Leakey's most dramatic discovery, made in 1978, and the one that she considers most important, has also been all but destroyed since she left the field. The footprints of Laetoli, an area near Olduvai, gave the world the first positive evidence of bipedalism. Three hominids (generally identified as Australopithecus afarensis ) had walked over volcanic ash, which fossilized, preserving their tracks. The terrain was found to be about 3.6 million years old. Although there had been suggestions in the leg bones of other hominid fossils, the footprints made the age of bipedalism incontrovertible. "It was not as exciting as some of the other discoveries, because we did not know what we had," she notes. "Of course, when we realized what they were, then it was really exciting."

Today the famous footprints may only be salvaged with the intervention of the Getty Conservation Institute. "Oh, they are in a terrible state," Leakey exclaims. "When I left, I covered them over with a mound of river sand and then some plastic sheeting and then more sand and a lot of boulders on top to keep the animals off and the Maasai off." But acacia trees took root and grew down among the tracks and broke them up.

Although Leakey steers clear of controversy in her answers and her writings, she has not entirely escaped it. She and Donald Johanson, a paleontologis t at the Institute of Human Origins in Berkeley, Calif., have feuded about the relation between early humans found in Ethiopia and in Laetoli. (Johanson set up his organization as a philosophical counterweight to the L.S.B. Leakey Foundation.) And some debate erupted about how many prints there were at Laetoli. Tim White of the University of California at Berkeley claimed that there were only two and that Leakey and her crew had made the other track with a tool during excavation. Leakey's response? "It was a nonsense," she laughs, and we are on to the next subject.

A subject Leakey does not like. "'What was it like to be a woman? A mother? A wife?' I mean that is all such nonsense," she declares. Leakey--like many other female scientists of her generation, including Nobel laureates Rita Levi-Montalcini and Gertrude Belle Elion--dislikes questions about being a woman in a man's field. Her sex played no role in her work, she asserts. She just did what she wanted to do. "I was never conscious of it. I am not lying for the sake of anything. I never felt disadvantaged."

Leakey just did her work, surviving bitter professional wars in anthropolog y and political upheavals. In 1952 Louis, who had been made a member of the Kikuyu tribe during his childhood in Africa, was marked for death during the Mau Mau uprising. The four years during the height of the rebellion were terrifying for the country. The brakes on Mary's car were tampered with, and a relative of Louis's was murdered. The house that Leakey lives in today was designed during this time: a low, white square structure with a central courtyard where the dogs can run at night.

These pets are very important to Leakey--a source of companionship and safety out in the bush. She admires the traits in them that others admire in her: independence and initiative. (Any small joy that I have about emerging from her house unbitten fades sadly when I reread the section in her autobiography about her telepathic dalmatian and learn that he died years ago.)

We seem to have covered everything, and so she reviews her discoveries aloud. "But you have not mentioned the fruits," she reminds me. One of Leakey's favorite finds is an assortment of Miocene fossils: intact fruits, seeds, insects--including one entire ant nest--and a lizard with its tongue hanging out. They lay all over the sandy ground of Rusinga Island. "We only found them because we sat down to smoke a cigarette, hot and tired, and just saw all these fruits lying on the ground next to us. Before that we had been walking all over them all over the place." She stops. "You know, you only find what you are looking for, really, if the truth be known."

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