Tag Archives: Origin of Life

Cross Of The Moment Film

In 2002, Jacob Freydont-Attie made the ok movie String Theory (decent camera work & acting, good characters, some compelling interactions, & non-sensical physics mumbo-jumbo). He’s now working on a non-fiction film Cross of the Moment, “on the greater philosophical issues of life on Earth.” He just posted a 24 minute draft of the first of five parts, on the Fermi Question. He interviews myself and Donald Brownlee and Peter D.Ward, authors of the book Rare Earth. The other two were interviewed indoors, I was outdoors. It seems to me that indoors looks better.

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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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Two Kinds of Panspermia

Caleb A. Scharf offers an interesting argument against interstellar panspermia:

You and I, or fluffy bunnies and daffodils are all unlikely candidates for interplanetary or interstellar transferral. The sequence of events involved in panspermia will weed out all but the toughest or most serendipitously suited organisms. So, let’s suppose that galactic panspermia has really been going on for the past ten billion years or so – what do we end up with? …

Life driven by cosmic dispersal will probably end up being completely dominated by the super-hardy, spore-forming, radiation resistant, chemical-eating, and long-lived but prolific type of critters. …

The problem, and the potential paradox, is that if evolved galactic panspermia is real it’ll be capable of living just about everywhere. There should be stuff on the Moon, Mars, Europa, Ganymede, Titan, Enceladus, even minor planets and cometary nuclei. Every icy nook and cranny in our solar system should be a veritable paradise for these ultra-tough lifeforms, honed by natural selection to make the most of appalling conditions. So if galactic panspermia exists why haven’t we noticed it yet? (more)

I see two rather different interstellar panspermia scenarios:

  1. Space-centered – As Scharf says, life might mainly drift from one harsh space environment to another. Yes sometimes life would fall onto and then prosper on someplace like Earth, but being poorly adapted to space such planet life would contribute less to future space life. Under this scenario life must on average grow in common space environments, and so we should see a lot of life out there in such environments.
  2. Planet-centered – Alternatively, space life might usually die away, and only grow greatly in special rare places like planets (or perhaps comets). In this scenario the progress of life would alternate between growth on planets (or comets) and then decay in space. A similar scenario plays out when seeds like coconuts drift between islands in the ocean – seeds die away during ocean journeys, and then multiply on islands. In this scenario life would be adapted both to grow well on planets, and to decay as slow as possible in space.

Scharf’s argument weighs against a space-centered scenario, but not a planet-centered scenario. Of course there is actually a range of intermediate scenarios, depending on how wide a range of environments let life grow.

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Why Complex Life Is Rare

I’ve said before that we have pretty good evidence for off-Earth bacteria life, suggesting that such life is common in the nearby universe. However, bacterial life might be common, yet complex multi-cellular life very rare. Here’s a plausible detailed theory about why:

Under conditions typical of alkaline hydrothermal vents, the combining of H2 and CO2 to produce the molecules found in living cells – amino acids, lipids, sugars and nucleobases – actually releases energy. … Life … is an inevitable consequence of a planetary imbalance, in which electron-rich rocks are separated from electron-poor, acidic oceans by a thin crust, perforated by vent systems that focus this electrochemical driving force into cell-like systems. The planet can be seen as a giant battery; the cell is a tiny battery built on basically the same principles. … The origin of life needs a very short shopping list: rock, water and CO2. … The universe should be teeming with simple cells. …

The problem that simple cells face is this. To grow larger and more complex, they have to generate more energy. The only way they can do this is to expand the area of the membrane they use to harvest energy. To maintain control of the membrane potential as the area of the membrane expands, though, they have to make extra copies of their entire genome – which means they don’t actually gain any energy per gene copy. …

Eukaryotes get around this problem by acquiring mitochondria, … containing both the membrane needed to make ATP and the genome needed to control membrane potential. … They were stripped down to a bare minimum. … Mitochondria originally had a genome of perhaps 3000 genes; nowadays they have just 40 or so genes left. For the host cell, it was a different matter. As the mitochondrial genome shrank, the amount of energy available per host-gene copy increased and its genome could expand. …

We know it happened just once on Earth because all eukaryotes descend from a common ancestor. The emergence of complex life, then, seems to hinge on a single fluke event – the acquisition of one simple cell by another. … The outcome was by no means certain: the two intimate partners went through a lot of difficult co-adaptation before their descendants could flourish. This does not bode well for the prospects of finding intelligent aliens. (more)

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Your existence is informative

Warning: this post is technical.

