February 23, 2009
Gregory Cochran and Henry Harpending examine the interaction between civilization and evolution, and in the process smash to pieces the Blank Slate hypothesis.
150 years ago Charles Darwin published the Origin of Species. Origin was not revolutionary because it proposed that lineages changed over time; the idea was in the air of the age, and evolutionary theories have an ancient pedigree, dating back to antiquity. No, what made Origin a sensation was that Charles Darwin assaulted the evolutionary question with an encyclopedic array of data making the case for descent with modification as an empirical fact, and introduced the process of natural selection as a compelling motive engine.
Darwin’s framework pointed the way for natural historians to move on from being describers to predictors. To this day, the ideas outlined in Darwin’s voluminous oeuvre influence evolutionary biologists, and his hypotheses are fruitful grounds from which formal theoreticians and empirical researchers launch their endeavors.
Origin explored some questions which were relevant to the natural history of our own species, but Descent of Man fully confronted the implications of the human animal shaped by nature. The intellectual reverberations of the tumult sparked by Charles Darwin’s unabashed naturalism echo down to our own age, as we have reassessed our position in the cosmos, with some arguing that the evolutionary paradigm completes the Copernican revolution whereby the human story became a sidelight on nature’s canvas. From the initial controversy evolutionary theory was assumed to have relevance for humanity. Francis Galton, Karl Pearson and R. A. Fisher pushed forward the Darwinian paradigm with a particular focus on human variation and evolution. While the first half of R. A. Fisher’s The Genetical Theory of Natural Selection is devoted to a mathematical model of evolutionary process, the second half probes the course of human history and its biological potentialities. Humanity was not an afterthought for Fisher and many of his contemporaries. On the contrary, they lived during the high tide of eugenics, a discipline which viewed itself as an applied field that naturally arose out of implications of Darwinian evolutionary theory.
Then came Adolf Hitler. There is no need for an epilogue of this era, humans soon became marginalized in the study of evolution, unless they were safely driven back into the deep past so as to have little relevance for the present. Human evolutionary biology became a matter of bones, not blood. Researchers in the age of molecular genetics fanned out across numerous “model organisms,” from the fly and mouse to the nematode, in the quest to understand the science of the gene. Great scholarship this might be, but it lacks something in catching the attention of the public.
But the same molecular methods which pushed aside thoughts of human evolution in the wake of the discovery of DNA by James Watson & Francis Crick ultimately led us back to Homo sapiens. While ethical constraints prevent scholars from engaging in experiments on humans and forced them to rely on pedigrees, molecular data is subject to no such limitations. Because of biomedical applications molecular analysis of human genetic variation has been at the forefront of research activity, resulting in funding for “Big Science” such as the Human Genome Project. In genomics, the mapping and analysis of the entire genetic code of organisms, humans are no longer an afterthought. It is arguable that over the last five years we have obtained more insight into the nature of the human evolutionary past than we were able to comprehend over the last 150 years.
As the data has crested the need for a deeper theoretical understanding of what it means has become critical. Though computers may help in mining data, the ultimate judgments of value and recognition of deep patterns remain a domain where humans reign supreme. In The 10,000 Year Explosion: How Civilization Accelerated Human Evolution, Gregory Cochran and Henry Harpending attempt to do just that. Through the welter of biostatistical data Cochran & Harpending see a vindication of the theoretical insights of Charles Darwin and R. A. Fisher.
While computational geneticists live in a world dominated by bytes, The 10,000 Year Explosion synthesizes seemingly disparate fields so as to generate a narrative of more insight than each part alone. With the long exile of biology from the human sciences there are many weeds to clear out, but the fallow period has left the soil fertile. The 10,000 Year Explosion starts with a set of facts derived from traditional historical disciplines, narrative history to paleoanthropology, and integrates them into a solid base of theoretical models supported by decades of research in animal breeding and population genetics, and finally sifts the resulting hybrid models through the filter of genomics. After converging upon a few likely probabilities they begin to turn the cranks on the machine to generate inferences and plausibilities in the hypothetico-deductive tradition of science.
