Why do people keep reposting this article. I must have seen it 10 times since yesterday.

The podcast was Unexplainable. Great show.

Unexplainable - Lost Worlds: Aliens from earth?

Maybe it was 400 million years. But even that is a pretty short time scale in universe terms.

I listened to a podcast the other day that suggested there could have been many intelligent forms on earth before us.

We'd only have to be gone for a few hundred thousand years for there to be no trace of us left and the next intelligent life be none the wiser to our existence previously.