 We have had six extinctions in the billion-year history of life on our planet. Each sharp spike in the chart below indicates one of these mass extinctions. There's a chart on the page. Occurring about 450 million years ago, the Ordovician slash Silurian mass extinction devastated marine life, which at the time dominated the planet. In a series of two extinctions, 60 and 70 percent of all life on the planet was taken respectively.