Chicago-born Sarich was educated at the University of California, Berkeley, where he gained his PhD in 1967. He has remained at Berkeley, being appointed professor of biochemistry in 1981.

Sarich was struck by the range of dates, from 4 million to 30 million years ago, within which anthropologists of the early 1960s placed the origin of the split between the hominids and the great apes. He began work, in collaboration with his Berkeley colleague Allan Wilson, to see if there was a more precise method of dating using the genetic relationship between man and apes. They chose to work with proteins, which closely reflect genes, choosing the blood-serum protein, albumin. As man and apes diverged further from their common ancestor, their albumins would also have diverged and would now be recognizably different.

Serum samples from apes, monkeys, and man were purified and then injected into rabbits to produce antiserums. A rabbit immunized against a human sample (antigen) will also react to other anthropoid antigens, only not as strongly. As antigenic differences are genetically based, response differences will therefore measure genetic differences between species. Sarich chose to work with a group of proteins found in blood serum known collectively as the ‘complement system’. Antigens tend to attract and fix some of the complement. The amount of complement fixed could be measured precisely. Thus differences in complement-fixation rates produced by the albumin of a human and a gorilla when injected into immunized rabbits would measure their immunological distance.

If it could be assumed that protein differences between species have evolved at a constant rate then immunological distance would also be a measure of evolutionary separation. It remained to calibrate the clock. Sarich and Wilson took as their base line the date 30 million years ago marking the separation between the hominoids and the old-world monkeys. Thereafter it was a relatively easy matter to turn immunological distance into dates.

The results were clear but surprising. Homo, on this scheme, separated from the chimps and gorillas only 5 million years ago. This was a bold claim to make in 1967 as orthodox opinion, argued for example by David Pilbeam, placed the split between hominids and hominoids closer to 15 million years. What is more they had the skull of Ramapithecus, dating from this period, to prove their point. Initially, therefore, Sarich's views were rejected out of hand by paleontologists. Slowly, however, Sarich made converts. He argued, “I know my molecules have ancestors, you must prove your fossils had descendants.” They found it more and more difficult to do so. Consequently, when it became clear that Ramapithecus was the ancestor not of man but the orangutan, opposition to Sarich largely disappeared.