The fossil record. There is little evidence of very complex animal life before about 541 million years ago, which is when the Cambrian explosion begun.
Before "modern" life evolved in the pre-cambrian era, there was the Ediacaran life forms that were complex multicellular life, but died out millions of years before the Cambrian explosion.
There's quite a few fossils showing single-celled organisms from 3+ billion years ago. I imagine if more complex life existed the fossils could've survived
Its possible there is a good explanation for why there would be no strong fossil record[0] for an advanced civilization preceding us.
>When it comes to direct evidence of an industrial civilization—things like cities, factories, and roads—the geologic record doesn’t go back past what’s called the Quaternary period 2.6 million years ago. For example, the oldest large-scale stretch of ancient surface lies in the Negev Desert. It’s “just” 1.8 million years old—older surfaces are mostly visible in cross section via something like a cliff face or rock cuts.
While I think its highly unlikely (I mean less than 0.00001% possible) the means in which we would could even detect it are complicated
> it’s based on last universal common ancestor estimates(LUCA) and supported by (lack of) fossil record.
I thought the Cambrian Explosion's fossil record was pretty sizeable - in fact, it's named after the place where the fossil layer was first discovered. I didn't know it was related to a common ancestor. Are you thinking of something else or am I missing something major?
I think we can see where different complex animals split in their evolutionary tree - so humans and starfish split a long way back. Then we use a standard mutation clock to estimate how long ago that was. If all complex animals split from simple animals an estimated 700m years ago...