14 DAY TRIAL // We look at a chimp and we imagine this is how our ancestors looked like and walked. But many researches show that the common chimp-human ancestor was more human-like. And now it appears that this ancestor was bipedal (it walked on two feet), not being a knuckle dragger. The concept of the humans as "upright apes" is challenged.
The ancestors of humans and the great apes could have actually walked bipedally too, thus the knuckle-walking of chimps and gorillas is the exception, not the rule.
"The other great apes we see now, such as chimps or gorillas or orangutans, might have descended from human-like ancestors," co-author Aaron Filler, a Harvard-trained evolutionary biologist and medical director at Cedars-Sinai Institute for Spinal Disorders in Los Angeles, told LiveScience.
He investigated how the spine was assembled in over 250 living and extinct mammals, some bones being 220 million years old. A series of changes pointed that walking upright, not with our knuckles, could have characterized the ancestors of modern great apes.
In most mammals, the body is roughly split in two by a tissue structure running in front of the spinal canal, the "horizontal septum" that marks the a dorsal part (the back side in man) and a ventral part (or the front part in man).
A mutation in the first direct human ancestor made this septum cross behind the spinal cord in the lumbar region of the lower back, a pattern that would have turned the horizontal posture inefficient.
"Any mammal with this set of changes would only be comfortable standing upright. I would envision this malformed young 'hominiform'-the first true ancestral human-as standing upright from a young age, while the rest of the mutant's family and species continued to walk around on all fours." said Filler.
The shift could have taken place "very abruptly, with just a few shifts in 'homeotic' genes, or ones responsible for how the body plan is laid out," added Filler.
Previously, the first known bipedal ape was believed to be about 6 million years old, but the new research suggests this was in fact the extinct hominoid Morotopithecus bishopi, which inhabited Uganda over 21 million years ago.
"Humanity can be redefined as having its origin with Morotopithecus," said Filler.
This pushes back the root of our two-footed walk about 15 million years, when we had a common ancestor with chimps, gorillas, orangutans and gibbons. A recent research pointed that upright walking could have emerged before our ancestors left the tree life.
"If you look at baby siamangs, which are a kind of gibbon, you'll see them walk bipedally on their own. It's just their natural way of walking. They never knuckle walk." said Filler.
"If bipedalism did evolve 21 million years ago, it more likely evolved to walk in trees than on the ground," said University of Chicago evolutionary anthropologist Russell Tuttle.
"Besides Morotopithecus, fossil vertebrae suggest three other upright ape species precede the 6 million year mark. So you have this fossil evidence for bipedalism, and you have apes such as gibbons. Perhaps humans represent the primitive condition, and knuckle walkers such as chimps and gorillas are modified. The ancestors of chimps and gorillas might have evolved knuckle walking as a speedier mode of travel," said Filler.
If bipedalism was a basic characteristic, then gorillas and chimpanzees evolved knuckle walking independent of each other. Genetic analysis could reveal if they developed this type of walking differently.
"I am getting the feeling that a revolution in our thinking about the origins of bipedality is now under way," said evolutionary anthropologist Robin Crompton at the University of Liverpool in England.