@Evelyn_475 Indeed we do not have a clue what came before the Big Bang. It is possible that the “stuff” of the Universe has existed eternally, and that it keeps cycling through Big Bangs and Big Crunches or that constant Expansion somehow triggers the next singularity. But we do know what conditions were like immediately after the Big Bang. We can use general relativity to project backward toward the singularity. General Relativity only fails us when we try to enter the singularioty. None of the know laws of physics apply there.
From projecting back, we know that the conditions immediately after the Big Bang .would make it difficult for even the most robust life forms to survive. The Universe was filled homogeneously and isotropically with an incredibly high energy density, temperatures 100,000 times as hot as our Sun, and enormous pressures. This homogeneous soup was very rapidly expanding and cooling. Approximately 10^−37 seconds into the expansion, a phase transition caused a cosmic inflation, during which the Universe grew exponentially. The Universe at this point had no elemental atoms. It consisted of quark-gluon plasma and the other indivisible elemental particles. It is very hard to conceive of a life form that could thrive in that environment and also adapt easily to the conditions on Earth.
@Evelyn_475 & @Odysseus Scientific theories are “just” theories, all right. But wild, unfounded speculation is wild, unfounded speculation as well. I’ll take my chances with theories, which fit large collections of observed data, accurately explain the observations and are falsifiable by predicting other observations which then turn out to exactly match the prediction—and have gone through rigorous peer review to verify that all that is the case.
That life evolved on Earth and humans evolved from that life is a theory. It has passed all the aforementioned tests. Aliens did it is wild, unfounded speculation. So far, it has none of the weight of evidence required to establish a postulate as a theory.
It’s great fun to speculate about over beers. But I wouldn’t base a cosmology on it.