Humans have 46 chromosomes. Chimpanzees, gorillas, and orangutans all have 48.

Allan Krill
Jan 3  
Edited Jan 3

Early in our evolution, a mutant baby was presumably born with a chromosome error in which the 2A and 2B chromosomes of apes were fused to form the long chromosome 2 of humans. This genetic mutation was probably associated with physical traits that made that mutant individual more successful. It produced offspring with similar genes, and eventually the entire population in that area had those mutant traits, and 46 chromosomes instead of 48.

Those traits would probably not have made arboreal apes more successful. If a chimpanzee is born with a significant mutation in the forest of Africa, that chimp will probably have no offspring and the mutation will be lost. However, in the case of humans, the genes and traits of chromosome 2 were presumably advantageous in the postulated marine habitat of Bioko. It is easy to see that human traits — bald body, longer legs, reduced canine teeth, subcutaneous fat, larger head and brain, weaker muscles — would not help a mutant chimpanzee be more successful in the African forest. 

This article explains the chromosome fusion that took place in human evolution. It suggests that geneticists study chromosome 2 near the fusion. It points out that genes involving human brains and gonads are near the join and were likely affected by the fusion.

(Thanks Annika, for sending me this article!)


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