I think an important question to research here is why p21 evolved in mammals. The article says knocking it out doesn't appear to increase cancer rates, which would have been my first and only guess.
My guess would be that regeneration is too big a drain on the metabolism. Compared to the animals with that ability, mammals have positively ridiculous energy needs already. It may be that better healing from wounds is not sufficiently useful to make up for the strain that the healing itself entails.
I seem to remember reading a while ago that the immune system in animals with regenerative ability needs to be stronger because the blastoma wound is "open" longer and more prone to infection. For humans today we can mitigate infection pretty well relative to a pretty much any mammal in the wild.
Maybe somehow it speeds up the system of evolution by keeping life expectancies shorter, thereby increasing turnover (death and new births)? I have no clue just throwing that out there.
To survive natural selection the gene must give the animal some reproductive advantage. I can understand that promoting longevity beyond the reproductive stage of one's life cycle affords no such benefit. But one still has to account for the fact that the lack of the gene appears to increase the survivability in the reproductive pahse as well. Something more must be going on.
Strictly speaking, it must give the _gene_ a reproductive advantage, not necessarily the animal. Often, these two advantages are aligned. But they need not be. This is the case in kin altruism. And this is also, why "promoting longevity beyond the reproductive stage of one's life cycle" affords sometimes very much such benefits. Example: grandparents can help raising their children's children (and their children's children's children ;)). Kin altruism becomes even more apparent in ants (or other social animals). Worker ants or bees are pretty much infertile but they help a lot.
It should be noted that the difference is much more extreme for ants than it can be for mammals due to a genetic quirk where a female ant shares 50% of her genes with her children and 75% with her sisters. (Therefore having more sisters is better than having children.)
> To survive natural selection the gene must give the animal some reproductive advantage.
Most probably, but not necessarily.
A gene can be present without being useful because it is linked to another gene that is itself useful; or it can be a leftover from an earlier moment of evolution where it was useful, and then it ceased to be useful but there was never an evolution pressure strong enough to have it disappear.
In summary, just because a gene is present and active does not mean it's useful today.
Not necessarily. The mutation could have survived for any number of reasons including genetic drift. As long as it didn't kill the organism right away there wouldn't be significant negative pressure to get rid of it.
Perhaps not having the ability to regenerate leads to more cautious behaviour, avoiding life threatening situations to a much greater extent? Puzzling...
