Even so it's not clear that discarding the body is a good strategy.
In most cases only a couple of offspring are going to survive the
rigors of infancy, with the challenges to their immune system and
the difficulties due to their small size and inexperience. In that
case keeping the adult alive will mean that the gene will have three
surviving instances rather than two, a considerable improvement.
The adult has already proven its survival abilities and should be
considered a valuable asset.
Plus, in each child any given gene has only a 50% chance of being selected.
In the adult it has a 100% chance.
The real mystery then is why genes allow the reproductive period to stop
rather than working harder to extend fertility. Actually men are often
fertile into their 60's and even beyond, so this discard-the-body theory
doesn't explain why so many symptoms of aging show up before then.
Is it just that the genes can't do any better, that despite their best
efforts, errors and waste accumulate and the cells just don't work
as well? Why then are egg cells kept so fresh and clean generation
after generation? Think about it: every cell in your body is the direct
offspring via fission of an earlier cell, and that line *literally* goes
back billions of years. Somehow it was able to keep itself clean and
avoid accumulated junk, until the last couple of dozen cell generations,
since it became part of your kidney or whatever. All of a sudden it's
just too difficult to prevent damage? After millions and millions
of generations? That's hard to believe.
Probably a better theory is that the genes could have kept people alive
and fertile longer, but that it didn't help because death by misfortune
was still so common. The healthiest man (or ape) in the world might
not have expected to live much beyond 50 due to the harsh and dangerous
world he lived in.
And actually, humans live pretty long compared to most mammals. Many of
us have experienced the sadness of a pet dog or cat dying after only 10
or 20 years. Maybe that was all they could expect in the wild even if
they stayed healthy. It could be that our own lifespan has increased in
the last few million years, as our increased intelligence allowed us
to avoid dangers. 70 years isn't that bad by this reasoning - it could
have been a lot worse.
Hal