Lactase is present in all mammals at birth but only persists throughout life in some societies. These societies typically have a history of dairy production and consumption. Lactase persistence (LP) has arisen multiple times in human history. Strong selection for LP occurred in both European and African populations which historically have relied on dairy products for a substantial portion of calories. Some African populations still rely heavily upon dairy products for a large portion of dietary needs. This represents an ongoing selective pressure for LP in these populations. The European allele for lactase persistence has been shown to differ substantially from the African alleles which also have variation within them. The presence of multiple alleles for lactase persistence in Africa indicates multiple selective events which were separate from each other. Based upon this, it is expected that ongoing selective forces would have a higher LP frequency than in European populations as the selective force on most European populations has since subsided. It would thus be expected that in groups with ongoing selection for LP would have a higher frequency of LP.
These are the frequencies which are actually found:
Northern Europe1 .75
Southern European1 .26
While the LP frequency of Maasai children is higher than that of Southern Europeans, it does not demonstrate the predicted frequency of approaching .75 which would be expected for a strong selective force. This means Maasai must overcome the lactose intolerance (LI) frequency by compensating with some behavioral modification relying upon less raw milk and more processed milk products or other dietary sources. This behavior could include the use of blood and processed foods into their diets instead of unprocessed milk which is high in lactose. The Maasai LP level is much lower than the predicted value and only slightly higher than Southern European populations and far lower than Northern European populations.1
While While LP frequency is not a reliable measure for dietary intake of dairy products, it may still prove useful in evolutionary phylogenies of human populations. Lactase persistence is not a requirement of societies which are dependent upon dairy products if the lactose intake is low enough in those individuals without LP alleles. The multiple alleles present do not indicate stronger selection for LP, but rather for multiple origins of lactase persistence in these cultures. Additional research could shed light on how exactly these populations get around the inability to digest lactase with dietary supplementation from processed milk products and non-dairy animal products.
1) Lactase Haplotype Diversity in the Old World.Hollox, E.J., et alAm. J. Hum. Genet. 2001. 68:160–172
2) Lactose malabsorption among Masai children of east Africa. Jackson, R.T., et al. American Journal of Clinical Nutrition