According to a study published Jan. 10 in Nature by geneticists from 4 universities including Harvard, “Analysis of 6,515 exomes reveals the recent origin of most human protein-coding variants.” By recent they mean really recent on evolutionary timescales:
We estimate that approximately 73% of all protein-coding SNVs [single-nucleotide variants] and approximately 86% of SNVs predicted to be deleterious arose in the past 5,000–10,000 years. The average age of deleterious SNVs varied significantly across molecular pathways, and disease genes contained a significantly higher proportion of recently arisen deleterious SNVs than other genes.
The authors explained this in evolutionary terms as the result of “explosive population growth” and that “selection has not had sufficient time to purge them from the population.” They claimed Europeans had more variants “consistent with weaker purifying selection due to the Out-of-Africa dispersal.” The last paragraph assesses the impact of their findings:
More generally, the recent dramatic increase in human population size, resulting in a deluge of rare functionally important variation, has important implications for understanding and predicting current and future patterns of human disease and evolution. For example, the increased mutational capacity of recent human populations has led to a larger burden of Mendelian disorders, increased the allelic and genetic heterogeneity of traits, and may have created a new repository of recently arisen advantageous alleles that adaptive evolution will act upon in subsequent generations.
As for advantageous mutations, they provided NO examples.
The findings depend on models and assumptions, but appear to support the thesis of John Sanford’s book Genetic Entropy and the Mystery of the Genome, which argues that the genetic load increases so rapidly that mankind could not have survived for tens of thousands of years, to say nothing of millions (see recent YouTube interview part 1 and part 2 [combined into one, above]). How can such rapid rate of degradation be sustained over evolutionary time?
The authors seemed a bit baffled by their findings. The following paragraph gives a hint of that (compare “expected” vs observed):
The site frequency spectrum (SFS) of protein-coding SNVs revealed an enormous excess of rare variants (Fig. 1a). Indeed, we observed an SNV approximately once every 52 base pairs (bp) and 57 bp in European Americans and African Americans, respectively, whereas in a population without recent explosive growth we would expect the SNVs to occur once every 257 bp and 152 bp in European Americans and African Americans, respectively (Supplementary Information). Thus, the European American and African American samples contain approximately fivefold and threefold increases in SNVs, respectively, attributable to explosive population growth, resulting in a large burden of rare SNVs predicted to have arisen very recently (Fig. 1b). For example, the expected age of derived singletons, which comprise 55.1% of all SNVs, is 1,244 and 2,107 years for the European American and African American samples, respectively. Overall, 73.2% of SNVs (81.4% and 58.7% in European Americans and African Americans, respectively) are predicted to have arisen in the past 5,000 years. SNVs that arose more than 50,000 years ago were observed more frequently in the African American samples (Fig. 1b), which probably reflects stronger genetic drift in European Americans associated with the Out-of-Africa dispersal.