The 10,000 Year Explosion Read online

  THE

  10,000 YEAR

  EXPLOSION

  Q2

  HOW CIVILIZATION

  ACCELERATED HUMAN EVOLUTION

  GREGORY COCHRAN • HENRY HARPENDING

  THE

  10,000 YEAR EXPLOSION

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  THE

  10,000 YEAR EXPLOSION

  HOW CIVILIZATION ACCELERATED HUMAN EVOLUTION

  Gregory Cochran and Henry Harpending

  BASIC

  B

  BOOKS

  A Member of the Perseus Books Group New York

  Copyright © 2009 by Gregory Cochran and Henry Harpending Published by Basic Books, A Member of the Perseus Books Group

  All rights reserved. Printed in the United States of America. No part of this book may be reproduced in any manner whatsoever without written permission except in the case of brief quotations embodied in critical articles and reviews. For information, address Basic Books, 387 Park Avenue South, New York, NY 10016-8810.

  Books published by Basic Books are available at special discounts for bulk purchases in the United States by corporations, institutions, and other organizations. For more information, please contact the Special Markets Department at the Perseus Books Group, 2300 Chestnut Street, Suite 200, Philadelphia, PA 19103, or call (800) 810-4145, ext. 5000, or e-mail [email protected] .

  Designed by Pauline Brown

  Library of Congress Cataloging-in-Publication Data Cochran, Gregory.

  The 10,000 year explosion : how civilization accelerated human evolution / Gregory Cochran and Henry Harpending. p. cm.

  Includes bibliographical references and index. ISBN 978-0-465-00221-4 (alk. paper)

  1. Human evolution. 2. Genetics. I. Harpending, Henry. II. Title. III. Title: Ten thousand year explosion.

  [DNLM: 1. Evolution. 2. Hominidae. 3. Civilization. 4. Genome— genetics. GN 281.4 C663z 2009] GN281.4.C632 2009 599.93'8—dc22

  2008036672

  10 9 8 7 6 5 4 3 2 1

  TO OUR FAMILIES

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  CONTENTS

  Preface ix

  OVERVIEW: CONVENTIONAL WISDOM 1

  THE NEANDERTHAL WITHIN 25

  AGRICULTURE: THE BIG CHANGE 65

  CONSEQUENCES OF AGRICULTURE 85

  GENE FLOW 129

  EXPANSIONS 155

  MEDIEVAL EVOLUTION: HOW THE ASHKENAZI

  JEWS GOT THEIR SMARTS 187

  CONCLUSION 225

  Notes 229 Glossary 243 Bibliography 253 Credits 267 Index 269

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  PREFACE

  For most of the last century, the received wisdom in the social sciences has been that human evolution stopped a long time ago—in the most up-to-date version, before modern humans expanded out of Africa some 50,000 years ago. This implies that human minds must be the same everywhere—the "psychic unity of mankind." It would certainly make life simpler if it were true. Unfortunately, a recent halt to evolution also implies that human bodies must be the same everywhere, which is obviously false. Clearly, received wisdom is wrong, and human evolution continued. In the light of modern evolutionary theory, it is difficult to imagine how it could have been otherwise.

  Since the social sciences—anthropology in particular— haven't exactly covered themselves with glory, we have decided to take a new tack in writing this book, one that takes the implications of evolutionary theory seriously while cheerfully discarding unproven anthropological doctrines. Our approach leans heavily on genetics—and with genetic information accumulating at an incredible rate, due to the ongoing revolution in molecular biology, it is an approach that we believe has been very fruitful. At the same time, we make use of paleontology, archaeology, and good old-fashioned history to support ourarguments. We think that it is mistake to neglect any relevant information.

  A lot of our work could be called "genetic history." It's a new kind of history: We share the usual facts, but we use a very different explanatory framework. Traditional historians tell the stories of battles and kingdoms and great men. Some study the history of ideas, or of science and technology. Quantitative historians examine commerce and demographic trends. We, however, are interested in the historical factors that have influenced natural selection in humans, particularly those having to do with the creation and spread of new, favorable alleles. This means that when a state hires foreign mercenaries, we are interested in their numbers, their geographic origin, and the extent to which they settled down and mixed with the local population. We don't much care whether they won their battles, as long as they survived and bred. We're not particularly interested in their cultural baggage unless it changes selection pressures or influences gene flow.

  Conventional social sciences, such as history and anthropology, chiefly concern themselves with brain software, by which we mean cultural developments such as mores, mythology, or social structure. Genetic history addresses changes in the underlying hardware, changes in body and brain, which also matter. If they didn't, dogs really could play poker.

  For an anthropologist, it might be important to look at how farmers in a certain region and time period lived; for us, as genetic historians, the interesting thing is how natural selection allowed agriculture to come about to begin with, and how the new pressures of an agricultural lifestyle allowed changes in the population's genetic makeup to take root and spread. We take this same approach whether we are looking at the humanrevolution of the Paleolithic period, the agricultural revolution of 10,000 years ago, or the modern development of the Ashke- nazi Jews of Europe, who rose to intellectual prominence in the West quite recently after experiencing unique natural-selection pressures during the Middle Ages.

