Thursday, October 22, 2009

Evolution, Design and the Fermi Paradox

What is the time scale for evolution of complex organisms such as ourselves? On Earth complex life evolved in about 5 billion years (5 Gyr), but one can make an argument that we were probably lucky and that the typical time scale T under similar circumstances is much longer.

There is an interesting coincidence at work: 5 Gyr is remarkably close to the 10 Gyr lifetime of main sequence stars (and to the 14 Gyr age of the universe). This is unexpected, as evolution proceeds by molecular processes and natural selection among complex organisms, whereas stellar lifetimes are determined by nuclear physics.

If T were much smaller than 5 Gyr then it would be improbable for evolution to have been so slow on Earth.

It seems more plausible that T is much larger than 5 Gyr, in which case we were lucky, in a sense I will explain. Inflationary cosmology predicts a very large universe (much larger than what is currently visible to us), so that complex life is likely to exist somewhere in the universe. Conditioning on our own existence (a use of the weak anthropic principle), we should not be surprised to find ourselves lucky -- the few Earth-like planets that manage to evolve life must do so before their suns die. Intelligent beings, while not likely to evolve on any particular Earth-like planet, are likely to observe an evolutionary history that took place over a fraction of 10 Gyr.

Why should T be so large? At present we are unable to make quantitative estimates for the rate of evolution from first principles. It is entirely possible that certain evolutionary steps were highly improbable, such as the appearance of the first self-replicating complex molecules. One can also imagine abstract fitness surfaces with local maxima that trap the system for exponentially long periods of time.

I would not be surprised to find that T is exponentially larger than 5 Gyr. Godel went so far as to propose: "... a mathematical theorem to the effect that the formation within geological times of a human body by the laws of physics (or any other law of a similar nature) starting from a random distribution of the elementary particles and the field, is about as unlikely as the separation by chance of the atmosphere into its components."

The framework described above makes the following predictions:

1. The overwhelming majority of Earth-like planets are devoid of life, thereby resolving the Fermi Paradox.

2. Improved understanding of evolution will uncover highly improbable steps -- that is, improbable even over billions (or perhaps 10^100 !) of years. The fact that life on Earth climbed these steps might suggest intelligent design or divine intervention, but is better explained by the anthropic principle.

See related post evolutionary time scales.

Note added: This idea came to me after reading some discussion of ID and the question of improbable steps in evolution. (Here improbable means, for example, that even in an Earth-size population the multiple simultaneous mutations required jump across a particular fitness valley are unlikely to occur in 5 Gyr given the known mutation rate.) It occurred to me that under certain assumptions what might appear to be ID could actually be due to selection bias -- not taking into account the possibility that complex life is rare even on Earth-like planets. It turns out that the idea is not new -- it goes all the way back to Brandon Carter's 1983 paper which (I believe) coined the term "anthropic principle"! See John D. Barrow and Frank J. Tipler, The Anthropic Cosmological Principle, p. 557 for more. Also see this 1998 paper by Robin Hanson (thanks to a commenter for pointing it out). If one assumes many improbable required steps, one can deduce an upper estimate on the remaining time over which favorable conditions can persist on Earth. Note it appears the remaining lifetime of the sun is less than 1 Gyr, so the coincidence is tighter than I suggested in the original post. As the number of required improbable steps increases the likelihood that we would evolve just before time runs out (favorable conditions end) becomes very high -- this is quantified in the two references above.

The point which I have (still) not seen discussed much is that biologists need not be so defensive about improbable steps in evolution. I sense an almost reflexive -- i.e., prior-driven -- response to any claims of improbability. (On the other hand, perhaps biologists just know more about the details: the claim would be that the historical record does not resemble a typical one that would be generated by a chain of improbable events. I think this question requires further study.) But the main takeaway from this analysis is that improbability does not imply design or intervention.

30 comments:

Carson C. Chow said...

