Matt Kaplan for the National Geographic:

As in cities today, the earliest towns helped expose their inhabitants to inordinate opportunities for infection—and today their descendants are stronger for it, a new study says.

In areas of ancient urbanization, it turned out, “we found very high frequency” for the TB-resistance gene, study co-author Thomas said. But, for example, “the Saami people from northern Scandinavia and the Malawi people from Africa, who have little history of urban living, did not have this frequency. (See our interactive Genographic atlas of ancient human migrations.)

Squint your way through it, but herein lies another explanation of ‘how the natives got smashed’ (see also Crosby, Diamond).

That link to the ‘interactive genographic atlas of ancient human migrations’ is worth a click too.