I was taking a quiz where it mentioned that the Earth goes into an extinction period every 26 million years. The problem I had with it was it was written factually (granted, it was a quiz answer/question), and, though I've heard of the idea, that's what I always thought it was: an idea, or rather an hypothesis. So, I went to http://scientificamerican.com
to see if they had anything on it, and they did, and they stated it was a hypothesis, I believe. The article said that there was no evidence therefore the whole idea is in limbo.
Then I thought if the Earth is estimated at 4.5 billion years, is it possible--by rate of decay, and other factors--that a similar humanoid (not necessarily "human") species could have existed? Not stating to the general degree or level in which we are at but just a generalisation of one?
Which, in turn, had me think that a few more extinctions (or more) could totally wipe out humans ever being here. I mean, the Earth has, I believe, about 2 billion more years, or something...and I once saw a documentary a few years back on how if no one's here to attend all man-made structures that eventually they will decay away, in time. Leaving little to absolutely no trace we were ever here (except perhaps the preservations of our fossils).
Likewise, in some future arena, could not another similar species exist?
Just late nite thoughts.