Re: Chromosomes 46 and 48

Dear Roger,

This is a very good question! In technical science terms "good question" means we don't have the answer yet. You are correct that humans have 23 pairs of chromosomes (2N = 46), that all of the other living primates that most recently shared a common ancestor with us have 24 pairs (2N = 48), and that the difference is located in the chromosome we call "#2" in humans.

The human chromosomes were numbered by size, with chromosome number 1 being the largest. But when people assigned the numbers to the chromosomes of the other "great ape" species; Gorillas, Chimpanzees and Orangutans; they found 2 chromosomes with the most DNA sequence and banding pattern similarity to human chromosome #2, one simlar to human chromosome #2 short arm (2p) and one similar to human chromosome #2 long arm (2q), so the Chimpanzees, Gorillas and Orangutans each have a chromosome named 2p and another named 2q. If the chromosomes were numbered by size in all species, perhaps 2p or 2q in chimpanzee would be #13, but I am not sure. The two explanations for the chromosome number diference are:

1: The common ancestor had 24 pairs and in the lineage leading to modern humans, two of them fused into one to create 23 pairs in the human lineage.

2: The common ancestor had 23 pairs and in the lineage leading to Chimpanzees, Gorillas and Orangutans one of them (#2) split in two.

Most people favor the fusion of 2p with 2q to create 2 in the human lineage, because there are quite a few different lines of evidence showing that Orangutans are the more distant relative, or split from the common ancestor sooner, with humans, chimpanzees and gorillas each splitting off later.

There are several other major chromosomal differences, like inversions and recombinations, between chimpanzees, gorillas and humans. But this is the only fusion or split event.

It is not necessary for each major event to have "an evolutionary advantage", in order for it to survive. It only has to not be so detrimental that it goes extinct. Think "elimination of the weakest" rather than "survival of the fittest". It is not just the one "most fit" individual who survives each generation, and we are not descended from the chimpanzees. We shared a common ancestor with chimpanzees and gorillas, and all 3 lineages survived so far. The chimpanzees and gorillas have evolved as much from that common ancestor as humans have, but all three lineages have elvoved in different directions.

Chromosome fusions and other major changes in chromosomes can create a genetic bottleneck. But it is not the case that this, or any other single major chromosomal change suddenly turned a chimpanzee into a human. It is clear from the fossil record, that the common ancestor of chimpanzees, gorillas and humans lived some 5 to 10 million years ago, and that as recently as 100,000 years ago modern humans co-existed with neanderthals.

There is still more that we don't know about human origins and evolution in general, than what we do know. We know for example that there can be competition within species, either for resources like food and shelter or for mating choices, as well as between species (only for resources and not for mates). But the year after year constant competition for a trait like long legs to run fast could be completely obliterated in a single year by a plague that kills off all but the members who carry a resistance gene, or who were just lucky enough to avoid infection. Likewise a flood or volcanic eruption or other event could sperate a group into two groups who no longer compete with each other.

Humans consider the differences between chimpanzees and humans to be huge. But to a fish probably all primates look alike. The rate of our change can arguably be said to be slower, taking 5 million years or so to go from knuckle-dragging to walking upright, than the rate of change among domestic dogs which have gone from wolf to thousands of different breeds as different as Saint Bernards and Dachsunds in just a few hundred years of breeding.