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

Allan Krill

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!)


Allan Krill

The '46-fix' probably originated in one mutant chimpanzee, and punctuated the speciation of humans. 

Chromosome differences are associated with marked differences in physical traits. A typical example of a chromosome abnormality is Turner syndrome, where an X-chromosome is lacking. This mutation is associated with special traits, including a short and webbed neck, low-set ears, low hairline at the back of the neck, and short stature. 

We can imagine a chimpanzee being born with 46 chromosomes and bizarre physical traits (perhaps a large head, weak neck, bald body, subcutaneous fat, hooded nose, ... ). If this chromosome fusion appeared in the African rainforest where chimpanzees are well adapted, such a baby chimp would probably have died. But if it appeared in a marine habitat, where some of the traits were advantageous, this chromosome abnormality might have been passed on to future generations. See my message 202.

Now geneticists have the technology to study chromosomes and try to understand how the 2A-2B fusion might be related to our unique human features. They will eventually be able to genetically engineer a chimpanzee with fused chromosomes 2A-2B, and see what physical features appear. ( Oof! )