[ycherkas (OP)]

There is a lot of interest in calculating the probability of life on earth, here are for example two very different views:
newbielink:http://evolutionfaq.com/articles/probability-life [nonactive]
newbielink:http://www.reasons.org/articles/probability-for-life-on-earth [nonactive]

So what is it? And what is the probability that life exists on some other planet?

[alancalverd]

As there is life on earth, the probability of its existence is 1, regardless of any preconceptions you may bring to the argument. Indeed the test of your preconceptions is whether they predict what has actually happened.

Given the enormous number and variety of other planets, and the very elastic definition of life, it is clear that the probability of life existing on at least one other planet is also 1.   

[Colin2B]

There is a lot of interest in calculating the probability of life on earth,

Are we asking the probability of life on earth, or intelligent life on earth?
I don't think the second one is at all likely []

Sorry, I'll read the article tomorrow, bed time here.

[alancalverd]

The "reasons" argument cited above is pure garbage. Someone has simply taken a lot of small numbers and multiplied them together, regardless of their relevance or validity, to produce a number which has neither.

Simple fact is there wasn't life, then there was, so the probability is 1. There's no point in estimating the starting odds after the horse has passed the finish line, and in biology as with gambling, betting stops once the race (or the first selfreplicating molecule) has started.

[ycherkas (OP)]

The replies remind me of the old joke:

- What is the probability that I go outside and see a dinosaur
- 50% because you either see one or don't

[alancalverd]

Bayesian statistics at its best.

Chichester advocated its inverse ("deliberate error reckoning") for trans-ocean navigation. If you aim at a specific destination, you probably won't hit it, but if you aim to the right of it, you know that as soon as you see land, your destination is on the left.

[Ophiolite]

To extrapolate the possibilities of life on other planets from a sample size of one is appropriate behaviour in the pub, but not in the world of science.

We do not know how life arose. All we have are marginally plausible possibilities. Therefore it is impossible to determine what conditions were necessary for it to arise, and therefore what the likelihood would be.

[alancalverd]

We don't know what conditions are necessary, but we have a very good idea of what conditions are sufficient, so our best guess is likely to be an underestimate.

[Ophiolite]

We don't know what conditions are necessary, but we have a very good idea of what conditions are sufficient, so our best guess is likely to be an underestimate.

If you truly believe this to be true, please specify the conditions that are sufficient.

We do not know the details of the process by which life arose. Therefore, how can you assert that there are no steps that require very specific and rare conditions, or for which the probability of success is very low.

We are simply, currently, to ignorant to make meaningful estimates.

[alancalverd]

If you truly believe this to be true, please specify the conditions that are sufficient.

Those present on Earth about 5,000,000,000 years ago. Or Divine Providence. There being no evidence for the latter, we must assume the former. In either case, the probability of life existing elsewhere is clearly nonzero.
 

[Ophiolite]

If you truly believe this to be true, please specify the conditions that are sufficient.

Those present on Earth about 5,000,000,000 years ago. Or Divine Providence. There being no evidence for the latter, we must assume the former. In either case, the probability of life existing elsewhere is clearly nonzero.

1. for the sake of accuracy, the Earth did not form until about 4.54 Ma. It is generally argued that conditions would not have been suitable for the development of life until the end of the Late Heavy Bombardment, around 3.8 Ma.
2. We do not know the detailed conditions obtaining at that time. In some regards we do not even know the general conditions that were present. (For example, debate continues as to the extent to which the atmosphere was reducing, or as to whether it was reducing at all.)
3. We therefore do not know if one or more of the necessary conditions are likely to be common on other planets, or unique to the Earth.
4. Since there is a possibility that one or more conditions were unique to the Earth, we cannot say that the probability of life elsewhere in the universe is non-zero.

[alancalverd]

The more extremophiles we discover, the wider the scope of potentially biogenic environments. The only requirement seems to be a temperature range within which the hydrogen bond is stable.

[Ophiolite]

Until we have determined, with a high degree of confidence and in comprehensive detail, a fully plausible path from organic molecules to a self-sustaining system that most biologists would be comfortable calling alive, then the wide range of environments in which we presently encounter life are irrelevant.

I lean quite heavily to Ward and Brownlee's notion that unicellular life is very common, but complex life very rare. But that can only be an opinion until the conditions noted in the prior paragraph are met.

[Don_1]

Reasons to Believe gives a whole stack of figures which are somewhat lacking.

To take the first figure "local abundance and distribution of dark matter 0.1" as being a required "Probability that feature will fall in the required range for physical life", where exactly did this figure come from? How was it calculated? Against which control was it compared?

Later in this list we find "ratio of ⁴⁰K, ²³⁵,²³⁸U, ²³²Th to iron in star-planetary system 0.02" Any scientist worth their salt will immediately note that this omits the ratio of K₉P to ²SiLi₄US.

The fact is, you could take any 'condition', known or hypothesised, and stick any figure you care to pull out the air against it as being a requirement for a "Probability that feature will fall in the required range for physical life".

The list does, however, omit the one 'probability' we know for sure: Life does exist on Earth. Therefore the probability is 100%, fact.

As for intelligence, whether it be a virus, a human or anything in between (Chimp, Shark, Dolphin, Dog, Bee), each has the knowledge to survive and reproduce, therefore, each has all the intelligence they require. We should not measure intelligence by human standards. A level of intelligence is relative to requirement.


As to the "Evolution - frequently asked questions", the first paragraph states,

Creationists often claim that the chances of a modern enzyme forming by random means are astronomically small, and therefore the chances of a complete bacterium (which is composed of hundreds or thousands of such enzymes & proteins) is so near to impossible that it would never happen in the 13 billion years or so since the universe took shape.

This does overlook the fact that some creationist purists still believe the Biblical age of the universe as being 6150 years, give or take 50 years or so. Not even enough time for the first oceans to form!