Monday, 13 September 2010

Why Evolution is True pt2

The main difference separating us from chimps is a fusion, a naturally arising event that occurs on chromosome 2. Other differences between human and chimp chromosomes are more subtle landmark events than whole chromosome fusions. The chimp equivalent of human chromosome 17 has undergone a rearrangement in which two breaks were made in the chromosome, the intervening segment flipped through 180 degrees, and the breaks rejoined. We know that the inversion occurred during chimp evolution, because the sequence is a genetic parasite (an ‘Alu’ insert, an ERV) occurring at one of the break points. It is undisturbed in the human chromosome 17, but has been broken in the equivalent chimp chromosome, in which the two halves are separated by millions of bases. Endogenous retroviruses have contributed the structural components of some genes to their animal hosts. A member of the HERV-W family (human endogenous retrovirus) resides in the DNA of all apes and must have entered primate DNA in an ancestor of the apes. A representative of the HERV-FRD family is present in all simians (including humans), demonstrating that it entered the primate germ-line in an ancestor of the simians. These ERV’s have retained their function in every species, and may have been co-opted to serve a role in reproduction.

Studies have shown that ERV’s are excellent markers of evolutionary relationships. The probability of finding two ERV’s independently integrated into the same DNA site in any two different species is exceedingly low. If multiple species posses the same ERV insert, they must have received it by inheritance from the same ancestor. If you’ve followed me so far (and I know that this is a lot to work to get through but it’s worth it), then you will be surprised to know that not only are there indeed two independent ERV insertions, but the genome project found 60 such events in total that we share with other primates, and thousands all together. This pattern proves, far better than any fossils could ever have, that we do indeed share common ancestry with each of the apes, and even demonstrates the order of relatedness I mentioned before.

Finally, there are plenty of instances in which two parents of the same skin color produced a baby with different tones. Suffice to say that the genetic information for varying tones of melanin in the skin is already present in every human’s genetic code. The dominant alleles and mutations that occur at the embryonic stage decide characteristics ranging from the skin tone to the susceptibility to certain viruses and diseases and everything in between. Funnily enough your next prediction is absolutely spot-on. The simplest organisms on this earth, the Eukaryote single celled Amoebae, carry in their genome 290 billion base pairs, while in contrast the human genome consists of only 3.2 billion. A genome is the organism’s entire hereditary information. Just out of interest, the largest genome for vertebrates belongs to the marbled lungfish with 130 billion base pairs. Since our ancestral line diverged from our fish cousins some 530 million years ago this is poignant. The simplest organism most commonly known is the Amoeba. It contains in its DNA the possible information for such a huge diversity of life that it is entirely likely that a similar organism is the ancestor of every species alive or dead.

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