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Dolphins 83 dence for the nature of their interaction with our ancestors; it may have involved competition or predation, or no direct contact at all. This discovery has intriguing implications. We tend to assume continu- ing progress in our own evolution, including the growth of our intellect. Now it appears that the expansion of the brain can be reversed under environmental pressures. The fact that these mini-humans made the same stone tools as their larger ancestors and may have been able to perform advanced cognitive tasks raises questions about the relationship between brain size and intelligence. Human brains (and bodies) have shrunk about 10% on average during the past 50,000 years. The reasons are unknown, but might be connected with our abandoning the mentally demanding foraging ways of life and settling down to be farmers.” A Close Call The survival of a particular intelligent species can be chancy. Some anthropologists, geneticists, and population biologists have concluded that humans were squeezed down to a small population (possibly as few as 10 thousand breeding men and women) some time during the past 400,000 years. “Our ancestors survived an episode where they were as endangered as pigmy chimpanzees or mountain gorillas are today,” said anthropologist Henry Harpending. That genetic bottleneck might have had a different outcome. About 73,000 years ago, a huge volcano on the island of Sumatra blew its top in a massive eruption. Scientists estimate that this volcano, now known as Toba, ejected 670 cubic miles of material into the atmosphere, more than 500 times the amount produced by Mount Pinatubo in the Philippines in 1991. Sulfuric acid particles scattered and absorbed sun- light, cooling the planet’s surface and reducing photosynthesis. Fine ash rained from the sky, penetrating animal lungs and fatally immobilizing birds. That eruption may have imposed a 6 year “volcanic winter” and a 1000-year ice age. Studies show a bottleneck of genetic diversity among humans at roughly the same time as Toba’s eruption.*” Homo sapiens was then a young species, sharing the Earth with Neanderthals and other homi- nids. The survival of this upstart may not have been a sure thing. Dolphins It is as if once the early human beings learned language they killed off all members of the next lower level, at least those that were on land. —Neurophysiologist John C. Lilly, 1961*!