What was the last common ancestor of humans and chimps like?

Factual Questions
1

We are all familiar with the popular misconception of the evolutionary chart, with a gibbon followed by a chimp followed by an Australopithecus followed by a neanderthal followed by us.

But even when we are told “it was actually more complicated than that”, I think the image in most of our heads of the last common ancestor of humans and chimps basically is a chimp.

However, my understanding is that there is a serious problem with this image, and that is knuckle walking. Chimps and gorillas appear to knuckle walk in totally different ways (in terms of how they shaped their hand bones to allow them to knuckle walk); but neither human no chimp ancestors nor any other known Miocene apes appear to have knuckle walked.

Which raises the question. If the human/chimp last common ancestor didn’t knuckle walk, how did it move around?

From what I can figure out, it may have been a chimp-sized but gibbon-shaped tree dweller that would have potentially been bipedal both on branches and on the ground; we think of humans as “coming down from the trees” and of chimps as staying in the trees, but in fact we are both terrestrially adapted from completely arboreal ancestors?

I was going to ask why we don’t have anything filling the “chimp sized gibbon” role today, but I guess that’s sort of what an orangutan is?

I guess it seems a bit counterintuitive that our closest relatives (chimps/bonobos - the Pan genus - followed by gorillas) are all so terrestrial but our most recent ancestor was probably more orangutan-like. But is that because the tree dwelling great apes died out while the land dwelling ones survived? Except that there used to be terrestrial apes in Asia who went extinct, and apparently arboreal apes in Africa who went extinct - what was different about the jungles of central Africa and southeast Asia that meant that different types of apes survived in each place?

2

The Ardipithecus genus sits near the divergence of the human-chimpanzee line, in the Late Miocene (5.5ish million years ago). If you think of that divergence as a Y, Ardipithecus is believed to sit on one of the arms near the junction and if you follow it you eventually get to the humans who contribute to this forum, as the apogee of hominid advancement, but there is enough debate and gaps in our knowledge of what else was happening that a new find could change that overnight.

Ardipithecus seems to have been a meld between chimp and human locomotion - competently bipedal when required but with long enough limbs with range of movement to be doing the tree thing too. A chimp is not the general precursor form but also adapted in its own way. Dunno about knuckle walking, not sure we have relevant skeletal material from the right time.

What we’ve learned from recent palaeontology, like Flores is that we have little good data from large chunks of the planet, esp. the bottom half of Asia, which might change our understanding of both range of diversity and also survival of different hominins.

3

This reconstruction from 2009 has one interesting caption that might be relevant;


I don’t know how well sourced that assertion is.