Someone asked my honest, somewhat informed opinion on the topic of whether there’s life anywhere else in the Universe, other than on this little blue dot. Although it wasn’t a question specifically directed at me as the owner of this blog, it seems like this is a good platform to address the issue. So… do I?
I don’t really know. There are lots of scientists who have calculated odds that there’s life elsewhere, and we know all kinds of things about our kind of life that might give an idea of how easy or hard it is for life to arise and evolve into complex forms. We know, for instance, that life seems to have had one single origin on Earth, and every living organism we know of (at the time of writing) is descended from the same line. We all use exactly the same building blocks, even though we’ve come up with different solutions along the way as far as body plan and even the basics of circulation go. (After all, right here on Earth there are molluscs and arthropods that use copper to transport oxygen in the blood, rather than the more familiar iron-based haemoglobin. You don’t need to go to Vulcan for that.)
Scientists also make guesses based on how long we think it took for life to arise. On the kind of building blocks we use and whether they’re common or not in the universe. On whether there’s a habitable zone around a planet where life is more likely to arise, and whether the right conditions can arise elsewhere. Some people say that single origin on Earth suggests that it’s hard for life to originate — others point to the diversity of life on Earth and say that it’s clearly easy for all kinds of life to evolve.
I’ve read a lot of these arguments, but I’m not convinced that any of them have more weight than any others. We’re arguing from one single sample: life on Earth. It seems like a pretty big sample, but as I noted already, all of those organisms on Earth originate from one event. That’s all the data we have: one event.
So I don’t think we can say we have evidence one way or the other about whether life is common or likely. The universe is massive beyond our comprehension, and we’re looking at the tiniest corner of it. One week you’ll find someone saying that obviously life must exist elsewhere, because we exist, and that means it’s possible, and we can’t happen to be on the one planet that does harbour life! (But of course we can. It’s not a coincidence that we’re on a planet that supports life; if we weren’t we wouldn’t be here to say anything different!) The next week someone will point out that the exact conditions on Earth are unreplicable: every meteorite strike, every molcule, every atom would have to be the same. And then someone will respond, well, the universe is infinite, so that must turn out the same somewhere else…
Whatever scientists say, or want to believe, I think it’s safe to say that we really just do not know. Maybe if the human race survives, leaves Earth and explores the universe and finds no other sign of life after a million years, we can safely say life is rare. Maybe we’ll explore dozens of Earth-like planets, and find life on each one, and then we’ll be able to say that life is common.
If you want my guess, then I would say it depends on what we find in our own solar system. There seem to be conditions in which bacteria-like organisms could thrive, within our own Solar System — I’ve written about this before in terms of Enceladus, one of Saturn’s moons — and that would be a good first test of whether the conditions need to be exactly the same as Earth in order for bacteria to both arise and thrive. (It wouldn’t be the same to put lifeforms from Earth into those conditions: even if they can survive or even thrive there, it doesn’t mean they could have arisen there.) If there’s no sign of anything in our Solar System, I wouldn’t give up all hope, but I’d be thinking on a much larger scale in terms of both time and space before we ever encounter other life.
At the moment, though? Everyone is guessing.