The 10,000 Year Explosion

by Gregory Cochran

  Already it's apparent that the Ashkenazi mutations were made common by natural selection, since there was sufficient genetic isolation for selection to occur, while nothing other thanselection can explain the existence of common genetic diseases that are concentrated in a few metabolic pathways. You can also see why this kind of natural selection is usually confined to geographically isolated human populations, since such strict rules against intermarriage with neighboring groups are fairly unusual. But what kind of selection occurred? What traits were more valuable among the Ashkenazi than among their neighbors?

  Well, we have some strong hints. What trait is accentuated among the Ashkenazim today? Are they as large as Samoans, as tall as the Tutsi, as milk-tolerant as the Dutch? No: Their special trait is intelligence.

  The mutations themselves suggest this: Some of them look like IQ boosters, considering their effects on the development of the central nervous system. The sphingolipid mutations, in particular, have effects that could plausibly boost intelligence. In each, there is a buildup of some particular sphingolipid, a class of modified fat molecules that play a role in signal transmission and are especially common in neural tissues. Researchers have determined that elevated levels of those sphingolipids cause the growth of more connections among neurons, the basic cells of the central nervous system (see page 221).

  There is a similar effect in Tay-Sachs disease: increased levels of a characteristic storage compound (GM2 ganglioside), which causes a marked increase in the growth of dendrites, the fine branches that connect neurons.41 This increased dendrito- genesis also occurs in Niemann-Pick type A cells and in animal models ofTay-Sachs disease and Niemann-Pick disease. These are the only known disease alleles that cause increased neural connections.

  We also have evidence—not definitive—that some of the mutations common among the Ashkenazim may boost intelli-

  Axon growth in Gaucher's disease. The top frame shows normal cultured rat neurons, the middle frame shows stunted axonal growth resulting from reduced levels of glucosylceramide, and the bottom frame shows increased growth and branching in an experimental model of Gaucher's disease with increased levels of glucosylceramide. Glucosylceramide is the storage molecule involved in Gaucher's disease. Elevated levels promote growth and branching in axons, the transmission lines of the nervous system.

  gence. We looked at the occupations of patients in Israel with Gaucher's disease, essentially all of whom were being treated at the Shaare Zedek Medical Centre in Jerusalem. These patients are much more likely to be engineers or scientists than the average Israeli Ashkenazi Jew—about eleven times more likely, in fact.42 There are similar reports on torsion dystonia, another Ashkenazi genetic disease. Ever since it was first recognized, observers have commented on the unusual intelligence of the patients who suffer from it.

  In 1976, Roswell Eldridge described early literature on torsion dystonia: One patient showed "an intellectual development

  far exceeding his age," and a second showed "extraordinary mental development for his age."43 At least ten other reports in the literature have made similar comments. Eldridge studied fourteen Jewish torsion dystonia patients and found that their average IQbefore the onset of symptoms was 121, compared to an average score of 111 in a control group of fourteen unrelated Jewish children matched for age, sex, and school district.44 There are also reports of individuals with higher-than-average intelligence who have nonclassic congenital adrenal hyperplasia (CAH), another common Ashkenazi genetic disease. CAH, which causes increased exposure of the developing brain in a fetus to androgens (male sex hormones), is relatively mild compared to diseases like Tay-Sachs. At least seven studies show high IQin CAH patients, parents, and siblings, ranging from 107 to 113. The gene frequency of CAH among the Ashkenazim is almost 20 percent.45

  HOW SELECTION HAPPENED

  Our picture of how natural selection favored higher intelligence among European Jews in the Middle Ages relies upon three key observations. The first is that prosperous individuals had considerably more children, on average, than nonprosperous individuals in those days, as was then typical in most societies.46 Second, Ashkenazi jobs were cognitively demanding, since the members of this group were essentially restricted to entrepreneurial and managerial roles as financiers, estate managers, tax farmers, and merchants. These are jobs that people with an IQ below 100 essentially cannot perform. Even low-level clerical jobs require an IQof something like 90.47 So, intelligence must

  have had greater rewards in those jobs than it does among farmers. This has to be true, really, since physical strength and endurance, which play a major part in success as a farmer, matter far less in finance and trade. If physical strength accounts for less of the variance, then cognitive and personality traits must account for more. Third, intelligence is significantly heritable. If the parents of the next generation are a little smarter than average, the next generation will be slightly smarter than the current one.

