Oct 02 2009

Ardipithecus ramidus

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.

Addendum:

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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9 responses so far

9 Responses to “Ardipithecus ramidus”

  1. Karl Withakayon 02 Oct 2009 at 12:51 pm

    “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 lack of understanding of evolution is very common. It’s the misconception that new species arise and split off from static, stagnant species that stay the same into modern times and never not evolve.

    Some people seem to think that the theory is that X million years ago there were chimpanzees, and some of them split off and eventually developed into modern humans and the rest stayed chimps, and there should be transitional forms from chimp to modern man rather than both chimps and humans having transitional forms back to a common ancestor that was neither chimp nor man.

    Indeed, on a pure plausibility basis, there’s no reason why modern chimps couldn’t have evolved from a common upright ancestor that evolved from a non-upright ancestor. Whales evolved from land mammals which evolved from sea creatures.

    Another common misconception is that evolution and natural selection necessarily lead to increasingly “sophisticated” species.

    I’ve heard people speculate what modern intelligent dinosaur descendants would look like today if they hadn’t been wiped out. I ask why should we assume they would have developed intelligence if they had not been wiped out. They were doing very well for themselves “fat, dumb, and happy” before something changed faster than they could adapt to it, and they died out.

  2. Karl Withakayon 02 Oct 2009 at 12:53 pm

    “and never not evolve.” was not intended to be a double negative. I was trying to decide between “do not” and “never” and flurbed them both together. :)

  3. jtpiatton 02 Oct 2009 at 3:42 pm

    Of course, the ‘scholars’ at DI have already begun the damage control on this find. Casey Luskin’s response uses a bizarre mixture of mutually exclusive criticisms to arrive at the conclusion that Ardi proves (ready for it…) that evolution is bunk. For example, after an initial salvo in which he attempts to cast doubt on the viability of using small fragments to reconstruct the Ardi skeleton for use in a study, Luskin then argues that those very same dubious fragments prove that Ardi couldn’t have been related to humans. The fact that he makes this breathless contradiction within the same paragraph is especially curious, as my guess is that most people’s skulls would have crumpled like a ripe melon in a trash compactor if subjected to the tremendous forces of cognitive dissonance generated by such a statement. My only hypothesis for Luskin’s survival is that he has a substantial tolerance level built up from years of conditioning.

    These few predictable goons aside, this research on Ardi is simply fascinating, and I can’t wait to read more about it as future work is published over the next few years.

  4. Watcheron 02 Oct 2009 at 4:05 pm

    This find actually lead to a great discussion on a message board I’m on. It was a great example of someone who is not a big follower of science, taking this information given, and coming to the same conclusion as anyone in the human evolution field. The only problem is that he had a gross misunderstanding of how evolution worked.

    http://www.dawgtalkers.net/showflat.php/Cat/0/Number/603782/page/0/fpart/1/vc/1

    Admittedly it is a Cleveland Browns message board, so if bad football is offensive to you, do not proceed :)

  5. Norwegian Shooteron 02 Oct 2009 at 5:22 pm

    Good post, Ardi is certainly exciting. And imagine waiting 17 years to publish! Science probably didn’t have a website when they started!

    It appears your web journalism skepticism wasn’t up to snuff when you saw the Examiner article. Calling it reporting is generous. The Examiner is just citizen blogging targeted at individual markets for advertising. He picked his own headline and he is certainly not a journalist.

    Please post some more of the best science reporting you’ve come across on this topic.

  6. Hyperionon 03 Oct 2009 at 1:12 pm

    This sort of leads to another question that I’ve often wondered (and I know that I’m not the only one who has wondered this), but where are the fossils of chimp ancestors?

    The general explanation has always been something along the lines that chimpanzee’s live primarily in wooded habitats that aren’t conducive to fossilization. This is true, but is a rather inelegant explanation. I do have to wonder whether some “hominid” fossils may be chimp/gorilla ancestors. Not Ardi, but I’m thinking more along the lines of boisei and robustus (whichever genus they’ve now been placed in, if I remember correctly they were moved out of Australopithecus).

    As you mention, there’s no reason why every hominid fossil necessarily represents a direct ancestor of ours, but there’s also no reason why they couldn’t also represent ancestors on the chimp or gorilla lines. This is especially interesting in light of the hypothesis that our common ancestor may have been more resemblant of humans (or at least, would not have resembled modern chimps or gorillas).

    Just food for thought.

  7. son 03 Oct 2009 at 3:25 pm

    A complementing overview can be found on John Hawkes web: http://johnhawks.net/weblog/fossils/ardipithecus/ardipithecus-faq-2009.html

  8. son 03 Oct 2009 at 3:28 pm

    Hyperion, John Hawkes recently wrote an overview at his blog: http://johnhawks.net/weblog/reviews/miocene_apes/late-miocene-african-apes-chronology-2009.html

  9. MarkWon 05 Oct 2009 at 10:18 am

    “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.”

    Technically they’re probably further. Chimpanzee generation-times are shorter than ours, so while we’re Ardipithecus’ great-to-the-power-of-200000-nephews and nieces, modern Chimps are the great-to-the-power-of-300000-nephews and nieces.

    (Numbers pulled from a hat, but you get my point.)

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