The overarching insight of the book is that contrary to popular belief, the rise of “civilization” did not result in the extinction of humans as animals subject to biological evolution and the emergence of a species which had untethered itself from the truisms of nature. In fact as evident in the subheading Cochran & Harpending argue that contrary to the conventional wisdom civilization has increased the rate of evolutionary change. There is no mystery if you assume that evolutionary change is somehow related to the change in a population’s environment, and human culture can be considered our species’ environment. At its most simple level all that is required for evolution are changes in the frequencies of genetic variants over time; this may be prevented by selective forces which constrain variation, but constancy has not been a feature of human history over the past 10,000 years. Add to this Cochran & Harpending’s insight that larger populations have more variation, like a more diverse palette for a painter, and one can see how the creative energies of evolution might be unleashed among on humans.
Over the past 10 years these a priori possibilities have been given plausibility by a series of papers which have examined the map of human genetic variation through combination of more powerful sequencing and robust computational analysis. In short, the results imply that the genome of the human species have been reshaped by natural selection within the past 400 generations, about 10,000 years.
A few specific examples clarify and extend the theoretical insights which serve as the preamble to The 10,000 Year Explosion. To many of us, the Neolithic Revolution, with the concomitant rise of agriculture and village life, might seem the first step on the long road out of Nature’s domain, but Cochran & Harpending argue that on the contrary this transition from the hunter-gatherer status quo resulted in a new bout of selective events and a radical shift in our species’ ecological niche. From the standpoint of today, both hunter-gatherer and agricultural society seem nasty, brutish, and short; but we shouldn”t forget that these two civilizations differed from one another is fundamental ways. First and foremost, the diet of farmers was less diversified than that of hunters & gatherers, who depended on a wide range of nutritional sources at any given time. Though grain provides sufficient calories for basic sustenance, there is a great deal of evidence that the spread of farming resulted in an increased misery index on the part of the average human because of the poverty of nutrients. Additionally, while the diet of farmers became progressively more monotonous, the diseases that afflict farmers became more virulent. It turns out that the environment for the pathogens that prey upon humans are the humans themselves, so with the greater density of living which accompanied agriculture, plague became the handmaid of human life. It is no surprise that immunity related regions of the genome are among those most subject to evolutionary pressures according to geneticists, and that hunter-gatherer populations often suffer increased mortality when faced with contact by settled populations.
The argument in The 10,000 Year Explosion is that culture did not just accelerate evolution, but that evolution can also accelerate cultural change. I have alluded to one aspect above, it seems entirely likely that the majority of the ancestry in the New World would not be of European provenance if not for the fact that the native populations were utterly defenseless against Eurasian pathogens. In contrast, in Africa and in Asia European colonialism did not result in population replacement, while in Australia it did, again likely for reasons of disease.
A more intriguing and less obvious candidate to illuminate the interplay of biology & culture takes us further back in history. Today about 50% of the world’s population speaks Indo-European languages, from India to England. These are languages whose affinities are clear, and linguistic methods that bear some resemblance to evolutionary techniques suggest that the common ancestor flourished on the order of 5 to 7 thousand years ago. The question is: how did it come to be that disparate populations came to speak such similar languages? Cochran & Harpending offer a biological hypothesis”that the original Indo-Europeans were the first people with the genetic mutation that allows adults to digest lactose sugars in milk. It turns out that dairying results in five times as much caloric output per unit of land as raising cattle for meat, so the economic logic for turning to dairy when ecological circumstances make that optimal is clear. But for many populations this is not an option because consumption of raw milk is not physiologically possible as an adult, at least to the point where one extracts sufficient nutritive value. The genetic data tell us that somewhere in Central Eurasia 7,000 years ago there lived an individual who carried a mutation which allowed them to digest milk as an adult, and that this mutation swept all across Western Eurasia. Simultaneously, there are hints that contemporaneously the Indo-European speaking peoples spread West and South, bringing with them the culture of cattle and cart. What this specific example illustrates is the co-evolutionary dynamic of human history, with changes in human ecology driven by cultural change leading to population increase and so driving further cultural and genetic diversification.
In the penultimate chapter, the authors show that the “The 10,000 Year Explosion” is not simply relevant to questions across the scale of thousands of years. In “Medieval Evolution: How the Ashkenazi Jews Got Their Smarts,” Cochran & Harpending wade into a topic which has piqued the interest of many thinkers across the generations, the possible reasons for the incredible contribution of Jews to modern intellectual production over the past 200 years. As usual the argument here is a synthetic one, drawing from both textual historical sources and population genetic theory. Surveying the ancient records Gregory Cochran notes that in the Classical world Jews did not have a reputation for cleverness, while at the same time in the medical literature contemporary Jews have a peculiar pattern of a high frequency of a variety of recessive diseases.
