What if Earth was a lifeless planet when humans arrived?

What would be the most important historical events the aliens would have to study? What would be the impact on Alien history? How would humanity’s presence be remembered in galactic history? What theories would they develop about how we came to be and what did we do with ourselves?<br><br>Let’s pretend there was no human or animal presence on Earth prior to the arrival of humans. Everything necessary for life: atmosphere, oceans, nutrient-rich soil, luckily arrived with the humans. Somehow we just perfectly functioned, unaware of our condition as we settled into our new home. One theory, called the 'artificial ecosystem theory' or AET, is that humans are not actually native to Earth but were put here and given everything they needed to succeed at some point in the distant past. An alternate perspective, the 'survival theory' proposes that early humans survived an event that destroyed the planet's ecosystem, and that the reason we find the planet so hospitable is that the ecosystem we see today is not natural but was nurtured by our ancestors.