Fewer Harder Steps

Somewhere between 1.75 billion and 3.25 billion years from now, Earth will travel out of the solar system’s habitable zone and into the “hot zone,” new research indicates. … In the habitable zone [HZ], a planet (whether in this solar system or an alien one) is just the right distance from its star to have liquid water. Closer to the sun, in the “hot zone,” the Earth’s oceans would evaporate. (more; source)

Fifteen years ago, the best estimates I found were that life appeared on Earth from 0.0 to 0.7 billion years after such life was possible at all, and that simple life would only continue to be possible on Earth for another 1.1 billion years. (Earth is now 4.5 billion years old.) These two numbers seemed close enough to be consistent with a simple model of Earth being very lucky to originate intelligence life.

This simple model says that a planet goes from no life to intelligent life by passing some “hard steps,” like inventing life, sex, multi-cellular bodies, and intelligence. The system had a constant chance per unit time of completing each new step, but these chances could be very different. That is, the steps could have very different difficulties; it might be much easier to invent sex than to invent life.

Even so, I showed fifteen years ago that that if all these steps were hard, i.e., if on a random planet each step would usually take longer than the time window for life on the planet, then given that intelligence eventually appears before the window closes, the actual distribution of durations observed between the steps (and the duration between the last step and the end of the life window) would be roughly equal. (To be precise, drawn from the same distribution with a modest variance.)

A standard account of five major evolutionary events by William Schopf roughly fit this model: his durations were 0.0−0.7,0.5,0.6,0.7,1.1, and 1.7−2.4 billion years. And that longest period is one we know little about, so it might really cover two steps.

However, this new result quoted above, of 1.75 or 3.25 billion years for time remaining on Earth, makes this simple model harder to accept. And it is actually worse than quoted above. Those two numbers are from two different models of how the Sun’s brightness is expected to increase with time. But both numbers assume few clouds on Earth. If we instead assume that the fraction of Earth covered by clouds will later be 50% or 100%, then the time left for life is 5 or 20 billion years.

In contrast, a best estimate now is that life appeared on Earth from 0.0 to 0.6 billion years after it was first possible. So even the best case ratio for these durations is 1.75/0.6 = 3, and a more believable ratio is 3.25/0.3 = 10. These seem hard to accept as a ratio of typical durations drawn from the same distribution. So how can we change the model to better fit this data?

First, this pushes us to give up the idea that life evolved on Earth at all, or that the origin of life was a hard step. If life evolved elsewhere, that could give a lot more time for hard steps to be achieved. After all, the universe is now 13.8 billion years old.

Second, this also pushes us, if a bit more weakly, to give up the idea that the evolution of intelligence was a hard step. Intelligence seems to have appeared only 0.6 billion years after the appearance of multi-cellular animals, and we seem to see a somewhat steady progression in increasing brain size, in contrast to the constant random search and random success of the model.

Third, if there is a hard step associated with our immediate future, it is not of the sort in this simple model, something we keep trying until we succeed. Instead, either something will destroy us soon, or not.

Finally, there seems to be only room for one or two hard steps so far in the history of Earth. And the more that some periods require easy but long steps, the less room there is. For example, it might be that Earth had to wait for its atmosphere to slowly fill up with oxygen before key further developments could be enabled. Or it might be that multi-cellular animals just took a certain slow delay to develop large smart animals.

The fewer hard steps there are, the harder each steps must be on average. So this news suggests should increase our estimate of just how hard is each hard step.

The best candidate for a hard step in the history of life on Earth seems to be the origin of Eukaryotes. Since the oldest eukaryotic fossil is approximately 1.5 billion years old, they appeared reasonably close to the middle of the window for life on Earth.

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  • Rpbert Koslover

    I may be simply misunderstanding this, but it seems to me that the type of reasoning you are employing could apply to an ensemble of planets in regard to understanding/predicting distributions of the times that pass before certain events occur. But are predictions of such time periods meaningful when your planet sample size is only one?

    • Garrett Lisi

      This same issue of the applicability of probabilistic reasoning for a singular case applies for the Doomsday Argument, and for the anthropic principle. I am curious to hear Robin’s take on it.

    • http://overcomingbias.com RobinHanson

      The reason we have the Fermi question is that we know of a great many planets without intelligent life, and only one planet with such life. Taken together we must conclude that the chances for intelligent life arising on any one planet must be small. So we need a model where that is true. My post is all about the implications of the need for such a model.

      • VV

        There are no planets other than Earth in the habitable zone of the Solar system. Some Jovian moons may have liquid water and thus possibly life, but there is no conclusive evidence.

        There are a few known terrestrial exoplanets within the habitable zone of their stars, plus some gas giant exoplanets which might have moons with liquid water, but currently we have no way to detect whether they harbour life, much less intelligent life.

      • IMASBA

        VV is right: it is hard to detect Earth-sized planets (or any rocky planets for that matter), so the fact that we mainly detect gas giants is biased, it’s even harder to detect life on those planets (although a civilization would make it easy for us) and it’s currently impossible to detect Earth-sized moons around exo-gas giants. It may be that life isn’t very rare IF you have the right planet: a rocky planet in the habitable zone, preferably with a large moon or itself a moon.

