A hypothesis for the chromosome content of the ancestors of (a) all placental mammals, (b) all living primates, and (c) carnivores. To derive the ancestral syntenic karyotype of primates (b) from the ancestral placental mammalian karyotype, three fissions and three fusions are necessary, whereas only four fusions are needed to derive the ancestral karyotype of carnivores (c). Numbers to the left and right of each chromosome represent homologous regions of the human and cat genomes, respectively. Numbers below each chromosome are identifiers for the chromosome in each ancestral genome. The colors represent the chromosome in the ancestral mammalian genome from which each region originated. The G-banded karyotype depicted for the ancestral placental genome is speculative.