#my opinion on that fungus: it was an species widespread across Eurasia and North America through Beringia that then had its habitat reduced

[elbiotipo]

I know aliens have never colonized Earth because I'm an ethnobotanist. Let me explain:

Since humans have domesticated plants, they had spread them everywhere. Maize was domesticated somewhere in the Americas (it's a big debate where and how exactly but I won't get into that) but by the time the Spanish came in 1492, it was spread from North America to Argentina. And after that, it spread all over the world. Potato spread to Europe, wheat to the Americas, and such with every cultivated plant.

Now, crops are maintained by humans, it stands to reason that if no civilization maintains them, they will be gone and perhaps go extinct from their introduced places. However, that's not the case for many, many other plants. Delonix regia, an ornamental tree we call the Chivato Colorado here, has adapted wonderfully to tropical and subtropical South America, despite being native from Madagascar. if tomorrow, humans were gone, it's believeable that chivatos would be still there and spread over the continent. Same with so many plants and animals, many of them introduced accidentally, coming in the mud of boots or water-logged ships. Surely you are aware of an amount of invasive species by current human activity.

This would even happen if aliens are not interested or unable (see below) to use earth species for agriculture. If there was a global alien civilization (not some limited enclaves), species would spread from here to there, even by accident, and we WOULD see some traces of it in the fossil record; there is no way even the most careful culture would avoid every single event of invasive species (as for alien invasive species, see below). While in fact, biogeography corresponds to continental drift (Gondwanan distribution, Eurasian distribution, etc.) and other spread such as floating rafts or island jumping. You don't see South American dinosaurs spreading to North America while it was an island continent, but crucially, also no plants spread. The spreading of plants is, in my opinion, one of the most lasting signs of our current civilization, and it will last long after we are gone.

Now, it's possible that alien cultures might have visited us in the billions of years of Earth's living history, after all, a planet with life is attractive. But unlike space strategy games, planets with alien biospheres would be "uncolonizable". Leaving aside the possibility of amnionia or silicon or such based life, even an alien species parallel to us, with DNA and water-based metabolism, would find our biosphere to be allergic or even toxic. We can't digest D-especular aminoacids or L-sugars, and in fact we only can digest a limited number of compounds. Most life on Earth is unpalatable and even downright toxic to us, actually: if it seems otherwise, it's because we domesticated and use the plants and animals that are the most palatable to us. If we found an alien planet where biochemistry is just a little different, the most likely result is that life there would be toxic, allergic, or totally incompatible to us (even alien microbes or parasites would be unable to harm us in the conventional way, after all, they are adapted to their own hosts and ecosystems). Instead of being places to colonize like in so much (manifest-destiny inspired but that's another thing) science fiction, they would be basically unliveable places. Perhaps you would have better chance to live in a planet like Mars or a self-contained space colony than to live in a world where everything is toxic. Any alien species left here (say, crops or pet animals) would starve to death by the simple fact Earth's biochemistry would be incompatible with them.

It is possible that there have been visitors to Earth in the billions of years it has been *alive* (the response is actually "how likely are complex life and civilizations to arise" which nobody has the answer to) But if they were, they probably were explorers, probably completely automated ones, not settlers, that left no trace in the fossil record. Perhaps somewhere there's an Animal Planet documentary about Earth, who knows... But... If Earth was settled long ago by an ancient alien civilization, we would see traces of it everywhere, and I'm sure whatever comes after us will see our own traces. But we haven't seen a single biogeographical trace for it.

Except for that fungus that grows in Texas and Japan. Maybe aliens really liked that one.