The philosophical answer to Milton's problem is, like so many philosophical answers, a counter-question. What do you mean by human life? This could involve further probing of a kind that a pre-Darwinian like John Milton would fail to comprehend. Does he include Neanderthal man, for example, in “human life”, or does he want to start with Homo sapiens?
A resurrected Milton might protest that in writing Paradise Lost he was not concerned with problems of evolution (of which he had not previously heard). His question suggests that he was thinking of the “life” of a particular “human being”; but he knew so little of what we now call biology that he would probably have been as nonplussed by the biological answer to his problem as one given in terms of prehistory.