Would you happen to have a link? I've been interested in toying around with this idea (just for fun) for a while, but I haven't started doing the actual research.

My instinct would be that (biological) immortality is (mostly) non-existant not only because there is no evolutionary pressure not to die, but because there is evolutionary pressure to die after begetting offspring. Very long lived progenitors would homogenize the gene pool, if not by incest then by sheer number of offspring. This would make them fragile in the face of both genetic errors and environmental shifts.

To counter this fragility, child birth might be infrequent. However, if child birth is infrequent, there is an increased probability of death before child birth.