CHROMOSOMES: Human 46 , Closest Primates 48

Apes and humans differ by one chromosome pair with humans having one less pair. Most scientists that study this subject favor humans fusing of two pairs to reduce the number and there seems to be evidence for this in chromosome 2. Some scientists favor the fission theory where apes got an extra pair from fission of one of their chromosomes...but I don't know of the evidence for this. By the way, primates closest to humans have 48 pairs while others like monkeys differ substantially...We are talking about Humans and their "closest relatives on Earth" if you will.



You can see it if you can find a picture of the chromosomes (called a karyotype) of a chimpanzee and a human and put
them side by side. It is assumed that in humans two chromosomes fused to produce a total of 46, because most gorillas also have 48. It makes more sense that the event happened in humans than that the event happened independently in the same spot at random in chimps and gorillas. WHen you look at a karyotype you note not only the number of chromosomes, but depending on how they were stained, you can see the pattern of bands (light and dark areas) on the chromosomes. When you take the two hromosomes that chimps and gorillas have, and put them end to end beside chromosome number 2 of humans, the banding pattern is almost identical. Click on this link to see a diagram of this:

 [link to www.indiana.edu] 

What you are looking at is a diagram of chromosomes 1-4 with the human chromosome on the left and the chimpanzee chromosome on the right. Note that for chromosome 2, humans have one long one, but the chimp has two short ones there. This also shows only one of each chromosome for each species, so when you add the other set, that gives humans 46 and chimps 48.