The world's greatest discoverer of the fossil remains of early human ancestors is not named Leakey or even Johanson. He is Kamoya Kimeu, a 46-year-old Kenyan, a former goatherd whose formal education ended with primary school.

Although largely unknown outside the small group of anthropologists who study the evidence of human evolution, Kimeu received the National Geographic Society's prestigious John Oliver LaGorce gold medal yesterday for outstanding accomplishment in exploration and the sciences. President Reagan presented the award to Kimeu (pronounced Ka-MAY-you) in a brief Oval Office ceremony.

Kimeu, who began his career 25 years ago as an unskilled laborer for Louis and Mary Leakey at Olduvai Gorge in Tanzania, joined such other recipients as Robert E. Peary, discoverer of the North Pole; Roald Amundsen, discoverer of the South Pole, and Jacques Yves Cousteau, the undersea explorer.

"Kamoya," said Gilbert M. Grosvenor, the society's president, "is an extraordinary modern-day explorer."

Kimeu, working first for the elder Leakeys and then for their son Richard, has personally found more of the physical evidence of human evolution than anybody else. Many of his discoveries, however, reached the scientific community and the world at large through the announcements of the Leakeys.

"My own success," Richard Leakey said yesterday, "has to a very large extent been built on the individual contributions of Kamoya." Leakey said that "virtually none" of the major finds with which he is often credited were really the result of his own searching.

Kimeu's most recent major find was the nearly complete skeleton of a human ancestor who lived 1.6 million years ago in what is now Kenya. The skeleton proved to be that of a Homo erectus boy about 12 years of age.

"When I started," Kimeu recalled of his early days with the Leakeys, "I thought it was a game. I couldn't understand why people would work so hard to find such tiny fragments of bone."

Only much later, he confessed, did he begin to appreciate the work's value. "Now it is because of the importance of the finds that I enjoy my work. There is no other way people can know the past of our species than through the kind of work we do."

In 1977, Kimeu became curator of prehistoric sites for the Kenya National Museums and administrator of the complex field expeditions in which a dozen scientists from around the world and scores of assistants scour the remote, arid wastelands of Kenya, living on rationed food and water, searching for fossils and carefully digging them up.