Tuesday, June 20, 2006

Mother's Little Gift to Darwin

Before we get entirely too enmeshed in Africa, I need to make a statement about Mitochondrial Deoxyribonucleic Acid, mtDNA for short. The interesting fact about mtDNA is its acquisition into the somatic line.

mtDNA resides outside the nucleus of every cell in the mitochodria. What is striking about mtDNA is that it is always inherited from the mother's ova. This is a product of what I have been hinting to previously dealing with Matt Ridley's Red Queen assertions. The female's ova is the larger of the two sex's gametes. You see, when the sperm fertilizes the egg, the smaller sperm cell sheds all nonessential material or in other words only the nucleus of the cell survives the process.

According to Ridley's assertion, this is a compromise on problems that occur on several fronts. First off, is one of the problems that I originally had with the whole concept of the transition from mitosis to meiosis as a viable method of reproduction. For a simpler single or multicelled organism it would seem much more convenient to divide and conquer. Certainly any organism that can reproduce by making exact copies of itself without the assistance of another organism should be able to crowd out sexually reproductive organisms in only a few generations when you consider the law of scarcity. So how is it that sexual creatures managed to evolve and flourish? Was it luck? The hand of some omnipotent god? To understand the reason requires that you take a gene's eye perspective. Evolution happens. Mutations occur and things change. What is in the gene's best interest is to make as many copies of itself as possible. While this is not so difficult to accomplish when you reside in a single cell, the competition increases greatly when dealing with more compicated organisms. Of course, this is a digression better left for later. Back to mitosis/meiosis. Given that asexual reproduction does not have the impediments that sexual reproduction faces, cells that reproduce asexually can reproduce at a much higher rate, but with the added condition that mutation will occur at an increased rate as well. Along with mutation, we need to take into account viruses which are another real world complication. Viral attachments can be deadly to organisms that cannot adapt their way around them. Viruses are an intangible in the genetic arms race. So while sexual reproduction is impeded by interaction and competition for reproductive rights, it gains in being able to propogate genes into the future by 'not putting all its eggs in one basket' so to speak.

The second point I'd like to make at this time has to do with another genetic arms race. This problem has to do with XY chormosomes and gender competition. For reasons I do not care to explain here, humans come in two and exactly two genders. Once again you have to understand it from the gene's perspective. When you are part of a 30000 pair of genes in the standard DNA, the problem is 'what to do to get ahead in life. Contrary to the opinion of some it is not all fun and games at this level. All genes 'desire' their organisms to thrust them into the future but only half will go. To make matters worse, if your human is a female you don't even get to decide the gender of your offspring. Perhaps your gene develops a mechanism that thwarts the activities of your male counterpart. This would be a good trick to get your way. Now perhaps your nemesis finds a way around your defenses. What we begin to have is the classic arms race that we can all remember from the '80s. At some point, the race becomes so expensive that someone has to call it quits and refuse to escalate any further. Or Maybe there will be a truce, but the point is that all arms races can only be carried to a point where it is economically feasible for both competitors. In the case of sperm/ovum armament, it was a unilateral move against aggression. It would appear that for the common good the sperm jettisons all but the nuclear DNA when combined with the egg. I'm not implying that chivalry is not dead here, merely that because this arrangement was made we get mtDNA from our mothers and not our fathers. In addition to stopping the arms race it solves another problem. Once again we return to viruses. By not bringing any non essential material into the union, the chances of viral mishaps are halved by one agent being frisked at the door.

Having said that, I'll move to my point. What I am prescribing in this series is known as the Genetic Replacement Hypothesis in Anthropology circles. This hypothesis basically states that modern human traits evolved from archaic human traits in one location (perhaps Africa) and because of natural selection prevailed over the traits which separate modern homo sapiens from archaic homo sapiens. The idea is that our ancestors all came from one place and the differences are merely superficial adaptations to the environments they settled in.

This theory is no the only one in competition for acceptance. It must compete with two other viable theories: The Multiregional Hypothesis that states that homo sapiens evolved in several different regions in isolation from archaic homo sapiens. While it may explain a few quirks it is highly unlikely that so many different lines could have evolved so much alike in complete genetic isolation. Then we have the Population Replacement Hypothesis. Which is similar to the genetic replacement idea except that it treats homo erectus as a separate species in direct competition with homo sapiens. The boost to genetic replacement came through mtDNA research. In 1987 a team at Berkeley did a study on 147 women from around the world which compared mtDNA among the members of the group. Cann, Stoneking, and Wilson, the study's authors found that the variation between women in the group from outside Africa was less than the differences between the women from Africa. From this study we can deduce that we share a common ancestry from within Africa.