I really do think people on both sides have failed to understand just how little selection is needed for the phenotypes to have changed so much. It's basic population genetics. I'm always struggling to understand how people on both sides of the "divide" seem to have failed to grasp this point.
I haven't failed to understand that, but just because it can happen, it doesn't mean that it did. We know that if you selectively breed Russian wild foxes for non-aggressive traits, after 3 generations you get pie-bald pooches with floppy ears. But nature rarely deliberately selects for specific characteristics or intensively breeds in any particular direction. Nature allows those best suited to their environments to survive and pass on their genes. Depending on the circumstances it can be an inexorable plod towards any change at all. If we all have flat paddle-like feet, it means that at some point in our past it became necessary to have them, or die out.
I also have learned enough about skeletal anatomy to understand that the joints and other traits show clear weight bearing in Homo erectus, just like us, if not more. As a medical practicioner, Marc should know this better than I do.
I'm thinking out loud here - trying to clarify my own ideas. Please bear with me:
Isn't it possible that our ancestors at one time, long ago, possibly between 5-7 mya, dangled from trees in swamps and perhaps waded a little, thereby gradually separating themselves from Pan who stayed in the trees? Isn't it also possible that at another later time, different ancestors preferred the swamp to the trees and spent their time wading in the shallows and eating on the shore? Isn't it possible that over differing periods of history, our ancestors dangled from the trees, waded in the shallows, swam in the water, and lived on the shore, gradually becoming more and more upright, as hominid fossils show, developing weight-bearing traits, until finally as Homo Erectus, they began to spend almost all their daylight hours diving and living off shallow food sources: shellfish, shrimp, etc., more or less giving up a land based existence except for coming ashore to sleep at night? If so, during that phase mightn't their bones have become heavier even though they still retained the weight bearing traits that they had already acquired from earlier generations? And wouldn't this also explain how we acquired so many unique aquatic traits and why they belong to all of us, not some of us? After all, the fossil and anatomical evidence at any time can only show what happened 'up to' that point in evolution.
I asked him, repeatedly, over several days, if he thought Homo erectus was or was not adpated to terrestrial bipedalism and how much terrestrial bipedalism he thought they would have done. When I met him, face to face, in London a few weeks ago, I asked him these questions again.
Just because HE had weight bearing features, didn't mean he needed them at that time. I have an appendix now, but I don't need it. So I don't understand the question fully either. Was HE adapted to terrestrial bipedalism, or was his ancestor? Am I adapted to a swimming / diving existence, or was my ancestor? Will future generations be able to tell, from looking at my skeleton, how much swimming or diving I might be doing? I don't think there is a clear yes / no answer unfortunately. It would be nice if there was, but then we wouldn't be talking about it.