What we REALLY evolved 'for'!

We have posted a few times about Just-So stories that take classical, perhaps obvious-seeming, explanations for how this or that trait evolved or what it was driven by natural selection 'for'.  Of course, these are often, perhaps usually, made up out of nearly whole cloth.  It is usually based on assuming that today's function was yesterday's adaptive reason: that what is good today, was good for sexual success yesterday (because that is the only thing that matters in regard to adaption by natural selection)

We have speculated that the gracile hominid hand found in some of the earliest fossils of our ancestors was not necessarily adapted 'for' tool use, or cooking, or hunting.  We gave it a different explanation that was even more plausible than these staid classical explanations (refresh your memory here).  

That was about adaptive evolution of human ancestors. But what about our closest relatives, with whom we share a reasonably recent (about 8 million years) common ancestors:  chimpanzees?  What did they evolve 'for'?  Let's concentrate where most anthropologists do, on locomotion.  Chimps can walk upright after a fashion, and although that's not what they do all the time, there have been various highly confidently asserted explanations about what chimp locomotion evolved 'for'.

Well, MT readers, we have another Bulletin of the Truth for you.  Only this time we are not going to rest our case on our own type of speculation.  We have direct, right before your eyes, evidence:

Finally, the truth will out!  Evolutionary explanations clear, simple, and genuine.  No more guesswork.