Suppose you know that there are a certain number of planets, N. You are unsure about the truth of a statement Q. If Q is true, you put a high probability on life forming on any given arbitrary planet. If Q is false, you put a low probability on this. You have a prior probability for Q. So far you have not taken into account your observation that the planet you are on has life. How do you update on this evidence, to get a posterior probability for Q? Since your model just has a number of planets in it, with none labeled as ‘this planet’, you can’t update directly on ‘there is life on this planet’, by excluding worlds where ‘this planet’ doesn’t have life. And you can’t necessarily treat ‘this’ as an arbitrary planet, since you wouldn’t have seen it if it didn’t have life.

I have an ongoing disagreement with an associate who suggests that you should take ‘this planet has life’ into account by conditioning on ‘there exists a planet with life’. That is,

P(Q|there is life on this planet) = P(Q|there exists a planet with life).

Here I shall explain my disagreement.

Nick Bostrom argues persuasively that much science would be impossible if we treated ‘I observe X’ as ‘someone observes X’. This is basically because in a big world of scientists making measurements, at some point somebody will make most mistaken measurements. So if all you know when you measure the temperature of a solution to be 15 degrees is that you are not in a world where nobody ever measures its temperature to be 15 degrees, this doesn’t tell you much about the temperature.

You can add other apparently irrelevant observations you make at the same time – e.g. that the table is blue chipboard – in order to make your total observations less likely to arise once in a given world (at its limit, this is the suggestion of FNC). However it seems implausible that you should make different inferences from taking a measurement when you can also see a detailed but irrelevant picture at the same time than those you make with limited sensory input. Also the same problem re-emerges if the universe is supposed to be larger. Given that the universe is thought to be very, very large, this is a problem. Not to mention, it seems implausible that the size of the universe should greatly affect probabilistic judgements made about entities which are close to independent from most of the universe.

So I think Bostrom’s case is good. However I’m not completely comfortable arguing from the acceptability of something that we do (science) back to the truth of the principles that justify it. So I’d like to make another case against taking ‘this planet has life’ as equivalent evidence to ‘there exists a planet with life’.

Evidence is what excludes possibilities. Seeing the sun shining is evidence against rain, because it excludes the possible worlds where the sky is grey, which include most of those where it is raining. Seeing a picture of the sun shining is not much evidence against rain, because it excludes worlds where you don’t see such a picture, which are about as likely to be rainy or sunny as those that remain are.

Receiving the evidence ‘there exists a planet with life’ means excluding all worlds where all planets are lifeless, and not excluding any other worlds. At first glance, this must be different from ‘this planet has life’. Take any possible world where some other planet has life, and this planet has no life. ‘There exists a planet with life’ doesn’t exclude that world, while ‘this planet has life’ does. Therefore they are different evidence.

At this point however, note that the planets in the model have no distinguishing characteristics. How do we even decide which planet is ‘this planet’ in another possible world? There needs to be some kind of mapping between planets in each world, saying which planet in world A corresponds to which planet in world B, etc. As far as I can tell, any mapping will do, as long as a given planet in one possible world maps to at most one planet in another possible world. This mapping is basically a definition choice.

So suppose we use a mapping where in every possible world where at least one planet has life, ‘this planet’ corresponds to one of the planets that has life. See the below image.

Which planet is which?

