From: Damien Broderick (damienb@unimelb.edu.au)
Date: Thu Sep 04 2003 - 00:21:13 MDT
>> Not so -- K-selection vs. R-selection will apply selection pressure
>> particularly if the "elders" of a group contribute significantly
>> to the survival of their children, grand-children, etc. This seems
>> likely to be true in humans, elephants and whales -- though there
>> is still a fair amount of academic debate on this topic.
>Yes, I see your point here. I hadn't considered that. I keep forgetting that
>"social" aspect of our nature. Maybe because I'm reclusive. :-)
>Still, I have to wonder what, if any, contribution "elders" would have that
>were significant enough to increase the likelihood of grand-children
>surviving to reproductive age in the early hunter-gatherer tribes of modern
>humans.
According to that splendid book THE LAST MORTAL GENERATION (which might be
out of date by now, four years later):
========
Die young, and leave a beautiful child
Menopause - `the Change' - occurs, you'll recall, when pituitary hormones
fail to activate repair and maintenance systems further down the track. It
seems an absurdly early point in human life to shut down the reproductive
system, since menopause begins thirty-odd years before many modern women
are ready to die of old age. Surely the Darwinian dogma tells us that the
sterile lose their place in the evolutionary queue. However healthy they
happen to be, their success as phenotypes (or bodies) cannot win their
genes a place in the next generation. They might even find fertile mates,
but they have already lost the contest. There are well-known exceptions to
this harsh economics, mostly among insects where sterile workers are
selected because they share more than the usual proportion of genes with
their fertile sisters, and aid the hive's prosperity. Humans don't have
that advantage - except on a cultural level, where single celibates,
dedicated homosexuals and other `non-breeders' enrich the survival chances
of their child-bearing fellows. Is that the clue we need to explain
menopause's peculiarities?
Recall that the endocrine cascade of menopause starts when the last of the
egg follicles in a woman's ovaries run out. This brute fact can be
interpreted in a several ways, some of them profoundly misleading. Without
eggs, it is sometimes said, a woman is no longer at the sharp end of the
natural selection process, so she can be discarded - left to die,
literally. This, as we'll see, is exactly what happens to other mammals
such as baboons and lions. A more accurate way of putting it is that, in
practice, bodies built from genes which managed to keep them healthy and
nimble until they were a thousand years old would have no reproductive
value over bodies that drop dead of heart attacks at 50 - if they cease
passing those genes on to descendants at 45. Evolution is a sieve for
groups of genes, not for bodies as such.
One trouble with that stark account, which seems sound as far as it goes,
is that it neglects the contribution of post- or non-reproductive family
members who share the same genes. Think of two imaginary human tribes, each
with several inter-marrying bands, who differ genetically in this respect.
In one, mothers simply die at about 50 or 55, seven years after giving
birth to their last child, conceived from one of their last eggs. The
moment the youngest child in the family is reasonably self-sufficient, the
mother (due to her characteristic blend of uncaring genes) gets cancer or
heart disease and dies. In the other tribe, mothers remain reasonably
healthy after an uncomfortable transition through menopause, and continue
as productive members of their community, providing food and, as custodians
of wisdom, hoarding and dispensing useful information that can make all the
difference between life and death to many of her grandchildren and nieces
and nephews for a couple more decades.
It seems intuitively obvious that the second evolutionary strategy will
prove more successful. Indeed, it is embodied in a recent anthropological
doctrine known as `the Grandmother hypothesis', advanced in 1997 by Kristen
Hawkes and James O'Connell, of the University of Utah, and Nicholas Blurton
Jones, of UCLA, based on research among the three hundred Hadza
hunter-gatherers in hill country near Lake Eyasi in northern Tanzania. The
healthy Hadza grandmothers foraged for food shared by their grandchildren,
which permitted their daughters to bear and suckle new babies rather than
expend energy and effort looking after their older children. It is true
that acting as wise elder repositories of knowledge (memes, in short) would
benefit the whole tribe indiscriminately. Hence, this benefit might not
have a direct and preferential effect on genetic reinforcement in the
blood-lineage along, and might not be selected for. Still, providing care
and food for their daughters' children certainly would benefit their own
gene line.
This is an endearing notion, and useful in offsetting the bombastic stress
some male anthropologists have placed on the supposedly supreme virtue of
hunting as a source of food (not that this has been taken terribly
seriously for decades anyway). The problem with the grandmother theory,
alas, is that it just doesn't seem to make mathematical sense. And in
genetics, that is a very important hurdle. There's nothing abstract and
arbitrary about the mathematical models used to test population genetics.
The equations are clear and unforgiving, if used appropriately.
