Oct 02 2009
I have been following the story of human evolution for about 30 years, and in that time the story has become increasingly interesting and complex. It has been a microcosm for our understanding of evolutionary history itself – we tend to start with simple A leads to B leads to C stories which may be parsimonious but do not reflect the tendency for chaos inherent in evolution. Over time we find a complexly branching bush, and our simple linear storylines fall away.
When I took a course on human evolution under Pat Shipman 24 years ago Ardipithecus was not known, now it is the oldest known member of the branch of primates that led to humans. I cannot say that it is the oldest human ancestor because because we do not yet know if it is a direct ancestor or a side branch. But if it is a side branch, it is still very close to our branching point with chimpanzees, and is likely very similar to our contemporary ancestor.
Scientists led by Tim White have completed a 17 year excavation, assembly, and assessment of specimens of Ardipithecus ramidus from about 36 individuals, including men, women, and children. The fossils come from the Awash region of Ethiopia – a location that has been a huge source of hominid fossils for years.
The analysis shows that Ardipithecus were fully upright bipeds, although lacked foot arches, and they showed no adaptations for knuckle walking, but did have adaptations for tree climbing. This supports other research which concluded that human ancestors were never knuckle-walkers, which seems to be a later development in chimps and gorillas.
Also, contrary to prior thought, our ancestors developed bipedalism while still living in the trees part time, and they were living in a wooded region – not on the open savanna.
The teeth of Ardipithecus reflect an omnivorous diet. Later Australopithecines had specialized molars for grinding hard food, like nuts and grains. This also reflects the fact that hominids were not evolving on a straight line to modern humans, but each genus and species were specializing to their niche.
The fossils date to about 4.4 million years ago. Australopithecus species (including the famous Lucy) date to as late as 4.0 million year. Ardipithecus is also more primitive than Australopithecus – closer to our common ancestor with chimps – in fact, so much so that our current specimens are probably late examples of a group that goes back even further (another reason why we need more fossils to make definitive judgments about how to draw the evolutionary lines). It is also possible that there was a rapid spurt of evolution between Ardipithecus and Australopithecus, but again, this is not a necessary hypothesis at this point.
Also of interest is how different Ardipithecus is from modern chimps. This has a bearing on how close Ardipithecus is to the common ancestor between humans and chimps – a species that has not yet been discovered. This could mean that the branching point between humans and chimps, thought to be about 6 million years ago, might be further back than that. Or it could simply reflect how far chimps have evolved themselves since that branching point. Our human biases lead to the tendency to imagine that the common ancestor between humans and chimps was very chimp-like, but chimps are just as far from that ancestor as we are.
This is an excellent fossil find and analysis that reflects 17 years of painstaking work by scientists. It adds to the growing evidence for the evolution of humans from primates that were common ancestors with modern great apes. It is also a beautiful transitional species – fully upright but lacking some adaptations for walking, with a chimp-like brain of 300-350 cc, retaining some adaptations for tree-climbing, but also with uniquely human features. This is the very definition of a transitional species.
In addition it demonstrates how new fossil finds can simultaneously reinforce our big-picture conclusions (evolution itself, and that humans and apes have a common ancestor) while surprising us in the details (humans probably evolved bipedalism in wooded areas and not on the savanna).
In all ways a beautiful addition to the human family tree.
BTW – if you want to see the worst science news reporting I found on this topic look here at the Christian Living Examiner. At first I thought it was just the nutty headline writer, but the body of the article itself is just crap. Chris Esparza writes:
The idea of the missing link is that somewhere way back when, there was a primate who almost seemed to be half monkey and half human, proving that there was at some point an evolutionary split. A recent discovery in Ethiopia disproves that theory.
Wow. Chris also gets a simple fact wrong, saying the specimen is 3.2 million years old when it is 4.4 – but that is just basic fact checking. But the bigger problem here is that the journalist obviously does not understand the first thing about evolutionary theory. They also completely misrepresented the scientist they quoted – essentially coming to the opposite conclusion.
Sloppy and stupid.
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