141-142: Variation in the human genome

Science -- Hinds et al. 307 (5712): 1072:

Individual differences in DNA sequence are the genetic basis of human variability. We have characterized whole-genome patterns of common human DNA variation by genotyping 1,586,383 single-nucleotide polymorphisms (SNPs) in 71 Americans of European, African, and Asian ancestry. Our results indicate that these SNPs capture most common genetic variation as a result of linkage disequilibrium, the correlation among common SNP alleles. We observe a strong correlation between extended regions of linkage disequilibrium and functional genomic elements. Our data provide a tool for exploring many questions that remain regarding the causal role of common human DNA variation in complex human traits and for investigating the nature of genetic variation within and between human populations.

These SNPs have distinct regional patterns because of evolution. There's no other sensible explanation. Understanding them tells us about the evolution of humans and our spread and interbreeding over hundreds of thousands of years.

There's also a nice commentary about "Harvesting Medical Information from the Human Family Tree." Human evolutionary information is a great tool for identifying cryptic genetic diseases.