  We arrived at this approach via different paths. Gregory Cochran was trained as a physicist—and following theory where it leads, no matter how odd, comes naturally to a physicist. Henry Harpending began his graduate career with enthusiasm for the social sciences. Years of disillusionment with the status quo in these fields led him to into demography and genetics, areas where he believes there is real theory with strong links to the rest of the sciences.

  Many colleagues have provided suggestions, ideas, and objections to the material in this book. We are grateful to all of them for their help and their criticism. Particularly prominent among these colleagues are John Hawks of the University of Wisconsin; Robert Moyzis and Eric Wang of the University of California at Irvine (Wang is now at Affymetrix Corporation); and Alan Rogers, Doug Jones, and Renee Pennington of the University of Utah. We have also benefited from discussions with members of the Human Biodiversity Internet discussion group run by Steve Sailer.

  We are indebted to Kristen Hawkes, James O'Connell, Dennis O'Rourke, and Jon Seger of the University of Utah; Gregory Clark of the University of California at Davis; Alan Fix of the University of California at Riverside; Montgomery Slatkin of the University of California at Berkeley; Kim Hill of Arizona State University; Bruce Lahn of the University of Chicago; Mel Konner of Emory University; Jeremy Stone of Catalytic

  Diplomacy; Razib Khan of the GNXP Web site; James Lee of Harvard University; Rosalind Arden of King's College London; Phil Rushton of the University of Western Ontario; and Balaji Srinavasan of Stanford University. Parts of this work were supported by the Unz Foundation.

  Homo erectus and descendants occupy much of temperate Old World about 1.8 mya to 500 thousand years ago

  Control of fire, prepared core stone tool technologies

  Modern humans leave Africa to populate much of Europe, Asia, Oceania about 50,000 years ago

  Modern looking humans in NE Africa, invade Middle East during warm period />
  Neanderthals develop in Europe and Western Asia

  Empires around Mediterranean and China 2000 years ago

  Development and spread of Indo-European speakers about 6000 years ago

  Last glacial maximum about 20,000 years ago

  Agriculture develops in several centers about 10,000 years ago

  Ashkenazi prosper in northern Europe 1200 to 400 years ago

  100,000

  10,000

  1000

  100

  1,000,000

  Years Before Present

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  1

  OVERVIEW:

  CONVENTIONAL

  WISDOM

  There's been no biological change in humans in 40,000 or 50,000 years. Everything we call culture and civilization we've built with the same body and brain.

  —Stephen Jay Gould

  Something must have happened to weaken the selective pressure drastically. We cannot escape the conclusion that man's evolution towards manness suddenly came to a halt.

  —Ernst Mayr

  We intend to make the case that human evolution has accelerated in the past 10,000 years, rather than slowing or stopping, and is now happening about 100 times faster than its long-term average over the 6 million years of our existence. The pace has been so rapid that humans have changed significantly in body and mind over recorded history. Sargon and Imhotep1 were different from you genetically as well as culturally. This is a radical idea and hard to believe—it's rather like trees growing noticeably as you watch. But as we will show in the following pages, the evidence is there.

  Scientists have long believed that the "great leap forward," some 40,000 to 50,000 years ago in Europe, marked the advent of cultural evolution and the end of significant biological evolution in humans. At this time, the theory goes, humans developed culture, as shown by the sophisticated new tools, art, and forms of personal decoration that emerged in the Upper Paleolithic. Culture then freed the human race from the pressures of natural selection: We made clothes rather than growing fur and built better weapons rather than becoming stronger.

  The argument that the advent of behavioral modernity somehow froze human evolution is dependent on the notion of a static environment.2 In other words, if a population—of humans, wolves, crabgrass, you name it—experiences a stable environment for a long time, it will eventually become genetically well matched to that environment. Simple genetic changes then do little to improve individual fitness, because the species is close to an optimum. An economist would say that all the $100 bills have already been picked up off the sidewalk. In that situation, evolution slows to a crawl. That's not to say that a stable species has reached perfection, but that its life strategy is well implemented. For example, hopping may not be as efficient as walking on all fours (four legs good, two legs bad!), but kangaroos are good at hopping; their bodies are well suited to their style of locomotion. The match of the population to its environment can never be exact, since environments fluctuate, but it can be quite close. For example, there are orchids that imitate a bee so closely in appearance and in odor that bees try to mate with them, and so pollinate the orchids. Some creatures that are well suited to their environment, such as horseshoe crabs,have managed to stay much the same for hundreds of millions of years. They're literally older than the hills.