  • VV

    Sexual reproduction doesn’t seem to be a prerequisite for multicellularity, since there are many multicellular species that either completely asexual, or can reproduce both sexually and asexually.

    Brain size over body size hasn’t been monotonically increasing over time.

    • VV

      Moreover, sexual reproduction evolved multiple times, multicellularity evolved at least twice (once for plants and at least once for animal and fungi), neurons might have evolved once, though cephalization evolved multiple times.

    • http://overcomingbias.com RobinHanson

      Even if brain and bodies sizes haven’t been monotonically increasing, there have been consistent trends. Even if sex isn’t needed for multi-cell bodies, it might be needed for the kinds of bodies that can make intelligence.

      • VV

        Even if brain and bodies sizes haven’t been monotonically increasing, there have been consistent trends.

        Honestly I don’t know. Do you have any reference.

        Even if sex isn’t needed for multi-cell bodies, it might be needed for the kinds of bodies that can make intelligence.

        Possibly, but that doesn’t seem intuitively obvious.

        There are several species of parthenogenic lizards that reproduce asexually. Since we evolved from lizards it would seem plausible that in an alternate world we could have evolved from these type of lizards.

        It could be pointed out that these lizards descend from sexual ancestors, and they are not completely asexual, since they can mate and produce viable offspring with males of closely related species, therefore perhaps sexual reproduction might be needed to evolve an animal with the complexity of a lizard, but AFAIK there is no compelling evidence to support that position.

      • IMASBA

        Asexual procreation in animals is usually a short term survival mechanism: their populations will always go extinct if asexual reproduction is maintained for longer periods. Sex is needed for them, as it is for us, to increase the genetic diversity of new generations, it offsets the slow reproductive cycles of animals and our lack of horizontal gene transfer.

      • http://overcomingbias.com RobinHanson

        A classic cite is: Harry J. Jerison (1991) Brain Size and the Evolution of Mind, American Museum of Natural History, New York.

  • Jan

    In 2010 you said:

    It seems to me that if the great filter is to consist of just one big
    step, the only plausible possibility is the development of
    multi-cellular life. All the steps before that one seem able to spread to other star systems via single-celled life hidden in dust, and it seems we haven’t had a big filter step since the multi-cellular innovation.
    http://www.overcomingbias.com/2010/11/at-least-two-filters.html#sthash.EfzIBbRl.9rRa9q7e.dpuf

    How exactly do you square this with

    > The best candidates for a hard step in the history of life on Earth seems to be the origin of Eukaryotes.

    Is there evidence for eukaryotic panspermia?

    • http://overcomingbias.com RobinHanson

      The post I linked to about eukaryotic life described it as the key to getting multi-cellular life.

  • arch1

    1) Robin, unless you consider the likelihood of intelligence surviving long term to be negligible, any estimate of earth’s future habitability which doesn’t factor in the desires of intelligent creatures seems irrelevant.
    2) That said, if you *were* to revise your future-earth-habitability model to include the effects of intelligent activity, this would appear to leave you in the position of believing that the likelihood of our descendants deciding to (e.g.) move Earth to a higher orbit before it fries affects, among other things, the relative likelihood of terrestrial life having arisen via in-place evolution vs.panspermia. Which seems a fairly non-intuitive connection to be making. Can you or someone else help me clarify my thinking here?

    • http://overcomingbias.com RobinHanson

      For the purpose of estimating the time distribution of events that lead to intelligence, what matters is how long is the window open for simple life that can’t restructure everything. Yes of course, once advanced life appears everything can change.

      • arch1

        Sounds plausible, thanks.

  • http://juridicalcoherence.blogspot.com/ Stephen Diamond

    Even so, I showed fifteen years ago that that if all these steps were hard, i.e., if on a random planet each step would usually take longer than the time window for life on the planet, then given that intelligence eventually appears before the window closes, then the actual distribution of durations observed between the steps (and the duration between the last step and the end of the life window) would be roughly equal. (To be precise, drawn from the same distribution with a modest variance.)

    It would help to have a conceptual summary of this argument.

    • http://overcomingbias.com RobinHanson

      You can’t be bothered to follow the link?

      • http://juridicalcoherence.blogspot.com/ Stephen Diamond

        I looked at it, but I didn’t find the explanation very conceptual. I think you’re a better writer today than you were then.

        I think I’d put it (very roughly) this way. If the time window is smaller than the average time for the first event, with additional required events the observed times for later events become indistinguishable.

        Then, while it’s true that our best estimates of the later events will be that they’re of equal (additional) duration, the reason is that we lack information to distinguish them.

        This says that the greater error involved in the respective inferences is what grounds warrant for belief that the intervals are equal. We are confident that the intervals are equal based on the absence of information to distinguish them.

        But (unless its systematic absence, which is itself information) it doesn’t seem possible to ground knowledge in information’s absence.

  • Alexander Gabriel

    I thought these hard steps were toward getting intelligent life. So what does it mean to have hard steps in our future? Also I’m not sure what evolution versus random search has to do with this–maybe it’s being implied intelligence/Cambrian explosion is a single step. While I see how having one or two hard steps leads to the origin of life being not likely a hard step, I don’t see why the spacing of 0.6 billion years between the Cambrian explosion and humans says anything about intelligence being a hard step since it might be that intelligence was hard and the explosion was not. If anyone understands this, maybe they can explain.

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