![Peter Ward: A theory of Earth's mass extinctions](https://pi.tedcdn.com/r/pe.tedcdn.com/images/ted/67420_800x600.jpg?h=200)
Asteroid strikes get all the coverage, but "Medea Hypothesis" author Peter Ward argues that most of Earth's mass extinctions were caused by lowly bacteria. The culprit, a poison called hydrogen sulfide, may have an interesting application in medicine.