Freethought & Rationalism Archive

Jesse · 04-15-2003, 11:07 PM

Some of you might be interested in the book Rare Earth, by Peter Ward and Donald Brownlee, two scientists who make the case that while the origin of life may be a relatively "easy step" that happens quite often, a whole bunch of very specific and unusual conditions may be present in our own planet and solar system without which the formation of long-lasting multicellular life could be very improbable. There's an online debate on this book here which includes the authors as well as Frank Drake and others. Ward and Brownlee's later book The Life and Death of Planet Earth is also interesting, suggesting for a number of reasons that the conditions needed for multicellular life on Earth are likely to end within a few hundred million years.

If true, the fact that intelligent life appeared "just under the wire" on Earth might itself have an anthropic explanation that tells us something about the likelihood of intelligent life appearing elsewhere--see the article The Great Filter by Robin Hanson, which looks for answers to "The Fermi Paradox" (the question of why, if there are intelligent extraterrestrial civilizations within the galaxy, they haven't colonized or at least visited the whole galaxy by now):

Quote:

First, let us distinguish between two different kinds of evolutionary steps. Let a "discrete" evolutionary step be one which must succeed within certain a short time period; failure then implies failure forever after. For example, if a certain type of solar system is required, then success here can only happen when the solar system forms. In contrast, let a "trial and error" step be something like search across a mostly flat fitness landscape, where failure today does not much affect the chances for success tomorrow. The main Great Filter implications are regarding trial and error type steps.

Consider a situation where a certain number of trial and error steps must be completed in a certain order within a certain total timewindow. That is, for each step there is some constant probability per unit time of completing that step, given that the previous step has been completed. If the probability of completing all the steps within the time window is low, then it turns out that for the cases where all the steps are in fact completed, the average time to complete each "hard" step is unrelated to how hard that step is!

For example, say you have one hour to pick five locks by trial and error, locks with 1,2,3,4, and 5 dials of ten numbers, so that the expected time to pick each lock is .01,.1, 1, 10, and 100 hours respectively. Then just looking at those rare cases when you do pick all five locks in the hour, the average time to pick the first two locks would be .0096 and .075 hours respectively, close to the usual expected times of .01 and .1 hours. The average time to pick the third lock, however, would be .20 hours, and the average time for the other two locks, and the average time left over at the end, would be .24 hours. That is, conditional on success, all the hard steps, no matter how hard, take about the same time, while easy steps take about their usual time (see Technical Appendix). And all these step durations (and the time left over) are roughly exponentially distributed (with standard deviation at least 76% of the mean). (Models where the window closing is also random give similar results.)

We can apply this model to the evolution of life on Earth, by examining the fossil record for roughly equally spaced apparent major innovations. Such an analysis can complement other attempts to find hard steps by intrinsic difficulty, necessity, and uniqueness in evolutionary history, such as attempted in [Barrow & Tipler 86]

The fossil record shows about five roughly-equal periods between major evolutionary changes since the Earth was formed [Schopf 92, Skelton 93]. Specifically, the earliest known clear fossils of simple single-cell life appeared 0.9 billion years after the earth cooled (4.5 billion years ago), though other evidence suggests life after only 0.5 billion years [Balter 96]). The earlist known large complex single-cell fossils ("eukaryotic" in appearance) then appear about 2.0 billion years after this early evidence. 0.8 billion years later the tempo of evolution picked up dramatically, perhaps with the invention of sex [Schopf 95], and then 0.5 billion years later we see the first substantial fossils of multi-cellular life [Knoll 95]. Finally, 0.6 billion more years brings us to where we are today.

While these periods are not exactly equal, they are roughly consistent with the (roughly exponential) distribution of actual durations between hard steps predicted by the above model of trial and error steps. Some important complications and caveats, however, must be considered.

It should be noted that the estimate for when the earth will likely become uninhabitable for multicellular life, because carbon dioxide levels will become low enough that plants go extinct, is also usually estimated at between 0.5 billion and 1 billion years from now, which would fit with the 'safecracking' model above.

Arbogast · 04-16-2003, 06:44 AM

Fermi Paradox is a lamer...

There is no evidence that the Universe is sparsely inhabited. That is idle speculation, with no knowledge existing on which to make such a bold claim. My claim is equally bold, but at least I admit it.

We are blind to what is beyond the Earth, and almost totally ignorant of the history of earth and the nearby stars.

""The Fermi Paradox" (the question of why, if there are intelligent extraterrestrial civilizations within the galaxy, they haven't colonized or at least visited the whole galaxy by now): "

Why is this always advanced as how things are. The human race has no significant knowledge pro or con on this issue...

If one assumes the Solar System doesnt contain the favored ecosystems of the local alien civilizations, how could we expect to know anything about them or their existence with our technology level???

The whole galaxy could be colonized, we would expect not to know at this point in our history if Earth was unimportant to them.

