In ancient times, Aboriginal people in southwestern Australia quarried fine grained rock on an ancient hill called Barlangi Rock to make very sharp tools. Dr. Timmons Erickson, a NASA scientist at the Johnson Space Flight Center, led a team which discovered that the rocks the Aborigines used to make tools were formed by the impact which produced the 46 mile diameter Yarrabubba Crater. Over the course of history asteroid impacts have caused mass extinctions of some species while improving the chances of other living forms like mammals to flourish. It is sobering to realize that human caused global warming is causing more extinctions than any asteroid in history.