In September, 1997, Natalie Angier reported in The New York Times that
`Using mathematical models, Alan Rogers of the University of Utah estimated
that a postmenopausal woman would have to double the number of children her
children bore, and eliminate infant mortality among those grandchildren, to
make menopause look like a sound strategy for propagating one's genes.'
That's an improbably improvement in life chances for any woman's
descendants, even a hardy Hadza's. I suspect, therefore, that in this raw
form, at least, the grandmother hypothesis has failed the test.
On the other hand, George C. Williams' model of inclusive fitness might
yet save the day. I find it impossible to believe that the presence of at
least some wise elders would make little or no difference to the survival
prospects of a typical foraging tribe. Today, sadly, that's not really
true. It is surely nice for small urban children to have the loving
attention of their grannies, and convenient for hard-working parents to be
able to turn over some of the task of child-minding. But none of this
worthy contribution makes any difference, I think, to the number of
children who survive to maturity and pass along the family's genes.
Everything has got much more complex and confused with the coming of
contraception, abortion choice, smaller families. If anything, it's those
with the least resources and most disrupted family arrangements who are
likely to have a larger brood.
Among other species, the evidence is ambiguous. A 1998 article in Nature
by Craig Packer, a biologist at the University of Minnesota, St. Paul, Marc
Tatar of Brown University in Providence, Rhode Island, and Antony Collins
of the Gombe Stream Research Centre in Kigoma, Tanzania, noted that:
reproductive cessation has also been documented in non-human primates,
rodents, whales, dogs, rabbits, elephants and domestic livestock. The human
menopause has been considered an evolutionary adaptation, assuming that
elderly women avoid the increasing complications of continued childbirth to
better nurture their current children and grandchildren. But an abrupt
reproductive decline might be only a non-adaptive by-product of
life-history patterns... a systematic test of these alternatives us[ed]
field data from two species in which grandmothers frequently engage in
kin-directed behaviour. Both species show abrupt age-specific changes in
reproductive performance that are characteristic of menopause. But elderly
females do not suffer increased mortality costs of reproduction, nor do
post-reproductive females enhance the fitness of grandchildren or older
children. Instead, reproductive cessation appears to result from senescence.
The animals studied were baboons and lions. The distinction in life span is
revealing. Female baboons who make it to old age become menopausal at 21,
and survive an extra five years - a couple of years longer than needed for
their last infant to outgrow dependency. Female lions live only 18 years,
with menopause at 14, and their cubs need about a year of maternal attention.
Kristen Hawkes argues that this might not be as water-tight an argument
against the `Grandmother Hypothesis' as it seems, in evolutionary terms.
Four-fifths of hunter-gatherer women, these days at least, survive beyond
menopause, often into their seventies, so their contribution is marked.
Lions, though, average only an extra three and a half years, and baboons
only five.
Tried and true
The basic argument here is that an organism that has stopped bearing young
- or, in the elaborated version, can no longer contribute to the thriving
of grandchildren - loses its relevance in the genetic struggle for
survival. That is, we die after we have fallen too far behind in the
struggle for genetic endurance. Accumulating errors exceed the ability of
repair systems to proofread fix them, even (to a lesser extent) the sex
cells. But this analysis, as we have seen, assumes part of what needs to be
demonstrated - that older organisms automatically cease breeding reliably.
While that is contingently true of complex creatures, as we have just seen,
what we need to ask is: why it should be so?
In other words, it is logically possible that an endlessly healthy,
self-repairing and fertile couple could run a production-line of their
joint genes until the end of time, with an inclusive fitness function (in
the jargon of the population geneticists) for their very own selfish genes
which is superior to that of their rivals' gene pool. Both Darwin and
Mendel would rub their hands with glee without a single censorious word of
reproof. The only reason our earlier imaginary tribes came to the same
grisly end was that the thousand-year-old women in the long-lived tribe
were assumed, rather arbitrarily, to become infertile at about 45.
Why, after all, shouldn't the machinery of the body's maintenance systems
keep working at peak efficiency forever? The accepted answer is that there
are better strategic trade-offs for available energy and coding, trade-offs
that allow the old to wear out and die. One reason this happens is that the
world is a dangerous place. It's a lethal place, most of the time, in the
wild - and the wild is where most humans have spent their time, until very
recently indeed.
Another reason is that its deadliness keeps changing lanes, as viruses,
bacteria and parasites frantically mutate. Sex happens to be a quite
effective method for generating novel immune defence codes, blending two
working systems into a fresh version, perhaps with some novel benefits.
That makes sex a kind of genetic-immunological luck dip, since many
combinations are just as likely to make the creature they specify worse off
- but evolution is a blind process, not a caring designer, and if one
strategy is more successful than another, it is preserved, at whatever the
cost to individuals.
========
Damien Broderick
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