Global warming will drive humans toward the poles. Their movement will be hampered by continual war. Many will perish while migrating, but their destinations will not be safe for long. The upcoming glaciation will drive the surviving humans toward the equator. Once again, their movement will be hampered by continual war. Many more will perish while migrating, but their destinations will be welcoming for the duration of the glaciation. The same holds for many other species of both animals and plants. Few and far between, the surviving humans will dedicate themselves to hunting and gathering close to the equator. All so-called races will become indistinguishable over many generations. The new interglacial period will lead to new migrations toward the poles, as well as to the gradual growth of the human population. The second chance will offer itself at long last. The only question is how many chances will humans get in the fullness of time? For the planet cannot bear too many false starts.

Addendum (November 28, 2016)

No matter how many chances humans get, the resources available on planet earth will be dwindling from chance to chance. And fast. With the possible exemption of wood to burn, as well as rivers and winds to churn the mills, everything else will be available in ever-smaller quantities. This applies primarily to coal and oil, the most important resources the last few centuries. It applies to uranium, as well. In the fullness of time, humans cannot possibly get more than a few chances—say, three or four at most. After that, the return to the apes will be the only option available. Looking back, this may well be the only meaningful option for the human species as long as the sun and the planets orbiting it keep spinning. And it is not such a bad option, after all. In the fullness of time, some apes may even boast that they have come from humans. Slightly different from their relatives, they will be proud of their origins. By then, humans will be remembered as gods of yesteryear. Many a myth told around the fire will entertain their progeny for eons.