During the most recent ice age, milk was essentially a toxin to
adults because — unlike children — they could not produce the lactase
enzyme required to break down lactose, the main sugar in milk. But as
farming started to replace hunting and gathering in the Middle East
around 11,000 years ago, cattle herders learned how to reduce lactose in
dairy products to tolerable levels by fermenting milk to make cheese or
yogurt. Several thousand years later, a genetic mutation spread through
Europe that gave people the ability to produce lactase — and drink milk
— throughout their lives. That adaptation opened up a rich new source
of nutrition that could have sustained communities when harvests failed.
This two-step milk revolution may have been a prime factor
in allowing bands of farmers and herders from the south to sweep
through Europe and displace the hunter-gatherer cultures that had lived
there for millennia...
Young children almost universally produce lactase and can digest the
lactose in their mother's milk. But as they mature, most switch off the
lactase gene. Only 35% of the human population can digest lactose beyond
the age of about seven or eight...
Most people who retain the ability to digest milk can trace their
ancestry to Europe, where the trait seems to be linked to a single
nucleotide... They proposed that the trait of lactase persistence, dubbed the LP
allele, emerged about 7,500 years ago in the broad, fertile plains of
Hungary. Once the LP allele appeared, it offered a major selective advantage.
Lots more interesting information about this at
Nature.
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