Transcript: Impacts from space have been implicated in several of the mass extinctions in the last half billion years. However, they are not the only possible cause. Geological change can profoundly affect life, particularly during periods of intense volcanism. Also, climate fluctuations can impact the survival of life, especially during times when the temperature non-linearly oscillated and led to extremes of heat and cold. There were times, for example, when the Earth was probably frozen down to its equator, or impacts could be responsible. On average, once every million years a one kilometer size impactor hits the Earth, devastating a continent sized region and causing global climate change. Once every hundred million years, a ten kilometer sized impactor can cause a mass extinction and a global catastrophe. How can we tell? The two key pieces of evidence are that the extinction occurs in an instantaneous interval as well as can be measured by geological techniques, but the fossil record is only accurate to about a hundred thousand years which is not the same as saying that the event was instantaneous. The second critical piece of evidence is a smoking gun, evidence of a crater of appropriate size that’s dated back to the same time as the extinction occurred.