04 October, 2009

Sunday Sensational Science

Cousin Ardi

You didn't think we'd got without Sunday Sensational Science when there's news like Ardi out there, did you?  Of course not!  A new addition to the family doesn't happen every day, after all.  And make no mistake: while we don't know if Ardi's in a direct line of descent or not, we can be sure she's part of our family tree:
Ardi, of course, is short for Ardipithecus ramidus, one of the earliest hominins found to date. Her skeleton (see image below), as well as bits and pieces of other skeletons of the same species, were described this week in a special edition of the journal Science. While a close relative of Australopithecus afarensis (made famous by "Lucy"), Ardipithecus ramidus is about half a million years older than the earliest Australopithecus afarensis and is a bit closer to the last common ancestor between living chimpanzees and humans.* As such the remains of Ardi and her kind give us a closer look at how some of the earliest humans evolved.
That's exciting stuff, my darlings. Lucy changed our thinking about our origins to a remarkable degree - after all, she proved bipedalism came before brains, among other things.  Ardi will show us how we got from tree-dwelling primates to A. afarensis, who could've given you a run for your money in a foot race on flat ground.

We've actually known about her for almost two decades now:

This introduction has been a long time coming. Some 4.4 million years ago, a hominid now known as Ardipithecus ramidus lived in what were then forests in Ethiopia. Fifteen years ago, Tim White of Berkeley and a team of Ethiopian and American scientists published the first account of Ardipithecus, which they had just discovered. But it was just a preliminary report, and White promised more details later, once he and his colleagues had carefully prepared and analyzed all the fossils they had unearthed. “Later,” it turned out, meant 15 years.

That's how careful science is done, my darlings.  Tim White and his colleagues weren't about to run their yaps without knowing what they're talking about.  As Brian Switek pointed out, there wasn't any Ida-like hype for this important find.  Just good, careful science.

Of course, their caution didn't prevent news organizations from mucking up the story with all sorts of silly headlines and confused interpretations of the science.  No, Ardi's not a missing link - we really need to stop thinking there's this Great Chain of Being with all sorts of links missing.  And as for this nonsense about "oldest human," well, she's not even human, folks:
These fossils are not "human." We limit "human" to the members of our Genus Homo, collectively called Hominidae often shortened to hominins.  The great apes, including humans are collectively called Hominids. "Ardi" is not a member of our genus, and is not a human. The notion of a "missing link" is falsely claimed by creationists to mean that there was a direct intermediate between modern humans and modern apes. This was never suggested by any evolutionary scientist, all the way back to Darwin. The discoverers of the newly reported fossils do think that even some of the common shared features of Chimps and Humans evolved independently. This is called convergent evolution, and is perhaps what Shreeve was refering to as "missing link" being falsified.
One of the most exciting things about her is what she can tell us about that common ancestor, and what traits did and did not come from that ancestor:
Much like Australopithecus afarensis, Ardipithecus ramidus had upper-body traits that exhibit adaptation to life in the trees while it had relatively broad hips more consistent with bipedal locomotion. The arms of Ardipithecus ramidus were very long (it could put its hands on its knees standing up; take a moment and try to do the same) and it had hands tipped in curved fingers well-adapted to grasping branches (see image to the upper left). It does not appear to have moved through the canopy by swinging from limb to limb, like a gibbon, but instead moved through the trees on all fours, grasping the branches below it rather than hanging from those above.
ArdipithecusThe hips of Ardipithecus ramidus, however, suggest that it probably spent a good deal of time walking upright. (See image to the left. Grey reconstruction is Ardipithecus ramidus. Yellow is Australopithecus afarensis.) In knuckle-walking apes like chimpanzees the blades of the pelvis are flat and come up over the back. In Ardipithecus ramidus the blades of the pelvis form are somewhat more bowl-shaped, a shape that helps hold the viscera of the abdomen in place in hominins that were constantly walking upright. The arrangement in Ardipithecus ramidus did not provide as much support as in our own genus, Homo, or even later australopithecines, but it offered more support than the same bones in chimpanzees. (It should be noted, though, that this interpretation is already controversial.)


The hypothesis that Ardipithecus ramidus was arboreally-adapted but did not knuckle-walk, however, is consistent with recent studies that suggest that knuckle-walking was not the ancestral mode of locomotion in the last common ancestor of humans and chimpanzees. In fact, slight differences in the way that gorillas and chimpanzees knuckle-walk may even mean that this "typical" ape mode of locomotion evolved more than once. And while most of the focus has been on Ardipithecus ramidus, the constellation of traits we see in its skeleton may suggest that living chimpanzees are more evolutionarily specialized than we previously thought. What we need to find are early fossil chimpanzees; their side of the family tree is practically blank. Being able to compare early humans with early chimpanzees would tell us much about the evolution of both groups.
Here's a nice comparison of everybody, starting with a chimpanzee on the left, Ardi in the middle (2 views shown), and Lucy at right:

So here we see the skeleton of a chimp, a front and side view of Ardi, and Lucy aka Astralopithecus afarensis, roughly to scale (Ardi is just over one-meter or a little over three feet in height). And low and behold, Ardi has the double curved spine of a hominid, long ape-like arms, while pelvis, legs, and feet are almost perfectly in between. Based on all that, Ardi could walk upright almost as well as Lucy, using her arms to carry back goodies from the grassland to her forest enclave. And with those long arms, fingers, and toes, including the partially opposing big toe, she could probably climb almost as well as an adult chimp or juvenile gorilla. That's about as good a transitional fossil between a more ape-like, knuckle-walking ancestor and a bipedal hominid like Lucy as you could ask for.
Now, as Brian mentioned above, whether knuckle-walking even existed in our last common ancestor is in doubt.  But it's striking to see just how easily Ardi slots in.

And since creationists are so easily confused, might we just repeat for the record: humans didn't evolve from chimpanzees.  Thank you.

Ardi's discovery is going to take us a long way toward understanding where we came from and how we got here, just as Lucy did.  It's so important, in fact, that Science was kind enough to make those papers free and easy to access.  I can't wait for the books, the lectures, and perhaps even the grand tour.  There's something special about looking on the bones of your relatively-close cousins.

Welcome to the family, Ardi!

(Tip o' the shot glass to Darksyde for several of the links contained in this post.  All images were filched from the various and sundry articles linked, should you want to track down their origins.  Science gets free drinks for the month for their great good sense in making so much science accessible to non-subscribers.  Raise a glass to our new family member, and to the scientists, bloggers and publishers who brought her by.)


Cujo359 said...

I sometimes wonder if most science journalists have even cracked open a book about biology in the last twenty years. If they'd read just about anything written by Gould, Dawkins, Wilson, Diamond, or Mayr, they'd know that the "missing link" thing is nonsense. Yet they persist with these themes.

Paul said...

It's very interesting that upright walking might have developed in forested areas rather than in savannas.

Chris said...

This morning I wanted to start a betting pool on how soon the Discovery Institute would spin the news on Ardi and who would spin it. But once again, the tireless Casey Luskin beat me to the punch:


constant gina said...

Ignoring, for a moment, the inevitable obfuscatory asshattery of the creonuts, that's one beautiful, beautiful fossil, and a beautiful piece of work on the part of the researchers.
Slow-cooked science for the win.