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a savvy handicapper would never have put money on the continued existence of this evolutionary dark horse. Nearly hairless, weak— no sharp claws or slicing teeth here— and slow, with a bumpy bipedal gait, humans might initially appear to be one of the unlikeliest survivors on earth. Except for the oversize brains. As the articles in this special edition collectively underscore, so much of the rise of our ancestors from humble beginnings to today’s worlddominant swell of humanity tracked the stunning growth of all that furrowed cortex. From roughly two million years to 250,000 years ago, the brain’s total volume expanded by a tablespoonful every 100,000 years, estimates Harvard University biologist E. O. Wilson. If we could stretch a modern person’s cortex fl at, it would occupy an area the size of four sheets of standard letter-size paper. In contrast, a chimp’s would cover one sheet; a monkey’s, a postcard; and a rat’s, a stamp. But size alone does not explain our matchless reasoning skills. One of the mysteries of human evolution is that other species with large brains (such as Neandertals) seemingly did not achieve comparable levels of cognition. Could a cultural innovation, perhaps driven by rapid environmental changes, have contributed to the rise of symbolic thought, language and cooperative group society? Ian Tattersall speculates along these lines in “How We Came to Be Human,” starting on page 66, and William H. Calvin explores “The Emergence of Intelligence,” beginning on page 84. As our primate ancestors’ intellects deepened, their bodies continued to morph. Their need to stoke the energy-consuming organ in their skulls with nutritious, calorie-rich fuel created selection pressure favoring features now characteristic of primates, such as grasping hands with opposable thumbs, relates Katharine Milton in “Diet and Primate Evolution,” starting on page 22. “To a great extent,” concludes Milton, “we are truly what we eat.” Other articles in the issue explore how and when early humans and our ape cousins began to sprawl around the planet. Even as recent discoveries answer some questions about our fascinating and complex history, they raise others. Alone among creatures alive today, we enjoy the ability to contemplate our species’ odyssey through time. Food for thought. letter from the editor A

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