That Dog Won't Hunt
I have a dog who had to have knee surgery on both hind legs. For several months, she did everything, and I mean everything, standing on just her two front paws. It was remarkable to watch. It was a temporary thing, and she survived because she had two squares a day, and a warm, safe place to sleep, plus people who would carry her up and down stairs. As soon as it no longer hurt to use her back legs, she went back to all fours.
Some might have thought after watching the video about "Faith the Dog," which James Barham referred to earlier, that this story is an argument for easy adaptation to walking on two legs. After all, if Faith did it, then a hominin ancestor surely could have. Let me assure you that there is no parallel.
The video about "Faith the Dog" was amazing, but painful to watch. This dog had no choice but to walk on its hind legs, and so it did. It would not survive in the wild in this state, though. It couldn't move fast enough to escape predators, and could not feed itself without the help of humans. I also imagine it must be exhausted after one short trip to the park, and if it could speak, would say, "Oh, my aching back. Somebody rub me on my shoulders, I think my neck is going to break!" Its posture was totally unsuited for its frame. Courageous? Yes. Remarkable? Yes. A path to bipedalism? No.
The story of our own journey to upright bipedalism, as recorded in the fossil record, is discontinuous. Lest we forget what it entailed, recall the last time you saw a chimp walking on its hind legs. The chimp's walking gait is lumbering, with the body swaying back and forth with each stride. Contrast that with how fast chimps can move when on all fours (knuckle walking).
Australopithecine fossils were ape-like in posture and gait. Their ribcage, hips, legs, spine, and feet were closer to chimp than human. While these hominins may have spent some time on the ground, they were not built for speed or running.
With the appearance of Homo erectus, though, many traits changed all at once. Below the neck, these hominins were virtually indistinguishable from a modern human. Their legs, lumbar spine, arms, shoulder girdle, pelvis and hips, rib cage and feet now were those of a long-distance runner with an efficient well-balanced gait1,2.
How did the new morphology arise? No one knows for sure. But one thing is certain. Faith the Dog is not evidence for the ability to evolve bipedalism, she is evidence for achievement in the face of adversity.
(1) J. Hawks et al., "Population bottlenecks and Pleistocene human evolution," Mol Biol Evol 17 (2000): 2-22.
(2) D.M. Bramble and D.E. Lieberman, "Endurance running and the evolution of Homo," Nature 432 (2004): 345-352.