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NEGATIVE ENERGY AND WARP DRIVE • MAGLEV TRAINS • SUPERHEAVY ELEMENTS Beyond ice ages: SNOWBALL EARTH A startling theory of our planet’s frozen past JANUARY 2000 $4.95 www.sciam.com We Were Not Alone Our species had at least 15 cousins. Only we remain. Why? Copyright 1999 Scientific American, Inc. January 2000 FROM THE EDITORS 4 LETTERS TO THE EDITORS 6 50, 100 AND 150 YEARS AGO 10 Vo l u m e 2 8 2 Numb e r 1 Once We Were Not Alone 56 by Ian Tattersall Paintings by Jay H. Matternes Homo sapiens is the only hominid that still walks the earth. Yet over the past four million years, 20 or more types of creatures similar to us and our ancestors may have existed, and often they shared their territory with one another. Perhaps the reason we are all that remains is on the tip of our tongues. THE NOBEL PRIZES FOR 1999 14 NEWS AND ANALYSIS IN FOCUS West Antarctica is disappearing, but flood fears are premature. 19 SCIENCE AND THE CITIZEN Sex in space. . . . Physics solves a Prisoner’s Dilemma. . . . Americans on the move. 22 Snowball Earth Paul F. Hoffman and Daniel P. Schrag 68 A revolutionary hypothesis suggests that hundreds of millions of years ago, ice up to a kilometer thick engulfed even the tropics, snuffing out most life. A runaway greenhouse effect ended the deep freeze but baked the planet. These brutal climate reversals might have encouraged the rise of multicellular organisms. PROFILE Molecular biologist Harold E. Varmus prepares for life after the NIH. 30 TECHNOLOGY AND BUSINESS New displays shake up aviation. . . . Old skyscrapers brace for modern hurricanes. . . . Reasons to kill elephants. 36 CYBER VIEW What on-line trading wo