Wednesday, April 09, 2008

Culture Can Change our Genes



http://www.edge.org/q2008/q08_11.html#christakis

NICHOLAS A. CHRISTAKIS
Physician and social scientist, Harvard

I work in a borderland between social science and medicine, and I
therefore often find myself trying to reconcile conflicting facts and
perspectives about human biology and behavior. There are fellow
travelers at this border, of course, heading in both directions, or
just dawdling, but the border is both sparsely populated and
chaotic. The border is also, strangely, well patrolled, and it is
often quite hard to get authorities on both sides to coordinate
activities. Once in a while, however, I find that my passport (never
quite in order, according to officials) has acquired a new visa. For
me, this past year, I acquired the conviction that human evolution
may proceed much faster than I had thought, and that humans
themselves may be responsible.

In short, I have changed my mind about how people come literally to
embody the social world around them. I once thought that we
internalized cultural factors by forming memories, acquiring
language, or bearing emotional and physical marks (of poverty, of
conquest). I thought that this was the limit of the ways in which
our bodies were shaped by our social environment. In particular, I
thought that our genes were historically immutable, and that it was
not possible to imagine a conversation between culture and genetics.
I thought that we as a species evolved over time frames far too long
to be influenced by human actions.

I now think this is wrong, and that the alternative — that we are
evolving in real time, under the pressure of discernable social and
historical forces — is true. Rather than a monologue of genetics, or
a soliloquy of culture, there is a dialectic between genetics and
culture.

Evidence has been mounting for a decade. The best example so far is
the evolution of lactose tolerance in adults. The ability of adults
to digest lactose (a sugar in milk) confers evolutionary advantages
only when a stable supply of milk is available, such as after milk-
producing animals (sheep, cattle, goats) have been domesticated. The
advantages are several, ranging from a source of valuable calories to
a source of necessary hydration during times of water shortage or
spoilage. Amazingly, just over the last 3-9 thousand years, there
have been several adaptive mutations in widely separated populations
in Africa and Europe, all conferring the ability to digest lactose
(as shown by Sarah Tishkoff and others). These mutations are
principally seen in populations who are herders, and not in nearby
populations who have retained a hunter/gatherer lifestyle. This trait
is sufficiently advantageous that those with the trait have notably
many more descendants than those without.

A similar story can be told about mutations that have arisen in the
relatively recent historical past that confer advantages in terms of
surviving epidemic diseases such as typhoid. Since these diseases
were made more likely when the density of human settlements increased
and far-flung trade became possible, here we have another example of
how culture may affect our genes.

But this past year, a paper by John Hawks and colleagues in PNAS
functioned like the staccato plunk of a customs agent stamping my
documents and waving me on. The paper showed that the human genome
may be changing at an accelerating rate over the past 80,000 years,
and that this change may be in response not only to population growth
and adaptation to new environments, but also to cultural developments
that have made it possible for humans to sustain such population
growth or survive in such environments.

Our biology and our culture have always been in conversation of
course — just not (I had thought) on the genetic level. For example,
rising socio-economic status with industrial development results in
people becoming taller (a biological effect of a cultural
development) and taller people require architecture to change (a
cultural effect of a biological development). Anyone marveling at
the small size of beds in colonial-era houses knows this firsthand.
Similarly, an epidemic may induce large-scale social changes,
modifying kinship systems or political power. But genetic change
over short time periods? Yes.

Why does this matter? Because it is hard to know where this would
stop. There may be genetic variants that favor survival in cities,
that favor saving for retirement, that favor consumption of alcohol,
or that favor a preference for complicated social networks. There
may be genetic variants (based on altruistic genes that are a part of
our hominid heritage) that favor living in a democratic society,
others that favor living among computers, still others that favor
certain kinds of visual perception (maybe we are all more myopic as a
result of Medieval lens grinders). Modern cultural forms may favor
some traits over others. Maybe even the more complex world we live
in nowadays really is making us smarter.

This has been very difficult for me to accept because, unfortunately,
this also means that it may be the case that particular ways of
living create advantages for some, but not all, members of our
species. Certain groups may acquire (admittedly, over centuries)
certain advantages, and there might be positive or negative feedback
loops between genetics and culture. Maybe some of us really are
better able to cope with modernity than others. The idea that what
we choose to do with our world modifies what kind of offspring we
have is as amazing as it is troubling.

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