Squares are possible worlds, each with two planets. Pink planets have life, blue do not. Define ‘this planet’ as the circled one in each case. Learning that there is life on this planet is equal to learning that there is life on some planet.

Now learning that there exists a planet with life is the same as learning that this planet has life. Both exclude the far righthand possible world, and none of the other possible worlds. What’s more, since we can change the probability distribution we end up with, just by redefining which planets are ‘the same planet’ across worlds, indexical evidence such as ‘this planet has life’ must be horseshit.

Actually the last paragraph was false. If in every possible world which contains life, you pick one of the planets with life to be ‘this planet’, you can no longer know whether you are on ‘this planet’. From your observations alone, you could be on the other planet, which only has life when both planets do. The one that is not circled in each of the above worlds. Whichever planet you are on, you know that there exists a planet with life. But because there’s some probability of you being on the planet which only rarely has life, you have more information than that. Redefining which planet was which didn’t change that.

Perhaps a different definition of ‘this planet’ would get what my associate wants? The problem with the last was that it no longer necessarily included the planet we are on. So what about we define ‘this planet’ to be the one you are on, plus a life-containing planet in all of the other possible worlds that contain at least one life-containing planet. A strange, half-indexical definition, but why not? One thing remains to be specified – which is ‘this’ planet when you don’t exist? Let’s say it is chosen randomly.

Now is learning that ‘this planet’ has life any different from learning that some planet has life? Yes. Now again there are cases where some planet has life, but it’s not the one you are on. This is because the definition only picks out planets with life across other possible worlds, not this one. In this one, ‘this planet’ refers to the one you are on. If you don’t exist, this planet may not have life. Even if there are other planets that do. So again, ‘this planet has life’ gives more information than ‘there exists a planet with life’.

You either have to accept that someone else might exist when you do not, or you have to define ‘yourself’ as something that always exists, in which case you no longer know whether you are ‘yourself’. Either way, changing definitions doesn’t change the evidence. Observing that you are alive tells you more than learning that ‘someone is alive’.

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Our Quiet Galaxy

Part of our surviving the great filter was our galaxy having especially few collisions with other galaxies:

The Milky Way and Andromeda are siblings, … we used to think they were near-twins. .. [But] the black hole at [Andromeda's] heart is more than a hundred times as massive as ours. And while our galaxy is strewn with about 150 of the bright galactic baubles known as globular clusters, Andromeda boasts more than 400. … Whereas Andromeda is a pretty well-adjusted spiral, the Milky Way is an oddball – dimmer and quieter than all but a few per cent of its peers. That is probably because typical spirals such as Andromeda are transformed by collisions with other galaxies over their lifetimes. …

The Milky Way must have lived relatively undisturbed. Except for encounters with a few little galaxies such as the Sagittarius dwarf, which the Milky Way is slowly devouring, we wouldn’t have seen much action for 10 billion years. Perhaps that is why we are here to note the difference. More disturbed spirals would have suffered more supernova explosions and other upheavals, possibly making the Milky Way’s rare serenity especially hospitable for complex life. (more)

So alien life is more likely to be found in our galaxy than in random galaxies. More generally, the more steps in the filter that are spatially correlated like this, the more likely that if life is anywhere out there, it is especially near to us.

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Turbulence Contrarians

A few months ago I came across an intriguing contrarian theory:

Hydrogravitional-dynamics (HGD) cosmology … predicts … Earth-mass planets fragmented from plasma at 300 Kyr [after the big bang]. Stars promptly formed from mergers of these gas planets, and chemicals C, N, O, Fe etc. were created by the stars and their supernovae. Seeded gas planets reduced the oxides to hot water oceans [at 2 Myr], … [which] hosted the first organic chemistry and the first life, distributed to the 1080 planets of the cosmological big bang by comets. … The dark matter of galaxies is mostly primordial planets in proto globular star cluster clumps, 30,000,000 planets per star (not 8!). (more)

Digging further, I found that these contrarians have related views on the puzzlingly high levels of mixing found in oceans, atmospheres, and stars. For example, some invoke fish swimming to explain otherwise puzzling high levels of ocean water mixing. These turbulence contrarians say that most theorists neglect an important long tail of rare bursts of intense turbulence, each followed by long-lasting “contrails.” These rare bursts not only mix oceans and atmospheres, they also supposedly create a more rapid clumping of matter in the early universe, leading to more earlier nomad planets (not tied to stars), which could then lead to early life and its rapid spread.

