Most alien civilizations probably destroy themselves. The Great Filter is probably technology combined with aggression and greed. It’s likely we will also destroy ourselves before we reach evolutionary maturity. Uniting humans as a species may not be possible without some clear and present immediate existential danger. There are too many different religions, governments, political parties, cultural and societal barriers which prevent unification. Humans are not equally educated and hold numerous belief systems which are contrary to species level unification. As long as political parties, money, and theistic religions exist, humans will likely never unite as one species. Nuclear weapons can make the planet uninhabitable for a very long time. It won’t kill off all humans in the initial war, but the prolonged global after effects of nuclear war will make the earth an apocalyptic wasteland where no food will grow, water and air will be contaminated, and those that do survive the war will die of disease, starvation, radiation poisoning, natural disaster, etc. Humans could become extinct. Some might live underground, but they’ll be forced to adapt and advancement will slow to a crawl if it exists at all. Survival will be the main priority. We’ll have created a Nuclear Dark Age. It’s not likely the survivors will recover even after many generations. And those that do may have genetic problems caused by radiation. They could be sterile, or birth defects could run rampant throughout the remaining populations. Populations will exist in pockets scattered about the world. At least for a little while…the earth will turn into a snowball as nuclear winter sets in for hundreds of years, if not thousands. The earth itself will recover in a few hundred thousand/million years. But humans will likely go extinct. Unless we colonize other planets and moons in our solar system, humans will not survive. And this is likely what happens throughout the galaxy and the universe at large. As alien civilizations gain technology capable of destroying themselves, cultural, political, economic, and religious divisions occur, conflict happens, and they wipe themselves out. Probably not all, but likely most. We’re not special. We’re a young species, a young civilization. We’re cosmic children playing with planet sized guns. Ignorant, irresponsible, immature, impulsive, aggressive, and greedy. And that will likely lead to our extinction.