Having considered how uncommon Earthlike worlds are, now we're going to look at potential Great Filters in the history of life itself.
The first is more of a meta-filter, and affects some of the possible stellar and planetary filters already discussed. It's simply this: what is the range of potential forms of life in the Galaxy?
The most restrictive answer is "life is like Earth life." But biologists and SF writers have come up with a vast range of potential kinds of "life" which are very unlike Earth life. (To thwart any pointless tail-chasing about the definition of life, I'll use this one: Life is any naturally-arising system capable of reproduction and evolution.) Here's a very quick rundown of types, from hottest to coldest.
• Plasma beings storing "genetic" information in magnetic fields, living on the surfaces of stars;
• Beings of fluorosilicone living in liquid sulfur oceans;
• Carbon-based life living in ammonia oceans;
• Carbon polymer life living in liquid hydrocarbon oceans;
• Beings of liquid helium.
There's no evidence that any kinds of exotic life like these exists, but there's no compelling physical reason why they couldn't — and in a vast universe of billions of galaxies, are they any less likely than ourselves? This is especially important because many of the Great Filters already described wouldn't catch exotic life. The high-temperature ones probably wouldn't care much about radiation, and the cold ones have much bigger "habitable" zones and more suitable star systems than we do.
Which makes this a big problem. We've been looking at Great Filters which make life like ourselves rare, but when considering exotic forms of life we either have to discover filters which could snare things we can only theorize about . . . or we run bang into the Fermi Paradox again. Where are they? Where are the fast-evolving high-temperature civilizations which could race to intelligence and Kardashev-III technology in half the time it took us to get here? Where are the vast and slow cold-life empires spreading from Kuiper Belt to Kuiper Belt across the Galaxy, ignoring the warm dry worlds circling close to the deadly stars?
We don't see them. To be fair, we're not really looking for them, either. That's because it's really hard to devise an experiment to detect biosignatures for completely hypothetical forms of life, and even harder to get funding for it.
All of which means this discussion is limited to Life As We Know It. Either our kind of life is the only possible kind of life, or there is a whole alternate set of Great Filters which apply to exotic life types and has so far prevented any of them from doing anything detectable across interstellar distances.
My personal opinion is that Earth-type life really is the only game in town. Carbon is a uniquely adaptable element, and I think it's entirely plausible that only carbon can form information-rich molecules like DNA, and only at liquid-water temperatures. The rest of this post will operate under that assumption, but note that it is an assumption.
Digression over.
The simplest Great Filter related to life is simply that life is rare. On Earth, the first living things date back nearly 4 billion years, out of the planet's 4.5 billion year history. That strongly suggests that life formed as soon as conditions were even marginally suitable, but we really don't know. Perhaps Earth just got really lucky. This question won't be settled for a while. Even if probes to Mars uncover traces of ancient life there, it's entirely possible that the two worlds traded primitive organisms and life only had to happen once.
Finding life on Europa or Enceladus would be a bit more suggestive. It's much less likely that organisms could make the voyage from Earth to the outer moons. If life can evolve on bodies as different as Earth and Europa, then presumably it arises anywhere conditions are right.
The early date of life on Earth is a strong clue that life is nearly inevitable. I'm going to make this one another 50-50 coin flip filter. If a world has suitable conditions, life arises half the time. I personally think that's pessimistic, but we'll stick to it for now. That cuts the number of potentially life-bearing worlds to just over 2 million in the Milky Way Galaxy.
The Drake Equation jumps directly from worlds-with-life to intelligence, and that seems to be a common fallacy both in discussions of SETI and in science fiction. Even now one can see films or TV shows depicting planets with trees and grass and furry critters, which characters describe as having "no life forms." But there are still some hurdles between some self-replicating goop in a warm little Archaean pond and Our Glorious Selves.
First, while life may be common, eukaryotic life, with a distinct cell nucleus, may not be. Most species on Earth are prokaryotes — smaller and simpler forms with no nucleus. Eukaryotes dominate Earth's biomass because they mastered the trick of symbiosis with other single-celled organisms. Every one of your cells includes mitochondria, which are apparently prokaryotes incorporated by some distant ancestor, making you capable of oxygen respiration. The cells in every green leaf include chloroplasts, which are basically blue-green algae co-opted by eukaryotes to allow photosynthesis.
How common are eukaryotes (or some analog) in the Galaxy? No way to tell, but here's a significant fact: they came along in the Proterozoic era, about 2 billion years ago. In other words, half the history of life on Earth consisted of nothing but prokaryotes living their single-celled lives. That gives us a useful number: assume another 50 percent chance of eukaryotic life (or an alien equivalent). That knocks us down to 1 million potential civilizations.
But we're not done yet. The next potential hurdle for life on Earth was multicellularity. Eukaryotes are all very well, but it's hard to have specialized tissues like, say, brains, when you've only got one cell. However, history suggests multicellularity is an easy hurdle to get over. It has evolved independently a whole bunch of times in Earth's history. There are even some multicellular prokaryotes. I'm going to declare that this filter isn't really a filter at all. If you've got cells, apparently at some point they're going to start clumping together, and the evolutionary advantages are strong.
Those are the life-related filters. There is a third — the emergence of life onto land, but I covered that one last time in the planetary filters, with the water world filter. I think it's otherwise pretty inevitable that organisms would colonize any environment they can reach.
Let's stop a moment and look at the Galaxy. It's full of stars, and many of them have planets. But there are huge patches where radiation from the galactic core or active star-formation regions has sterilized every world. Squeezed in between those deadly zones are safe areas, with Sun-like stars separated by about 10 light-years. But potentially habitable worlds are more like 50 light-years apart. Most of those have oceans full of single-celled organisms or clumps of simple cells, but nothing more complex. You'd have to go more than a hundred light-years in the right direction to find a planet with complex organisms.
And how many of those worlds have someone on them looking back with curiosity? I'll take that up next time.
The Star Trek Horta image is almost certainly copyrighted by Paramount or Viacom or CBS or whoever owns the rights to it.
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