  However, modern humans have experienced a storm of change over the past 50,000 years. We left Africa and settled every continent other than Antarctica. We encountered and displaced archaic humans like Neanderthals—and probably picked up genes from them in the process. An ever-accelerating cultural explosion from the Upper Paleolithic to the Neolithic and beyond led to new technologies and new social forms. Indeed, culture itself has been an increasingly important part of the human environment.

  Geographic expansion (which placed us in new environments) and cultural innovation both changed the selective pressures humans experienced. The payoff of many traits changed, and so did optimal life strategy. For example, when humans hunted big game 100,000 years ago, they relied on close-in attacks with thrusting spears. Such attacks were highly dangerous and physically taxing, so in those days, hunters had to be heavily muscled and have thick bones. That kind of body had its disadvantages—if nothing else, it required more food—but on the whole, it was the best solution in that situation. But new weapons like the atlatl (a spearthrower) and the bow effectively stored muscle-generated energy, which meant that hunters could kill big game without big biceps and robust skeletons. Once that happened, lightly built people, who were better runners and did not need as much food, became competitively superior. A heavy build was yesterday's solution: expensive, but no longer necessary. The Bushmen of southern Africa lived as hunter-gatherers until very recently, hunting game with bows and poisoned arrows for thousands of years in that region. They are a small, tough, leanpeople, less than five feet tall. It seems likely that the tools made the man—the bow begat the Bushmen.

  With the invention of nets and harpoons, fish became a more important part of the diet in many areas of the world, and metabolic changes that better suited humans to that diet were favored. Close-fitting clothing provided better protection against cold, allowing people to venture farther north. In cool areas, people needed fewer physiological defenses against low temperatures, while in the newly settled colder regions they needed more such defenses, such as shorter arms and legs, higher basal metabolism, and smaller noses. With the advent of new methods of food preparation, such as the use of fire for cooking, teeth began to shrink, and they continued to do so over many generations. Pottery, which allowed storage of liquid foods, accelerated that shrinkage. Complex biological functions tend to slowly deteriorate when they no longer matter, since mutations that interfere with the function no longer reduce reproductive fitness, and you might think that this would explain these dental changes. However, this trend, which we call "relaxed selection," happens too slowly to be the explanation. Instead, the changes in tooth size must have been driven by positive advantages—possibly because small teeth are metabol- ically cheaper than large ones.

  As the complexity of human speech approached modern levels, there must have been selection for changes in hearing (both changes in the ear and in how the brain processes sounds) that allowed better discrimination of speech sounds. Think of the potential advantages in being just a bit better at deciphering a hard-to-understand verbal message than other people: Eavesdropping can be a life-or-death affair. We have evidence of this,since a number of genes affecting the inner ear show signs of recent selection.3 Such genes are easy to recognize, since radical changes in them cause deafness. Combined with an increased capacity for innovation, complex speech must also have entailed an increase in the capacity for deception—and must have resulted in selective pressures for changes in personality and cognition that helped people resist Paleolithic con men.

  There's a common impression that evolutionary change is inherently very slow, so that significant change always takes millions of years. A more detailed look at the fossil record, combined with evidence from contemporary examples of natural selection, makes it clear that natural selection can proceed quite rapidly, and that the past consists of long periods of near-stasis (in populations that were well matched to their environments) interspersed with occasional periods of very rapid change. Those brief periods of rapid change are poorly represented in the fossil record, since fossilization is rare.

  Stephen Jay Gould's position that 50,000 or 100,000 years is an "eye blink," far too short a time to see "anything in the way of evolutionary difference," is simply incorrect.4 We are surrounded by cases in which selection has caused big changes over shorter time spans, often far shorter; everything from the dog at your feet to corn on the cob is the product of recent evolution.

  The most accessible examples are the products of domestication. Domesticated animals and plants often look and act very different from their wild ancestors, and in every such case, the ch
anges took place in far less than 100,000 years. For example, dogs were domesticated from wolves around 15,000 years ago; they now come in more varied shapes and sizes than any other mammal.

  Chihuahua and Great Dane

  Their behavior has changed as well: Dogs are good at reading human voice and gestures, while wolves can't understand us at all. Male wolves pair-bond with females and put a lot of effort into helping raise their pups, but male dogs—well, call them irresponsible. There have been substantial changes in dogs in just the past couple of centuries: Most of the breeds we know today are no older than that.

  In an extreme example, the Russian scientist Dmitri Belyaev succeeded in developing a domesticated fox in only forty years.5 In each generation he selected for tameness (and only tame- ness); this eventually resulted in foxes that were friendly and enjoyed human contact, in strong contrast to wild foxes. This strain of tame foxes also changed in other ways: Their coat color lightened, their skulls became rounder, and some of them were born with floppy ears. It seems that some of the genes influencing behavior (tameness in this case) also affect other traits—so when Belyaev selected for tameness, he automatically got changes in those other traits as well. Many of these changes have occurred as side effects of domestication in a number of species—possibly including humans, as we shall see.