A quote from:
http://www.nature.com/ng/index.html

[In an advance publication article in 'Nature Genetics'] Sarah Tishkoff and
colleagues report that lactase persistence due to variation upstream of the
gene encoding lactase-phlorizin hydrolase (LPH) has arisen independently in
Africans and Europeans. Their results confirm the strong selective pressure
of adult milk consumption in human evolution.

Nature Genetics
Published online: 10 December 2006;   doi:10.1038/ng1946

Convergent adaptation of human lactase persistence in Africa and Europe
Sarah A Tishkoff et. al. [very long author list omitted]

Abstract
A SNP in the gene encoding lactase (LCT) (C/T-13910) is associated with the
ability to digest milk as adults (lactase persistence) in Europeans, but
the genetic basis of lactase persistence in Africans was previously
unknown. We conducted a genotype-phenotype association study in 470
Tanzanians, Kenyans and Sudanese and identified three SNPs (G/C-14010,
T/G-13915 and C/G-13907) that are associated with lactase persistence and
that have derived alleles that significantly enhance transcription from the
LCT promoter in vitro. These SNPs originated on different haplotype
backgrounds from the European C/T-13910 SNP and from each other. Genotyping
across a 3-Mb region demonstrated haplotype homozygosity extending >2.0 Mb
on chromosomes carrying C-14010, consistent with a selective sweep over the
past approx7,000 years. These data provide a marked example of convergent
evolution due to strong selective pressure resulting from shared cultural
traits?animal domestication and adult milk consumption.

Abstract online at (also full-text access at):
http://www.nature.com/ng/journal/vaop/ncurrent/abs/ng1946.html

Related online news story, 'Science' magazine:

Milk Tolerance Evolved More Than Once
By Ann Gibbons
ScienceNOW Daily News
11 December 2006

Free online access (for a limited time) at:
http://sciencenow.sciencemag.org/cgi/content/full/2006/1211/1

1st paragraph of Science news story:

Milk may do a body good for some, but for half of the world's adults, it
causes cramps and diarrhea. Now, a new study indicates that the ability to
digest milk arose more than once in humans descended from cattle herders--a
finding that sheds light on how culture can have a rapid and dramatic
effect on our genome.

Tom Billings