Most animals have the ability to regenerate lost parts, but not most of the tetrapods (the descendants of the four-legged creatures that invaded land; tetrapods include amphibians, reptiles, birds, and mammals).
Salamanders are the tetrapods best at regenerating lost parts: some can replace lost limbs, eyes, hearts (!) and tails at any stage of their life. Other salamanders can regenerate parts only when young, before metamorphosis into adults. Frogs can replace lost limbs, but only the limbs they have when tadpoles. As adults they lose this ability. Fish can replace some fin rays, but not lost fins. Lungfish (freshwater fish in the subclass Dipnoi) can apparently replace lost entire front or rear fins. Another primitive freshwater fish, Polypterus, can apparently regenerate its pectoral fins, though it’s not known whether this ability is limited to juveniles. Here’s a Polypterus:
Finally, and I bet you didn’t know this (neither did I), we humans can regenerate our fingertips, though this happens more readily in children than adults. I’ve added this photograph of fingertip regeneration suggested by reader Mark Sturtevant in a comment, which comes from a nice page summarizing animals’ ability to regenerate. It’s a bit grisly, but hell, we should be able to stand looking at this for the sake of education:
Other than that, though, humans can’t regenerate anything other major parts. The study of regeneration in salamanders has thus become a sort of cottage industry in biology, for if we could figure out how they do it—and they’re making some progress here—maybe we could help amputees regrow their limbs, something that God has proven incapable of doing—though He’s said to be able to cure many other ailments.
The information on regeneration across groups that I just gave came from a new paper in the Proceedings of the Royal Society (Series B) by Nadia Fröbisch et al. (link and free download below). But the main point of the paper was a striking new finding: fossil amphibians from 300 million years ago apparently had the ability to regenerate limbs, too. This, I think, is the first time that any fossil has been shown to have the ability to regrow lost body parts.
The authors studied many specimens of the primitive amphibian Micromelerpton credneri, from 300-million-year-old deposits in Europe. This is what one looks like; the preservation is remarkable (the scale bar is 1 cm., and there are about 2.5 cm per inch). The authors note that in this specimen you can see the shadow of the skin, the pigments in the retinas, the external gills, and the pattern of scales. This is not really a salamander, but a primitive amphibian whose placement in the tree of tetrapods will be shown shortly:

Looking at many specimens, the authors found that in some of the limbs there were signs of regeneration that resemble those seen in modern salamanders when they lose their limbs. They claim (and I can’t judge this, but take their word for it) that these anomalies are not simple deformities in the limbs of salamanders that have not lost their limbs.
Here is one sign of regeneration, two fused “phalanges” (fingers, if you will); normal specimens have four fingers on their front “hand,” this one has a bifurcated finger so there are five digits:

Another putatively regenerated limb, a foot this time. Feet normally have five digits, this one has six, with both central digits being thinner than normal. This, the authors say, is also a sign of regeneration and not just a deformity.
One potential problem with the results, which the authors discuss, is that while these are likely signs of regeneration, they don’t show that the regeneration happened in adults. The digits could have been lost and regrown as juveniles, and the signs of regeneration simply persisted in the adult, which may not themselves have lost the ability to regenerate limbs. In other words, these primitive amphibians may be like frogs or some salamanders, having regenerative abilities only when young.
So what does this mean for the evolutionary history of regeneration? The authors included a nice phylogenetic diagram of fish, amphibians, and other tetrapods, both primitive and modern, showing their regenerative capabilities. The placoderms (extinct armored fish) and chondrichthyes (fishes with cartilage: rays, skates, and sharks) can’t regenerate. Polyptera and Dipnoi can (the asterisks supposedly indicate regeneration only in juveniles, but we don’t know that for these two groups). We don’t know about the fishapod Tiktaalik, Acanthostega (one of the first tetrapods with limbs), or Eryops, an early and extinct amphibian. In fact, we don’t know the regenerative capabilities of anything without a gray box (indicating some regneration) or yellow box (those species whose phylogenetic placement is unsure; frogs and salamanders thus appear in two places).
Amniotes (birds, reptiles, and mammals) can’t regenerate anything.
What the authors suggest from this is because the “outgroup” of tetrapods—the lungfish and Polypterus, which are fish—have regenerative abilities, as well as some of their descendants (frogs and salamanders), it is possible that the ancestor of all tetrapods, an early fish, also had the ability to regenerate body parts. In the descendants that can no longer do it, like us, we might have lost that trait. In the language of cladistics, regneration is “symplesiomorphic”: an ancestral trait.
An alternative hypothesis is that the groups in gray independently evolved the ability of regenerate: it would then be a “synapomorphy” (shared derived character).
We can’t decide between these hypotheses at present though the authors favor the former one. The kind of evidence we’d need to decide between these hypotheses would be a bunch of early fossils showing the ability to regenerate, particularly in very primitive tetrapods or their precursors like Tiktaalik. If those had that ability, and it was seen in several early tetrapod species, it would support the notion that the very first tetrapods could all regenerate their limbs, but that the ability has been lost in some groups.
And that would raise the question: if groups like reptiles and mammals lost their ability to regenerate parts, why? It would seem to be a terrific advantage to be able to regrow lost parts. One possible answer is that the developmental system of these groups evolved in such a way that regeneration became physiologically impossible.. But of course we won’t know any of these things until we have better fossil evidence as well as some molecular data on exactly how limbs regenerate. If the molecular and developmental basis of regeneration were similar in all tetrapods, it would suggest that they have inherited that system from an ancestor, as it would be unlikely that such similarity could evolve independently. Biologists are working feverishly on the developmental basis of regeneration.
Maybe God can’t heal amputees, but perhaps science can.
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Fröbisch N. B., C. Bickelmann, and F. Witzmann. 2014. Early evolution of limb regeneration in tetrapods: evidence from a 300-million-year-old amphibian. Proc R Soc B 2014 281: 20141550
h/t: Dom


















