Apes vs Human Chromosome Relationship
Country: United States
Date: January 2006
Please could you advise me about the 48 chromosomes
in chips/apes and the 46 chromosomes in humans that it is now being
suggested is because humans split off from Chimps/apes at a point
in the evolutionary tree. Is it possible that the apparent match
and combination could be accounted for by any ohter reason than
that it was a split in a common ancestor? Is this the only possible
There is indeed good genetic evidence to support this. The other
explanation is that the starting number is 46 and that the chromosomes split
in two, but that would mean that it happened independently many times in all
the other primates that have a chromosome number of 48. The probability of
that is very low, so it makes more mathematical sense that primates other
than humans have 48 and that in humans two chromosomes came together to make
our #2. One of the ways that geneticists pair chromosomes in a picture is
to look at the banding pattern. If you line up a human chromosome #2, and
the two chromosomes from chimpanzees that match it, it appears that these
chromosomes attached head to head (tails pointing out). Now that both the
human and chimpanzee genomes have been sequenced, comparing the sequences of
these chromosomes provides even better evidence of this.
The ends of
chromosomes have stretches of repetetive DNA called telomeres. These seem
to be "caps" on the ends of the chromosomes that prevent the DNA from
"unraveling". This telomeric DNA has a specific sequence. When studying
the middle of chromosome number 2 of humans, there is DNA that has the
signature of telomeric DNA on both sides of the centromere, which the
"pinched" portion of a chromosome. Why would telomeric DNA be found here?
The explanation that makes the most sense is that two shorter chromosomes
joined head to head. The fact that chimpanzees and humans have different
numbers of chromosomes immediately causes a reproductive barrier and would
be an immediate speciation event.
Evidence suggests that humans evolved from a common ancestor of apes and
human by the fusion of two pairs of chromosomes that reduced the chromosome
number from 48 to 46. How this happened is not known.
Ron Baker, Ph.D.
We could always come up with all kinds of fantastic explanations. As a
rule, scientists work with the simplest explanations first, because
simple explanations require us to make few assumptions. This principle
is known as "Occam's Razor". For the same reasons, scientists always
look for explanations that are based on natural, observable, testable
In the case of the striking similarities between human and chimp
genomes, by far the simplest explanation anybody has ever proposed is
that humans and chimps share a common ancestor, and a fairly recent one
at that. Because nobody has ever produced compelling evidence that this
explanation is wrong, it is the one that scientists tentatively accept.
If such evidence were to emerge, then scientists would look to modify
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Update: June 2012