to find aliens, we need to know when and why life began on earth
How old is life on Earth? It’s a pretty simple question but the answer is both very complicated and extremely important. Since we keep discovering new alien worlds that may be habitable on an almost monthly basis, knowing if they actually could harbor life and when it might take hold becomes pretty fundamental to achieve one of the ultimate goals of space exploration: find out if we’re alone in the universe for certain. Sure, considering the vastness of space and the number of worlds that are surely out there, we can assume that we’re not, even if we’ll never meet aliens face to face due to distance, or timing, or both. We can even do some math and speculate what the typical inhabited wold may look like, and why it will be nothing like our own, but it would still be an educated guess. Until we find markers of biological activity on other worlds, there will always be room for doubt and we can always say that it’s terribly unlikely it’s just us out there, but be unable to offer ironclad evidence that life is truly universal. And this is where the question of exactly how and when the only life we know of so far started on the only planet we know for fact has spawned it provides us with a guideline for when it might arise on other worlds and how to detect it when looking into space.
For the longest time, scientists viewed life as a fragile thing. It needed just the right amount of water, just the right temperatures, just the right kind of climate, a stabilizing moon to keep its rotational axis stable, and just about the right everything. Forget the Goldilocks Zone, we needed the 4.5 billion year old Goldilocks planet to even have a shot at finding bacteria. Before we could accurately date radioactive isotopes, the planet was thought to be only 500 million years old at most and numerous scientific sages thought of life beginning elsewhere and seeding Earth because the planet should have been way too young, too hot, and too prone to cataclysms to spawn organisms by itself. Darwin himself was a big fan of the idea, and there’s something to be said for the theory of panspermia, which often gets a bum rap despite quite a bit of evidence to indicate that comets and meteors probably played a big part in delivering the chemicals needed to kick start life as we know it. But as we’ve learned more and more about what organisms actually thrive just out of sight, the nothing of life as a fragile thing that needs a lush paradise to have a chance at survival seems very, very mistaken.
We’ve been finding life everywhere we go and as long as there’s water and a source of energy, there’s some organism that has no problem reproducing in that niche for millions, if not billions of years. Toxic hot springs? Acidic lakes and pools sealed by ice, devoid of all oxygen? Nuclear waste dumps? Pick an environment that sounds utterly uninhabitable and there’s a microbe colony feeding off things that would instantly kill you. It seems that when life takes hold, it takes a star dying and literally baking the planet like an autoclave to wipe it out because even constant meteor bombardments and volcanic blasts weren’t a hindrance to its emergence. In fact, the more we look for fossils of the first organisms on Earth, the older they are. A recent discovery managed to set the timeline for planetary habitation back by 100 million years, which means that 3.8 billion years ago, the first oceans, which were green with an abundance of iron, not blue like they are today, were full of microorganisms living around hydrothermal vents, feasting on the many nutrients stirred by near constant volcanism. It’s also possible that the first precursors to cells, made of little more than an envelope of fats with bits of what would become RNA, existed just half a billion years after the Earth was born.
So the main takeaway here is that if your planet is habitable, life takes hold very quickly. It may draw on organic matter brought by debris from space or erupted from volcanoes, or both, but once it finds a stable configuration, it’s going to spread and start evolving. It might never become multicellular, but there’s some evidence that this leap may be a lot easier than we think, and it might not become intelligent in the same way we are, building cities, doing science, engineering spaceships to explore other worlds, but it may last for a very long time, growing enough to drastically alter its planet’s atmosphere, much like plants did 350 million years ago, and we’re doing right now. Since we know how to estimate a star’s age, we can also figure out how old planets around it are, and if some of them are in the habitable zone, we can use the processes we know take place on Earth as a guide for what kind of life we’re likely to find at this stage of the game, and what to look for when analyzing the starlight shining through its atmosphere. Unless we see light from alien megacities, that will be our best bet to find another living world.