Comparative mapping, which compares the location of homo- logous genes in different species, is a powerful tool for studying genome evolution1. Comparative maps suggest that rates of chromosomal change in mammals can vary from one to ten rearrangements per million years1–4. On the basis of these rates we would expect 84 to 600 conserved segments in a chicken comparison with human or mouse. Here we build comparative maps between these species and estimate that numbers of con- served segments are in the lower part of this range. We conclude that the organization of the human genome is closer to that of the chicken than the mouse and by adding comparative mapping results from a range of vertebrates, we identify three possible phases of chromosome evolution. The relative stability of genomes such as those of the chicken and human will enable the reconstruction of maps of ancestral vertebrates.