One of the insights of population genetics is that only a few conditions can result in a high frequency of extremely deleterious diseases. One way that one could obtain this state is through inbreeding and population bottleneck, where random forces overwhelm the ability of selection to “purify” the genome of deleterious alleles. Another is for positive selection to throw off deleterious byproducts because one gene generally is implicated in a larger number of biochemical processes. The latter dynamic is especially evident after bouts of recent selection, as over time natural selection tends to “perfect” adaptations. A range of genetic data implied to Cochran that contrary to conventional wisdom Jews were not particularly inbred, nor did they go through a population bottleneck. This naturally leads to the conclusion that the high frequency of recessive diseases is due to selection, and likely recent selection. As the ancient historical data do not imply that Jews during this period had a peculiar cognitive profile or suffered strange ailments, it would seem that the time frame implied that selection events might have occurred during the Middle Ages. While some scholars have thought that the rabbinical tradition explained Jewish scholarly attainment, Cochran shows that from a population genetic standpoint there were simply not enough rabbis for this to be a viable source of selective pressures. So theory compels the hypothesis to settle upon the likelihood that the peculiar Jewish cognitive profile emerged through selection on intelligence while Jews were merchants & money-lenders.
Got that? The hypothesis of the origins of Ashkenazi IQ illustrate the strengths and weaknesses of the methodology which Cochran & Harpending outline in The 10,000 Year Explosion. First, pile up an enormous stack of facts from an array of disciplines. Second, master theoretical evolutionary biology so that you have a machine to reprocess the facts and produce plausible descriptions and predictions. Third, comb the empirical genomic data to see if predictions and descriptions derived from theory & observation actually match what the empirical results are. Easy, no? The reality is that this is a peculiar skill set. Most historians don’t know the difference between mitochondria and microsatellites, while few geneticists are aware that Fiddler on the Roof took extreme liberties with the nature of Jewish life in Europe. How many people could offer an opinion on the functional genomics of lactase persistence and the validity of Marija Gimbutas or Colin Renfrew’s models for the origins of the Indo-Europeans?
But onward and upward. There was an age when chemists did not need to take a course in calculus in their education, while today one can not obtain a degree in biology with at least that minimal level of mathematical fluency. Disciplinary toolkits change and evolve, and The 10,000 Year Explosion is in some ways a preview of what is to come, as E. O. Wilson’s vision of consilience becomes more than just a vision.
These are not just academic questions. In Why Some Like It Hot: Food, Genes, and Cultural Diversity, Gary Naban reports that Navajo who received milk from the federal government in aid programs simply thew it away. The Navajo of course are not lactose tolerant. People differ, and they differ in large part because of the diversification of human cultures over the last 10,000 years. Sickle-cell anemia, blue eyes, the thick straight hair of East Asians, and lactose tolerance are just a few of the new traits which our species developed in the recent past. Cochran & Harpending would argue that these changes are almost certainly due to the fact that we humans altered our own environment and the resultant modulation of selection pressures.
Sometimes the consequences are unpredictable, if the theory of how Jews developed their cognitive profile is correct, it is obvious that this is not an adaption to winning the Nobel Prize in physics or economics! Rather, that is just a side-effect of selection for rapid abstraction among medieval money-lenders, just as a host of neurologically degenerative diseases also are. The shape of human variation is an essential background condition which we must take into account whenever we formulation public policy, from something as prosaic as food subsidy programs to more fraught topics such as differential academic performance of ethnic groups in diverse modern societies.
Newtonian mechanics resulted in a sea change in the efficiency of modern warfare, removing the guesswork from the parabola which defines the arc of a cannon ball, and the emergence of engineering as the dominant vocational background of successful military men such as Napoleon. To a great extent we live in the pre-Newtonian age when it comes to public policy, totally uniformed by the sciences of human nature, shooting blindly at targets and relying on guess work and horse sense. In the next few decades the type of thinking on display in The 10,000 Year Explosion will perhaps become relatively banal, and likely guide our expectations about the efficacy of public policy meant to shape the course of human events.