FOR ALL THE ANCIENT skulls and prehistoric-stone tools that Mary Leakey chiseled out of the rocks of East Africa, what this accidental anthropologist will be best remembered for are feet. Feet prints, actually. One day in 1978, on the arid Laetoli plain of Tanzania, Leakey bent over an impression that looked as flit had been made by a human heel. With a dental pick and brush she painstakingly cleaned away the 3.75 million-year-old, hardened volcanic ash that encased the print. Three hours later, convinced that the print had indeed been left by human ancestors, she stood up. Lighting one of her trademark cigars, she announced, "Now this really is something to put on the mantelpiece."
Or in a museum. The 75-foot-long trail of crisp footprints had been made by three lithe hominids (members of the human family) who ambled across the volcanic plain at the dawn of humankind. One of them seemed to pause and turn left, briefly, before continuing north. This relic of a behavior from eons back brought the find to life in a way that mere bones could not. As Leakey wrote, "This motion, so intensely human, transcends time.
A remote ancestor... experienced a moment of doubt." The find helped overturn the prevailing wisdom that the seminal event in human evolution was the development of a big brain. Instead, it was standing up, which freed the hands to make tools. Toolmaking stimulated growth in the size and complexity of the brain. "This new freedom of forelimbs posed a challenge," Leakey wrote soon after the discovery. "The brain expanded to meet it." And humankind was born.
Without Leakey, who died last week in Nairobi at 88, the family tree of mankind would have been quite short of branches. …