“There’s a reason why we have laws against incest,” says Jeff Wall, assistant professor of epidemiology and biostatistics at University of California, San Francisco. Every time we have a kid, says Wall, there’s a chance things can genetically go wrong. That chance increases substantially if there are fewer genetically diverse individuals to mate with.

“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.

Still, it’s worth try. Our world deserves to end in the same way it started—with a bang.