Mary Leakey, 83, the renowned archaeologist and paleoanthropologist whose discoveries of prehistoric bones and artifacts in East Africa led to a deeper understanding of the origin and development of the human species, died Dec. 9 in Nairobi.

She helped establish the earliest known traces of human history at 3.6 million years ago, about 3 million years earlier than previously believed. With her husband, Louis S.B. Leakey, she confirmed the theory of Charles Darwin that at least some of mankind's earliest ancestors could be traced to the continent of Africa. Their work also demonstrated that humanity did not evolve along a solitary line but that there were two, three or more types of pre-humans.

Known for her "meticulous attention to detail and her dedication to scientific interpretation of fossils," Mrs. Leakey "played a pivotal role in rewriting the history of early humans," said Gilbert M. Grosvenor, the chairman of the National Geographic Society, which supported much of her work with research grants.

Mrs. Leakey's best-known discoveries included:

* A trail of 3.6 million-year-old hominid footprints in hardened volcanic ash in 1978 at Laetoli on the Serengeti Plain in Northern Tanzania. In a 1979 article in National Geographic magazine, Mrs. Leakey described them as "tracks so sharp they could have been left this morning." The footprints demonstrated that this prehistoric human walked on two feet, freeing his hands for "myriad possibilities -- carrying, toolmaking, intricate manipulation. From this single development, in fact, stems all modern technology. Somewhat oversimplified, the formula holds that this new freedom of forelimbs posed a challenge. The brain expanded to meet it. And mankind was formed."

In an interview with the Associated Press at her home outside Nairobi three months ago, Mrs. Leakey said of the footprints: "I think it's the most important find in view of human evolution. I was really looking for tools, but we never found any at the site."

* Skull fragments of an early hominid, 1.75 million years old, in the Olduvai Gorge in Tanzania in 1959. Working with dental picks, paintbrushes and sieves, Mrs. Leakey collected 400 fragments of the skull, and she worked over a period of 18 months to piece them together. This prehistoric creature came to be known as Zinjanthropus and later Australopithecus. Its discovery changed the prevailing views about the time scale of human evolution, which previously had been thought to have originated 500,000 years ago. Mrs. Leakey concluded that the skull belonged to someone who had lived at the same time as a human ancestor but was not in the direct line of evolution.

* The skull of Proconsul africanus in 1947 on the island of Rusinga in Africa's Lake Victoria. This was an apelike ancestor of apes and humans who lived about 25 million years ago. This discovery focused worldwide attention on the Leakeys and on East Africa as a possible cradle of mankind.

Mrs. Leakey was born in London, the daughter of a landscape painter who traveled extensively in France and Italy. As a child, she became fascinated with archaeology, and her father encouraged her in this interest. Independent-minded as a teenager, she was expelled from two convent schools, and she decided to pursue drawing and archaeology. "I had never passed a single school exam, and clearly never would," she wrote in her autobiography, "Disclosing the Past."

By age 20, she was doing the drawings for archaeological digs in England. This brought her into contact with Louis Leakey, who was then a Cambridge University professor with an established reputation for fieldwork in East Africa. In 1935, they traveled to East Africa together, and they married in 1936 after he divorced his first wife.

For most of the next 60 years, Mrs. Leakey would remain in East Africa, where she spent most of her time in the field, searching the ground for the tiniest of fragments and scraps of bones and tools that might shed light on how the human race came into being.

By the mid-1960s, they established a permanent camp at Olduvai Gorge, where Mrs. Leakey spent most of her time. She loved to study the fossil scraps she collected, spending days with them in a hot tin shed at her camp on the edge of the chasm, meticulously drawing them for scientific publications and wondering about the almost-human minds of the creatures who made the stone tools. As the sun set, she loved a good glass of bourbon and a cigar in the glare of a hissing gas lantern that hung under a nearby thatched roof that served as her dining room.

Over the years, she would have bouts with pneumonia, malaria, appendicitis and bilharzia, a disease caused by parasites, which incapacitated her for months. In a statement announcing her death, her son, paleontologist Richard Leakey, said only that she died peacefully. He did not give a cause.

Louis Leakey died in 1972, but the couple's partnership and marriage had disintegrated several years before. During the 1960s, while Mrs. Leakey continued to do research in the field, he stayed mainly in Nairobi while in East Africa. He also traveled frequently to the United States to lecture, raise money and speculate at news conferences about the significance of his wife's discoveries, often leaving the impression that he, personally, had made the finds. Because Mrs. Leakey hated publicity, she did not object.

But there came a time when she began to question the validity of her husband's work. On the finding of the Zinjanthropus skull fragment, Louis Leakey claimed at first that it was from a direct human ancestor, then had to retract the statement. "I ended by losing my professional respect for Louis; and it had been very great indeed," Mrs. Leakey wrote in her autobiography.

Three months before her death, Mrs. Leakey agreed that it was impossible for scientists to pinpoint exactly when prehistoric man became fully human. "We shall probably never know where humans began and where hominids left off," she told the Associated Press.

Mrs. Leakey's interests also included prehistoric rock paintings. She agreed to write her autobiography only after obtaining an agreement that a book she wrote on rock paintings at Kandoa, Tanzania, would also be published. She also wrote "Olduvai Gorge: My Search for Early Man" in 1979.

In addition to her son Richard, survivors include two other sons, Jonathan, a herpetologist, and Philip, an assistant minister for natural resources in Kenya. Richard Leakey is the secretary general of Kenya's opposition Safina Party.