I didn’t understand turbulence well enough to judge these theories, so I set it all aside. But over the last few months I’ve noticed many reports about puzzling numbers and locations of planets:

What has puzzled observers and theorists so far is the high proportion of planets — roughly one-third to one-half — that are bigger than Earth but smaller than Neptune. … Furthermore, most of them are in tight orbits around their host star, precisely where the modellers say they shouldn’t be. (more)

Last year, researchers detected about a dozen nomad planets, using a technique called gravitational microlensing, which looks for stars whose light is momentarily refocused by the gravity of passing planets. The research produced evidence that roughly two nomads exist for every typical, so-called main-sequence star in our galaxy. The new study estimates that nomads may be up to 50,000 times more common than that. (more)

This new study was theoretical. It used a best fit power law fit to the distribution of nomad planet microlensing observations to predict ~60 Pluto sized or larger nomad planets per star.  When projected down to the comet scale, this power law actually matches known bounds on comet density. The 95% c.l. upper bound for the power law parameter gives 100,000 such wandering Plutos or larger per star.

I take all this as weak support for something in the direction of these contrarian theories – there are more nomad planets than theorists expected, and some of that may come from neglect of early universe turbulence. But thirty million nomad Plutos per star still seems pretty damn unlikely.

FYI, here is part of an email I sent the authors in mid December, as yet unanswered: Continue reading "Turbulence Contrarians" »

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The SETI Game

When listening for signals from aliens, it isn’t enough to just point an antenna at the sky. One must also choose details like directions, angles, frequencies, bandwidths, pulse widths, and pulse intervals. Apparently most SETI searches assume that for a given signal power density, aliens would pick details to make it as easy as possible for us to detect their signals. So standard SETI searches are optimized for such easily-seen signals. Two excellent papers, published back in July, instead consider what sort of signals would be sent by “beacon” building aliens, who seek to create the maximum possible power density at any given distance away from them.  (One of the authors is SF author Greg Benford.) Such signals are quite different, and most of today’s SETI searches are not very good at seeing them:

Minimizing the cost of producing a desired power density at long range … determines the maximum range of detectability of a transmitted signal. We derive general relations for cost-optimal aperture and power. … Galactic-scale beacons can be built for a few billion dollars with our present technology. Such beacons have narrow “searchlight” beams and short “dwell times” when the beacon would be seen by an alien observer in their sky. … Cost scales only linearly with range R, not as R2. … They will likely transmit at higher microwave frequencies, 10 GHz. The natural corridor to broadcast is along the galactic radius or along the local spiral galactic arm we are in. …

Cost, spectral lines near 1 GHz, and interstellar scintillation favor radiating frequencies substantially above the classic “water hole.” Therefore, the transmission strategy for a distant, cost-conscious beacon would be a rapid scan of the galactic plane with the intent to cover the angular space. Such pulses would be infrequent events for the receiver. Such beacons built by distant, advanced, wealthy societies would have very different characteristics from what SETI researchers seek. … We will need to wait for recurring events that may arrive in intermittent bursts. …

A concept of frugality, economy. … directly contradicts the Altruistic Alien Argument that the beacon builders will be vastly wealthy and make everything easy for us. An omnidirectional beacon, radiating at the entire galactic plane, for example, would have to be enormously powerful and expensive, and so not be parsimonious. … For transmitting time t, receiver detectability scales as t1/2. But at constant power, transmitter cost increases as t, so short pulses are economically smart (cheaper) for the transmitting society. A 1-second pulse sent every 10 minutes to 600 targets would be 1/600 as expensive per target, yet only *1/25 times harder to detect. Interstellar scintillation limits the pulse time to >10-6 s, which is within the range of all existing high-power microwave devices. Such pings would have small information content, which would attract attention to weaker, high-content messages. …

