Mutating to Perfection
We learned that species adapt to their environment through mutations and
inherited traits. If that's true, why, after hundreds of thousands of years of eating red meat
and being in the sunlight, do scientists say both of those things are bad for a person?
Wouldn't you think that the body would adapt to a diet of meat and high cholesterol?
And wouldn't the body build up defenses against harmful UV light?
Yes, and we have! First of all, evolution is not goal oriented. Mutations happen
randomly, but then natural selection takes over and those with the favorable mutations
survive long enough to reproduce.
Dark skin is dark because it contains melanin which is a natural form of "sunscreen".
The initial guess as to why there are different skin colors is to protect against skin
cancer. This was because it was noticed that the darkest skin occurs in areas with the
most direct sunlight. But, skin cancer usually occurs after the age of reproduction.
So evolutionarily speaking, that is not a viable answer. Now, the current hypothesis
is that it protects against the loss of folate which is necessary during the development
of the nervous system in an embryo. But skin that is too dark doesn't let in enough
Vit D. So as people moved away from equatorial areas toward the poles, sun intensity
decreased. Skin color is a balance between having enough Vit D and protecting folate
As far as eating red meat, most heart disease occurs later in life past the age of
reproduction, so it shouldn't be a selection pressure. The meat we eat today is raised
to contain more fat because it tastes better, and we eat a lot more meat today than our
ancestors were able to. Our teeth show that we have evolved to be omnivores, whether
you choose to eat meat or not.
Organisms develop responses/defense to things that harm them through a
process known as natural selection. There's natural variation in the
gene pool, some of which is beneficial, and some of which is harmful.
The helpful traits increase reproduction, and so the beneficial traits
tend to accumulate over time. But that doesn't mean the harms are no
longer harmful -- it just means organisms have found ways to manage or
mitigate those harms. Moreover, some things that are harmful don't
affect (or happen after) reproduction, so simple models of natural
selection don't necessarily apply.
Let's start with a simple example of wounds (cuts, scrapes, etc.). The
body has platelets to make scabs, and the immune system to protect
against infection, and these have evolved over time. But if someone
stabs you with a sword, it's still really bad. People who survive
wounds live to reproduce -- people who don't survive wounds as well
(for example, people with hemophilia), tend not to reproduce as much.
Humans have evolved methods of dealing with wounds -- but getting
stabbed is still bad.
Referring to your example of sunlight -- sunlight is actually both
helpful and harmful. The body has build up defenses against sun to
some degree -- peoples' skin darkens after prolonged exposure to
sunlight, and there are genetic components to skin color too. People
in tropical zones tend to have darker skin than people in polar zones.
Sun can still harmful, but people with darker skin have some added
defenses. However, sun is also important in making vitamin D -- if you
don't get enough sunlight, you can have a deficiency of vitamin D
(that's bad). In terms of natural selection, as long as the person
makes it to reproductive age, then the selective pressure to change is
gone (or nearly so).
Your example of diet is a little different. Human's diet has changed
dramatically since industrialization (the last 100-200 years) adding
much more meat. Before that, stable crops (wheat, potatoes, etc.) were
the primary source of calories. In developing countries, meat is still
a relatively minor component of diets compared to the US. Humans have
had very little time to respond to this change on a global scale.
However, there is a lot of evidence of pre-existing genetic variation
that affects how peoples' bodies handle meat. For example, some
peoples' bodies convert HDL ("good cholesterol") to LDL ("bad
cholesterol") inefficiently, resulting in lower risk of heart
problems. Other people convert LDL very efficiently -- and have
problems because of it. This example is also interesting because heart
problems tend to emerge *after* reproductive age is reached. So the
simple models of natural selection don't work as well for long-term or
late-life health issues.
Hope this helps,
How long do you suppose humans have had a diet high in fatty meat? Probably
not for many generations. We have evolved to crave salt, fat, and sweets
because those nutrients are rare in Nature. We haven't had a chance to
adapt to abundant supplies.
Sunlight is a double-edged sword. Populations who have long lived in high-sun
areas consistently develop dark skin as partial protection. But we don't want
to be too well-protected, because UV is needed for us to generate Vitamin D
from its precursors. So evolution has come up with a pretty good compromise
based on those conflicting needs.
Richard E. Barrans Jr., Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming
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Update: June 2012