Joined: Jan 2009 Gender: Male Posts: 676 Location: Loveland, Colorado Karma: 14
Human Versus Chimp Chromosomes « Thread Started on Feb 9, 2009, 7:10pm »
Human Versus Chimp Chromosomes
* One of the interesting facts in human genetics is that the human genome map is so extremely similar to that of the chimpanzee -- to be sure humans have 23 pairs of chromosomes while a chimp has 24, but no big deal, because one human chromosome, "chromosome 2", is a combination of two chimp chromosomes. The similarity between the genomes is regarded as fairly good evidence for common descent of humans and chimps. In fact genetic analysis shows humans are more closely related to chimps than gorillas or orangutans are.
Some have even claimed the genetic similarity is so close that humans are the "third chimpanzee" -- the second being the bonobo or "pygmy chimp", somewhat different in appearance from the chimp itself, very different in behavior, being noted for a level of sexuality that would embarrass a free-love communalist. However, bonobos and chimps being clearly more closely related to each other than they are to us and the "third chimpanzee" notion is just seen as theatrical.
* Not surprisingly, there are critics who take exception to this reading of the genetic record -- yes, believe it or not, evolutionary science has its critics. There's been a discussion of the matter at the Panda's Thumb evo science blog, but it's written at a graduate bioscience level and hard to follow. However, it did link to a nice essay by Paul Zachary ("PZ") Myers, a biologist at the University of Minnesota, on the Pharyngula blog. Since the arguments actually raise some interesting technical points, it's worthwhile summarizing Myer's essay here.
Some critics have claimed that there is no real similarity between the chimp and human genomes -- they have the same basic components, but that's about it. As one put it, the comparison between the two is about the same as the comparison between two entirely different books, they use the same words but they have nothing otherwise to do with each other. The reality that the similarity is close and comprehensive -- they're more like two different modern editions of the same ancient book. It should be noted that nobody says that humans evolved from chimps, we both came from a common ancestral species, and both have genomes modified from that of the ancestral species.
So that criticism is nowhere -- but the critics have other arguments. How do we know, they ask, that we have one chromosome that is the merger of two chimp chromosomes? Actually it's obvious. Chromosomes often have a restriction or "centromere" in the middle and distinctive "caps" know as "telomeres" at the ends. Not only do we see the same genes in our chromosome 2 that are found on two other chimpanzee chromosomes; we can see the structure of two centromeres in our one chromosome, and also the relics of telomeres embedded in the middle. As Myers put it: "It's an open-and-shut case."
The critics then go on to ask why we would assume a chromosomal fusion along the line of descent from the human/chimp ancestor. Why wasn't it a case of a chromosome splitting to give the chimp 24 chromosomes? That is superficially plausible, but:
First: that's not what the chromosome 2 structure, particularly the embedded telomeres, suggests. The chromosomal fusion leaves its fingerprints.
Second: humans and our relatives are intolerant of chromosomal duplications. The most infamous example results in Down's syndrome, which results in an oversized head and some variable degree of mental retardation. Down's syndrome is actually a fairly mild consequence of a chromosomal duplication -- most of the time the embryo dies well before coming to term.
Third: This is just a red herring anyway, since the fusion isn't evidence in itself for the common descent of humans and chimps. The evidence is the similarity of the two genomes, and on first sight the difference in number of chromosomes would work against the idea. The fusion simply explains why it doesn't. The only mildly interesting information in the fusion is that it took place after the split between the chimp and human lines.
* OK, all this is nowhere again, but now the critics raise an honestly interesting question, which Myers does a pretty good job of answering. That chromosomal fusion only took place in a single individual -- but only one, necessarily of one sex, which meant a 23-chromosome individual mating with a 24-chromosome individual. Can that work? Will they match up? Doesn't it cause problems?
As already mentioned chromosomal duplications do tend to be disastrous in our bloodline. Not so for fusions. As Myers put it, what we see in humans is a "classic instance" of a "Robertsonian translocation". They're by no means unusual, about one in 900 human births have a fusion of this kind, but they don't generally cause much trouble. They may (or may not) result in reduced fertility; except for some specific cases, they do not result in a genetic disease.
