It started 160 million years ago

At least, it now seems to have according to a paper in the latest Nature covered on NPR yesterday as: Dinosaur-Killing Asteroid Traced to Breakup Event. According to a new analysis by asteroid researchers Bill Bottke and colleagues, it appears that that fateful meteor that struck the earth 65 million years ago resulting in the extinction of dinosaurs originated as a fragment of a larger (170 Km wide) asteroid that broke up during a collision in the asteroid belt 160 million years ago. That collision produced a rain of debris much of which went on to produce a cluster of pockmarks on our moon (at a rate above the background average, which is what led these researchers to analyze it more closely) and one large chunk which took out the dinosaurs, cleaning the vertebrate evolutionary slate up a little bit for the eventual emergence of us! Yay!!

Here's the tantalizing tidbit tracing the chain of events from the abstract of the Nature paper:

Fragments produced by the collision were slowly delivered by dynamical processes to orbits where they could strike the terrestrial planets. We find that this asteroid shower is the most likely source (>90 per cent probability) of the Chicxulub impactor that produced the Cretaceous/Tertiary (K/T) mass extinction event 65 Myr ago.

That Nature link above gives you the abstract, with the rest hidden behind a pay firewall, of course (and we don't yet have Nature available online here at CSU-Fresno!). I don't understand why Nature doesn't make their editorials and News & Views features more available, but there is a brief editorial summary of this paper here. And Bottke's website may have the reprint up at some point, but meanwhile you can start by listening to that NPR story.