According a recently published paper by researchers Abraham Loeb (Harvard), Rafael A. Batista (Oxford), and David Sloan (Oxford), we may be amongst the first percentile of life in the universe. Here is the link to the paper: http://arxiv.org/pdf/1606.08448v2.pdf
I find this paper’s conclusions very fascinating. Keeping in mind that the universe is estimated to be around 13.7 billion years old and our planet about 4.5 billion years old (so about a third the age of the total universe). That would imply that there could have been life perhaps billions of years older than we are. Yet this paper suggests otherwise.
The very first stars that would have formed in the universe would have been giant Population III stars, that were at, or exceeded the Eddington Limit. Beyond the Eddington limit, stars eject their mass due to the outward pressure of radiation. Their lives would have been exceedingly short and they would not have been able to have formed much in the way of life. They formed in the unique, metal poor environment in the aftermath of the big bang. Their remains would have set the precedence for smaller stars, including the one that became our Sun.
The paper explores the idea that the age of the universe is such that the age of the universe is such that only now do the more moderately-sized stars (which have exponentially greater lifespans) begin to form. It may very well be that our sun is amongst the upper limits of stars with habitable life. Consider that it took around 4 billion years from the time our planet formed to the Cambrian Explosion that led to the formation of complex life and it easy to understand why it is probable that a star substantially heavier would not be able to form any complex life forms. Perhaps it is possible for other planets to form a Cambrian Explosion faster? It has been hypothesized before that the Cambrian Explosion was caused by the availability of free oxygen in the atmosphere. Oxygen is a very reactive element, and it took billions of years of photosynthesis from simpler lifeforms to reach the levels that they were before the Cambrian explosion.
Could that process have speed up? Unfortunately we have no way to know. We only know the history of our own planet, not life on other planets, and even then, our understanding of the evolution of complex life is far from complete. We can only guess what percentage of planets even give rise to simple life forms and of those, what percentage give rise to complex life. It has been suggested before that simple life (like prokaryotic life on earth) may be common in the universe, but that more complex life, (like eukaryotic life) may be far more rare. I would hesitate to guess that some planets will be faster at reaching their equal to a Cambrian explosion and that some will be slower.
Yet another possibility is that the process took even longer. Researchers Alexei A. Sharov and Richard Gordon have proposed the idea that life may have formed before earth. They believe that life on earth formed from less complex organisms that may have evolved from a previous star and that over time, life has become exponentially more complex. They base their hypothesis on an extrapolation of the complexity of life. Here is their paper. In that case, we are a continuation of a process that began billions of years before our sun, assuming of course their hypothesis is correct.
What does this all mean?
I have often wondered if the Great Filter is before us or behind us. The Great Filter was the original explanation to the question raised by Enrico Fermi and his famous question “where are they” that led to what became know as the Fermi Paradox. If there are so many stars in our universe, where are the aliens? Why do we not see evidence of complex life throughout the universe that we can detect?
This is, in my opinion, the most important question that we humans must answer. If the Great Filter is ahead of us, then we must do everything possible to minimize the risks to our own destruction, because the fate of intelligent life is to destroy itself. If even 1% of all life has formed, that would imply to me that a great many intelligent species have come and gone in the Milky Way. That we do not see other evidence of extremely advanced species is troubling. It only takes one after all to colonize the entire Milky Way. So far as we have discovered more and more extrasolar planets, it would suggest that number of planets at least is not the bottleneck to intelligent life spreading.
Carl Sagan famously speculated that it was the fate of species to destroy themselves. If so, this has dire implications about the future. We would be well advised as a species to carefully address our challenges and to minimize our risks of self-annihilation.
I will end this on an optimistic note. I have often expressed my admiration for Carl Sagan and I consider him a true visionary. Society needs more such visionaries. I hope that we as a species can learn from our own mistakes and that we can advance someday to the next level as a species.