Steven Levy ARTIFICIAL LIFE Steven Levy is the author of two previous books: Hackers: Heroes of the Computer Revolution, and The Unicorn’s Secret. He has written for Rolling Stone, Es- quire, Harper’s, and currently writes a monthly col- umn for Macworld magazine. Helives in New York City and western Massachusetts with his wife and three-year-old son. ALSO BY STEVEN LEVY Hackers: Heroes of the Computer Revolution The Unicorn’s Secret: Murder in the Age of Aquarius ARTIFICIAL LIFE ARTIFICIAL LIFE A Report from the Frontier Where Computers Meet Biology fda. Steven Levy VINTAGE BOOKS A DIVISION OF RANDOM HOUSE, INC. _ NEW YORK FIRST VINTAGE BOOKS EDITION, AUGUST 1993 Copyright © 1992 by Steven Levy All rights reserved under International and Pan-American Copyright Conventions. Published in the United States by Vintage Books, a division of Random House, Inc., New York, and simultaneously in Canada by Random House of Canada Limited, Toronto. Originally published in hardcover by Pantheon Books, a division of Random House, Inc., New York, in 1992. Library of Congress Cataloguing-in-Publication Data Levy, Steven. Artificial life: a report from the frontier where computers meetbiology / Steven Levy.—1st Vintage Books ed. p- cm. Includes bibliographical references and index. ISBN 0-679-74389-8 (pbk.) 1. Neural networks (Computer science) I. Title. [QA76.87.L48 1993] 006.3—dc20 92-50600 CIP Manufactured in the United States of America 10 9 8 7 6 To Andrew CONTENTS Prologue: In Silico 1 The Promised Land 11 Playing by the Rules 47 Garage-Band Science 85 God’s Heart 121 The Genetic Algorithm 153 Alchemists and Parasites 189 CONTENTS Artificial Flora, Artificial Fauna, Artificial Ecologies 231 Real Artificial Life 271 The Strong Claim 309 Notes and Sources 349 Acknowledgments 371 Illustration Credits 373 Index 375 Vill PROLOGUE: IN SILICO Ifpatterns ofones and zeros were “‘like’’ patterns ofhumanlives and death, if everything about an individual could be represented in a computerrecord by a long string of ones and zeros, then what kind of creature would be represented by a long string of lives and deaths? Thomas Pynchon Thecreatures cruise silently, skimming the surface of their world with the elegance of ice skaters. They move atvarying speeds, some with the variegated cadenceofvacillation, others with what surely must be firm purpose. Their bodies—flecks ofcolors that resemble paperairplanes or pointed confetti—betray their needs. Green ones are hungry. Blue ones seek mates. Red ones wantto fight. They see. A so-called neural network bestows on them vision, and they can perceive the colors of their neighbors and something ofthe world around them. They know something about their own internal states and can sense fatigue. They learn. Experience teaches them what might make them feel better or what might relieve a pressing need. They reproduce. Twoofthem will mate, their genes will merge, and the combination determines the characteristics of the offspring. Over a period of generations, the mechanics of natural selection assert them- selves, and fitter creatures roam the landscape. They die, and sometimes before their bodies decay, others of their ilk devourthe corpses. In certain areas, at certain times, cannibal cults arise in whichthis behavioris the norm. Thecarcasses are nourishing, but not as muchas the food that can be serendipitously discovered ontheterrain. The nameofthis ecosystem is PolyWorld,andit is located in the chips and disk drives of a Silicon Graphics Iris Workstation. Thesole creator of this realm is a researcher named Larry Yaeger, who works for Apple Computer. It is a world inside a computer, whose inhabitants are, in effect, made of mathematics. The creatures have digital DNA. Someof these creatures are more fit than others, and those are the ones who eventually reproduce, forging a path that eventually leads to several sorts of organisms whosuccessfully exploit the peccadilloes of PolyWorld. ‘The species have their own unique behaviors and group dynamics,” notes Yaeger. One group seems on the edge of psychosis—the “‘frenet- ics,’ who, zipping compulsively through the landscape, constantly de- sire food and sex and expend energy onlittle else. Then there is “‘the cannibal cult,” members of which seek their own to mate with, fight with, and eat. They form grotesque clumps from which they need not ARTIFICIAL LIFE movein orderto fulfill any of those needs. A third species is the “edge runner.” Owing to a peculiarity in the landscape—unlike our own spherical planet, PolyWorld can be programmedto have a distinct end ofthe world—thereis a benefit in lurking on the brim of oblivion. Once a respectable numberoffellow creatures adoptthis behavior,there will always be an ample supply of conjugal partners, as well as old carcasses now turned to food. Yaeger is cautious about sweeping statements; he prefers to describe what he has done and what might immediately follow from it. “So far what PolyWorld has shownis that successful organismsin a biologically motivated and only somewhat complex environment have evolved adaptive strategies for living in this environment,” he