Reviewer 2's Review (Conrad Istock)

“Initial comments and observations after reading the Abstract.

It is likely that most men and women in early human populations did not live to the age of 50. Some group leader males may have lived much longer and reproduced, but would likely have many deleterious mutations in their sperm, probably reducing the viability and survival of offspring. There is also the possibility that selection in these early populations might have favored longer, post-menopausal, survival of some women who performed behavioral roles that enhanced group survival. Such has been claimed for post reproductive female elk who guide their groups over routes they know will efficiently move them between summer and winter habitats. This is akin to the "grandmother" scenario cited in the introduction. At some point we will also have to face the fact that traditionally women live longer then men on average, though the gap is closing.

General comments.

I like the problem addressed here and the approach is intriguing. It is truly provocative and for that reason alone I hope it will be published.

I wonder how realistic the stability assumptions, for example p. 11, are. Early humans may have experienced incessant fluctuations in their demography. They, like the more recent analogues, probably lived in small groups innocent of stable age distributions and so on.

I found the jumping back and forth between this new approach and Hamilton's awkward. Why not just give the Hamilton review early on with figure 1 and S=0 and then move on logically (unbroken) with the two sex development? Returning once more to the contrast in the discussion.

Why do females come with a finite supply of eggs?”

N.B. These are the general comments made by the reviewer when reviewing this paper in light of which the manuscript was revised. Specific points addressed during revision of the paper are not shown.