Afarensis

A commonly encountered creationist argument asks why there are still monkeys if they evolved into apes and humans. This is the stripped down version of an argument by Casey Luskin. Casey has discovered the MSNBC article concerning the recent study of quadrupedalism and bipedalism in chimpanzees and bipedalism in humans. Jason over at EvolutionBlog discussed the first part of Casey's whine and Chris at NorthState Science has taken Casey to task for the second part (drawing my attention to it in the process) which specifically focuses on anthropology.

Here is Luskin:

The second article, "Why we quit aping around, began walking," hypothesizes that humans began walking upright because "[t]raveling upright takes a quarter the energy of 'knuckle-walking'." It's a nice anthropocentric story, but if falls into the same predicament as the first article. Our upright-walking species, Homo sapiens comprises one out of about 7 species of the species of the family Hominidae. This means that only 14% of living species of Hominidae were naturally selected to walk upright. So think about this from the apes' perspective: If upright walking is so energetically favorable, why do apes still "knuckle-walk"? I'm sure that some armchair Darwinian paleoanthropologist would be happy to oblige us with a just-so story as to why living ape species did not evolve bipedal locomotion and instead found knuckle-walking more advantageous for survival. We would then have a theory which can explain both (a) complete bipedal locomotion, and also not (a). In cases like these, one cannot help observe that Neo-Darwinism is like a theory which can explain anything, and therefore actually explains nothing.

This is rather like arguing that Einstein's theories were wrong because they can explain (a) gravity, and also not (a), or arguing chemistry is wrong because it can explain why some (most) elements can bond to others and not (a) some - such as the "nobles" can't.

So think about this from the apes' perspective: If upright walking is so energetically favorable, why do apes still "knuckle-walk"?

Or, if apes evolved into humans why are there still apes?

Primates move about in a wide variety of different ways, there are vertical clingers and leapers, quadrupedal leapers, arboreal quadrupeds, terrestrial quadrupeds, knuckle-walkers, fist-walkers, brachiators, suspensory climbers (some of whom hang by their tails), and bipeds. In reality, no one form of locomotion predominates among a given primate species. Chimpanzees, for example, are terrestrial quadrupeds - most of the time. Unlike some of their primate cousins they do not place their palms flat on the ground, unlike other primates they don't support their weight on the palmer surface of their fingers (like some baboons) rather they rest their weight on the third and fourth digits of their hand (second row of phalanges to be exact). In the trees they engage in both arboreal quadrupedalism and suspensory behavior. Starting in the Eocence, most primate species were arboreal quadrupeds or quadrupedal leapers. Oligocene primates seem to have been leapers and arboreal quadrupeds as well. We don't see terrestrial and suspensory species until the Miocene when cercopithecoids and hominoids arrive on the scene. Brachiators and bipeds are Plio-Pleistocene developments. These are some the data points the theory of evolution must explain

Casey seems to be basing his criticism entirely on the MSNBC article, I guess actually reading the PNAS article by Sockol et al he was criticizing makes entirely too much sense. It's a shame because he could have prevented himself some embarrassment if he had. It's actually a pretty good paper. Here is what it is about (there is an excellent discussion of the paper at Anthropology.Net):

Here, we compare human and adult chimpanzee locomotor energetics and biomechanics to determine links among anatomy, gait, and cost. Our study focuses on two primary questions. First, do adult chimpanzees follow the pattern of costs found previously for juveniles (6)? Second, do differences in anatomy and gait between bipedal and quadrupedal walking, as well as between chimpanzees and humans, explain observed differences in cost? Using this biomechanical approach to link differences in anatomy and gait to cost, we then examine what changes, if any, would lower the cost of bipedalism for an early hominin, such that bipedalism would be more economical than the ape-like quadrupedalism of the last common ancestor.

Based on previous research by other workers, the authors were also able to make two predictions, which they then preceded to test by collecting data on metabolic, kinematic, and kinetic variables. They were then able to relate the anatomy and gait of chimps and humans to energetic cost. This allowed them to make a further prediction about the fossil record:

Results of our biomechanical analyses linking anatomy and gait to cost generate two testable predictions for the hominin fossil record. If locomotor economy was a selective force behind hominin bipedalism, then early hominin lower limbs should be longer than those of apes, and the ischia of early hominin pelves should be more dorsally projecting.

Fossils being one thing Luskin neglected to mention (along with all that empirical stuff like data collecting and testing predictions). As I mentioned here and here there is abundant evidence that the hindlimb has been lengthening. There is one more interesting piece of information in the Sockol et al article. One chimp in the sample was able to extend the hindlimb more fully and hence had a smaller energetic cost during bipedal locomotion. This is fascinating and it would be interesting to this type of study carried out on a larger sample of chimps.

Most primates (a) give birth to one offspring. Prosimians, tamarins and marmosets give birth to twins. So any theory that explains (a) no twins and not (a) twins must be wrong...unless you actually consider all the relevant facts, gather data perform experiments and test predictions. What makes this particularly amusing is that Casey seems to be saying that if we explain the fact that humans are bipedal and chimps and gorillas are still quadrupedal then our theory means nothing because it can explain everything. Then he dares us to explain why chimps and gorillas remained quadrupeds and humans became bipeds.

P. S. One of these days I will get back to work on my primate locomotion series. I promise. Scout's Honor. Really. I'm serious.