Exobiology was the original term given to the sciences central to the question of life-in-the-Universe. It’s now been largely replaced by Astrobiology, but I’ll stick with the original. To investigate life-in-the-Universe one needs to look at what the most likely sort of extraterrestrial life will be apart from Little Green Microbes. One on up the evolutionary ladder and we have complex life forms, at least terrestrially. However, unlike Little Green Microbes, complex (plant and/or animal) life will be rare.
If it took 0.5 billion years for the unicellular origin of life on Earth, it then took nearly another 3.5 billion years between the appearance of that ‘simple’ single cell life and the eventual explosion of multicellular (complex) life. For some reason(s), it appears to be biologically difficult to go from the single cell to a complex multicellular organism based on the only example we have to judge such things. Probably the overall obstacle to the early quick-smart establishment of multicellular (complex) life is that heretofore microbes (single cells) were 100% self-sufficient generalists. Complex organisms require the evolution of single cells to give up being generalists, become specialists, and work as part of a team. That degree of organization apparently takes lots of time, especially to the stage of where it can leave a large fossil presence, if it takes place at all.
Apart from that, planetary environments tend to be dangerous and rarely stable, and thus you need a lot of factors in place to ensure that simple life even survives the long term and get the chance to evolve into multicellular life. Or, if you have evolved multicellular life, the odds are pretty good it’s going to get the Big “E” – Extinction. I mean complex life is very vulnerable to environmental forces. A tornado probably isn’t going to bother bacteria, but it sure could tear you apart. That’s not to say bacteria can survive everything the cosmos can throw at them, but when nasties come, you stand a better chance of survival if you’re a microbe. So, in order to get to the first proto multicellular critter, and from that to us, you need a lot of Goldilocks factors operating in your favour.
But there are apparently just too many planetary Goldilocks factors at play to grant the probability that complex, multicellular, plant and/or animal, life is common throughout the cosmos. You need to be in a quiet part of the galaxy – no nearby supernovas, black holes to suck you in, gamma ray bursters, etc. You need a long lived stable single star. You need a gravitationally stable solar system so that planets are not gravitationally disturbed out of their orbit and either plunged into the parent star or cast out of the parent system altogether. You should have a good Jupiter(s) to absorb and/or gravitational deflect comets/asteroids that would otherwise slam into your otherwise environmentally-friendly planet causing havoc to established life forms. You need a planet that’s in a pretty circular orbit, one that doesn’t stray too close or too far from the habitable zone surrounding the parent star. The planet must have a fairly stable temperature range over geologic time periods, and so you must have an atmosphere, and thus has to be massive enough to retain an atmosphere, without being so massive that you end up with a brown dwarf. The axis can’t have an extreme tilt, and it would greatly assist if the planet had a large moon around it to assist its long term stability. You need some sort of plate tectonics to ensure land building and the recycling of materials. If it’s intelligence, with technology you seek, the planet can’t be an all water world. The planet must have formed in a region abundant in the heavier chemical elements (oxygen, sulphur, carbon, silicon, nitrogen, various metals, etc.). When you take all those factors (and more) into account, the number of suitable abodes where simple life can slowly evolve into complex life decreases rather quickly. And there’s no guarantee that there is really any directed purpose to evolution in that evolution doesn’t require simple life to become complex life. Survival and leaving offspring is what it’s all about, and if single cell critters do that what more is needed?
One note about planetary disasters/catastrophes is that these cut both ways. They can’t be frequent enough and/or large enough to wipe out the entire biosphere in total, especially the biosphere comprised of complex life forms, but on the other hand, infrequent small disasters can spur on evolutionary change by opening up environmental niches, but depending on who/what you are, when you are, and where you are, a disaster can be a double edged sword. I mean if you’re a T-Rex sixty five million years ago, its bad news. On the other hand, without the bad news for T-Rex, we wouldn’t be here, so for us, an asteroid impact 65 million years ago turned out to be good news!
It’s also difficult to naturally transport complex life around the galaxy, unlike microbial life. If a meteor hit Earth and blasted a chunk of terra firma off towards Mars, pity the poor cockroach going along for the ride. Cockroaches are tough, but not that tough. And even if a cockroach did make it alive to the surface of, say Mars, it wouldn’t survive long.
The main proponents of what is now called the ‘Rare Earth Hypothesis’ are the scientists Brownlee and Ward (see below), and they have certainly stirred up an astrobiological hornet’s nest with the idea. That’s good for science in the long term. The belief in an unproved but accepted scientific proposition, in this case that that there are lots of complex alien critters out there, needs to be challenged if fields of inquiry aren’t going to stagnate. However, make no mistake, it is the ‘Rare Earth Hypothesis’, not the ‘Unique Earth Hypothesis’, so religious fundamentalists who have taken this hypothesis to their hearts; who still need Planet Earth, and human beings, as some sort of religious special creation, should really not take comfort from these ideas. The Copernicus Revolution is still alive and well.
Further reading:
Brownlee, Donald & Ward, Peter D.; Rare Earth: Why Complex Life Is Uncommon in the Universe; Copernicus Books, New York
Burger, William C.; Perfect Planet, Clever Species: How Unique Are We? Prometheus Books, Amherst , New York
Morris, Simon Conway; Life’s Solution: Inevitable Humans In A Lonely Universe; Cambridge University Press, Cambridge
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