I remember I read or was told in a conference in a Buddhist center that the gap between Tathagata Buddhas is always six thousand years in this Eon at least, and thus between Kassapa and Sakiamuni there were 6000 years, the Dharma wheel moves for 5000, Dharma is then forgotten for 1000 and a new Buddha appears, next been Maitreya. However it was a Vajrayana school thus I guess they might have different texts than other schools.
What it’s obvious, for me at least, is that different versions of the time gap exist. But lets assume this text is correct, I think there are two options:
-Whether time in the Tushita heaven is not the same as here, and a few years here are millions of years there, thus the life between Kapassa and Sakiamuni is not that long or
-When Buddhism talks about humans is not always referring to literal homo sapiens, the term is more about “a creature that is able to understand and practice Dharma”, thus, although science have not discover it yet it might be that some life form back then was capable of doing so.
On the other hand the existence of alien life is very clearly confirm on Buddhist texts as they constantly speak of other realms and worlds that are clearly not on Earth.