| HigHurtenflurst said: Well I got started on a little bit of it, but a lot of it comes down to time constraints... As I (and some others) have mentioned, life on this planet has spent more time in unicellular form alone than not, and the good conditions for complex life such as ourselves will degrade in another billion(ish) years. (more simple life can probably last another 2-3 billion before the effects of the dying Sun render this planet uninhabitable) So on Earth simple life has so far been around ~4 billion years, and could last another 3 billion. Complex life has existed for less than a billion years, and probably doesn't have much more than a billion years left (though some forms may be able to continue existence in deep oceans). Sol isn't a particularly special star, stars with more mass than the Sun go through life quicker and would allow even less time for life to evolve past the simplest stages... and stars with significantly more mass might even be too violent to form a decent planetary system in the first place. There are so many factors to take into account to allow for life (at least life as we know it)... for life in general lots of planets will go through stages of being adequate for simple life to form, but the universe is ever changing, and the time most planets will spend in even an "adequate" (rather than ideal) stage is limited. More complex life needs more time and more ideal circumstances than most planets will be able to give. |
That's what we assume from our experience. Maybe in other systems it doesn't need this much time to evolve more complex life. Perhaps because Earth is so stable and protected it got stuck in a local maximum of boring bacterial life for billions of years. We could simply be in a boring low density part of the galaxy where no one would expect complex life to show up, hence no one is checking us out. As far as we've come scientifically, the feeling that we must be special is still tainting our conclusions.
We can only start to make generalizations about how life develops after we have examined more star systems, a sample size of 1 is pretty poor. All we know for now is that life manages to survive everywhere on the planet where we least expect it and organic material can hitch a ride on asteroids between planets. At least simple life should be very common in the universe.
