National Geographic has an article about a Kurdish family where five out of the nineteen siblings in one family walk on all fours instead of upright.
Scientists who have studied the three sisters and two brothers insist that it is not a hoax.
Uner Tan, a Turkish neurophysiologist, has studied them and believes they are "evolutionary throwbacks" to our ancestors. A team of German scientists led by Stefan Mundlos believes they have found the precise gene which has knocked out their ability to walk upright.
It's not clear what they mean by "the" gene that controls bipedalism: walking on two legs successfully requires many features, both anatomical and mental, and knocking out just one of them will cause the whole system to break. To take an extreme example, it's hard to walk upright if you have no legs. Less extreme example: the move to bipedalism would have required changes to our hips and backs. Our backs are still not completely evolved to suit our upright stance, which is why people are prone to back problems. Comparing us to our cousins (gorillas, orangutans, chimps and bonobos) shows that we have significantly longer legs than they do, relative to our body-size. The evolution of bipedalism would have required all these features to evolve more-or-less in lockstep (albeit presumably in fits and starts), and it isn't credible that there is a single gene that controls them all.
Nevertheless, certainly there could be a single gene -- or many single genes -- that the lack of could disable bipedalism. If you remove the accelerator cable from a car, the car won't move, but that isn't to say that the accelerator cable is the thing that makes cars move forward.
After studying the family, the British evolutionary psychologist Nicholas Humphrey pointed out that the genetic mutation alone wouldn't be sufficient to cause the lack of bipedalism. He gives equal credit to a family that was accepting of the children's strange gait, and making no efforts to cajole them into standing upright. Sensible, so far. But then he goes on:
"This is for real," Humphrey said. "You only have to look at the calluses on the hands of the young man [Huseyin] to see he's been on his hands for a very long time."
Why single out Huseyin? Don't the other four siblings have calluses on their hands? And walking on your hands is hardly the only way to make them callused.
I don't have any specific reason to expect a hoax, but calluses on the hands of one of the five siblings is hardly a reason to give the all-clear. Nevertheless, in the absence of any specific reason to expect a hoax, I think it is worth treating it as genuine.
The behavior could potentially reveal much about our own evolution, Humphrey says.
"Here we've got a living example of how it might be for a member of our species to walk on four legs," he said.
Most experts assume that the quadruped ancestors of humans walked in a similar way to apes such as gorillas and chimpanzees.
But the Turkish brothers and sisters walk on only their wrists or the heels of their hands, with their fingers held off the ground, the researchers say. This position appears to have saved their fingers from damage; the sisters, for instance, engage in both crochet and embroidery.
Chimps, our closest living relatives, use their whole hands and fingers for walking.
"Chimpanzees basically wreck their fingers by walking on them," Humphrey said.
"I think this new evidence, suggesting that [early-human ancestors] walked on their wrists, is much more plausible and interesting," Humphrey said.
I question this completely. Chimpanzees wreck their fingers? Chimps have very good manual dexterity, I'd need to see some support for this astounding claim before accepting it.
But even if correct about chimpanzees, Humphrey's claim that the siblings walk only on the heels of their hands isn't supported by the evidence available. The article has one photo of four of the siblings walking. In it, you can clearly make out just two hands on the ground, and in both of them, the people are clearly using their entire hand, fingers and all.
The photographic evidence contradicts Humphrey's claim, and calls into doubt Uner Tan's conclusion that this is a viable model of pre-bipedal human movement.
If you try it for yourself, I'm confident you'll find that it is impossible to do what the researchers claim. With the heel of the hand, or the palm, flat on the ground, it is virtually impossible to bend the fingers back far enough to keep them off the ground. There's maybe a couple of millimetres give in the finger joints, and it is quite tiring. There are basically three ways of hand-walking: with open hands flat on the ground; on closed fists; or on the knuckles of the fingers, as apes do when knuckle-walking. I do not believe there is any evidence at all, either in anatomically modern humans or any plausible ancestor, for a mode of quadrupedal walking where the fingers are held up off the ground.
There is no doubt at all that human development is complex. The five siblings in question all display a range of congenital deficiencies, include mild mental retardation and lack of balance. It is absolutely possible that whatever genetic damage the five siblings have -- and it might be as little as a single gene -- could lead to them doing the "bear crawl" into adulthood.
But I am extremely skeptical that this gives us any insight into human evolution. For Uner Tan to describe this as "backward evolution" is as absurd as it would be to describe Tay-Sachs disease as backwards-evolution, or sufferers of Huntington's Disease as "evolutionary throwbacks" to an otherwise unsuspected ancestor. The siblings are clearly broken. They're not a throwback to "primitive Man". The most one could say is that, possibly, early pre-bipedal ancestors of human beings may not have had the gene which the siblings are missing. But that's not the same thing: if you rip out the computer chip from a modern Ford Fiesta you don't get a Model T Ford, you get a broken Fiesta.
(Unlike modern cars, biological organisms are astonishingly good at continuing to work with bits missing. As fragile as living things are, we're also incredibly resilient.)
Update, January 3rd: More information here.