Cost-optimized beacons … can be found by steady searches that watch the galactic plane for times on the scale of years. Of course, SETI literature abounds with consideration of the trade-offs of search strategy (range vs. EIRP vs. pulse vs. continuous (continuous wave, CW) vs. polarization vs. frequency vs. beamwidth vs. integration time vs. modulation types vs. targeted vs. all-sky vs. Milky Way). But, in practice, search dwell times are a few seconds in surveys and 100–200 seconds for targeted searches. Optical searches usually run to minutes. And integration times are long, of order 100 s, so short pulses will be integrated out. …

Behind conventional SETI methods lies the assumption that altruistic beaming societies will send persistent signals. In searches to date, confirmation attempts, when the observer looks back at a target, in practice usually occur days later. Such surveys have little chance of seeing cost-optimized beacons. … Distant, cost-optimized beacons will appear for much less time than as assumed in conventional SETI. Earlier searches have seen pulsed intermittent signals resembling what we (in this paper) think beacons may be like, and may provide useful clues. We should observe the spots in the sky seen in previous work for hints of such activity but over year-long periods. (more)

Of course both the usual assumption that aliens will pay any cost to make a given power density signal easy for us to see, and the new assumption that aliens ignore our costs and merely seek to maximize power density, are both somewhat unsatisfactory. It would be better to model this interaction as a game, where each side has a limited budget and seeks to maximize the probability of at least one successful communication, holding constant the behavior it expects from the other side. Each side would of course also have to integrate over possible locations and budgets for the other side.

I’m very interested in working with (sim, math, or physics) competent folks to more formally model this SETI communication game.

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On Berserkers

Adrian Kent is getting a little publicity for posting his ’05 paper on the berserker hypothesis, “that evolution has very significantly suppressed cosmic conspicuity”, i.e., that many aliens are out there, but hiding from each other. He advocates taking the hypothesis seriously, but doesn’t actually argue for the coherence of any particular imagined scenario. Kent’s excuse:

It would be very difficult to produce a model that convincingly predicts the likelihoods and spatial distributions of the various strategies, since the answer surely depends on many unknowns.

He instead just claims:

The hypothesis is certainly not logically inconsistent and it seems to me not entirely implausible.

So what then is Kent’s contribution? Apparently it is a bunch of strategy fragments, i.e., strategy issues that aliens might consider in various related situations. It is not clear that these are much of a contribution, at least relative to the many contained in related science fiction novels. But, well, here they are: Continue reading "On Berserkers" »

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Earth Is Not Random

The great filter is whatever obstacles stand in the way of simple dead matter eventually giving rise to a visibly expanding interstellar civilization. It is now confirmed that a non-trivial chuck of that filter is in planets having special orbits that let climates be stable over time:

Planetary anthropic selection, the idea that Earth has unusual properties since, otherwise, we would not be here to observe it, is a controversial idea. This paper … [compares] Earth to synthetic populations of Earth-like planets … [for] high (or low) rates of Milankovitch-driven climate change. Three separate tests are investigated: (1) Earth-Moon properties and their effect on obliquity; (2) Individual planet locations and their effect on eccentricity variation; (3) The overall structure of the Solar System and its effect on eccentricity variation. In all three cases, the actual Earth/Solar System has unusually low Milankovitch frequencies compared to similar alternative systems. All three results are statistically significant at the 5% or better level, and the probability of all three occurring by chance is less than 10^-5. It therefore appears that there has been anthropic selection for slow Milankovitch cycles. This implies possible selection for a stable climate, which, if true, undermines the Gaia hypothesis and also suggests that planets with Earth-like levels of biodiversity are likely to be very rare. Continue reading "Earth Is Not Random" »

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