I actually agree and blogged about this recently. I think the probability of life arising is highly unlikely and the answer to how we could be so lucky is simply because the sample space is much, much bigger than the visible universe (e.g. string theory landscape) so the chance of life occurring somewhere (some time?) is nonzero and like you said we can only condition on our own existence.

Steve Hsu said...

We don't need string theory -- inflation is enough, or even many worlds ;-)

SD Scientist said...

10 GYr is the expected lifetime for a star the size of Sun. It's a wide range. A star with half the mass can last 50 GYr or more. A 2M star can last less than a billion years. As far as stars go, Sun is below median.

It appears more probable to me that 5 GYr is the characteristic time it takes for intelligent life to evolve. We could probably come up with some kind of dimensional handwaving argument to justify this number, just like we can use dimensional arguments to calculate characteristic dimensions and velocities of carbon-based intelligent lifeforms.

That puts your argument on its head. We live next to a Sun with a steady-state lifetime of 10 GYr because it takes O(10 GYr) for life to evolve, and not vice versa.

The correct solution of the Fermi Paradox is, of course, technological singularity.

Clark said...

Discussion like this don't provide any acceptable definition of "improbable", hence I believe you're pulling the estimate of plausible T values out of your nether regions.

In fact, to make an analogy, I think you sound just like the econo-physicists who gravely tell us that a 20 standard deviation move in markets is "highly improbable", every time there is a crisis. That is, about every 5 years.

SD Scientist said...
This comment has been removed by the author.
SD Scientist said...

A further thought. Even if T >> 5 GYr, the first intelligent civilization to emerge would be likely to do that on a star that formed shortly after the Big Bang. It is highly unlikely and it requires severe fine-tuning to explain (without resorting to singularity & such) why we're apparently first civilization in the neighborhood, and yet we're on a relatively young star, and there are many stars in our own galaxy that are 5-9 Gyr older than the Sun. For example, Tau Ceti, just 12 light years away, has luminosity 0.52 of solar, and it's two times older than the Sun. Why are we here and not there?

DB said...

SD Scientist: Early on, there weren't many heavy elements around, so those first stars didn't have rocky planets. Only after the first generation of stars went supernova was there enough iron and carbon and oxygen for life.

DB said...

5 Gyr is remarkably close to the 10 Gyr lifetime of main sequence stars...
Ha! Only a physicist would call a 5,000,000,000 year difference "close"!

Thras said...

To be fair, Godel didn't believe in evolution. And your argument doesn't make the majority of Earth-like planets "devoid of life", but instead devoid of complex life.

In any case, I don't think the position is necessarily borne out by the geologic record. The Cambrian explosion of complex life happened just about as soon as it could -- after the anaerobic organisms prepared an oxygen-rich atmosphere that could support respiration. As soon as that happened, complex life was more or less instantaneous in the fossil record.

The basic chemistry to prepare that atmosphere doesn't seem wild enough to justify a T "exponentially" greater than 5 Gyr.

Now, as for the initial appearance of life, things are a lot more murky. But the evidence suggests that it happened just about as soon as it could have happened.

Carson C. Chow said...

I will go on to say that estimating the probability of life appearing is extremely hard. Even if we knew all of the rules governing our universe, it would still be at least NP hard and the question is undecidable if you ask it for all possible (computable) universes since it amounts to solving the halting problem.

Steve Hsu said...

"We could probably come up with some kind of dimensional handwaving argument to justify this number, just like we can use dimensional arguments to calculate characteristic dimensions and velocities of carbon-based intelligent lifeforms."

Uh, no, we can't.

"In any case, I don't think the position is necessarily borne out by the geologic record. The Cambrian explosion of complex life happened just about as soon as it could -- after the anaerobic organisms prepared an oxygen-rich atmosphere that could support respiration. As soon as that happened, complex life was more or less instantaneous in the fossil record.

The basic chemistry to prepare that atmosphere doesn't seem wild enough to justify a T "exponentially" greater than 5 Gyr.

