This video says about itself:
30 May 2013
Few people have heard of the Ordovician Period, but it was one of the most important periods in Earth’s history. Many familiar sea creatures evolved, and life took first steps onto land.
From Science News:
Asteroid barrage, ancient marine life boom not linked
New dating debunks idea that bombardment created eco-niches needed to diversify
By Thomas Sumner
11:00am, January 24, 2017
An asteroid bombardment that some say triggered an explosion of marine animal diversity around 471 million years ago actually had nothing to do with it.
Precisely dating meteorites from the salvo, researchers found that the space rock barrage began at least 2 million years after the start of the Great Ordovician Biodiversification Event. So the two phenomena are unrelated, the researchers conclude January 24 in Nature Communications.
Some scientists had previously proposed a causal link between the two events: Raining debris from an asteroid breakup (SN: 7/23/16, p. 4) drove evolution by upsetting ecosystems and opening new ecological niches. The relative timing of the impacts and biodiversification was uncertain, though.
Geologist Anders Lindskog of Lund University in Sweden and colleagues examined 17 crystals buried alongside meteorite fragments. Gradual radioactive decay of uranium atoms inside the crystals allowed the researchers to accurately date the sediment layer to around 467.5 million years ago. Based in part on this age, the researchers estimate that the asteroid breakup took place around 468 million years ago. That’s well after fossil evidence suggests that the diversification event kicked off.
Other forces such as climate change and shifting continents instead promoted biodiversity, the researchers propose.