In the 1970s, there were popular published books selling the proposition that aliens visited Earth and taught humans how to build cities and monuments and things. The idea was that mankind couldn't have developed civilization on our own, so we had to have extraterrestrial help. Those theories were soundly discredited over time. But Kurzgesagt looks at the idea from a different direction. All kinds of creatures and civilizations, including those from another world, could have come and gone and we just don't know about them because they existed too long ago, or maybe their artifacts did not remain to be found, or are something we wouldn't even recognize. Of course, it's all just conjecture. This video explains how the vast timeline of the Earth makes it difficult to know what may have happened before humans came along, and how events beyond our knowledge can slip by us. It's an exercise in thinking about how little we actually know about the past. A couple hundred million years from now, another civilization may arise that knows nothing at all about us. Oh also, the last minute of this video is an ad. #Kurzgesagt #earthhistory #alien #civilization