|MadSci Network: Evolution|
Human DNA is actually close to 98% identical to chimpanzee DNA. The human genome has now been completely sequenced, and the chimpanzee genome has not been sequenced, but large sections of it have been. Some recent papers that address this issue are: Kaessmann H, Wiebe V, Paabo S. Extensive nuclear DNA sequence diversity among chimpanzees. Science. 1999 Nov 5;286(5442):1159-62. PMID: 10550054 Kaessmann H, Heissig F, von Haeseler A, Paabo S. DNA sequence variation in a non-coding region of low recombination on the human X chromosome. Nat Genet. 1999 May;22(1):78-81. PMID: 10319866 Samonte RV, Conte RA, Verma RS. Localization of human midisatellite and macrosatellite DNA sequences on chromosomes 1 and X in the great apes. J Hum Genet. 1999;44(1):57-9. PMID: 9929980 Zhao Z, Jin L, Fu YX, Ramsay M, Jenkins T, Leskinen E, Pamilo P, Trexler M, Patthy L, Jorde LB, Ramos-Onsins S, Yu N, Li WH. Worldwide DNA sequence variation in a 10-kilobase noncoding region on human chromosome 22. Proc Natl Acad Sci U S A. 2000 Oct 10;97(21):11354-8. PMID: 11005839 Metzler-Guillemain C, Usson Y, Mignon C, Depetris D, Dubreuil G, Guichaoua MR, Mattei MG. Organization of the X and Y chromosomes in human, chimpanzee and mouse pachytene nuclei using molecular cytogenetics and three- dimensional confocal analyses. Chromosome Res. 2000;8(7):571-84. PMID: 11117353 As far as I know, there is no region of the human genome that is really distinct from chimpanzees. A good discussion of human/chimp evolution is the book: "The Third Chimpanzee" by Jared Diamond. But it is much more about cultural evolution and changes in behaviors than about changes in the DNA. In my opinion, the most likely source of the major differences between humans and chimpanzees come not from "new genes" or significant changes in existing genes, but from chromosomal rearangements. If my memory is correct (I don't have time to look this up right now) there have been something like 8 or 9 non-homologous chromosomal crossover events between the last common ancestor of chimps and humans, and modern chimps and humans. These are events where something like chromosome 14 fuses with chromosome 66 to create a new larger chromosome, or chromosome 3 crosses over with chromosome 5 to create twon new chromosome that are 3/5 and 5/3 hybrids. Each of these events can instantly create a new species if the individuals with the new chromosomes can no longer mate with the individuals with the old chromosome arrangement. As far as I know, it is not yet know how many of the 8 events occurred on the human lineage vs how many occurred on the chimp lineage. For example all 8 might have been on the human lineage and the chimps have had no chromosomal rearrangements since the common ancestor with humand, or 5 might have happened in the human lineage and 3 in the chimp lineage. All I know is that there are 8 total differences, and sorting out which happened in which lineage is difficult without complete sets of chromosomes from ancestors which no longer exist. For a number of different reasons, I suspect that the chromosomal rearrangements are very important. One is the observation that chromosomal rearrangements observed in humans almost always result in large changes. For example, trisomy 21 (also known as Down syndrome) which introduces no new genes and no chromosomal rearrangement, but just one extra copy of one small chromosome, results in many changes in both looks and behavior. Likewise, many chromosomal translocations result in distinctive syndromes in the carriers. A second reason is the "genetic bottleneck" that such an event creates if it results in a speciation event. If just a few individuals carry the new chromosomes and they can only mate with each other and not with the parental population, they will be founding members of a new species and any traits they carry will not get "diluted out" into a huge population. Just as with point mutations and other small changes in genes, most chromosomal rearrangements would tend to be detrimental rather than clearly beneficial to the carriers. With some 6 billion people on the planet today, there are tens of thousands who currently carry chomosomal translocations, and it is not likely that any of them are the founders of a new and improved species of human. It also takes more than just good genes to survive in any society, and ever more so in the increasingly complex human societies. I really smart chimpanzee might be killed of by his troop if he was not also strong, or if he was just strange looking to the members of his troop. Maybe the super-intelligent chimp would be "too smart for his own good" and die because he was curious about what the leopard looked like up close. To found a new species, the individuals would most likely have to leave their parental group and go off to start a new group. In almost all competition-driven evolution, it is within-species competition that is more intense than between-species competition. The elk's large antlers are not primarily good at fighting off wolves, but primarily good at fighting rival male elk for mating rights with female elk. Anyway, there is actually much more that we still don't know about evolution and speciation, than we know. It is certainly not like Darwin had all the details figured out in the mid-1800s. Studying molecular genetics, evolution, and other areas of biology are still very exciting and ripe for new discoveries every day. It will take a combination of genetics, paleontology, archeology, sociology, and many other disciplines working together to uncover the origins and explanations of human evolution. In my opinion, what make us "human" is not in our genes but in our behaviors. It is social evolution, which is largely Lamarkian rather than Darwinian, that has so greatly influenced the last 50,000 years of our journey. We have learned to cooperate and work together to solve problems that individuals or smaller groups could not solve. Language and other tools have helped that to happen. Brian Foley, PhD
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