this new scientist article explains that there was a population bottleneck which the chimpanzee and homo sapiens' common ancestor went through at which there were around 10,000 individuals in the species. as a result, mutations in DNA regions which code for proteins have been more resilient in our species than in others which don't have such a bottleneck, for example mice. my attempt to understand this follows.

disadvantageous mutations are those which affect a carrier's chances of reproduction. not all disadvantageous mutations are going to be absolute: perhaps they increase the likelihood of developing a certain form of cancer by a certain amount.

if a disadvantageous mutation occurs in an individual, there's a chance that the mutation will be weeded out over generations as non-carriers mate with carriers, giving non-carrier children on average 50% of the time (based on a shaky assumption, that bit), and as carriers fail to reproduce slightly more often than the non-carrying population.

another possibility is that through chance the mutation manages to spread through the population. for this to happen, the harmful effect of the mutation would have to be low enough that carriers are still able to beat the spread and produce offspring which spread the mutation.

in a relatively small population, like that of the bottleneck, i think that the odds against a harmful mutation achieving a foothold in the population are lower than those in a large population with high rate of exchange of genetic material.

so, the idea is that we can interpret this tendency to "weed out" harmful mutations as a phenomenon that you're only going to see operating successfully in populations above some size which varies with the severity of the harmful effect of the mutation. that's what the article is referring to when it talks about the mice species' better performance relative to humanity's.

so, i'm not sure whether i've got the right end of the stick - are we saying that the 140,000 observed mutations are preserved from the time of the bottleneck, or that the majority of them are and the period of heightened sensitivity to mutation is past, or that the mutations have just been more likely to accumulate over the time since the bottleneck?