Myers went into detail on this. Suppose we have a chromosome named {A} with one set of genes and a chromosome named {B} with another. In a normal "diploid" cell, with twinned chromosomes, we have pairs of these chromosomes: {A}{A} and {B}{B}. In meiotic division, a gamete -- germ cell, with only one set of chromosomes, awaiting the match of another set from the other parent -- gets one of each of these chromosomes, resulting in {A} and {B}.
Now suppose a Robertsonian fusion takes place that results in a merged chromosome {A:B} in a diploid cell. Since there's only one merger, the diploid cell has the chromosome match of {A}{B}{A:B}. When meiosis occurs, the chromosomes could be sorted out in several different ways:
{A}{B} Normal, no problem. {A:B} Normal, no problem. {A}{A:B} LETHAL. {A} LETHAL. {B}{A:B} LETHAL. {B} LETHAL.
That means that an individual with a Robertsonian fusion can carry on the fusion successfully. It may inhibit fertility to a degree, but there are still plenty of opportunities to bear healthy offspring. Once the fusion starts spreading through the gene pool, then eventually there will be offspring with the diploid combination [A:B][A:B] and all offspring will be normal. The relative improvement in reproductive success tends to branch populations with duplicate fused chromosomes off as separate species.
Theoretical hand-waving? Not hardly, because the process has been observed several times with other species. In short, the evidence is that humans acquired a merged chromosome after branching off from our common ancestor with chimps -- and the important matter is just that the human and chimp genomes are so similar. The merged chromosome 2 is merely an incidental detail.
* Somebody working on animal behavior once pointed out that there are two different mindsets on the close relationship of humans and apes. Some people find the idea absolutely appalling. Others simply think: "Yeah, sure. So what's the problem?" I'm clearly in the second camp. I sometimes wonder how anyone could observe how we behave and not believe we're another species of primate. Believe me, I don't consider myself an exception, either, though I consider myself better off for realizing it.
« Last Edit: Sept 23, 2009, 8:07pm by Greg Goebel »
Joined: Jan 2009 Gender: Male Posts: 676 Location: Loveland, Colorado Karma: 14
Re: Human Versus Chimp Chromosomes « Reply #1 on Feb 9, 2009, 8:58pm »
The PT argument continues and has its interesting insights into chromosome number:
BEGIN QUOTE:
Wild Horses (Equus ferus przewalskii) have 66 chromosomes. Domestic horses (Equus caballus) have 64 chromosomes, and Donkeys have 62 chromosomes. The cross of a domestic horse and a donkey produces a mule or hinney with 63 chromosomes. Crossing a wild horse with a domestic horse produces a horse with 65 chromosomes.
END QUOTE
I'll update this reply as more interesting comments pop up.
Joined: Jan 2009 Gender: Male Posts: 516 Location: Bangalore, India Karma: 11
Re: Human Versus Chimp Chromosomes « Reply #3 on Apr 9, 2009, 7:56pm »
The conventional wisdom is that matings between animal species result in non-fertile hybrids--mules and hinnies from horse-donkey matings, tigons and ligers from lion-tiger pairings.
According to this article from The Washington Post, however, one species of animal living today, may be the descendants of a long line--1.2 million years worth--of hybrids. The reason why this is thought to be so, is because this animal shows a discrepancy in the dates of divergence between its X and other chromosomes when compared with the corresponding chromosomes of its closest living relative. The comparison shows that the female X chromosome is around 1.2 million years younger that the other chromosomes. From the article:
It appeared the two species shared a common ancestor who gave them both their X chromosomes, and did so more recently than the ancestors who gave them all the other chromosomes.
The best explanation, the scientists think, is that the 2 species:
broke away from each other not once, but twice. The first time was more than 6.3 million years ago. The second time was at least a million years later.
I recommend reading the article for the remaining fascinating details.
Joined: Jan 2009 Gender: Male Posts: 516 Location: Bangalore, India Karma: 11
Re: Human Versus Chimp Chromosomes « Reply #4 on Sept 7, 2009, 2:35pm »
A nice video produced by UC, Riverside to highlight how similar Human and Chimp mt-DNA really are. The technique used to produce the visualization is simple (How stupid of me to not have thought of that!) and would make a nice little OpenGL project for II/III year Comp. Sci. students.
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