The Science of Existence (125-4) Comparative Chromosomes

Comparative Chromosomes

The number of chromosomes varies widely between species. Humans have 46.

Most organisms carry 10 to 50 chromosomes. A salamander has 20 times more DNA than a human. A mosquito has 6 chromosomes, but a silkworm has 56. A mouse has 40, a duck 60, a goldfish 94, and a toucan 106. One species of fern has 630 chromosome pairs per cell.

Humans are most closely related to chimpanzees and bonobos, which have 48 chromosomes: 2 more than people. But 1 human chromosome has the information stored in 2 chimp chromosomes.

Genetic drift of humans from chimps and bonobos began 5 mya. Yet the DNA sequences differ by less than 1%. Chimp blood can substitute for human in transfusions.

Chimps and bonobos diverged after humans left the lineage. Chimp-bonobo genetic drift started 2 MYA.

Humans lack 510 DNA sequences that chimps, macaques, and mice share. Most of those sequences are thought to be genetically unimportant, if not entirely vacant of genes.

Though genetic differences between humans and chimpanzees may be statistically slight, they are phenotypically significant. One lost sequence allows expansion of certain brain regions in humans during development. Another controls production of sensory facial whiskers and penile spines, which humans lack.

Penile spines help males ejaculate quickly during intercourse. Quick impregnation increases the immediate prospect for reproduction. Lacking penile spines results in longer copulation times, affording emotional bonding between mating partners; something quite instrumental in human evolutionary success.