Yeah I heard on a podcast with a biologist that it only happened once and was a mystery. I'm glad to hear that's probably wrong as this is one of the bottlenecks suggested in limiting the possibility of alien life.
We can ballpark most of the parameters for the possibility of intelligent life capable of being detected at interstellar distances--and they add up to why haven't we already seen it?!?!
Three parameters remain which in combination must add up to billions to one against it. The development of multicellular life, the development of intelligence and the lifespan of that intelligence.
By observation the first two took a considerable amount of time, between them using up 99% of the clock. (Earth's environment will not remain suitable for the development of a species like ours for more than about another 50 million years. The sun is slowly warming, Earth's ability to compensate will peg at that point and Earth's temperature will start rising--favoring things which can evolve faster.) This research suggests that multicellular life isn't as high a barrier as it looked like.
Thus the odds that the limiting factor is behind us go down. That means the odds that it's in front of us go up. And if it's in front of us it's not far in front of us.