  We can construct a scenario using IQ scores that illustrates this principle. We'll assume that parents of each generation averaged a single IQ point higher than the rest of the Ashkenazi adult population. In other words, let's suppose there was a modest tendency (mediated through economic success) for intelligent parents to have more surviving children than average parents— a tendency that certainly would not have been noticed at the time. If we assume a heritability of 30 percent for IQ, a very conservative assumption, then the average IQof the Ashkenazi population would have increased by about a third of a point (0.30 point) per generation. Over forty generations, roughly 1,000 years, Ashkenazi IQwould have increased by 12 points. If we assume that the Ashkenazim began with a typical European IQ_of 100 in the year AD 600, they would have reached an average IQof 112 by 1600, just about what we see in the Ashkenazim today. This picture is consistent with observations of high verbal and mathematical scores among Ashkenazi Jews, paired with average or lower-than-average visuospatial scores. Verbal and mathematical talent would have helped medieval businessmen succeed, whereas visuospatial abilities were irrelevant.

  There may well have been some selection for IQ among Europeans in general over this period. Christian merchants in

  London or Rotterdam may have experienced selective pressures similar to those of the Ashkenazi Jews, but there was an important difference between those merchants and the Jewish population: Christian merchant families intermarried. The mixing would have caused extensive gene flow with the general population, the majority of whom were farmers. It seems that if IQ increased in the general European population, there was a greater increase among the Ashkenazim.

  Our hypothesis also explains why certain things didn't happen—in particular, why we don't see high IQscores and unusual intellectual achievement among other Jewish groups today. Although they, too, had very low rates of intermarriage, they never seem to have that high concentration of white-collar jobs that would have led to strong selection for verbal and mathematical intelligence. In part, this was because there were many more Jews in the Islamic world than in Christian Europe: With less persecution, there were more Jews than there were white-collar jobs. Our picture also explains why there's no real sign of unusually high intelligence among the Jews back in the days of the Roman Empire: The required events simply hadn't happened yet.

  CONCLUSION

  Cultural innovation has been a driving force behind biological change in humans for a long time—certainly since the first use of tools some 2.5 million years ago. Natural selection acting on the hominid brain made those early innovations possible, and the innovations themselves led to further physical and mental changes.

  Biological and cultural co-evolution was slow at first, at least by modern standards, but gradually things sped up. The archaeological record shows that our capacity for innovation continued to increase until, about 40,000 years ago, we were primed for what has been called the "human revolution" or the "creative explosion" of the Upper Paleolithic in Europe and northern

  Asia. This su
dden spurt in technology and art occurred shortly after modern humans expanded out of Africa—and it too must have involved biological changes, changes that we suspect were driven in part by genes stolen from the Neanderthals and other archaic humans, the previous occupants of Eurasia. Behavioral modernity led to even more change: Men made better tools and then, in turn, were reshaped by those tools over many generations.

  With the development of agriculture, both cultural and biological evolution accelerated even further because that way of life made new demands on humans. Before agriculture humans had always been foragers: The huge population increase associated with agriculture resulted in more favorable mutations as well as more new ideas. This rapid evolution of our species following the spread of agriculture is indeed a 10,000 year explosion.

  The explosion is ongoing: Human evolution didn't stop when anatomically modern humans appeared, or when they expanded out of Africa. It never stopped—and why would it? Evolutionary stasis requires a static environment, whereas behavioral modernity is all about innovation and change. Stability is exactly what we have not had. This should be obvious, but instead the human sciences have labored under the strange idea that evolution stopped 40,000 years ago.

  All this means, has to mean, that biological change has been a key factor driving history. It has certainly not been the only factor, and it has been strangely intertwined with more traditional influences. Genetic changes like lactose tolerance have arisen and spread because of cultural innovations (such as the development of agriculture) as well as the random occurrence of the right mutations, and those genetic changes have in turn hadtheir own cultural consequences. The expansion of the Indo- Europeans, the successful European settlement of the Americas and Australia, the failure of the "scramble for Africa," the entry of the Ashkenazi Jews onto the intellectual stage, possibly even the industrial revolution and the rise of science—all appear to be consequences of this endless dance between biological and cultural change.

