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EDIT: As many have pointed out, the whole Lunar Collision aspect I originally envisioned is probably more extreme than I predicted. I don't know where to start as far as the math goes, although my gut instinct says that a series of smaller collisions could make it work, but I could take or leave the Moon getting enough mass to be habitable. That's just a thought I had mostly for spectacle. The important part is that in modern times, a K-Pg level impact or series of impacts occurs, so what organisms are likely to survive that?

Original Main Body:

An origin I'm considering for the Science Fantasy world I'm building is that in the not too distant future, next Sunday A.D., a foreign celestial body or cluster of asteroids/comets joins Earth's orbit, then collides and merges into the Moon, adding enough mass to it that it eventually gets oceans and an atmosphere, and spreading debris around that peppers the Earth with meteorites for the eons to follow. The first wave of meteor crashes causes a mass extinction event comparable to the K-Pg impact.

So here's my question: What notable modern organisms will have died out or diversified after a new K-Pg type event, which also includes the moon's mass drastically increasing?

I of course searched for more generally related threads before asking. I'm not talking about:

Would animals really mutate in the post-apocalypse? This thread is just talking about radiation-induced mutation.

What is the next dominant species? This is asking what non-primates might become like humans, not quite what I'm looking for.

What aquatic creatures would survive a large-end mass extinction? This is too specific to the ocean, and also assumes that no life larger than an insect survives the impact. My apocalypse isn't quite that extreme.

In my scenario, humans just barely survive, evolving into at least 8 species and reverting from modern/futuristic to stone-age tech until about 10,000 years before the actual story begins, and they make some pretty drastic adaptations to make it through, even with their intelligence. So with that as a benchmark, what other life forms will make it, and which ones won't?

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    $\begingroup$ an impactor large enough to add that much mass to the moon will blow a huge amount of material out, the moon will start over as a molten ball or magma. The amount of material raining down on earth will make the dinosaur killer look like a spit ball. the question is not what life survives on earth the question is IF life survives on earth. $\endgroup$ – John Sep 6 at 18:32
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    $\begingroup$ I mean, some bacteria will almost definitely survive somewhere, but I'd be very surprised if anything multi-cellular did. $\endgroup$ – Gryphon Sep 6 at 18:36
  • $\begingroup$ @John I think its not so much an asteroid colliding with the moon, but more of another moon-sized planetoid being gravitationally captured by the earth, and then the moon colliding with that. What's left won't really be the moon anymore. $\endgroup$ – Starfish Prime Sep 6 at 18:57
  • $\begingroup$ You’re pretty much rewriting Seveneves by Neal Stephenson, and should certainly read it before going any further with your own story. $\endgroup$ – Mike Scott Sep 7 at 9:20
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    $\begingroup$ @Gryphon What makes you so sure bacteria would survive? The amount of energy we're talking about here is enough that I would be surprised if less than half of the Earth's crust was turned molten. $\endgroup$ – forest Sep 7 at 11:25
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Since you mentioned K-Pg: All animals above a certain (small) weight went extinct. They could not hide fast enough nor could meet their food demands after the event.

But your event will be much worse then K-Pg was, I don't think anything above bacteria, some tough bugs like the tardigrades or fungi will survive. It won't be a single rock, but thousands of huge, Sudbury Basin level impactors.

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With the amount of energy in a collision required to drastically increase the mass of the moon, the amount of matter raining down on Earth would be enormous. A significant portion of the Earth's crust would become molten (bye bye Oceans). We say life always finds a way, but there are no reservoirs of life in the mantle. The chances are that no life would survive, unicellular or otherwise.

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