Friday, October 09, 2009

Ardipithecus ramidus in Science Magazine

Science Magazine has a series of articles on the Ardipithecus ramidus remains that have been fully described. The articles are behind a subscription wall, so I cannot link to them. I will hit the high points, though. These quotes are necessarily short in order to conform to fair use guidelines. From Owen Lovejoy et al's Careful Climbing in the Miocene: The Forelimbs of Ardipithecus ramidus and Humans Are Primitive1:
This discovery ends years of speculation about the course of human evolution. Our ancestors’ hands differed profoundly from those of living great apes, and therefore the two must have substantially differed in the ways they climbed, fed, and nested. It is African apes who have evolved so extensively since we shared our last common ancestor, not humans or our immediate hominid ancestors. Hands of the earliest hominids were less ape-like than ours and quite different from those of any living form.
From Combining Prehension and Propulsion: The Foot of Ardipithecus ramidus2:
One of the great advantages of our more rigid foot is that it works much better as a lever during upright walking and running (as it also does in monkeys). However, Ar. ramidus still had an opposable big toe, unlike any later hominid. Its ability to walk upright was therefore comparatively primitive. Because it had substantially modified the other four toes for upright walking, even while retaining its grasping big toe, the Ardipithecus foot was an odd mosaic that worked for both upright walking and climbing in trees. If our last common ancestor with the chimpanzee had not retained such an unspecialized foot, perhaps upright walking might never have evolved in the first place.
From The Pelvis and Femur of Ardipithecus ramidus: The Emergence of Upright Walking3
Ardipithecus ramidus now unveils how our skeleton became progressively modified for bipedality. Although the foot anatomy of Ar. ramidus shows that it was still climbing trees, on the ground it walked upright. Its pelvis is a mosaic that, although far from being chimpanzee-like, is still much more primitive than that of Australopithecus.
From The Great Divides: Ardipithecus ramidus Reveals the Postcrania of Our Last Common Ancestors with African Apes4:
The hominid descendant of the last common ancestor we shared with chimpanzees (the CLCA), Ardipithecus, became a biped by modifying its upper pelvis without abandoning its grasping big toe. It was therefore an unpredicted and odd mosaic. It appears, unlike Au. afarensis, to have occupied the basal adaptive plateau of hominid natural history. It is so rife with anatomical surprises that no one could have imagined it without direct fossil evidence.
The takeaway message here is that just as human evolution has proceeded by fits and spurts, the science behind it has done the same, like most other scientific endeavors. Five years ago, we were looking for an ancestor that looked something like a primitive chimpanzee. Now we know that is not how it went. The other cherished model that will go by the wayside is that bipedalism was an adaptation to the expansion of the savanna and grasslands and the retreat of the forests. The morphology of Ardipithecus seems, instead, to strongly suggest that bipedalism arose while our ancestors were still living in a forest environment. When the forest retreated, however, it is clear that Ardipithecus ramidus or something like it had the necessary adaptations to survive the changes and thrive in the new environment.

Some will say that because we were so wrong about our early hominid models, we don't know much about evolution. Those arguments have been put forth before. A cursory look at Marvin Lubenow's dreadful book Bones of Contention shows that. Ardipithecus doesn't tell us that human evolution didn't happen. It tells us that some of our models were wrong. In any scientific field, each new discovery ought to result in a slight rewrite of some cherished theories. If it doesn't, the science is stagnant. This is the critical difference between science and creationism. creationism cannot change its theories or models. It has only one explanation for how things happened, which must be correct. Any evidence that does not comport with those models must be explained away, no matter the cost to intellectual or scientific integrity. We found this out first-hand with the RATE project. The discovery of Ardipithecus ramidus should be fascinating and exciting to any evolutionary biologist. It not only shows us where we were wrong, it shows us what we got right and where our models need to be adjusted. More importantly, though, it shows us a glimpse of our deep past in an breathtakingly new and exciting way.

1Lovejoy, C.O., Simpson, Scott, White, Tim, Asfaw, B, and Suwa, G. (2009) Careful Climbing in the Miocene: The Forelimbs of Ardipithecus ramidus and Humans Are Primitive. Science 326: 70

2Lovejoy, C, Latimer, B., Suwa, G., Asfaw, B., and White, T.D.(2009) Combining Prehension and Propulsion: The Foot of Ardipithecus ramidus. Science 326: 72

3Lovejoy, C., Suwa, G., Spurlock, L., Asfaw, B., White, T.D. (2009) The Pelvis and Femur of Ardipithecus ramidus: The Emergence of Upright Walking. Science 326: 71

4Lovejoy, C., Suwa, G., Simpson, Scott W., Matternes, Jay H., White, T.D. (2009) The Great Divides: Ardipithecus ramidus Reveals the Postcrania of Our Last Common Ancestors with African Apes. Science 326: 73

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