Message #113662

[Janna Britton]

Boreas
Member
Posts: 433
From: Namsos, Norway
Registered: Jan 2004
  posted 02-22-2005 13:07              
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HEAVY MILK-ADDICT?
There is always someone making it differently. Some time ago they started squeezing the milk out of cows, - and drink it, - just like babies sipping from mothers breasts...

For some reason this culture become very strong - at least among some societies. So strong that they eventually went through thick and thin to develop a resistence towards the poisoning effect that milk-proteins create when digested, leaving the milk-sugar (lactose) behind - in the digestive process. One may wonder what the motivation could be...

For some unbelivably funny reason they carried on drinking cow-milk. While the rest of the worlds adult population quited during youth - the agricultural societies must have continued, - since the result is part of our present reality; namly populations where the larger majority is used to butter, cheese and rockefort. By the accumulation of persistance did these populations start to develop very specific, intestine enzymes, able to split the milk-sugar - and make it part of normal digestion.

But, - thats like asking for a brand new inner organ! The adaption required to gain this ability must have been very demanding -since it ended up as the most clear and typical European haplogroup. Consequently there is a link between the agriculture and genetic development. The early adaption to an agricultural diet would be linked to the haplogroup with the highest tolerance (persistance) of the lacto-diet. Finally this question are about to be answered. One may just wonder - did the Atlanteans drink milk? Here`s the expertise;

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Fresh lessons in the history of milk drinking By
Edward Hollox,
Institute of Genetics, University of Nottingham.

Most people cannot drink milk as adults without the symptoms of lactose intolerance, and most lactose intolerance is due to absence of the lactase enzyme in the gut. This presence/absence is a genetic polymorphism commonly called lactase persistence/nonpersistence, depending on whether or not lactase activity persists from childhood into adulthood.1 In Northern Europe, lactase persistence is common and many people not only drink milk, but culturally it is seen as a healthy and nutritious food. How this happened is now becoming clearer.

Definition
Lactase nonpersistence is the ancestral state, and lactase persistence only became advantageous after the invention of agriculture, when milk from domesticated animals became available for adults to drink. As expected, lactase persistence is strongly correlated with the dairying history of the population. This genetic ability to digest milk has been regarded as a classic example of gene-culture co-evolution, where the culture of dairying creates a strong selective advantage to those who can drink milk as adults, for only they can nutritionally benefit from the milk. A recent paper confirmed this link by analysing the diversity in bovine milk protein genes and showing that the highest gene diversity (and by implication the largest historical population size) is in cows from areas of the world where dairy farming is practised and the people are lactose tolerant.2 In humans, epidemiological analysis has shown that the cultural development of dairying preceded selection for lactase persistence.3 Since dairying is thought to have originated around 10 000 years ago, the selective pressure has been only for the past 400 generations. Despite this short time, there is suggestive evidence of recent positive selection: lactase persistence is associated with one haplotype, which is very common only in northern Europeans, and is distant from the ancestral haplotype.4, 5 Discovery of the possible molecular basis of this polymorphism - a single nucleotide change 14 kb away from the gene, has allowed further analysis of genetic variation associated with lactase persistence/nonpersistence.

Reliability
Proving that the lactase gene has been under recent positive selection in Northern Europe is difficult. As it is a recent regulatory change, codon-based methods that examine the different substitution patterns across a gene are not suitable. Instead, methods relying on allele frequency must be used - which are vulnerable to the fact that frequency patterns produced by selection can also be produced by demographic processes such as changes in population size and genetic drift. A statistic called 'relative extended haplotype homozygosity' (REHH) has been developed, which relies on the fact that a selected haplotype (ie a haplotype on which a relatively recent beneficial mutation has occurred and has risen to high frequency) will have an extended range of linkage disequilibrium (LD) compared with other haplotypes in the population.9 This is because the selected haplotype is young, and hence there has not been enough time for recombination to break it down. We infer that this young haplotype has been driven to a high frequency by positive selection. It is not an ideal method: since it relies on the length of linkage disequilibrium on one haplotype in relation to the frequency of that haplotype, it may be vulnerable to different sampling strategies that could alter the apparent frequency of that haplotype.10 Allele-specific recombination rates could also produce a similar effect. Nevertheless, since it compares variation on different haplotypes across the same region, it is less vulnerable to demographic changes than other population genetic measures.

Positive selection
REHH was used by Joel Hirschhorn's lab to provide further support for positive selection in Northern Europeans.8 It confirms that the haplotype carrying lactase persistence is almost identical for nearly 1 Mb, is therefore young and must have been positively selected to reach the observed frequency of 77% in Northern Europeans. Analysis of markers across this region showed very high genetic differentiation between European Americans (dairying) and Asian/African American (nondairying), suggesting that these markers had hitchhiked on the haplotype carrying lactase persistence. By considering the Asian Americans and African Americans to have a diversity representative of a pre-dairying 'European' population, a selection coefficient of 1.4-15% was calculated - consistent with the 5% previously predicted using a gene-culture co-evolutionary model.11 Did early farmers, who practised mixed farming, really rely on milk so much? There is now genetic evidence that they did, although it is still not clear why milk was so important (for discussion, see Hollox and Swallow12).