Mustrum_Ridcully
Legend
Tell that to the Native Americans, who were decimated by smallpox from Europe not just before airplanes, but before the invention of the internal combustion engine.
And what I read on the topic suggests that we find the greatest genetic diversity on the African continent, and losing the American and European continent wouldn't affect genetic diversity much.
The way smallpox was differently effective across the world is tied to this, or at least that's what I read.
And the Native Americans are still just a part of humanity, they are not its own species.
Except that those humans had a long tradition of functioning in a world of that nature. We do not. Er, well most of us. Even someone like myself who does have quite a few skills in that arena wouldn't last because of a lack of modern devices. Once my contacts ran out and my glasses were broken all of my hunting skills become moot. Like it or not modern convenience and, especially, modern medicine, have removed quite a bit of humanity's survival ability.
Unless someone suddenly takes away all technology we have in an instant, this won't be a problem. You'll have an acclimiation phase that won't be nice.
And not to forget under what kind of conditions a vast majority of humanity actually lives in China and India. So what, I as pampered European citizen might not be able to survive the hardships of not being able to go online to order a video game on Amazon and drive the car to the nearest convenience store (because neither do I have fuel for my car, nor does the convenience store still get deliveries, nor could Amazon still get deliveries to me, and I might not even get the electricity to run my computer...), but some peasant in China won't notice such a big difference.