As I mentioned in my previous post, we also have information as to the SPEED at which life evolved on earth.

The earth formed about 4.6 billion years ago. Between 4.2 and 3.8 billion years ago there was a heavy asterodial bombardment of the earth. Its unlikely that life could have prospered under these conditions. The environment did not stabilise until around 3.8 billion years ago. The first evidence of life dates from around 3.8 billion years ago.

So as soon as conditions existed that favoured life, life developed. This makes it very unlikely that the development of life is a rare event. As I see it the default assumption has to be that life starts to develop as soon as condition favour it.

It tells us that the probability is not exactly zero. It tells us that, given enough time, life will form again under similar conditions. The question is, what is the average time for life to form once the conditions are present? Could this be so much longer than the lifetime of a star that life in the universe was a one shot deal?

We have a sample size of one, but fortunately, it does tell us some things. Let's set the window of opportunity for life to arise on an earth-like planet in a sun-like solar system to be about 8 billion years. We have evidence that the earliest life on Earth formed within two to three hundred million years of the time it was first possible. That means that on Earth, life formed in about the first 3% of the window of opportunity.

This certainly lends evidence to support the idea that the average time for life formation per planet is not very large. It would have to be exceedingly large for life to be a once in a universe phenomenon, given any reasonable estimate of the number of planets with early earth-like conditions in the entire history of the universe.

I am in agreement with you. But there are many others who argue that the odds for life elsewhere are very long or very short.

Let me put it this way. The empirical probability of life in the universe is clearly (at least) one over... Well, that's the damnable problem. We don't know the denominator so we can't assign a probability.

Or here's another: We know that lifeless molecules exist in the cosmos naturally, and we know that much more complex molecular structures exist within living cells. What we don't know is (1) How likely it is that lifeless molecules can become living entities, and (2) How great is the opportunity for that to happen.

As far as (1) goes there are many people who believe that matter is self-organizing and that the likelihood of lifeless molecules becoming lifeful is almost a cinch given the right circumstances. As for (2) "the right circumstances" plus time is what is required; and there are many who believe that there exist throughout the cosmos many, many places that constitute the right circumstances. What we do know from Big Bang theory is that the universe is approximately 13.7 billion years old, which appears to be plenty of time considering that life appeared here on earth merely some hundreds of millions of years after it developed.

However, according to the notion of panspermia it is possible that the really, really hard part of life-building took place prior to the earth's coming into existence.

One more however: maybe matter is not sufficiently self-organizing and that living matter can only arise by chance. Then the odds look long (although we have no idea what the actual probability is), but however long they look we know that we exist.

And from that I know which way to bet.

We have no idea. None. Everyone making claims here is just blowing smoke. We can't even go 50/50, because we don't even have a context in which to cast our guess.

I like this a lot, but don't forget the possibility of panspermia.

I tend to agree with this except for one beautiful (or ugly) fact: we exist.

Applying the cosmological principle of "mediocrity" it would appear that it is more likely that we are not alone.

However the principle of mediocrity is from induction and like all arguments from induction it "proves" nothing. Tomorrow will be much like today--probably. The sun will rise--probably. But we don't know for sure.

QFT

Add in the likely fact that life began from scratch only once on Earth, where conditions were ideal at that time, and it starts to look even less likely to occur again elsewhere.

And if science doesn't recreate abiogenesis inside a couple dozen years you could consider it occuring randomly a very slim proposition indeed.

Right, and with a sample size of 1, how valid is an inductive argument?

False. This is senseless, and moreover it indicates a lack of understanding of both the biological sciences and statistics.

One more thing: we are presumably talking about life as we know it, carbon-based life. We have no examples of other possible forms of life. Of course strictly speaking this does not really address the question of whether life is or is not likely elsewhere.

Here's another notion that strictly speaking also does not really address the question: over 90% of the universe consists of matter and energy we know nothing about. What kinds of life may or may not be possible among the dark matter and dark energy is unknown.

Poker players know one thing that others may not: sometimes you have to bet on the basis of incomplete information. I choose to bet that we are not alone.

I think the heart of the disagreement is in justifying your bolded claim. I dont think looking at how the world is in a one-off situation can tell us anything about how likely it is that would happen. All it tells us is that it did happen. Life did arise very quickly, sure - was that a lucky fluke or was it because it was bound to happen as soon as it could? We really wont know until we know more about those initial conditions and/or processes.

We only choose between inductive arguments. They are never "valid" in the same sense that logical arguments are. I think that tiny sample of one gives the edge, however small, to our not being alone. But again it proves nothing.

The "likely fact" is not a fact at all as it is entirely possible that life began on this planet and died, began and died, etc., many times before it became established. The early environment of the earth may suggest that.

I don't agree. And if the sun had only risen once through human history, I would hardly expect it to rise again. Maybe it would, maybe it wouldn't. I certainly have much stronger evidence that the sun will rise again based on the fact that the sun has risen every day of my life, and that there is significant observational evidence explaining why it rises and predicting that it will rise again.

That an event happens once doesn't imply that multiple occurences of the event must happen. If that were true, it would imply multiple universes. Which would remove the validity of our example by providing a greater context within which other examples of life may exist. Any way you look at it, there's no way to talk about any "edges" or likelihoods with a sample size of 1. We don't know anything about the probabilities for "life exists in many places," "life exists but only in one place," and "life doesn't exist." We know that the latter is not the case, but this knowledge is irrelevant because we are only able to ask the question in the possible minority of cases where life actually exists.

If the probability of life existing in the average star system is 1 in 10^200, and there are exactly 10^200 star systems, then we can talk about the likelihood of other life existing. But until we have the information necessary to provide a framework, we have nothing.

no, but how about - when it deals with chemical interactions ( no magic involved) the fact it occurred once means the likelihood of it occurring again can't be zero? ( even though it may have reoccurred zero times).

luckyme