How Close Life’s Birth?

Did life here begin on Earth, or did it start elsewhere in our solar system, the galaxy, or further?  Polymaths like myself love big questions like this, where many disciplines are relevant.  This particular question is also a good test case for the "absurdity heuristic."   Intuitively to most people the scenario where life here started elsewhere seems radical and weird.  But stepping back it is not at all obvious why we should think such a scenario unlikely, especially since most now accept that single cells can survive long journeys in space. 

A recent Washington Post reported life that seems well-adapted to space, offering further support: 

  • An electron beam meant to clean up a bioterrorism site transforms a mild-mannered microbe into a life form able to withstand radiation doses hundreds of times stronger than would kill a person.
  • Altered by the absence of gravity, an everyday bacterium aboard a spacecraft mutates into a highly lethal bug that poses a surprise threat to astronauts. …

Astronaut[s] … grew salmonella bacteria, …  Back on Earth, the space-grown bugs were fed to mice. They proved to be nearly three times as likely to cause disease and about twice as deadly as they were before the flight, the team reports in this week’s online edition of the Proceedings of the National Academy of Sciences, released yesterday.  Tests found 167 genes that were either more or less active in the shuttle bugs than in their earthbound counterparts — including many under the control of a single genetic regulator called Hfq. …

A second study, described in the October issue of Radiation Research, involves single-cell organisms known as archaea, which share some traits of bacterial and human cells. …  Scientists have long been aware that some bacteria are remarkably resistant to radiation. The most resilient of all, Deinococcus radiodurans, grows happily while basking in gamma-ray doses of 5,000 grays, hundreds of times as high as a common E.coli bacterium can handle. (One gray is the amount of radiation in about 5,000 chest X-rays.)  … [They] took cultures of an archaeon called Halobacterium and exposed it to the 20 million-electron-volt Idaho [accelerator] beam. … The mutant microbes that survived that experiment are unfazed by doses exceeding 11,000 grays.

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  • I think one should be careful not to read too much into those new findings. Even if there had once been an adaptation to space, one would have to expect that adaptation to have been lost through mutation accumulation over the four billions of years that have lapsed since the last hypothesized instance of selection for such an adaptation.

    When it comes to the related question of explaining the absence of observed extraterrestrial life, it appears to me that the common tendency (bias?) is to assign too much probability to one or another among all sorts of far-flung hypotheses, at the expense of the simplest hypothesis which is that intelligent life, and perhaps any life, is extremely unlikely to evolve on any given planet. (With enough planets and an obvious observation selection effect, this seems to fit the evidence perfectly.) Maybe that simplest hypothesis is too boring to hold people’s attention. Or maybe people overlook the observation selection effect. A crude misapplication of the representativeness heuristic may be at work: “We’ve looked at the Earth, and, lo, it has life. We’ve also looked at the moon and glanced at a few other planet, and they did not have life – so the odds seem like about 1/5 that any given planet has life.”

    However, I’m not sure that either this or the pansperima hypothesis is a good test case for the “absurdity heuristic”, since in these cases we can’t independently verify whether the heuristic worked or not…

  • There’s a bounded amount of information in the genome that can be maintained against the pressure of degenerative mutation. For mammals, this bound is on the order of 100 million bits: If 2 parents have 16 children, and on average all but 2 children die or fail to reproduce, this imposes an upper bound of roughly 3 bits of selection pressure per generation, which we can approximate as ~1 bit. (If the average number of surviving children is more or less than 2, the population rapidly goes to infinity or zero.) And among mammals the average copying error in DNA is 1 per 10^8 bases per generation. So at most 12 megabytes or so of information can be maintained in the genome against the pressure of degenerative mutation. This was calculated from first principles before the public was “surprised” by junk DNA and the discovery that there are only 30,000 protein-encoding genes in humans.

    As bacteria lack sexual reproduction and have a higher rate of copying errors, the amount of information maintainable in them is less.

    Unused adaptations will rapidly disappear. As George Williams points out in the classic “Adaptation and Natural Selection”, life probably hit the upper bound on information storable in the genome hundreds of millions of years ago, and since then evolution has been replacing adaptations, not accumulating adaptations.

    A more virulent bacterium is not necessarily more fit. Diseases evolve to leave their hosts alive long enough to infect others. Bird flu kills so many victims because it lives deep in the lungs, which makes it hard to transmit between humans. It’s not like a contest of strength in a boxing ring – a disease that becomes ultra-fatal in space may actually have lost adaptive qualities.

    So the only really interesting bit of evidence here is that life can rapidly evolve to tolerate high radiation doses. But I’m not sure that implies even a single floating bacterium being able to survive, slowly floating across light-years. That part of the question exceeds my competence to guess, apart from the point that there is the Great Silence to consider.

