Something I think has been forgotten.
There seems to be a picture of a sharp line in time and space between the old and new. I don't know if it's been determined but I'm sure the forest to savannah change would have taken some 1,000's of years.
Also there would be some intermediate environment. There is room for some preadaptation and, as has been pointed out, there is room for a life style involved with both.
There seems to be some picture, also, that as soon as our ancestors came out of the trees they'd be on a savanah with lion and leopards after them just as you'd have today. I don't think it would be like that.
If the forests were retreating quickly it is possible that the new environment would be a challenge for the predators too. They would need some time to adapt as well. It is possible that getting away from the forest would be an advantage until the predators adapted to it as well.
I think the real challenge for all of us here is that we would like more resolution in the fossil record. Right now we have a few 100 specimens spread over 5 or 6 million years. This isn't enough to track the change from forest to savanah that could well have taken place over time scales of around 100,000 years (or even 10,000's). It may take centuries to find some 10's of samples from a very specific time frame around 6 million or so years ago.
I think that fossils don't form well in a forest environment anyway. Is that correct? The chemistry isn't good for preserving the bones. WIthout those details we will have a tough time being sure of what happened in detail.
One other line of research will arise as we understand exactly which genes do what and the difference between us and chimps (as a stand in for what we may have evolved from). We do know that our physical from (walking up right for example) evolved early, very early. Our brains came in stages, later and more slowly. We have some evidence of the brain changes.
I think that we understand that a significant fraction of the difference between our and chimps genomes are in the area controling brain development (that shouldn't be a surprise). What may be found is that the changes required to get us upright and much more adapted to the open savanah aren't all that large. If we knew the exact genetic changes we might well be able to hypothosize that only 100's of generations are enough to lift us to our feet.
™ Version 4.2
