Last week I wrote the following sentence:
” I have the impression that as soon as you find a fossil ape that probably walked on two legs, its being put into the human lineage without asking further questions about its true relationships.”
I think this needs some further explanation:
One of the key features for the classification of hominins is bipedalism, simply because we are bipedal. But are those very early, probably bipedal, fossil Apes really hominins? There are some arguments which reject theses hypothesises. Molecular genetics nowadays put the divergence between Chimpanzees and humans somewhere between 4 and 5 million years. Most of the fossils (Sahelanthropus tchadensis, Orrorin tugenensis and at least Ardipithecus kadabba) are older than this date. There are strong reasons to suggest, that you cannot move the date of divergence very far into the past. For more information on this issue I suggest reading John Hawks Post "Reviewing the clock and phylogenomics". Also some traits which were used to classify those fossils as hominids are far from being unequivocal, as you can see at the discussion of Ardipithecus ramidus last week.
Ok, let’s assume that all those early hominins in fact are not hominins. They still show some traits that could be related to bipedalism aren' they? How can we explain that? The answer could be that bipedalism evolved more than once.
Is there any evidence for the convergent evolution of bipedalism?
Well, there is no direct evidence, but here I will present you a hypothesis where parallel evolution of bipedalism could be possible.
Reynolds (1985) showed that Apes in general (some in a larger degree than others) try to put more weight on their hind limbs. The main reason for this seems to lay in the more laterally oriented glenoid fossa of recent Apes. The lateral orientation of the glenoid fossa results in more flexible upper limb and enables suspensory Locomotion while in trees (Aiello, Dean, 1990). On the other Hand it generates higher stress on the shoulder-joint while terrestrial locomotion. This problem could be one of the reasons for the evolution of knuckle walking in Gorillas and Chimpanzees.
Now there is some evidence which points to the possibility that knuckle walking in Chimpanzees and Gorillas evolved twice (Kivell, Schmitt, 2009; Wunderlich, Jungers, 2009; Lovejoy et al., 2009), which means that hominid bipedalism didn’t evolved from a knuckle walking ancestor. What does that mean for the evolution of bipedalism?
Let’s assume we have a more generalized, not fully suspensory Miocene Ape with a flexible shoulder. Let’s assume further that this Ape has to climb down from his tree. Because the lack of strong flexors in the Forearm, he isn’t able to do efficient knuckle walking and his flexible shoulder makes it impossible to walk in a “normal” quadrupedal gait. The consequence would be that this Ape completely avoids walking on his upper limbs and instead will start walking bipedally. Looking at Ardipithecus ramidus this scenario seems not that unlikely. According to the Authors, Ardi was an above-Branch Quadruped while in the trees. On the other Hand, she had a flexible shoulder, at least of what could be told from the remains.
Under the light of this model, the evolution of traits related to bipedalism would be the logical consequence of the functional morphology of said Ape. This means, that if you put similiar selection pressures on different populations of this hypothetical Ape, it would result in the parallel evolution of bipedalism in these populations.
Well okay, everything I said above is more or less hypothetical, but what could be done to put it on a more solid ground?
Well momentarily I can think of two things: First you have to look how Apes exactly stress their forelimbs during terrestrial locomotion, there is some research done with it, but for what I know is that it’s not enough.
Secondly I think one should try to simulate this above mentioned “hypothetical Ape” and look what kind traits it needs to have that bipedalism is the most efficient option for terrestrial locomotion. With this set of traits you can start looking at the fossil record and try to falsify this hypothesis. The last thing is, in my opinion, the most important one, because not many models in human evolution are directly falsifiable through the fossil record.
I’ve been working on this thing for several months but two weeks ago I’m stuck. So I thought the best Idea would be to tell this stuff to a wider audience, which I have done now.
What do you think of it? Is it rubbish? Is it some old Idea which has been given up long time ago and I simply didn’t realized it yet? And if not, what else could be done to build it up to a real model and not some simple thought-experiment by an undergraduate student who has way to much time.
So, if you have any suggestions, please tell me. You will help me a big deal with it.
P.S.: If anyone got accsess to the "Journal of Zoology" and could mail me the following article (Email: edgarneubauer[at]gmx.de), I would be very, very happy:
Larson, S. G., Stern, J. T. (2009). EMG of chimpanzee shoulder muscles during knuckle-walking: problems of terrestrial locomotion in a suspensory adapted primate. Journal of Zoology, 212 (4). S. 629-655.
Aiello L., Dean, C. (1990) An Introduction to Human Evolutionary Anatomy. Elsevier Academic Press, London.
Kivell, T., Schmitt, D. (2009). Independent evolution of knuckle-walking in African apes shows that humans did not evolve from a knuckle-walking ancestor Proceedings of the National Academy of Sciences, 106 (34), 14241-14246 DOI: 10.1073/pnas.0901280106
Lovejoy, C., Simpson, S., White, T., Asfaw, B., Suwa, G. (2009). Careful Climbing in the Miocene: The Forelimbs of Ardipithecus ramidus and Humans Are Primitive Science, 326 (5949), 70-70 DOI: 10.1126/science.1175827
Reynolds, T. (1985). Stresses on the limbs of quadrupedal primates American Journal of Physical Anthropology, 67 (4), 351-362 DOI: 10.1002/ajpa.1330670407
Wunderlich, R., Jungers, W. (2009). Manual digital pressures during knuckle-walking in chimpanzees American Journal of Physical Anthropology, 139 (3), 394-403 DOI: 10.1002/ajpa.20994