  If researchers in the human sciences continue to ignore the fact of ongoing natural selection, they will have thrown away the key to many important problems, turning puzzles into mysteries. Cortes, with 500 men, conquered and held an empire of millions. Try to explain this without invoking biological differences in disease resistance caused by ongoing evolution—it can't be done.

  Thucydides in the fifth century BC said that human nature was unchanging and thus predictable, and many scientists today believe that human nature stopped changing tens of thousands of years ago. Historians seem to make the same assumption. In so doing, they're ignoring tremendous opportunities: not just in decoding the past, but in shaping the future as well. Continuing evolution over human history has been a vast natural experiment, an experiment that promises big payoffs in understanding, and then fighting, disease and mental illness. Limone sul Garda hid an important clue about human disease. With a million villages in the world, there must be many more such clues. Some of the results of history's experiments may even aid us in more ambitious efforts aimed at increasing human life spans and cognitive abilities.

  It's time for researchers in the human sciences to shrug off the chains of dogmas like evolutionary stasis and "psychic unity." There's no time to lose—and there's a world to win.

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  NOTES

  chapter one

  Sargon of Akkad, about 4,000 years ago, built one of the first empires in what is now Iraq. Imhotep was an early Egyptian architect, engineer, and physician.

  "Behavioral modernity" is anthropological slang for the cultural creativity thought to be characteristic of modern humans.

  John Hawks, "Adaptive Evolution of Human Hearing and the Appearance of Language," 77th Annual Meeting of the American Association of Physical Anthropologists, April 11, 2008, Columbus, Ohio.

  Stephen J. Gould, "The Spice of Life," Leader to Leader 15 (Winter 2000): 14-19.

  Lyudmila N. Trut, "Early Canid Domestication: The Farm-Fox Experiment," American Scientist 161 (1999): 161.

  Jack C. Schultz and Ted Floyd, "Desert Survivor," Natural History 108, no. 2 (1999): 24-29.

  John Tooby and Leda Cosmides, "On the Universality of Human Nature and the Uniqueness of the Individual: The Role of Genetics and Adaptation," Journal of Personality 58, no. 1 (1990): 17-67.

  John Tooby and Leda Cosmides, "Evolutionary Psychology: A Primer," University of California Santa Barbara, http://www.psych.ucsb .edu/research/cep/primer.html (accessed October 1, 2008).

  M. Clifton, "Dog Attack Deaths and Maimings," 2008, from http://www.dogbitelaw.com/Dog%20Attacks%201982%20to%202006% 20Clifton.pdf.

  Yali Xue et al., "Spread of an Inactive Form of Caspase-12 in Humans Is Due to Recent Positive Selection," American Journal of Human Genetics 78, no. 4 (2006): 659-670.

  George H. Perry et al., "Diet and the Evolution of Human Amylase Gene Copy Number Variation,"Nature Genetics 39 (2007): 1256-1260.

  Vincent Sarich is a professor of anthropology who played a key role in estimating the date of the divergence between humans and chimpanzees, Frank Miele is a senior editor at Skeptic magazine, and Chuck Lemme is a longtime reader of Skeptic.

  Chuck Lemme, "Race and Sexual Selection," Skeptic, http:// www.skeptic.com/eskeptic/05-03-22.html (accessed October 1, 2008).

  Richard Lewontin, "The Apportionment of Human Diversity," Evolutionary Biology 6, no. 1 (1972): 381-398.

  Mark Rieger, Introduction to Fruit Crops (New York: Food Products Press, 2006).

  Koh-ichiro Yoshiura et al., "A SNP in the ABCC11 Gene Is the Determinant of Human Earwax Type," Nature Genetics 38 (2006): 324-330.

  Laurent Keller and Kenneth G. Ross, "Selfish Genes: A Green Beard in the Red Fire Ant," Nature 394 (1998): 573; Michael J. B. Krieger and Kenneth G. Ross, "Identification of a Major Gene Regulating Complex Social Behavior," Science 295, no. 5553 (2002): 328-332.

  John Hawks et al., "Recent Acceleration of Human Adaptive Evolution," Proceedings of the National Academy of Sciences 104, no. 52 (2007): 20753.

  chapter two

  Roy C. Anderson, "The Ecological Relationships of Meningeal Worm and Native Cervids in North America," Journal of Wildlife Diseases 8, no. 4 (1972): 304-310.