says. Whenit comes to describing the creatures themselves, Yaeger is less tentative. “I see them,” he says, “‘as artificial life.” In September 1987, more than one hundredscientists and technicians gathered in Los Alamos, New Mexico, to establish the new science of artificial life. The event celebrated a technological and scientific water- shed. A deepened understanding of biological mechanisms, along with the exponentially increasing power ofdigital computers, had brought humankindto the threshold of duplicating nature’s masterpiece,living systems. The pioneers were both thrilled at the prospect and humbled by previous speculations ofwhat lay ahead. The legacy ofMary Shelley, who wrote of Frankenstein and his monster, as well as the dark accom- plishments hatched onthevery site of the conference, hovered over the proceedings like, as one participant put it, a bugaboo. Nevertheless, the mood was exuberant. Manyofthe scientists drawn to Los Alamos had long dreamed of an aggregate effort to create a new form oflife; their individual labors had looked toward that day. Now it had arrived. Thesignificance of the moment was later framed by a physicist named James Doyne Farmer, who coauthored a paper about the implications of this new science. Its abstract alone was perhaps as striking a description of nascent technology at the lab as any since the developmentof the atomic bomb. Those whoread it would have been In Silico well advised to take a deep breath—and perhaps suspenddisbelief until resuming aspiration—before reading the following prediction: Within fifty to a hundred years a new class of organismsis likely to emerge. These organismswill be artificial in the sense that they will originally be designed by humans. However, they will reproduce, and will evolve into somethingotherthan their original form;theywill be “alive” under any reasonable definition of the word. The advent ofartificial life will be the most significant historical event since the emergence of human beings. Artificial life, or a-life, is devoted to the creation and study oflifelike organisms and systems built by humans. Thestuff ofthis life is nonor- ganic matter, andits essenceis information: computers arethe kilns from which these new organisms emerge. Just as medical scientists have managed to tinker with life’s mechanisms in vitro, the biologists and computerscientists of a-life hopeto create life in silico. The degree to which this resembles real, ‘‘wet” life varies; many experimenters admit freely that their laboratory creations are simply simulations ofaspects of life. The goal of these practitioners of “weak” a-life is to illuminate and understand moreclearly the life that exists on earth and possibly elsewhere. As astronomer A. S. Eddington hassaid, ‘‘the contemplation in natural science of a wider domain thantheactual leads to a far better understanding ofthe actual.”’ By simulating a kind oflife different from that with which wearefamiliar, a-life scientists seek to explore paths that no form oflife in the universe has yet taken, the better to understand the concepts and limits oflife itself. Hopingthat the samesorts ofbehavior foundin nature will spontane- ously emerge from the simulations, sometimes scientists attempt to model directly processes characteristic of living systems. Biologists treat these artificial systems as the ultimate laboratory animals; their character- istics illuminate the traits ofknown organisms,but, since their composi- tion is transparent, they are much moreeasily analyzed thanrats,plants, or E. coli. Physicists pursue a-life in the hopethat the synthesis oflife will shed light on a related quest: the understanding ofall complex nonlinear ARTIFICIAL LIFE systems, which are thought to be ruled by universal forces not yet comprehended. By studying phenomena such asself-organization in a-life, these mysteries may soon be unraveled. The boldest practitioners of this science engage in “strong”’ a-life. They look toward the long-term developmentofactual living organisms whose essence is information. These creatures may be embodied in corporeal form—a-life robots—or they may live within a computer. Whichever, these creations, as Farmerinsisted, are intendedto be “‘alive under every reasonable definition of the word”—as muchas bacteria, plants, animals, and humanbeings. Many might consider this an absurd claim on the face of it. How could something inside a computer ever be considered alive? Could anything synthesized by humansever aspire to such a classification? Should not the term “‘life’’ be restricted to nature’s domain? The questionis difficult to answer, largely because we have no “reason- able definition” oflife. Nearly two thousand years ago, Aristotle made the observation that by ‘‘possessinglife,” one implied that “‘a thing can nourishitselfand decay.’ Most everyonealso agreedthat the capacity for self-reproduction is a necessary condition for life. From there, opinions diverged andstill do. One could devise a laundry list of qualities charac- teristic of life, but these inevitably fail. They are either overly dis- criminating or excessively lenient.