August 11, 2011 | 4
When we think of adaptation we often imagine responses to variations in exogenous parameters. To be plain and concrete, the weather changes and sea levels rise. A less obvious case would be random acts of nature which can alter the course of evolution. New World monkeys are likely to have radiated from a small founding group which somehow floated across a far narrower Atlantic ocean from Africa 40 million years ago. In response to an environment which lacked many competitors there was the incredible biological explosion into the diverse forms which we see today in this clade. This is evolution on the broadest canvas of natural history.
But evolution does not occur simply via changes in environment. Life forms themselves compete with each other in a co-evolutionary arm’s race, and there is always a jostling within species for scarce resources. Even assuming an environment of quotidian sameness evolution by adaptation would still continue. Life is always in a race with itself, and no equilibrium is safe from the protean innovations of mutation.
What the above suggests is that the division between evolution and adaptation driven by environmental pressures and biological competition is an artificial one. On a fundamental level the earth and and life sciences just specialize in different branches of the same natural phenomenon. Just as geological conditions shape life, so life itself can affect geology. Consider coal and limestone. This insight is old. James Lovelock implicitly elaborated upon it when he formulated his Gaia hypothesis, which elided the distinction between life and non-life, and also the divisibility of individual organisms.
A similar case for unity where we impose separation can be made for our own species when we speak of biological and cultural evolution. Just as organisms are subject to co-evolutionary pressures, an eternal back and forth, so our cultures and biology are always in dialogue. The distinction between nature and nurture is only a figment of our imagination, albeit one which is convenient for conceptual “bookkeeping.” Since the “Great Leap Forward” 40,000 years ago the rate of human cultural change has been increasing. Whereas the Oldowan toolkit persisted for hundreds of thousands of years, Holocone cermanic cultures don’t exhibit continuity such cohesion for most than a few thousand years at most. This difference is almost certainly due to some biological change which renders human cultural expression infinitely flexible.
But I’m not here to write about how different ceramic styles have evolutionary implications. Rather, the biggest recent global cultural shock to the genetic architecture of our species is agriculture. Agriculture is often termed a cultural ‘revolution,’ even if further digging into the most recent literature indicates a longer series of incremental and gradual changes. No matter the truth of how it played out, we can’t deny that this cultural revolution had likely demographic and biological ramifications. Man the hunter and gatherer became man the peasant.
As far as demographic ramifications, these have implicit biological side effects. For example, it seems likely that during the Ice Age much of Southeast Asia was dominated by dark-skinned peoples with curly hair, the ancestors of what are today termed ‘Negritos.’ These pre-agricultural populations were demographically swamped by rice farmers who expanded from fringes of southern China. The wholesale demographic shift had biological implications because the farmers were physically different from the hunter-gatherers whom they replaced, a difference visible today in the physical features of Malays as opposed to the Negrito tribes of the interior. This may be just the most recent and apparent shift of this sort. Many of the ancient DNA extractions in places like Europe seem to lend some support to Peter Bellwood’s thesis in First Farmers that a few ancient agricultural hearths were the demographic engines of the past 10,000 years. All across the world the transition to farming may have been deeply biocultural.
But the most intriguing aspect of the impact of agriculture upon human biological diversity is not one of demographics, but of direct adaptation to the new mode of production. Lactose tolerance is the most prominent case of this. Several independent mutations which allow adults to digest lactose sugar have been documented. One in Europe and Central Eurasia, another in the Middle East, and others in Africa. In all of these cases unrelated populations adapted to a common cultural innovation, animal husbandry, by shifting their biology. But this is not the only case. Animal husbandry varies in frequency across agricultural societies (e.g., Northern Europeans made much greater use of mixed-farming techniques than the Chinese), but grains of some sort are ubiquitous. Not only do humans have a much better ability to digest starch than our chimpanzee relatives, but some human populations seem to have higher levels of amylase in their saliva than others due to a genetic variation. Not surprisingly the populations which have higher normal levels of amylase are those which have practiced agriculture the longest.
This is important “pure science” information which has relevance for optimal nutrition. Many of the diets which presume to take to heart the insights of human biological diversity are based upon flimsy science, such as the blood type diet. In contrast, other diet regimes which are heavy on an understanding of the human evolutionary past, such as the Paleolithic diet, act as if the recent history of the human species is an irrelevant epilogue. Obviously there’s only so much evolution that can occur in 10,000 years. Populations all around the world seem to have a taste for sugar. But these human universals are the basis for further understanding, clarification, and specification. They shouldn’t be where our knowledge base ends.
We are all variations on the common template, not replicas. In modern developed nations we are faced with an “obesity epidemic.” That epidemic has elicited an outcry from public health officials. Though many recommendations are easy to generalize in terms of their applicability, it seems likely that a “one size fits all” regime will exhibit diminishing marginal returns. Everyone knows individuals who can eat “all they want” and remain trim, and others who have to monitor their intake vigilantly lest they begin to pack on the pounds. We know this intuitively, so public health officials who make catchall assertions without caveat often lose credibility in the eyes of the public.
To achieve greater personalization in our health, we need to take into account the recent past of our species, and our families. When thinking about this I remember in particular a friend who was adopted from Korea in the late 1970s. Her parents gave her a glass of milk everyday, because that’s what considerate American parents did then. But she reacted very negatively to this regime. Their response was to constantly switch milk varieties through her elementary school years. Finally my friend’s parents gave up when she approached her teen years. In adulthood she realized that like most Koreans she was lactose intolerant. This is a trivial and obvious example, at least today, but there are likely many more where this came from.
Bellwood, P., 2005, First Farmers: The Origins of Agricultural Societies
Voight BF, Kudaravalli S, Wen X, Pritchard JK, 2006 A Map of Recent Positive Selection in the Human Genome. PLoS Biol 4(3): e72 doi:10.1371/journal.pbio.0040072
Enattah, Nabil Sabri; Jensen, Tine G.K.; Nielsen, Mette; Lewinski, Rikke; Kuokkanen, Mikko; Rasinpera, Heli; El-Shanti, Hatem; Seo, Jeong Kee; Alifrangis, Michael; Khalil, Insaf F.; Natah, Abdrazak; Ali, Ahmed; Natah, Sirajedin; Comas, David; Mehdi, S. Qasim; Groop, Leif; Vestergaard, Else Marie; Imtiaz, Faiqa; Rashed, Mohamed S.; Meyer, Brian; Troelsen, Jesper; Peltonen, Leena, Independent Introduction of Two Lactase-Persistence Alleles into Human Populations Reflects Different History of Adaptation to Milk Culture, American journal of human genetics doi:10.1016/j.ajhg.2007.09.012
Itan Y, Powell A, Beaumont MA, Burger J, Thomas MG, 2009 The Origins of Lactase Persistence in Europe. PLoS Comput Biol 5(8): e1000491. doi:10.1371/journal.pcbi.1000491
Haak W, Balanovsky O, Sanchez JJ, Koshel S, Zaporozhchenko V, et al. 2010 Ancient DNA from European Early Neolithic Farmers Reveals Their Near Eastern Affinities. PLoS Biol 8(11): e1000536. doi:10.1371/journal.pbio.1000536
Perry GH, Dominy NJ, Claw KG, Lee AS, Fiegler H, Redon R, Werner J, Villanea FA, Mountain JL, Misra R, Carter NP, Lee C, Stone AC., Diet and the evolution of human amylase gene copy number variation, Nat Genet. 2007 Oct;39(10):1256-60. Epub 2007 Sep 9.
Image credit: José-Manuel Benito Álvarez
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