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Based on various methods of geological dating, particularly radiometric analysis, Earth is estimated to be around 4.5 billions years old. This is around the time the Earth's Moon was thought to be created by the impact of Theia, a Mars sized planet, or through a series of impacts.

These impacts would have melted the crust of Earth, extinguishing any signs of previous life or civilisations.

What would be the possibility of life, or even civilisations of some sort, existing on this early Earth before the meltdown?

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    $\begingroup$ It is a cool idea. $\endgroup$ – Willk Jul 2 '18 at 11:51
  • $\begingroup$ With life, anything is possible. We only have one example to go by which obviously took longer to develop (see answers here), but if you postulate some very fast evolving form of life that works under (for us) extreme conditions, well, who can prove you wrong? Nobody. If you're smart, you won't try to work out all the details, you won't be able to construct a completely new form of life on paper - and just leave it at a description: Faster evolving life that ended up producing some kind of civlization. $\endgroup$ – Raditz_35 Jul 2 '18 at 12:46
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    $\begingroup$ While many things are possible, recall that the early Earth had a very different surface (no oceans), atmosphere (no oxygen), and was subject to rather heavy bombardment (many mass-extinction events). We have fossilized stromatolites only a couple hundred million years after initial ocean formation, so that's a pretty narrow window for a much-bigger-than-Cambrian explosion that has left no fossil evidence. $\endgroup$ – user535733 Jul 2 '18 at 13:25
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    $\begingroup$ Could down voters please provide feedback? $\endgroup$ – James Jul 2 '18 at 17:17
  • $\begingroup$ Why was my question downvoted???? $\endgroup$ – IlludiumPu36 Jul 3 '18 at 1:20
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The leading theory is that Earth had a surface with liquid water before Theia hit and possibly some very primitive single celled organisms in it, they had at most 100 million years to evolve, they didn't get far. Theia vapourised a large percentage of the rocky surface of proto-Earth, that blasted all the water that didn't turn into plasma off into space. The impact completely liquefied the rocky crust of our planet. Nothing on Earth survived that.

Any insterstellar colonists should know better than to tackle a young system still in it's heavy bombardment phase, they'd be doomed just about before they set foot on any world in the system.

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  • $\begingroup$ No the interstellar colonists could track all the planetesimals and asteroids and planets and their super computers could calculate that Earth would not have a dangerous impact for 10,000 years or 100,000 years or 1,000,000 years or whatever, and thus consider it safe to colonize. When and if calculations indicated a body too massive to divert would eventually hit Earth they would begin packing up and leaving for a safer world. When in human history have any humans ever settled someplace they knew would be safe for a hundred generations like those colonists could have calculated? $\endgroup$ – M. A. Golding Jul 2 '18 at 17:46
  • $\begingroup$ @M.A.Golding I suppose that depends on your underlying assumptions about colonisation, one of mine being that it's a one-way trip, as such you can't "pack up" you have to avoid systems with a high likelihood of local extinction. $\endgroup$ – Ash Jul 2 '18 at 17:50
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None native. Given what we know about development of life since the great impact, it would've taken way too long for intelligent life to develop on our planet back then.

If any aliens tried to colonize Earth, they must have also tracked the incoming planet long before it could take them with the whole planet and must have left. Whatever pre-existing infrastructure was vaporized by the impact

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