M and I were watching something on Discovery about the Neanderthal and there was a commercial for another show about “the Real Eve”. Something like “Was there really a single common ancestor for the whole human race? Find out how she might have lived, etc.”

This led to a discussion about how evolution really happens. Neither M or I have studied it in detail, so perhaps there is something fundamental here we’re not getting.

M’s point was “There can’t have been just one single ancestor, there were probably thousands of people living in a group and they all evolved together”. She finds it hard to believe that there was just one individual that we all descend from. My point was “Well, that’s how mutations happen, to individuals, and they pass on the mutated trait to their children. So if there is a trait that differentiates humans from non-humans, there must have been one individual who gained that trait through mutation – therefore that was the first human.”

It seems like evolution works in one of two ways. One way is a mutation that makes a person different, but not different enough to prevent mating. Then the mutated trait can be passed on and mixed in to the population, and if it is a desirable trait, people that have it may be more likely to survive, and over hundreds of generations, either everyone will have it (because they are descended from the original mutant) or they will have died out, but that would be less common. However, since the offspring of either type can still mate, there isn’t a new species formed, this is just variation within the species. This explains how humans from different parts of the world can look so different, if they have lived mostly isolated from each other for a time (thousands of years). Such variations are probably not significant in the larger scheme of things; as long as the individuals from various tribes are still able to mate, the variations don’t mean a new species.

However, that doesn’t explain how a new species was formed in the first place. According to the definition, a species is a group of individuals who are capable of mating with each other. If a new species is formed, there must be a group of individuals who are no longer capable of mating with the larger population. After that point, the family tree is forever diverged; there will never again be any sharing of genetic traits between humans and other primates. But how actually does that work? If there is one individual mutant, who is unable to produce offspring with any of her own kind, that individual would die childless, and no new family tree would form.

Here is one possible theory. There must be a halfway-state where the new individual2 and the old family1 can still mate with each other, and the offspring of that union are either family1 or family2, depending on whether they inherited the trait. Up to this point, it’s the same as the first scenario. But, then suppose there is yet another mutation, another half-step, that makes individual3 incompatible with family1 but still compatible with family2. The offspring of family3 would mate with the others in family2 and their children would either be family2 or family3. The family3 individuals might also have sex with people from family1, but they would be incompatible and no children would come of it. Therefore all surviving members of family3 would have both individual2 and individual3 as common ancestors.

Perhaps this is similar to the relationship between donkeys and horses? I believe donkeys can mate with horses but they produce only mules, which are always sterile, so there is no “breed” of mules which has its own population. They are similar enough to breed, but the cross-breed is not viable on its own, so they are still considered to be separate species.

I should go and read about origins some more…