  • Martin

    Organisms can mutate to withstand radiation, but that doesn’t address the more serious question of dessication. Only a seed-like organism could withstand long trips through space (possibly, although even seeds may not make it). That requires an already complex organism, a eukaryote, which means simpler life (bacteria, viruses) would have to have DEvolved. Once you acquire complexity, it’s a lot harder to lose it.

    In other words, the terrestrial theory of life’s origin is more parsimonious and probable.

  • “Intuitively to most people the scenario where life here started elsewhere seems radical and weird”

    This is true, but I suspect to most people the idea of life starting at all is radical and weird, probably to the extent that intuition about its particulars (like where in the universe it happened) is impossible. Believing life started down the road or on the other side of the universe probably requires similar imaginitive capacity, so the absurdity heuristic is probably not at work (except perhaps in persuading people there are more satisfying things to think about).


    The survivability and adaptability of life is well documented.

    In fact, one of the easiest explanations of life’s start on this planet so long ago is that a comet or asteroid impacting here had numerous hardy microscopic life forms on it–jump starting evolution/life on this fertile soil.

  • Once you acquire complexity, it’s a lot harder to lose it.

    Um, that’s simply not true as a matter of evolutionary biology. It’s enormously easier to lose information than acquire it. Acquiring information takes thousands of generations of natural selection. Losing it requires one cosmic ray strike.

  • Here are the apparent propositions:

    1) Life started on some planet.
    2) Life started on some planet and then traveled to another planet.

    I agree #2 seems plausible, given what we know about life. But isn’t #1 more probable?

    I don’t think exogensis is “radical and weird” given current evidence, just unnecessary.

  • Martin

    Eliezer: As an all-or-nothing event yes, but it’s a lot harder to selectively lose adaptations. If you want to lose all the information in a seed-like eukoryote, you just have to kill a seed-like eukaryote. But if you want to evolve into a bacterium, you have to selectively lose the nuclear envelope, introns, the sexual reproduction machinery, the machinery that builds seed coats, and so on. That’s unlikely, especially since each of those adaptations is fitness-enhancing.

  • Martin

    Actually, a better example would be spores. Some single-celled eukaryotes in the fungi class can sporulate, withstand dessication, and go dormant for several years. The idea that they could survive a multimillion-year space hike is unlikely, though. My argument stands.

    There’s also the minuscule probability of spores or anything coming from a solar system light years away. We found a few Martian rocks, but Mars is practically hugging us in terms of space distances. We haven’t found any rocks from Pluto. The probability of a rock from another solar system landing here is infinitesimally small.

  • Nick, we can’t verify it yet, but presumably we will eventually know if our life started on Earth or elsewhere.

    Eliezer, modular unused adaptations will quickly degrade, but unused adaptations that are tied into other used adaptations may last a lot longer.

    Martin, the scenario people have in mind is organisms deep inside rocks in space.

    Thomas, if the chance is p that life spreads to N planets, and (1-p) that it stays on one planet, then the chance Earth is where our life started is (1-p)/(1+(N-1)p).

  • J Thomas

    Expose a microorganism to ionising radiation and its descendents will be more resistant to ionising radiation for some generations with no adaptive mutation involved. They turn on repair mechanisms that are usually turned off. But those repair mechanisms work when the organism is growing, not while it’s dormant.

    Most of the organisms that are adapted to an evolved planetary system will be unready to deal with a place where there’s nothing alive. After a billion or more years adapting the environment to suit themselves, they mostly won’t know what to in an abiotic environment. The ones that do best might be things that grow slowly in places that hardly anything lives. If there’s a little bit of life growing in comets etc, those might do well colonising comets etc in another system, and expand from there. I don’t know how they’d get accelerated to nearly lightspeed so they could get to other systems in a reasonable time, or how they’d survive getting decelerated, etc.

  • Astronaut[s] … grew salmonella bacteria, … Back on Earth, the space-grown bugs were fed to mice. They proved to be nearly three times as likely to cause disease and about twice as deadly as they were before the flight

    But that’s not evolutionary progress; that’s just adaptation to a new set of circumstances: spreading fast and killing hosts is something most diseases evolve to avoid (syphilis used to be a pretty nasty disease that killed all victims within a few years, but benign versions spread farther). I suspect, though, that something adapted to spreading from one planet to the next would be pretty likely to last on any given planet, so a weaker but broader form of the same hypothesis (like “Most planets with life have life that formed on another planet”) would be valid.

  • Paul Ewald and Greg Cochran gave a good explanation of how the virulence of germs is determined by how easily they spread here.

