Feb 17 2011

Lucy’s Feet

Lucy is one of the most famous fossil specimens of a human ancestor we have. It is a fossil of a female Australopithecus afarensis, a species that lived from 3.7 to 2.9 million years ago. This is soon after the split from our common ancestor with the chimpanzees (about 6-8 million years ago). It therefore tells us a great deal about the evolutionary forces that were shaping the hominid line.

One question of great significance is the extent to which A afarensis was bipedal. Was it bipedalism that defined the hominid line and made humans what they are, or was bipedalism a later adaptation?

The consensus has been that A. afarensis was indeed bipedal. This comes from multiple independent lines of evidence – the shape of the pelvis, the articulation with the femur, and details of the spine, for example. However, at the same time examination of the upper extremities reveals retained adaptations for life in the trees, such as strong curved fingers for gripping branches.

This makes A. afarensis a nice transitional species. We can easily imagine that this is a species that has partly come down from the trees and has evolved bipedalism in order to wander the expanding African savannah to hunt and scavenge. But at the same time these creatures were probably returning to the trees for shelter and safety. They are on their way to being fully bipedal, but have still not left the trees behind completely.

One question that has remained open was the anatomy of A afarensis feet – how adapted were they to bipedalism? The more similar they were to modern human feet, then the more time it is likely that A. afarensis spent walking rather than swinging from branches.

A newly published analysis of a 3.2 million year old afarensis fourth metatarsal (foot bone), in conjunction with examination of other afarensis foot specimens, indicates that A. afarensis likely had arched feet. An arched foot is a key adaptation to bipedalism and is a marker of the hominid line. The authors report:

A complete fourth metatarsal of A. afarensis was recently discovered at Hadar, Ethiopia. It exhibits torsion of the head relative to the base, a direct correlate of a transverse arch in humans. The orientation of the proximal and distal ends of the bone reflects a longitudinal arch. Further, the deep, flat base and tarsal facets imply that its midfoot had no ape-like midtarsal break. These features show that the A. afarensis foot was functionally like that of modern humans and support the hypothesis that this species was a committed terrestrial biped.

Pretty cool. This new find supports the other lines of evidence that A. afarensis was not only bipedal, but was committed to bipedalism.

For laughs I like to see what the creationists have to say (if anything) when a new find like this is published. I was referred to Ken Ham’s Answers in Genesis page. The discussion of this news item was particularly weak, which is saying a lot given the ordinarily thin standards of creationist propaganda. I can sum up the entire intellectual content of Ham’s analysis in two words: “Oh, yeah!”

He writes:

Scientists have pointed out that most of Lucy’s features, especially her head and jaw, are distinctly apelike, with wrist bones suggesting she walked on all fours. These observations are difficult to reconcile with the claim that her pelvis and leg bones are evidence of upright walking, although creationists have attacked that claim as speculative. And, of course, the general incompleteness of Lucy’s skeleton casts doubt on the certainty of any interpretation.

This is not hard to reconcile at all – Lucy has a mixture of ape-like and human features because she is part of  the way evolutionarily between our ape common ancestor and humans. Lucy is what all creationists dread and try desperately to deny, a transitional fossil.

Ham also characterizes the Lucy specimen as incomplete. While this is technically true in that all fossil specimens are incomplete, Lucy is actually a relatively complete specimen. That is part of what makes Lucy famous. She is complete enough that she provides a great deal of fossil information, but Ham would have you believe she is a poor specimen that “casts doubt on the certainty of any interpretation.” This is classic denialism.

Take a look at the picture of the Lucy fossils. This is a fabulous specimen. Also keep in mind the bilateral symmetry of the human body – so every bone can be reflected in the other side. For example if we found a specimen that was the entire left side of a hominid, only 50% complete, that would be as good as a 100% complete specimen.

And Lucy is not the only afarensis specimen we have. There are many specimens, including one rich site in Ethiopia, AL 333, with over 200 bones from an estimated 17 individuals. This is the source of the metatarsal bone in the current analysis, along with 35 other foot bones.

Of course paleontology is an inexact science that attempts to make the best inferential reconstruction of the past from incomplete data. It is like a horrifically complex puzzle and we don’t have all the pieces. There is therefore room for contrary opinions and doubt. But that is not the same thing as saying that we know nothing and does not allow for the evidence we do have to be brushed aside.

The denialist strategy is to exaggerate doubt and declare any evidence as being insufficient. Ham writes:

As with many high-profile fossils, a layer of interpretation lies between what the creature was really like and our idea of what the creature was like. This interpretive layer thickens the less complete a fossil is, and Lucy is a perfect example of that. Even if australopithecines sported an arched foot, it does not mean they were our ancestors; even if the outer portion of some australopithecines’ feet were arched, it does not mean their entire foot was; even if certain foot bones were twisted in a certain manner, it only “suggests” the outer portion of their feet was arched; and so on—at each step, the scientists have made a jump from the evidence to the conclusion. Moreover, how partial and interpretation-laden are the 35 fossils used in this analysis, and how certain is their connection to Lucy?

The is pure denialism. The fact is we have a large amount of hard and compelling evidence. This allows us to make some very confident broad brush-stroke conclusion – for example that humans and modern apes share a common ancestor, that the hominin line leading to modern humans has been evolving from our common ancestor for 6-8 million years, and that bipedality was an early adaptation of our line. As we try to make finer and finer details claims, it gets more tricky, and we need more lines of evidence (and always more specimens) to resolve differences of interpretation.

But uncertainty at the finer levels of detail does not call into question the big picture. That again is a denialist strategy – focus on tiny details as if they cast doubt on the big picture.

Ham also ignores the massive problems that the A afarensis specimens represent for the young-earth creationist position. According to that mythology there is no reason for a creature like A. afarensis to exist at all. But evolution predicts that a species like A afarensis should have existed at some point in the past few million years. What exact twists and turns evolutionary history actually took is still a matter to be determined, slowly, as we accumulate more specimens. But something with a mixture of human and ape features had to exist in the past if humans share a common ancestor with apes, as evolution predicts.

Ham’s caviling in nothing but pathetic denialism. Meanwhile, paleontologists continue to pull more and more fossils out of the ground, each one a little validation of the fact of evolution while teaching us more about the small details of the course of evolution.

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