this seems to scale. humans didn’t use to travel more than 30 km from their place of birth in historical times.

When Combs looked closer, distinct rat subpopulations emerged. Manhattan has 2 genetically distinguishable groups of rats: the uptown rats and the downtown rats, separated by the geographic barrier that is midtown. It’s not that midtown is rat-free—such a notion is inconceivable—but the commercial district lacks the household trash (aka food) and backyards (aka shelter) that rats like. Since rats tend to move only a few blocks in their lifetimes, the uptown rats and downtown rats don’t mix much.

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