Now, as for the initial appearance of life, things are a lot more murky. But the evidence suggests that it happened just about as soon as it could have happened."

It is true that the first replicators appeared pretty fast (within 100 Myr?) after the Earth was formed. If this was one of the improbable steps, and T >> 5 Gyr, the probability per unit time is roughly constant over 100M-timescales, so the record somewhat disfavors this assumption, but not below the few percent level.

What is your 99% confidence interval for T?

Marcos said...

What I think is quite hard to grasp is that the first replicators appeared almost as fast as it was possible (that 100My figure), but they appeared only once. Or, at least modern DNA based replicators appeared only once (we'd know if there were other families of RNA compilers, our encoding is quite "arbitrary").

Now, that could be solved by panspermy, that's why I always supported that idea, but can also be explained by extraordinary luck, like you imply we had.

Thras said...

"What is your 99% confidence interval for T?"

It depends on your definition of "Earth-like conditions" of course.

We have three pieces of real evidence:

1) The fossil record, as discussed above.

2) Our biochemical knowledge of life and ourselves.

3) The fact that we haven't found life elsewhere yet.

Two is still expanding and is hard to interpret. Three has a substantial problem: there are a lot more variables in the Fermi equation than T, and any of them could be responsible. What if we're a lot closer to the upper limit of a technological civilization to accomplish things than we realize? What if the exponential trend, "progress," was a short-term, early and mid 20th-century phenomenon?

Regardless, I certainly don't know which variables in the Fermi equation have kept the aliens from turning us into galactic food-beast/slaves yet.

So we're left with 1), the fossil record. And that tells me that life, and complex life, seems to appear quickly whenever the conditions are right.

So T is probably very close to 5 billion years under sufficiently Earth-like conditions. To a high (if not 99%) degree of confidence.

An analysis of exactly what Earth-like conditions are and how likely they are can be as pessimistic as you'd like, however. See the book "Rare Earth."

Thras said...

"But they appeared only once"

Or they appeared a billion times and complex DNA-based life eats the simple replicators for breakfast every time.

Support for the argument? Well, we know that's what happened to whatever the original replicators were. Nothing like that is with us anymore. At one point they should have covered the globe.

SD Scientist said...

"SD Scientist: Early on, there weren't many heavy elements around, so those first stars didn't have rocky planets. Only after the first generation of stars went supernova was there enough iron and carbon and oxygen for life."

That is a good point - however, we didn't have to wait 10 GYr for that. Only the heaviest stars go supernova, and those have relatively short lifetimes, usually 10^7 to 10^8 years.

anon said...

The use of "probability" here is meaningless.

The use of "complex life" here is meaningless.

Once there is a single cell the rest is easy. At the level of the cell humans aren't much more complex than paramecia.

I'm waiting for the first total synthesis of a cell. There's no money for it though I'm sure.

Luke Lea said...

Isn't the speed of light by itself enough to solve the Fermi paradox. The amount of time and energy required to travel to the nearest star, let alone another Galaxy, are both, well, astronomical.

Steve Hsu said...

"Isn't the speed of light by itself enough to solve the Fermi paradox. ... "

Robots.

Mariano M. Chouza said...

Some thoughts & links (sorry for my somewhat broken English):

The biosphere is expected to become uninhabitable in 1-2 Ga (see also this very interesting book by the same James Kasting - 20 MB).

The expected times between "evolutionary steps" are a function of their relative hardness, so a "fast" origin of life doesn't imply a high probability, as it's explained in this paper by Robin Hanson.

Thanks for your blog!

zzzhou said...

5Gyr is not the evolution time scale; it's the global geological/chemical time scale. Evolution time scale is 2% to 20% of that. Whenever conditions become ready for life to take a major step, evolution does the job in a ~100MYr. Three times it has done it already.

The first time, as soon as the Earth formed a crust, the first cellullar life appeared. There was negligible time gap. The second time, when free crustal iron was exhausted and O2 starts to accumulate in the oceans, mitochondria, oxygen metabolism, and eukaryotes appeared. Then the third time, as Thras pointed out, when O2 levels reached the level for multicellular metazoans to function, they just appeared, in more forms than you can imagine. It's scary how fast evolution works.

