As freaky as this is, an atavism - the occurrence of an ancestral trait otherwise lost in the extant species - is good evidence for evolution. Given the right mutation(s), an organism may develop a trait as it was in all the ancestors of the species. There are many examples of this, such as humans with tails.
So what caused this? Contrary to what the snake expert on the site said, an autopsy won't reveal the cause. For that the snake's genome has to be sequenced. Was it a mutation in the germ-line (either egg or sperm), or a somatic mutation during early development? The fact that the snake only had one leg suggest that it would be somatic, or it would most likely have had two. (My youngest son has two different eye-colors, and no doubt that is caused by a somatic mutation.) What kind of mutation (single nucleotide, or a larger scale event)? And note, that if it is a somatic mutation, offspring of this individual would not have had legs. Shame that. I'm sure lots of people would fancy havinga rare legged snake, for a pet or for dinner.
Update 9/16/09:
Other good hypotheses have surfaced. Ironically, PZ Myers is guessing that pleiotropy is to blame (the irony being that I should have thought of that), by way of some gene(s) that control limb development also have other functions, and that constraint then prevented the gene(s) from being destroyed. Instead it could be silenced specifically for limb development, and it is this repressor that PZ guesses has been suppressed during early development by an environmental factor, leading to activation of the genes that make the limb. Another hypothesis is that the whole thing is a photoshop hoax, and yet another that it is the leg of the snake's last meal protruding through the skin. I highly doubt this last idea, seeing that the limb and digits are very well defined, which seems unlikely if the skin belongs to the snake.
There was also a discussion in germany (schlangengrube.de) about this case, with many people assuming that the snake ate a lizard and a prey foot poked through the body wall. This explains both the strange position and the swollen snake body.
ReplyDeleteConcerning atavisms, I'd not expect a forelimb to look that well at all (AFAIK forelimbs did not shrink gradually during evolution, but were switched off suddenly very long ago (maybe something with Hox genes), whereas hindlimbs *did* shrink). So the genes controlling the details (fingers, joints etc.) of the limbs should be rather obliterated by mutations and dysfunctional after 1e8 years sitting around unused and unselected. But being just a physicist, I might be wrong.
That's an interesting idea. It doesn't look like it could be a leg of a lizard from the inside to me, but at least an autopsy will settle that question. I did notice the swollen body, though...
ReplyDeleteHopefully we'll hear from that autopsy soon.
Sorry, but that is very definitely the leg of a lizard consumed by the snake. First, note that there is a very obvious bulge along the body, and the leg is in the right place to be the forelimb of that bulge. Note also that the soles of its foot face towards the head of the snake, which is what you expect from a prey item eaten head first.
ReplyDeleteHow it came to protrude through the body wall I don't know - either because the lizard was eaten alive and ruptured the body wall through its struggles (possible but unlikely - although I am not sure what that snake is or what its foraging methods are), or, more likely, the body ruptured when the snake was clubbed to death. This is the sort of thing you can sometimes see in roadkilled snakes.
If it were an atavism, you would expect the leg to have grown in the "correct" place, namely the pectoral girdle (much further forward, and very unlikely, since no snake has a remnant of one), or, more likely, from the pelvic girdle, at the base of the tail, much further back. It certainly would not have grown at midbody, out of the stomach. Note that in the tailed child, the tail is in the anatomically correct position.