Our Comet Ancestors?
A few years ago PCW Davies persuasively argued that Earth life more likely started on Mars. Last year, Napier and coauthors argued that comets are an even more likely source:
The recognition that life has an information content too vast to be assembled by random processes has led to many discussions of possible evolutionary routes, starting from a simpler self-replicating organic system and ultimately leading to the present-day protein- DNA-based life. … The clay model … uses the repeating lattice structures of clay particles and their catalytic properties of converting simple organic molecules in aqueous solution into complex biopolymers. …
The volume of clay on the Earth is vastly surpassed by that in comets. A single comet of radius 10 km and 30% volume fraction of clay contains as much clay, to within a factor of around 10, as that of the early Earth. However, our Solar System is surrounded by about 10^11 comets forming the Oort cloud … Whereas the average persistence of shallow clay pools and hydrothermal vent concentrations of clay on the Earth can range from 1 to around 100 years, a cometary interior provides a stable, aqueous, organic-rich environment for around 10^6 years. … mechanisms for interstellar panspermia have recently been identified, and we may
have to multiply this number by the number of Oort cloud analogues in the Galaxy.
(See also comments.) The entire paper is very short and qualitative – I’d have preferred quantitative discussion of rates of comet collisions with each other and with early Earth, to help us estimate how fast comet life could spread across comets, and how far it would have needed to spread to give it a decent chance of spreading to Earth.
But my core reaction is to marvel at how little work like this gets done. Figuring out the origins of life usually comes near the top of important scientific questions, yet in fact few resources go into this area. One reason, I suspect, is that for now the best way to approach this subject is qualitative and integrative, while academia mainly rewards impressive displays of ways with words, math, and tech. Does the topic also just seem silly?
J, the article claims comets are great for clay.
Sure, and another expert I never heard of before commented on this very blog to say they aren't.
But here's my tenuous reasoning. There's strong reason to think that life used enzymes made of RNA before it developed proteins. And the genetic code is an effective gray code -- many mutations have little effect. It appears to be tailored to the vagaries of ribosomes, which get the second base right almost all the time, occasionally miss the first base, and miss the third base pretty often.
For life to evolve on earth, we need to first evolve a set of RNA enzymes that work well enough. Then they have to evolve proteins as enzymes and genetic codes. And if they develop multiple genetic codes and one of them has to win out -- that's pretty extreme to happen so quickly. When you switch genetic codes you give up your library of working code. It's worse than changing computer languages after you have a lot of tools built in the old language, it's like rewriting all your tools from scratch, after you forget how they were made or what they did.
If the genetic code evolved, it should have taken a long time to do that -- or else it happened in a very large place.
This reasoning is not ironclad. Maybe there's a reason for the genetic code to be a gray code that didn't require it to evolve. Maybe it's a coincidence. But it isn't something I'd expect to happen in a single comet within a million years, to randomly spread to other comets that happen to be nearby when it approaches the sun. And I'm real unclear about temperatures. Frozen clay is not particularly good for life, nor is steamed clay.
But it does look good to start with an environment a billion times as large as the earth to evolve life in. Right now there isn't all that much water in the solar system. Planets can slow their water loss by holding it in with gravity. But they still lose water. There must have been more water in the past than there is now. So if there was once a whole lot of water around the sun, maybe a whole lot of water at the right distance that it neither boiled nor froze, water droplets that coalesced and split, that might do it. If it was just in the volume that the earth traces out today, that's more than 5,000 times the volume of the earth -- and life may have evolved on a thin layer of the earth's surface. If we have life spreading through an environment that's billions as times as large as the earth's ecology, it might evolve very fast. A million years might be quite fast enough to get things that can survive on the earth's surface, that keep going when the bigger environment is gone.
You can't get liquid water lasting very long in vacuum. The water would spread outward with the solar wind etc, it wouldn't last very long unless there was a tremendous amount of it to start. But our galaxy is a very bog place, and there might be somewhere that could keep a whole lot of water for a very long time. A billion times the size of the earth for ten million years is not out of the question.
But it might also work to evolve life on comets that spend a thousand years out in the cold dark before they spend one year getting close to the sun. I can't say it wouldn't work.
econ: "Frozen microscopic life in deep Antarctic ice (which receives cosmic radiation) degrades almost entirely rather quickly."
http://en.wikipedia.org/wik...
"While a dose of 10 Gy of ionizing radiation is sufficient to kill a human, and a dose of 60 Gy is sufficient to kill all cells in a culture of E. coli, D. radiodurans is capable of withstanding an instantaneous dose of up to 5,000 Gy with no loss of viability, and an instantaneous dose of up to 15,000 Gy with 37% viability. A dose of 5,000 Gy is estimated to introduce several hundred complete breaks into the organism's DNA."
This bacteria didn't evolve to survive the rigors of space. Space life might be far tougher than would be predicted from average Earth life forms.
Panspermia is an interesting explanation for how complex replicators could arise soon after the Earth surface cooled enough for liquid water to accumulate.
However, if life is common in the Oort Cloud then I think scientists would already have evidence. E.g., spectrographs of comets should show distinctive trace compounds.
If life were common in our galaxy, scientists should have seen evidence of intelligence.
At this stage I don't believe there is enough supporting evidence for either panspermia or Earth-origin replicators.