To realize that all of Europe once looked like this is startling. Most unexpected of all is the sight of native bison. Just 600 remain in the wild, on both sides of an impassable iron curtain erected by the Soviets in 1980 along the border to thwart escapees to Poland’s renegade Solidarity movement. Although wolves dig under it, and roe deer are believed to leap over it, the herd of the largest of Europe’s mammals remains divided, and thus its gene pool. Belarus, which has not removed its statues of Lenin, has no specific plans to dismantle the fence. Unless it does, the bison may suffer genetic degradation, leaving them vulnerable to a disease that would wipe them out.
“If the whole world were founded by two people, you would have to get lucky in the genetic lottery many times,” Wall says, and the odds for that lottery are not good. A sudden shift in the environment could snuff out the two human survivors before a single child was born. Even if they avoid external conflict, their genes may combine in a way that passes down a congenital heart condition to their children, which, without medical care, would probably be the end for all of them. These genetic constraints are so extreme that Wall doubts humanity would survive more than a couple generations.