By studying mitochondrial DNA from samples over 1000 years old to the present, scientists have good evidence that human genetic diversity is on the decline. You can see the article from the Royal Society here (a great source of many forms of long term science in general).
“In a study covering five different periods of history, from 300 AD to the present day, and geographically spread across much of Europe, scientists have extracted the mitochondrial DNA from a sizable number of individuals in an effort to examine changes in diversity. The results, published in the Royal Society journal is intriguing to say the least. 1700 years ago, three out of every four individuals belonged to a different haplotype. In modern Europe, the number is only one in three. The researchers blame a combination of plague, selection of dominant lineages and culturally-inflicted distortions. The researchers say more work needs to be done, but are unclear if this involves archaeology or experiments involving skewing the data in the local female population.” -from Slashdot
