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Studying recent human … (1)… can be frustrating.
Take lactose tolerance—the ability of some people, particularly of European
…(2)…, to digest this sugar, found in milk, even into adulthood. In the
evolutionary past the …(3)… for lactase, the enzyme that does the digesting,
normally got switched off in adults. But one variant (known as an allele) of
the gene that does the …(4)… instead leaves the system running, …(5)… adult
lactose tolerance. That lets this allele’s bearers consume a
diet rich in dairy products, a useful trick for a species beginning to …(6)…
milk-producing animals. Genetic …(7)… shows that lactose tolerance emerged
between 5,000 and 10,000 years ago. In evolutionary terms, it is …(8)… extremely
recent. In human terms, though, that …(9)… means how it spread and how this
process …(10)… with the rise of dairy farming are lost in the mists of
prehistory.
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