The real speed governor has always been the slow process of O2 photosynthesis changing the global chemistry.

As for intelligence, we are maybe 10~100MYr ahead of chimps, parrots, ravens, whales, etc.

Solution for the Fermi paradox? The Earth is unique, one in a trillion or more.

SD Scientist said...

Isn't the speed of light by itself enough to solve the Fermi paradox. The amount of time and energy required to travel to the nearest star, let alone another Galaxy, are both, well, astronomical.

Age of the Universe is even more astronomical.

It's been calculated that a single technological civilization with sufficient desire to expand could colonize the Milky Way in a few million years.

Since we haven't seen any evidence of such colonization, there are several possibilities. Either we're alone in the Milky Way, or they are around but they've been hiding (perhaps treating the Earth as a zoo or something like that), or the naive picture of Star Trek - like extraterrestrials colonizing the galaxy as legacy individualistic "skin sacks" is false.

anon said...

"The probability of life arising..."

What does that mean? Really?

You're just putting words together. You're not actually saying anything. Do "colorless green dreams sleep furiously"?

Probability is a model only. It is only a tool. It is not the reality.

The only exception, or supposed exception, to this is quantun mechanics, where randomness is "irreducible".

kackermann said...

Here's the thing:

The optimality of the genetic code is puzzling.

Its fault-tolerent arangement is tough to ascribe to luck.

By some metrics, it is a million to one lucky roll of the dice.

It had to settle on this coding early. In fact, the progenitor species, right?

Do you buy million-to-one?

Is there not room for the possibility that the building blocks were common enough to select the optimal arrangement?

Mark said...

"What I think is quite hard to grasp is that the first replicators appeared almost as fast as it was possible (that 100My figure), but they appeared only once."

Well, RNA/DNA is universal, but a whole host of other cellular machinery appears to have appeared at least twice - once in bacteria and once in Archaea. Also, as another commentator pointed out, it may have happened many more times, but the other designs were simply out-competed.

There's a very nice recent article on emergence of the original replicators at New Scientist (an article of unusual quality for that magainze!)

Todd White said...

Prof. Hsu:

I'm curious about your subhead. What do you mean by, "Pessimism of the Intellect, Optimism of the Will?"

anon said...

Steve does not know what he means when he uses "improbable" in the above.

Don said...

"improbability does not imply design or intervention"

Well, no. But it does strongly suggest it, which is all that should be expected. See page 5 of this book about the argument from design:

"http://books.google.com/books?id=0TSqizxUv-YC&dq=raymond+smullyan+who+knows&printsec=frontcover&source=bn&hl=en&ei=H0_jSrPDDon-sQO-ktCwBA&sa=X&oi=book_result&ct=result&resnum=4&ved=0CBgQ6AEwAw#v=onepage&q=&f=false"

Don the libertarian Democrat

Chris said...

Strongly suggests it to who, and why?

Chris said...

PS: I've listened to this kind of argument—perhaps one should just say assertion—for a long time, and I've concluded that unless one is willing to define "design" as an activity of the sort practiced by the designers we actually know (ie, ourselves) then we should avoid appeals to design as an explanation.

If we honestly examine our own efforts as designers, we are forced to confront how much trial and error is involved—that is, how much our efforts as designers actually resemble biological evolution and the often convoluted and halting process of scientific discovery. I think this realization is necessary, if not sufficient, to give appeals to design some explanatory value. Explanations based on the assumption of an omniscient and omnipotent designer, or a designer whose ways we cannot possibly understand, strike me as inescapably vacuous. They explain everything and nothing, and betray their advocate's indifference to, or contempt for, genuine explanation.

Edward said...

This article--Stars May be Cosmic Road Signs to Intelligent Aliens --reminded me of your post.

Blog Archive

Labels