It's interesting to consider how the ancestors of modern humans who first had 46 chromosomes became dominant. It could have been due to simple genetic drift, ma

It's interesting to consider how the ancestors of modern humans who first had 46 chromosomes became dominant. It could have been due to simple genetic drift, maybe the family with the translocation had a high number of offspring by chance for generation after generation, until it became a significant portion of the population of the early hominids. More likely, (in my opinion), is that the translocated chromosome itself contained some beneficial alleles (gene versions) that gave its carriers an edge. This has nothing to do with the translocation itself, it just so happens that that particular chromosome could have a version of a gene (or versions of several genes) that increased the carrier's fitness in the environment. Some of theories regarding early hominid evolution cite a movement away from heavily forrested regions into more open grasslands. Maybe this early translocated chromosome contained alleles that were beneficial in the savannas. At this point you'd have a number of individuals with one copy of the translocated chromosome and 47 total chromosomes (meaning the other homologous (http://en.wikipedia.org/wiki/Homologous_chromosome http://en.wikipedia.org/wiki/Homologous_chromosome) chromosomes are not joined together, "normal") and a number of people who still had the original 48 chromosomes. As the article mentions, only 1/3 of the pregnancies of a 48-chromosome individual with a translocation individual would survive. However, if there was inbreeding present individuals with the translocated chromosome could be mated with each other. Here we have an even less likely chance of a successful pregnancy (8/36 or only 25%) but on the other hand we now have the possibility of individuals with just the translocated chromosome. Instead of a joined chromosome and 2 unjoined chromosomes, these individuals have 2 joined chromosomes and a total of 46 chromosomes. Now remember that I postulated that the joined (translocated) chromosome could have carried beneficial alleles as opposed to other, unjoined versions. Some of those alleles may be recessive, meaning they'd be eclipsed by other alleles. For an individual with two copies of the translocated chromosome they would only have those recessive alleles, so they wouldn't be as likely to be eclipsed. Again, these recessive alleles may have brought about beneficial traits that weren't expressed in individuals who only had one joined chromosome. Thus, individuals with 46 chromosomes could have had an even higher chance of survival and finding other 46-chromosome individuals, thus continuing this trend. Anyway, I admit that much of the above is really just guesswork so take it with a grain of salt. I'm primarily exploring the possibilities that could have resulted in the original 48->46 chromosome shift. It was probably a mix of genetic drift, having beneficial alleles, and the founder effect (http://en.wikipedia.org/wiki/Founder_effect http://en.wikipedia.org/wiki/Founder_effect). Hope you enjoyed it!