James Castle and John Rodgers of Clemson University put the blame on microscopic algae, which produce toxins and consume oxygen. In their work, presented at the annual meeting of the Geological Society of America in Portland, the scientists reported their findings of spikes in the number of algae fossils – stromatolite structures – coinciding with mass extinctions of the past.

Events like cosmic impacts may have not directly killed plants and animals, the theory goes, but the nutrient-rich fallout would trigger the rapid growth of algae. Their population exploded, and they released large amounts of toxins which other species had no way to deal with.

Castle warns that a similar scenario could repeat:

"Algae growth is favored by warmer temperatures. You get accelerated metabolism and reproduction of these organisms, and the effect appears to be enhanced for species of toxin-producing cyanobacteria."

Presently the population of toxic algae is slowly increasing and expanding. It already poses a danger to fish and other water wildlife, and can be dangerous for humans as the algae invade reservoirs of drinking water.