LCA of humans and chimps/bonobos

 

I'm curious about the LCA of humans and chimps/bonobos. I take it no fossils are known because it was rare and lived in a place where fossilization is unlikely. It would not have been a chimp, but more like them than like us?

Yes — that is essentially the right picture, with one important qualification: we don't know exactly what the last common ancestor looked like, and “more like a chimp than us” is probably directionally right but can be misleading.

The LCA of humans and chimpanzees/bonobos probably lived roughly 6–8 million years ago, although the molecular estimates have considerable uncertainty. It wasn't a chimpanzee that somehow started becoming human. Rather, chimpanzees and bonobos have evolved just as we have since the split. So the LCA would have been its own ape species, ancestral to both lineages.

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And I think your fossilization argument is quite plausible. A small population of forest-dwelling apes living in warm, wet environments has a terrible fossilization probability. You need rapid burial, protection from scavengers and weathering, suitable sediment chemistry, and then millions of years without the remains being destroyed. The African environments in which the crucial populations lived have also undergone enormous geological and climatic changes. Finding any fossils from that particular population would be a remarkable stroke of luck.

We do have a few candidates from around the right period, though. Sahelanthropus tchadensis, from about 7 million years ago, is particularly interesting; Orrorin tugenensis is around 6 million years old. Then somewhat later we have things such as Ardipithecus. But none can confidently be declared the LCA.

As to its appearance, here's where it gets fun. It probably had a small brain, a relatively projecting face, substantial jaws and teeth, and long arms, much more in the general anatomical direction of an African ape than a modern human. But it may already have possessed some capacity for upright walking, at least intermittently. The really interesting question is whether habitual bipedalism began before the human lineage separated or evolved very shortly afterward.

And it probably wasn't living exactly like either modern chimpanzees or modern bonobos. Those species have had six or seven million years of their own evolution. In fact, the common ancestor may have been less specialized than either. Modern chimps, for example, are superbly adapted to climbing and knuckle-walking, while bonobos have their own distinctive adaptations.

So if you could somehow take a photograph of the LCA, I suspect your first reaction would be something like:

“Oh, that's an ape.”

But if you then watched it walking, climbing, feeding, and interacting socially, you might occasionally think:

“Wait a minute—that thing is doing something surprisingly human.”

That's one reason the LCA is such a fascinating creature to imagine: it wasn't half chimpanzee and half human. It was the third thing from which both eventually emerged.

The pre-humans did better on the grasslands because they could stand more easily and longer?

Yes, partly — but I would turn the causal arrow around a little.

Early hominins probably didn't succeed on open grassland simply because they could stand longer. Rather, whatever advantages early bipedalism provided became increasingly valuable as they spent more time moving and feeding in more open environments.

Standing and walking upright could have helped in several ways. It freed the hands for carrying food or infants, gave a better view over vegetation, and made walking relatively efficient over long distances. It also reduces the amount of body surface directly exposed to the midday sun, while allowing greater cooling from air movement — potentially quite useful in hot open country.

But there's an important complication: the earliest hominins probably weren't grassland specialists at all. The picture of our ancestors abandoning the forest and marching onto an African savanna is now considered much too simple. Several early hominin sites were mosaics of woodland, bush, wetland and patches of open country.

And bipedalism itself probably predates extensive savanna living. That's one of the really interesting clues about the LCA and its immediate descendants. They may have been climbing animals that increasingly adopted an upright posture in trees and on the ground, with full-time terrestrial walking coming later.

There's also a nice evolutionary chicken-and-egg problem here: did changing environments favor bipedalism, or did bipedalism allow our ancestors to exploit changing environments? The answer is probably both, interacting over millions of years rather than one sudden transition.

And I suspect the thing that ultimately mattered enormously was walking efficiently, rather than merely standing. A creature that can comfortably walk several miles on two legs has a very different ecological repertoire from one that must move around on all fours.