  • douglas

    The experimental evidence would indicate that life can only come from life. This seems to be the only valid scientific hypothesis available. (As far as I can tell from my researches the hypothesis of abiogenesis has a mountain of failed experiments as its evidence). I have trouble with this and would like to over come this bias. Where is Kurt Godel when I need him?
    Does anyone know of an author or a book that accepts this hypothesis as true? (It seems that to make a world view that agrees with the evidence, this is a major stumbling block)

  • g

    Douglas, that view used to be universally regarded as true, so just about any author or book before about 1800 would do. It was rejected when scientists started being able to synthesize organic chemicals. I don’t understand why the fact that no one has yet made life from scratch should be taken to show that “life can only come from life”, and it’s hard to see *how* that could be so. Especially as it implies an unattractive infinite regress.

  • If we have independent reasons (e.g. Fermi) to believe abiogenesis is improbable, then the fact that life popped up on Earth very soon after conditions allowed is evidence for an extraterrestrial origin:

  • douglas

    g- that result comes from Pastuer.
    From TH Huxley (1860)
    “The proposition that life may, and does, proceed from that which has no life…remained the accepted doctrine of learned and unlearned Europe down to the 17th century.”
    To say the production of urea is the evidence for abiogenesis (which I believe was the chemical used to disprove biogenesis) seems to me an example of the cached thought winning over the evidence. Certainly no one has produced life (or anything like it) from non-life as of today.
    Steven-Putting the process in space doesn’t deal with the main issue.
    It seems this leads to a paradox– something that I wish I was better able to deal with. Certainly if life is an irreducible much thinking needs to be re-evaluated– my attraction to this site in the first place.
    Is there a contemporary author that deals with this without getting into religious texts?

  • Nick Tarleton

    Life is a particular arrangement of matter. Why should we assume a particular arrangement of matter is impossible to produce naturally, just because we haven’t yet been able to reproduce it artificially?

  • douglas

    Nick-“life is a particular arrangement of matter”
    This is the assumption that is being questioned.
    Why should we assume anything other than the evidence in this case?

  • g

    Douglas, what result comes from Pasteur and why does it matter that it did? (The synthesis of urea was Woehler, not Pasteur.)

    How is the fact that no one has yet synthesized life from non-life evidence that it’s impossible? It’s evidence that it’s not easy, but no one ever expected it to be easy. (Well … of course it’s evidence that it’s impossible, just as the fact that I don’t have a child called Bartholemew is evidence that it’s impossible to have a child called Bartholemew, but it seems to me that it’s *extremely weak* evidence. Even the simplest living things are very complicated, and our technology is limited. Why *should* we expect to have been able to synthesize life from non-living ingredients?)

    The evidence the other way isn’t the mere fact that some guy synthesized urea. It’s the fact that scientists looking at the processes of life have consistently failed to find any way in which they are anything other than ordinary physical processes. (The nearest thing to a counterexample is consciousness. I consider most of what’s said about the mysteries of consciousness to be mere obfuscation, but even if it were to turn out that consciousness is fundamentally nonphysical that wouldn’t indicate that *life* is.)

    To put it differently, the evidence that life is a physical phenomenon is rather like the evidence that gravity is: that the planets aren’t kept in their courses by angels pushing them in the right directions, etc. And arguing that we should believe life can only come from life because no one has managed to synthesize a living thing from scratch is like arguing that we should believe only angels can keep planets in their orbits because no one has managed to make a solar system and have it work correctly without angels.

  • douglas and g and Nick, this post is about where life arose, not about whether it is possible for life to arise. You are off topic.

  • douglas

    Robin- I’m sorry, I didn’t mean to suggest life could not arise.
    I also recognize that my desire to question certain biases is unusual.
    I would like to continue with g–as he has been very helpful to me recently, what is the protocol for this? (assuming he wouldn’t mind)

  • Douglas, g has a website and lists his e-mail address there.

  • J Thomas

    G, if you look at what organisms do, they take a wide variety of chemicals and find ways to convert them into a much narrower variety of chemicals first. Then they take these building blocks and build all the things they need. It kind of looks like a pair of funnels facing each other.

    Simpler, earlier life presumably started with a much narrower set of precursors and built up a narrower set of products. We now have a system to build proteins out of amino acids — tremendously flexible and efficient for enzymes etc. But strings of nucleic acids can make enzymes too. It would be simpler for life to start out as nucleic acids that didn’t yet make proteins but only catalysed chemical reactions that let them reproduce themselves etc.

    And we have had laboratory examples of RNA that reproduced itself in a particular environment, one that provides the precursors it needs. I argue that this is life.

    Further, some vaguely similar environment is almost certainly where our kind of life started. A place that had lots of nucleic acids freely available and one where they spontaneously linked into chains, and some of those chains had random enzymatic activity, and then some started to reproduce themselves. This environment could have existed on earth — but it’s long gone now, everything that can be used has been re-used many many times since then. Or it could have been somewhere else. But it was probably nucleic acids that did it, and it happened in a place that had most of the building blocks handy and that needed almost no construction work beyond just a rather inefficient replication.