Digit Evolution

The answer given was incorrect. More digits is primitive. The earliest animals with legs, like Icthyostega and Acanthostega from late Devonian Greenland, had more than five digits. Acanthostega had EIGHT fingers per hand. There was no three-fingered reptile ancestor, and a fin is not one finger. The earliest reptiles kept five of the original set, the mammal-like reptiles kept five, advanced therapsids had five because their ancestors did, and we have five because evolution hasn’t caused us to lose or gain any either. If you look at the fossil record, you won’t see tons of cases where digits are added, but they are often lost (a well-known example is five fingers per hand in the earliest dinosaurs, two fingers per hand in Tyrannosaurus rex). Icthyosaurs, fishlike aqautic reptiles that decended from land reptiles, has up to eight or nine digital rows in the fin, but they were just rows of bones in a fin and not really fingers. Extra-fingered mutations do not cause an evoultionary trend to more fingers, but lost digits do affect evolution. If you want to see the Acanthostega’s weird 8-fingered paw, there’s a diagram of it in the May 1999 issue of National Geographic.

And the Staff Report in question is, Why do we have five digits on each limb?

Eh, Lego, I went back and read it and you’re right. This:

–does sound like he’s implying that the trend in Evolution is towards more fingers, that life forms tend to add fingers as they go along the evolutionary path, rather than lose them.

But I don’t think that’s what he meant to imply. Doug, care to respond? [look of polite inquiry]

Lego is right; I was wrong - but I shift the blame to the *^$!#&)@ web page that I referred to doing my research, which stated it as I wrote it. I checked the latest textbook (published a few months ago) and it states that the first tetrapods had five digits, meaning it apparently went straight from fins to five digits. I get the impression from this that people don’t consider Acanthostega to be the ancestor of the remaining tetrapods, but an evolutionary dead end.

Apologies for trusting the first answer I came across, instead of digging a little deeper. If we need to change the answer, Dex, can it be done?

Not too long ago, it would have been OK, but if it’s as recent as this year, it’s sadly behind the times.

Is Aconthostega gunnari the real-life model for the Darwin Fish? I adopted my sigline from those badges and bumper-stickers I’ve seen.