Also, with the huge variety of possible ecosystems beyond the earth, it stands to reason that Earth Biology is an obscure subset of the Science of Biology in the Universe.
Earth Biology is a subset, and Jerusalem is not the center of the Universe!!!

Arbogast · 04-16-2003, 07:06 AM

If a person is knows nothing about a subject, it is ignorant to assume that nothingness is a reflection of how much there is to know on the subject.

Just because a person was ignorant 8000 years ago about the solar system, it does not follow there is nothing to know about the solar system.

Just because we know nothing of the history of the races living in the stars, it does not follow there is no history.

Jerusalem was not the center of the Inhabited Universe, neither is the Planet Earth!!! (Article of Faith)

SULPHUR · 04-16-2003, 08:55 AM

How many here lent their PC's to the SETI program?

BioBeing · 04-16-2003, 09:09 AM

Abrogast - did you read the review from Amazon for the book Jesse linked to:

Quote:

Ward and Brownlee admit that "It is very difficult to do statistics with an N of 1...."

Of course they admit they really don't know!

Thanks for the link, Jesse. It looks like an interesting book. If the Universe Is Teeming with Aliens... Where Is Everybody? Fifty Solutions to Fermi's Paradox and the Problem of Extraterrestrial Life is also in my wish list. Now if only I had time to read...

And Sulphur - I do Folding@Home, not SETI.

Arbogast · 04-16-2003, 09:10 AM

It runs sometimes on 1 Windows and 2 Linux PCs for 2.5 years...

SETI at Home is a great early step in SETI research, and who knows, it might actually be adequate to determine an answer.

I will look at book tonight...

But I always thought Fermi's Paradox was a lamer...

BioBeing · 04-16-2003, 09:27 AM

Reading through that discussion that Jesse linked to - here is something I hadn't thought of before...

Quote:

Don Brownlee: My prediction is that the nearest alien neighbors live in feces and food scrap left on the Moon by the six Apollo missions. Even though it�s been three decades, there is a good chance that hearty bacteria live and can reproduce inside encapsulated small damp places and survive the monthly cycles of heat and cold as well as the effects of solar flares, ultraviolet light, and hard vacuum.

If born-on-the-Moon organisms are not living in food scraps (and worse) there are probably dormant terrestrial organisms trapped inside vast numbers of components -- wire harnesses and tape interfaces that are parts of the lunar lander, back packs, surface experiments, rover, etc. Somewhere out there is Allan Shepard�s unsterilized golf ball, which is likely to carry a small zoo of terrestrial microorganisms. Beyond our Moon, my great hope is that microbial life or at least fossil evidence for its prior existence will be found on Mars, Europa, or some other solar system body.

04-16-2003, 06:44 AM	#12
Arbogast Junior Member Join Date: Mar 2003 Location: Athens, Ga, USA Posts: 61	... Fermi Paradox is a lamer... There is no evidence that the Universe is sparsely inhabited. That is idle speculation, with no knowledge existing on which to make such a bold claim. My claim is equally bold, but at least I admit it. We are blind to what is beyond the Earth, and almost totally ignorant of the history of earth and the nearby stars. ""The Fermi Paradox" (the question of why, if there are intelligent extraterrestrial civilizations within the galaxy, they haven't colonized or at least visited the whole galaxy by now): " Why is this always advanced as how things are. The human race has no significant knowledge pro or con on this issue... If one assumes the Solar System doesnt contain the favored ecosystems of the local alien civilizations, how could we expect to know anything about them or their existence with our technology level??? The whole galaxy could be colonized, we would expect not to know at this point in our history if Earth was unimportant to them. Also, with the huge variety of possible ecosystems beyond the earth, it stands to reason that Earth Biology is an obscure subset of the Science of Biology in the Universe. Earth Biology is a subset, and Jerusalem is not the center of the Universe!!!

04-16-2003, 07:06 AM	#13
Arbogast Junior Member Join Date: Mar 2003 Location: Athens, Ga, USA Posts: 61	PS If a person is knows nothing about a subject, it is ignorant to assume that nothingness is a reflection of how much there is to know on the subject. Just because a person was ignorant 8000 years ago about the solar system, it does not follow there is nothing to know about the solar system. Just because we know nothing of the history of the races living in the stars, it does not follow there is no history. Jerusalem was not the center of the Inhabited Universe, neither is the Planet Earth!!! (Article of Faith)

04-16-2003, 08:55 AM	#14
SULPHUR Senior Member Join Date: Nov 2002 Location: hobart,tasmania Posts: 551	seti How many here lent their PC's to the SETI program?

04-16-2003, 09:10 AM	#16
Arbogast Junior Member Join Date: Mar 2003 Location: Athens, Ga, USA Posts: 61	I do SETI It runs sometimes on 1 Windows and 2 Linux PCs for 2.5 years... SETI at Home is a great early step in SETI research, and who knows, it might actually be adequate to determine an answer. I will look at book tonight... But I always thought Fermi's Paradox was a lamer...

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