A recent piece of news is that it is now almost certain that there is a vast ocean of salty water under the ice sheets of Ganymede, and that this makes Ganymede a good place to go looking for extraterrestrial life. This is exciting stuff. If we found life there, or anywhere else in the Solar System, it would have a profound effect on our understanding of ourselves, how we came to be here and about the chances of finding life, especially intelligent life, elsewhere in the Universe. This has got me thinking as I think that finding aliens would be about the coolest thing we could possibly do “out there”. But this begs a question, “What is life?” and also would it look ike Earth life or would it be something completely different? let’s start by looking at how we define life, here on planet Earth. First of all it has to metabolise, that is to say it has to take stuff in and use that stuff to make energy for it to use for growing, reproducing etc. This is in the form of things containing chemicals that can be converted into storable, usable energy. This is also known as food. There is also stuff like, reacting to its environment and reproduction, but once you start getting into a really detailed discussion of what is living and what isn’t, and here I mean what is not a living thing, not what has been living but is now dead, it starts to get a bit hazy and vague. For example, a virus can’t reproduce on its own, it needs a host cell to do that. It also doesn’t really do a hell of a lot, it just floats around waiting to find a cell it can invade and force into making more viruses. On the other hand there are plenty of humans who fit this description pretty well: But although beings like the one above are more or less immobile and, if we are brutally honest, unlikely to reproduce, they tend to make up for it when it comes to the consumption of nutrient matter! One thing that is common to life on Earth, and therefor to all life that we know about, is that it is based on Carbon and needs liquid water. The Carbon is necessary because it can form long and complex molecules with other elements and water because it is an excellent solvent, which basically means that it can transport chemicals around in cells or in a multicellular organism. It is also bipolar which means it can form different kinds of bonds with different other atoms, and has nothing to do with mood swings. So can life be made of other things than Carbon and water? Weeelllll, no. Or yes. Or maybe. You see there are alternatives to Carbon. Silicon is similar and can do a lot of the neat stuff that Carbon does, but it isn’t as readily reactive as Carbon, so basically Carbon does the job better than Silicon. As for alternatives to water, we have a chance to see if that is possible in Saturn’s moon Titan. Here everything is just like on Earth with seas, rivers, rain etc except that the incredibablly low temperatures on Titan mean that these are made of Methane, not water. water is a rock on Titan so it has been proposed that methane or ethane could do the job of water in Titanian life. But this leads us to a purely practicle problem. If there is life on Titan and it uses Methane instead of water, then because of this, and the low temperatures, it is unlikely that we wouldn’t expect it to be life in the fast lane, so maybe we would could sit and stare at something for weeks or months or years without anything happening. Hmmm, that sounds a lot like this guy again: So you might think, “Nah, that’s just a wierd rock, it’s not moving or metabolising or reproducing so it’s not living.” But maybe by the time your grandkids came back it had moved a millimeter or maybe there were two of them. So something could be living and we could walk right past it and not even notice. It could also be somewhere where we don’t think life can exist and this has been the case on Earth until the 70’s when we started finding life in places we thought impossile, the bottoms of deep oceans, hot springs etc.and suddenly we discovered that life can exist in almost impossible conditions. But even if life is Carbon based and uses water, would it look like life on Earth? Well here we have two basic theories, Convergence and Divergence. Convergence means that organisms that fill the same ecological niche, will end up looking similar. For example, a predator will need good, 3D vision to pinpoint its prey and to determine the distance to the prey, so its eyes will be at the front of its head and this is indeed what we see. But so do the apes are not all apes are predators. The thing about apes is that we climb trees, well mostly, some of us have moved on from there, but our bodies show the signs of our tree climbing history and one of those is eyes at the front of the head. This is actually for exactly the same reason as predators, it is to give 3D vision but in our case this is so we can judge the distance to the next branch, a pretty vital ability for tree climbers! Prey animals have their eyes at the side of their heads in order to give a view which covers as much of 360 degrees as possible in order to avoid any of those nasty fronty-eyed predators sneaking up on them and using them as nutreants in their metabolic processes. Apart from Spiders and Insects and similar creepy-crawleys, just about everything with eyes has two of them so why couldn’t aliens have three, or four? Well that’s basically because nature is stingy, it doesn’t want to waste energy so why bother with three eyes when two will do the job just as well?
OK then, let’s look at Divergence. This states that if life appears on two planets then it will be extremely unlikely that there would be any similarities at all because they are totally unrelated. There would be no common points of evolution because basically, how could there be? This would not only mean that they would look nothing like us, but that all our SciFi films were wrong about them speaking American English, which would make communication a lot harder! So there we have two very different possibilities, either we will find life that has many similarities with life on Earth, or we find life that is nothing like anything we know, or maybe we won’t find it because we won’t recognize it as life at all. So what will we find? Things that look at lot like Earth life, or something so alien that we have to redefine what we mean by life? My guess is that we will one day find both and some things that are in between. One day, someone will get the chance to say those immortal words: