits of evidence published recently have allowed scientists to flesh out the picture of human evolution a bit more. The first is a collection of fossils of Australopithecus amanensis found in Ethiopia. This is a significant find because it fills in a previous gap in the fossil record.

The big picture that we have so far is that human ancestors split from chimp ancestors about 5-7 million years ago, and seem to have quickly become bipedal after they diverged.  One candidate for the earliest fossil species on the human line is Ardipithecus ramidis, which dates to about 4.4 million years ago. Fossil remains of this species are scant, but they appear to have had a dentition more similar to apes like chimps than to later hominids. They also have derived characteristic homologous with Australopithecines and cranial bones suggesting that the skull rested vertically on top of the spinal column, suggesting they were bipedal.

The next genus thought to be in the line that led to humans is Australopithecus. These include the famous Lucy specimen that is an incredibly intact fossil. Australopithecines were clearly bipedal, although less adapted to bipedalism than humans, and have other features nicely transitional from non-human apes to humans. The oldest Australopithecus is an afarensis from 3.4 million years ago. Hence the gap from 3.4 to 4.4 million years ago.

The new fossils are from an older species in the genus Australopithecus, called anamensis and dates to 4.1 million years ago. This nicely fits in the gap between afarensis and ramidis. Even better, anamensis is anatomically transition between the two – more primitive that afarensis but with derived Australopithecine characteristics not seen in ramidis. An better still, all three species exist in the same place. So we have a temporal and morphological sequence in the same geographic region.

All of this is compelling evidence that these three species represent an actual evolutionary sequence. However, the case is far from closed on that question. This is merely the most straightforward interpretation of the data. Evolutionary sequences, however, tend to be complex and bushy. So it is possible that amanensis and afarensis are not in a line but are separate branches from ramidis, or even from a yet undiscovered ancestor. We have too few specimens to say with any confidence what the ultimate relationship among these various species will be.

However, whether these species are in an actual sequence or are side branches, there existence is still powerful evidence for the evolution of humans from ape common ancestors.

The next new bit of evidence is also very cool – a hominid footprint from 1.5 million years ago.  Footprints are great evidence because they preserve the soft-tissue anatomy of the foot in a way that bones along cannot. They also give functional information – for example they are more direct evidence for bipedality.These new footprints are probably from a Homo erectus- a probable human ancestor that was fully upright and bipedal (hence the name).

The new footprint, found in Kenya, shows a foot that has an arch similar to human feet and a big toe in a very modern position. By contrast, the oldest hominid footprint, found in Laetoli Tanzania, is probably from an Australopithecus afarensis and dates to 3.7 million years ago. This footprint shows a more flat structure and a big toe that is still angled out to the side – more adapted for grasping than bipedality.

Cool and cooler.

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