JimMcGinn wrote:Another point. If sophisticated tool usage underlies bipedalism then we'd expect head size to have increased at the beginning of hominid evolution. The fact that head size increases later indicates that if tool usage underlies the selective origins of bipedalism then these tools must have been very crude, sticks and stones. And if these tools were very crude then this severely limits the types of scenarios we can envision.
That is what happened. To give you some numbers, the Cranial Capacity/Body Mass ratio for the apes considered human ancestors increases gradually along human evolution, from around 6 with Ardipithecus, to 12 with Australopithecines, to 17-20 over Homo Habilis, Erectus and Sapiens. At the same time, the ratio for extant Pongo, Gorilla and Pan is at 3-9.
Full list: http://www.hornshire.com/AAH/Great%20Ap ... pacity.pdf
JimMcGinn wrote:IMO, an even bigger impediment is the fact that chimps are so plainly ineffective with tools.
I wouldn't say they are ineffective. I think that chimps are listed as number two on the list of tool using species, after humans, and higher ranked than eg. sea otters. I have seen zoo experiments, where chimps stacked boxes on top of each other to reach high hanging fruit.