Man, gorilla, and chimpanzee likely shared an ancestor in whom the fine genetic organization of chromosomes was similar to that of present man. A comparative analysis of high-resolution chromosomes from orangutan, gorilla, chimpanzee, and man suggests that 18 of 23 pairs of chromosomes of modern man are virtually identical to those of our ``common hominoid ancestor,'' with the remaining pairs slightly different. From this lineage, gorilla separated first, and three major chromosomal rearrangements presumably occurred in a progenitor of chimpanzee and man before the final divergence of these two species. A precursor of the hominoid ancestor and orangutan is also assumed.