The expression global extinction frequently conjures images of all life being snuffed out by an act of celestial hubris.  Asteroids, bolides or comets can be lobbed our way at whim.  Earth, in its first billion years or so was probably hammered by extra-terrestrial bits of rock and ice.  The cratered surface of our moon attests to this.  Some argue that it was these early collisions that provided at least some of our water and possibly even the organic compounds that eventually gave rise to life itself. Continue reading →