The most harmful mutations in humans arose recently

A new study shows that over 86% of the most harmful mutations to humans actually arose only 5,000-10,000 years ago. 

These mutations are single nucleotide polymorphisms (SNPs), and while some of these are, as we know, not dangerous or sometimes even beneficial, they are more harmful than not.

Its posited that the Out-of-Africa Bottleneck led to an inefficient elimination of the most harmful mutations, rather than ridding our old genetic makeups of the harmful mutations. Furthermore, while these mutations existed before the bottleneck, they were essentially unseen or ineffective because of the large population of humans. The bottleneck diminished the human population to the point where the mutations became more manifest and more effective at affecting a significant portion of the population.

The study also found that "the older the genetic variant, the less likely it was to be deleterious." Additionally, about 81% of SNPs in Europeans and about 58% of SNPs in Africans arose in the past 5000 years. Older variants, from over 50,000 years ago were more frequent in Africans. These statistics serve as a testament to the out of Africa Bottleneck, and show how these harmful mutations became, essentially, more popular in the past 5,000-10,000 years.

http://www.eurekalert.org/pub_releases/2012-11/uow-mot112812.php