Astrophysicists estimate that the prevalence and strength of events known as Gamma Ray Bursts (GRBs) throughout the Universe may have prevented life (as we know it) from forming until about 5 billion years ago. Furthermore, they point out the likelihood (~60% chance) that Earth was assailed by a GRB within the past billion years, meaning one of the great extinction events in history may have been a direct result of the GRB. Upon hitting Earth, a GRB would have ionized the ozone layer, leaving the surface susceptible to life-destroying UV radiation.
Finally, the authors note that Earth's location on the outskirts of the Milky Way is unsurprising given that higher probabilities of GRB occurrence exist in the internal regions of galaxies.
So, life may be even rarer and more precious than we thought!
In earlier epochs, when galaxies were smaller, the low-density outskirts were not far enough away from the center to provide much protection from GRBs, Piran and Jimenez say. This problem, combined with the higher frequency of GRBs in the past, led them to speculate that life would have had trouble getting started in the Universe until 5 billion years ago.