Nov 07 2019

The Evolution of Bipedalism

Published by under Evolution
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The evolution of human bipedalism is one of the great events in the history of life that we still need to flesh out. We tend to focus on big transitions because of their implications for the story of life – the evolution of flight, moving out onto land, and the development of intelligence. There is still much we don’t know about the exact path taken in the development of human bipedalism, because we lack good windows into that time and place of our past.

Scientists have now described a new hominid species, which I’ll get to below, but first some background. The first evidence we have of hominid bipedalism is in Sahelanthropus tchadensis, which lived between 6 and 7 million years ago. For context, the last common ancestor with chimpanzees was between 8 and 6 million years ago, so this is pretty close to the transition. We only have skull specimens, but we can tell the species was bipedal because the foramen magnum, the opening for the spinal cord at the base of the skull, opens below like in humans, rather than toward the back like in all other apes.

About 6 million years ago we have Orrorin tugenensis, from which we have a thigh bone which suggests bipedalism. But the earliest human ancestor with extensive evidence of bipedalism is Ardipithecus ramidus, dating to 4.4 million years ago. We have several specimens, the best of which, “Ardi”, contains foot and limb bones. Ardi has clear adaptations to bipedalism, but also for tree climbing.

Keep in mind, we did not evolve from chimpanzees – chimps and humans share a common ancestor. Chimps have diverged from that common ancestor as much as humans have. We don’t have any specimens that are candidates for that common ancestor, so we don’t really know what it was like. Chimps can walk on two legs part of the time, but they are not adapted to it. Chimps (and gorillas) knuckle walk – the walk on four limbs with their forelimbs resting on their knuckles.

So here’s the big question – how did the common ancestor with chimps get around? It’s probably a safe bet that they were not already fully bipedal. Originally it was assumed they were knuckle-walkers, and that was the path to bipedalism, but that theory is now out of favor because it doesn’t fit the evidence. So chimps probably evolved knuckle-walking after they split with humans. The remaining possibilities are some version of being ground-dwellers vs being arboreal. Ardipithecus is evidence in favor of the arboreal  theory, but this is far from conclusive.

Now we have a report of a new hominoid species, based on four specimens found together, a male, two females, and a juvenile. The BBC reports:

The fossils of Danuvius guggenmosi, which lived 11.62 million years ago, suggest that it was well adapted to both walking upright on two legs as well as using all four limbs while climbing like an ape.

This is an incredible find. First, it’s right in the gap of our current fossil record, so it fills in a space in our knowledge nicely. Second, there are multiple specimens with cranial, spinal and limb bones. This species seems to have evolved bipedalism while still living in the trees, another piece of evidence in favor of the arboreal theory.  Perhaps bipedalism first appears in species walking upright through the tree branches, or they lived in the trees but came down to the ground and walked upright to see over vegetation or carry food.

But no one specimen is going to answer these questions definitively. This is because evolution does not occur in a straight line. It is a branching bush, and we are finding random specimens spread out among the branches of that bush. We don’t know if Danuvius is on the path to humans, or was a side branch that has nothing directly to do with human bipedalism. Just like knuckle walking is not transitional to full bipedalism, perhaps “tree walking” is also not transitional. But it may be – we don’t know.

We need a much more complete picture of the bush to know what was happening at that time. Further, we need to explain the evolution of not only humans but of chimps. Any human ancestor from before the split to chimps evolved into humans and chimps. So if Danuvius is an early step toward human bipedalism, it must have also evolved into knuckle-walking along the chimp line.

To complicate things further, the split between humans and chimps may go back further than 6-8 million years but the split wasn’t clean because of later interbreeding. Human and chimp populations may have already separated, but still shared some genes from time to time for even millions of years (until chromosomal changes made us incompatible). While this was happening, other hominoid cousins may have been undergoing adaptive radiation in many directions.

This is analogous to the evolution of flight in theropod dinosaurs. Once we discovered some good fossil windows and started finding all kinds of feathered dinosaurs, it was clear that dinosaurs did not evolve in a straight line to modern birds. There was incredible adaptive radiation, will all kinds of feathered dinosaurs and even flyers that did not lead to modern birds.

In both cases we are still in a phase where each new find makes the picture more complex, and we actually expand what we don’t know (the known unknowns). It is only realistic to expect that once we start really filling in the time between 20 and 5 million years ago with more and more hominoid fossil species, the picture is going to get horrifically complex. But eventually, hopefully, a clear enough picture will emerge to give us a good idea what path our ancestors took on the way to full bipedalism, and how those ancestors relate to our closest living relatives.

What recent specimens, most notably Ardi and now Danuvius, are telling us is that a closely related branch of the hominoid family tree evolved early bipedalism and tree climbing together. Whether this is the branch that lead to humans remains to be seen. We await more specimens.

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