In the conversations I’ve heard about potential-future-human-extinction, people usually look around the universe and say “well, if it’s so easy to survive, where is everyone?” (a slight variation on the Fermi paradox, which asks “if life is so common where is everyone?”) They assume life is easy enough to start, but difficult to sustain. They sketch a scenario where life, once started, reaches some stage like our own and then kills itself. The whole ‘we’ll destroy ourselves’ answer to human extinction.
But there’s a new theory published (or at least a new discussion of an older theory) of what is called a “Gaian bottleneck.” I’ll link you to the phys.org page that gives a light summary of it; a link to the journal article is found at the end, though I haven’t had time myself to get very far into the actual article at all.
Basically, the theory suggests that life on earth has already passed the most difficult part of its evolution:
”‘Most early planetary environments are unstable. To produce a habitable planet, life forms need to regulate greenhouse gases such as water and carbon dioxide to keep surface temperatures stable.’
“About four billion years ago Earth, Venus and Mars may have all been habitable. However, a billion years or so after formation, Venus turned into a hothouse and Mars froze into an icebox.” (While life on Earth successfully stablized Earth’s atmosphere.)
So it’s not a direct answer to your question, just a vague optimism. Life made Earth habitable back when life was beginning, and we’ve just got to maintain it. I think that’s a pretty cool idea.
No natural disaster to this point in Earth’s history has wiped out all existing life. Since humans are found basically anywhere there is land, I think it would take quite a natural disaster to wipe us all out entirely, one like Earth hasn’t yet seen.
That said, we will eventually lose the oxygen in Eath’s atmosphere (at least, so said some end-of-earth nat geo show I saw a while back). And some billion years later the sun, as it dies, will disintegrate Earth back into smaller components. So it goes back to the whole ‘we can only live as long as the Earth does, unless we go elsewhere.’
I had an anthro professor who suggested that when you look at how poorly our genus has survived (we’re the last of it), it’s only a matter of time until we’re gone.
And (if I remember my earth-creation-shows well enough) the bacteria that originally produced the atmosphere the rest of us enjoy suffocated itself in its own byproducts. In a way, then, the theory of life-destroys-itself exists simultaneously with the theory of life-sustains-itself. So really, who knows?
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