About 35 million years ago the Earth was impacted by two large asteroids creating the more than 50 mile diameter Popigai [pop a gay i] Crater in Russia and a similar crater in Chesapeake Bay in the USA.  Analysis of fragments indicate that the impacting objects were not made of the same material. This finding has led scientists to speculate that there may be an astronomical process which changes conditions in the inner asteroid belt and causes the Earth's orbit to change slightly.  This combination could have triggered the ice age which caused the first significant ice sheet in Antarctica to form and led to the last major extinction event in the Earth's history.