Not much to start life with.

I suspect this is not actually that common. Giant impacts are more common in early solar systems; things eventually settle into nice circular orbits like we have now. Whereas intelligent life does seem to take a while to evolve, so probably more common later in a solar system's life cycle.

https://en.wikipedia.org/wiki/Theia_(hypothetical_planet)#Co...

Then again, how well do we know of stuff in these spaces today? It seems to me we barely have a clue of the space junk we ourselves sent up orbiting in our backyard.

And is not even that the impact erased their trace from earth, is that if they got extinct long enough (more than 100 million years ago), with today's technology, we cannot infer their presence, given the fossil (and other types of) record.

A great write up about this is: https://archive.is/https://pacificklaus.com/the-silurian-hyp...

They'd have had to be watching from one very high orbit, and even then I wouldn't bet favorably on their chances considering the sheer gargantuan volume of debris Theia tossed into the space above earth.

Anywhere terrestrial and they'd be dead far too quickly to watch much of anything. I've seen some fairly detailed models on the presumed effects of this collision and it would have rapidly super-heated the whole Earth's atmosphere while vaporizing or melting the crust of the entire world down to a depth of at least several hundred meters. Good luck finding a bunker that can handle that.

He makes no claim to what life was before the formation of the moon, but rather than the cataclysm of formation of the moon is what changed life on Earth - of course he wraps the whole idea up in his mysticism - but his 1950s writing on this was not far off what happened.

There's good reason why this first era of Earth's history is called the "Hadean eon" - as in the fires of hell.

> Throughout part of the eon, impacts from extraterrestrial bodies released enormous amounts of heat that likely prevented much of the rock from solidifying at the surface. As such, the name of the interval is a reference to Hades, a Greek translation of the Hebrew word for hell.

https://en.wikipedia.org/wiki/Hadean

https://www.britannica.com/science/Hadean-Eon

The Earth-Theia collision has been only the last of a great number of collisions.

The original Earth had been much smaller and it had grown by colliding with many similar bodies.

The reason why the last collision is likely to have been also the greatest is that both Earth and Theia had grown previously through many collisions.

This has been similar to the "Highlander" movies, where each victorious immortal took the force of the decapitated one, until only 2 remained for the final confrontation, which were stronger than any of the previous immortals, because these 2 had gathered the forces of all the others.

In the same way the planets close to Earth have grown after each collision, at the expense of the smaller impactor, so the impacts have been between bigger and bigger planets, until the final collision between Earth and Theia, which both were bigger than the participants to earlier impacts from their history.

Before this final impact, the time intervals between 2 successive big impacts might have been too short for life to appear.

It turns out that what I was missing is that an object on the surface of the moon is much closer to the center of the moon than the same object would be on earth, because the radius of the moon is much less than the radius of the earth, and gravity is inversely proportional to the square of the distance between the two objects.

mass_ratio / radius_ratio^2 = gravity_ratio

0.0123 / 0.273^2 = 0.17

https://www.wolframalpha.com/input?i=mass+of+moon+%2F+mass+o...

https://www.wolframalpha.com/input?i=gravity+on+moon+%2F+gra...

https://en.wikipedia.org/wiki/Gravity#Characterization

https://www.wolframalpha.com/input?i=radius+of+the+moon+%2F+...

That mass has come from bodies which have collided with Earth and which had a chemical composition similar to that of "carbonaceous chondrites". (That is a kind of meteorites; most of the Earth had a composition similar to "enstatite chondrites", another kind of meteorites.)

The carbonaceous chondrites are not made of carbon, they only have more carbon than the Earth and similar planets, because in the early Solar System the carbon was present mostly as carbon dioxide, which is volatile so that most of it does not condense into the planets at the higher temperatures of the zones closer to the Sun, where the inner planets have condensed, but only at lower temperatures, farther from the Sun, where it reacts with metallic oxides, forming solid carbonates, like limestone.