Comment by adrian_b

If life had appeared before the Earth-Theia collision, it could have been terminated by any of the earlier impacts.

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.