What's most curious about this is why less complex creatures have an enormous ability to regenerate but more complex ones don't. If it is a matter of a few genes, you would expect that random mutations would impart the self-regeneration trait onto us but evolution has chosen not to.
I can only surmise that for complex creatures, self-regeneration is not only worthless, but is undesirable (since no complex creatures seem to have self-regeneration but many less complex creatures do). This, of course applies to complex creatures as a species anyways. I think I'd find it extremely valuable for myself.
I don't know the answer but perhaps it has to do with the thinking aspect of complex creatures and how that affects mating. I'd be interested in hearing others hypothesize about this.
However, it just says that other pressures have been greater than the pressure to (keep the ability to) regenerate. Or the costs of being able to regenerate are probably prohibitive.
The competing pressures might include (for example) a pressure to be smart or strong enough not to lose body parts in the first place, or a pressure to develop coping strategies when a limb is lost. Or the pressure to give food and resources to offspring, over attempting immortality. Or the pressure to have more complex tissues (even if they are more difficult to regenerate), although the article sheds a shadow of doubt on this last one. If these competing pressures are great enough, and more importantly, the pressure to keep the regeneration trait is low enough, the trait will simply drift away (randomly mutate) into nonfunctional genetic code. It doesn't mean it is completely undesirable.
More "complex" animals like humans don't lose a lot of body parts on a day to day basis. And those who do, have their (evolutionary) fitness determined by their ability to cope with the loss, rather than by their ability to regain those parts.
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