New mass extinction event identified by geologists

Now consider that the solar system is 4.6 billion years old, and shortly thereafter were the terrestrial planets. That means there were 18 chances for a mass extinction like this. Now granted many of these planet embryos have fused into the current planets or turned into asteroids and meteors, but what we're left with is the possibility of the rise and fall of intelligent life in our own solar system on different planets (or our own) without a trace.

Fascinating and frightening.