(continued from first part...)

Your conception of evolution seems to be of a process that is
additive--that is, one newly evolved trait gets piled on top of previous
traits which are retained with the same level of efficiency in functioning
they previously had, with no trade-offs involved. But evolution doesn't
work that way. There are two reasons for this.

The first is that when new traits are evolved, they do so in response to
selective pressures. When our ancestors of the genus Homo began adapting to
higher levels of meat in the diet, it was because they were actually eating
higher levels of meat and lesser levels of plants (according to what
evidence we have from which we can deduce anything). Therefore the
selective pressure for extracting the highest levels of nutrients possible
from plants was relaxed somewhat as the pressures for adapting to more
animal food in the diet increased, as humans started getting more of their
key nutrients from animals than from plants. The key point to be made here
is that when selective pressures for a trait are relaxed or reduced, a
certain amount of dis-adaptation occurs, or in physiological terms a
lessening of efficiency in the functioning of that trait.

We can see this in a few concrete things about human metabolism such as the
fact that humans are not as well-adapted to synthesizing taurine in their
diets as they could be--for instance, vegans have lower plasma levels of
taurine than do omnivores [Laidlow SA (1988) "Plasma and urine levels in
vegans." American Journal of Clinical Nutrition, vol. 47, pp. 660-663],
presumably because humans came to get more of their taurine directly from
animals over the eons and thus became more dependent on this source.
Another more general example of this process of dis-adaptation can be seen
in the fact that most mammals can synthesize vitamin C from their diets,
but somewhere along the way, primates and humans lost that ability--due to
the fact their diets have been so consistently vitamin-C rich, there was no
longer a significant need to be able to synthesize it, the selective
pressure for retaining the gene was no longer present, and the ability was
eventually lost. Now we *have* to get vitamin C in our diets.

The second reason for evolutionary trade-offs and disadaptions is due to
the metabolic costs involved. An organism has a finite pool of metabolic
energy to devote to maintaining its various physiological systems. I
believe this has already been discussed on the list before, but a good
example of this can be seen in the relationship between the relative size
of the brain and the size of the digestive system. It is now thought that
the reason the human gut became smaller as the brain was enlarging is that
both are very energetically costly to the organism (the brain using 20-25%
of the BMR in humans), and to enlarge the metabolic budget of one, the
other has to be reduced. [Leonard WR et al. (1994) "Evolutionary
perspectives on human nutrition: the influence of brain and body size on
diet and metabolism." American Journal of Human Biology, vol. 6, pp. 77-88;
Aiello, Leslie C.; Wheeler, Peter (1995) "The expensive-tissue hypothesis:
the brain and the digestive system in human and primate evolution." Current
Anthropology, vol. 36, no. 2 (April 1995), pp. 199-221.]

As the gut became smaller over evolutionary time, it became more dependent
on denser foods with higher energy and nutrient quotients that were easier
to assimilate with less digestive energy expended to get them. This meant
not only that foods denser in terms of pure energy (calories) would have
been favored (not only animal foods, but also more concentrated plant foods
such as nuts, for example; also tubers), but also those more nutrient-dense
in terms of higher proportions of long-chain fatty acids (available in
animal foods primarily in the environment in which they lived) to support
the enlarging human brain.

The evolutionary tradeoff on this issue has been that with a smaller, less
energy-intensive gut, humans are not as efficient as our ape cousins such
as the chimp and gorilla--who continued down a different evolutionary
track--in breaking down plant matter for energy and nutrients. (Both have
guts that support a larger and more varied population of intestinal flora
and fauna, for instance, to help with more efficient breakdown of fibrous
plant matter.) Sure, we can certainly still eat plants, and they are an
important part of our diet for certain things, but the point here is what
is the balance between animal and plant foods that's the most *optimum* and
*efficient* for our species.

The point of all this is: There is a continuum of efficiencies and
tradeoffs in absorption and utilization of nutrients, depending on what the
nutrient is. And these are determined by which proportions of which
foodstuffs prevailed in the diet over the particular stretch of
evolutionary time during which our species evolved. Certain nutrients
(carbohydrates for example), are more plentifully and easily gotten from
plants. Others from animals. Certainly we can and do get nutrients such as
iron and zinc, for instance, from plants as well as animals, but they are
more plentiful and better absorbed from animal foods, reflecting an
adaptation and a metabolism that is more efficient in getting those
nutrients from animal foods, and thus more *dependent* on getting them from
animal foods in a natural environment. Even in a more artificial
environment such as modern society where one doesn't have those some
constraints of environment, if you are going to go by evolutionary
reasoning, you would still have to expect that it is more *efficient* and
*better optimized* for such a balance.

Now one may say that in today's artifical, human-engineered environment, if
they plan their diet right, they can get all the nutrients needed from a
diet higher in plant foods and lower in animal, which certainly may be
possible. Again, however, what is at issue here is the question of
*optimal* adaptation. If one subscribes at all to evolution, or is using
that as one's justification, there is no way around the fact that it makes
the most sense, that is, it is more *optimal* and more *efficient*, to try
to match the general profile of food intake that humans evolved on, and
what they have more recently evolved to.

It is inherently risky and mathematically improbable to assert that a diet
*different* in proportions of foodstuffs to the one we evolved on is better
or can somehow be just as good. Just as the vast majority of genetic
mutations are detrimental and have to be winnowed through the sieve of
evolution over eons of time to select out the ones truly advantageous, the
probabilities are just as high that any behavioral or dietary change
different from the evolutionary behavior pattern is going to be
detrimental, or at the least not as optimal.

--Ward Nicholson <[log in to unmask]>