  Daniel M. Tompkins et al., "Parapoxvirus Causes a Deleterious Disease in Red Squirrels Associated with UK Population Declines," Proceedings of the Royal Society, B: Biological Sciences 269, no. 1490 (2002): 529.

  Richard Klein, The Human Career (Chicago: University of Chicago Press, 1999), 524.

  Richard Klein, The Dawn ofHuman Culture (New York: Wiley, 2002), 270.

  Christopher S. Henshilwood et al., "Emergence of Modern Human Behavior: Middle Stone Age Engravings from South Africa," Science 295, no. 5558 (2002): 1278-1280.

  Carleton S. Coon, The Origin ofRaces (New York: Alfred A. Knopf, 1963).

  Chris Stringer and Peter Andrews, "Genetic and Fossil Evidence for the Origin of Modern Humans," Science 239, no. 4845 (1988): 1263-1268.

  D. Curnoe et al., "Timing and Tempo of Primate Speciation," Journal of Evolutionary Biology 19, no. 1 (2006): 59-65.

  9. Hilde Vervaecke and Linda Van Elsacker, "Hybrids between Common Chimpanzees (Pan troglodytes) and Pygmy Chimpanzees (Pan paniscus) in Captivity," Mammalia (Paris) 56, no. 4 (1992): 667-669.

  Trenton W. Holliday, "Speciation by Distance and Temporal Overlap: A New Approach to Understanding Neanderthal Evolution," in Neanderthals Revisited: New Approaches and Perspectives, edited by T. Harrison and K. Harvati (New York: Sinauer, 2006).

  J. Sweeney and T. Samansky, "Elements of Successful Facility Design: Marine Mammals," in Conservation of Endangered Species in Captivity: An Interdisciplinary Approach, edited by E. F. Gibbons, B. Durrant, and J. Demarest (New York: State University of New York Press, 1995).

  On the supposed insignificance of Neanderthal admixture, see, for example, Chris Stringer and Peter Andrews, The Complete World of Human Evolution (London: Thames and Hudson, 2005), or Klein, The Dawn of Human Culture.

  M
ilford Wolpoff et al., "Modern Human Ancestry at the Peripheries: A Test of the Replacement Theory," Science 291, no. 5502 (2001): 293-297.

  It's conceivable that we already have: Martha Hamblin and Anna Di Rienzo of the University of Chicago found an extremely unusual version of a region near the Duffy gene in a few Italians, very different from that seen in other humans. M. T. Hamblin and A. Di Rienzo, "Detection of the Signature of Natural Selection in Humans: Evidence from the Duffy Blood Group Locus," American Journal of Human Genetics 66, no. 5 (2000): 1669-1679.

  Amos Zayed and Charles W. Whitfield, "A Genome-Wide Signature of Positive Selection in Ancient and Recent Invasive Expansions of the Honey Bee Apis mellifera," Proceedings of the National Academy of Sciences 105, no. 9 (2008): 3421.

  H. C. Stutz and L. K.Thomas, "Hybridization and Introgression in Cowania and Purshia," Evolution 18, no. 2 (1964): 183-195.

  Brian Hare et al., "The Domestication of Social Cognition in Dogs," Science 298, no. 5598 (2002): 1634-1636.

  Henry Harpending and Jay Sobus, "Sociopathy as an Adaptation," Ethology and Sociobiology 8, no. 3 (1987): 63-72.

  Richard W. Bulliet, The Camel and the Wheel (Cambridge: Harvard University Press, 1975).

  David W. Frayer, "Evolution at the European Edge: Neanderthal and Upper Paleolithic Relationships," Prehistoire Europeenne 2 (1993): 9-69; David W. Frayer, "Perspectives on Neandertals as Ancestors," in Conceptual Issues in Modern Human Origins Research, edited by G. A. Clark and C. M. Willermet (New York: Aldine de Gruyter, 1998), 220-234.

  Cidalia Duarte et al., "The Early Upper Paleolithic Human Skeleton from the Abrigo do Lagar Velho (Portugal) and Modern Human Emergence in Iberia," Proceedings of the National Academy of Sciences 96 (1999): 7604-7609; Eric Trinkaus, "Early Modern Humans," Annual Review of Anthropology 34 (2005): 207-230.