Some scientists believe that if the conditions on a planet are similar to the ones that existed on the earth a few billion years ago, the emergence of life is inevitable. But since exact copies of the early earth are likely to be few and far between, it is difficult to predict how much change in these conditions life can tolerate and still emerge.

Since the processes required for life as we know it can take place in quite a broad temperature range (over one hundred degrees), I think it is indeed possible that any planet with liquid water, or maybe even some other proper solvent, could develop life.

Again, if we get so far that multicellular organisms emerge, it is reasonable to speculate that they can develop as far and even further than us humans. I would still assume that if there is life on other planets, it is in vast majority of cases single-cellular or something "basic", because for mutlicellular organisms (or very complex organisms if they are not made of cells), the requirements for their surrounding conditions are likely to be much more strict than for simpler organisms.

It is currently impossible to even estimate probabilities. We simply do not have the data.

There is a major fallacy in a lot of thinking on this subject. That is : to assume that, because life on Earth survives and even thrives in a wide range of environmental conditions, that it can do the same on other planets. The problem is that, while living things can adapt to these widely variable conditions, after 3 billion years of evolution, there is absolutely no way of knowing what conditions life needs to originate in the beginning.

There is a widely held theory that life began on Earth in geothermal waters. If so, in spite of the fact that it can now grow in Antarctica, it will not come into being under Antarctic conditions. Thus, cold planets will not originate life. However, that logic is probably pretty thin, since we still do not know what range of conditions can originate life.

In other words, you can speculate all you like about life away from Earth. Everyone’s guess is about as good as everyone elses.

I merely pointed out that processed required for life as we know it can take place within ~100 degrees of Celsius. That is the minimum prerequisite for this kind of life to emerge. I also said that exact replicas of the young earth are probably extremely rare, so if life emerged elsewhere it may have had to happen in different conditions. At the moment, of course, we can just speculate. But there is nothing wrong with well-thought speculation when we do not have the evidence to go further.

Also, the human mind is very prone to take this model of ours as the only possible one. However, completely other systems of life can exist, not only this nucleic acid-based one we know. There are scientists who think that even liquid methane oceans in some planets/moons could harbour some kind of life forms. Silicon-based life has also been proposed several times. Of course, it has been very challenging to propose a chemical model for that kind of life to function, but the simple fact that we do now know how it could happen does not mean that it cannot happen.

And I disagree with your last line. Not every guess is as good as every other. You can make well-grounded guesses and then guesses that do not have any grounds. For example, the search for life from the Martian soil is based on some very carefully thought of ideas, not just some wild guesses. Because the search for the possible extra-terrestial life is very expensive, good guesses could save much money, time and resources when compared to bad guesses. Even when the most likely outcome is that we find nothing…

biohazard

There is a very big difference between highly trained minds finding good ways to search for life, and accurately predicting where life exists.

I have a personal prediction about life on Mars myself. I predict that, if we find it, it will prove to be similar to Earth bacteria. That is based on the fact that conditions for abiogenesis and evolution of life on Mars would have had the appropriate liquid environment for a geologically short time only, making it unlikely for life to form and evolve from scratch on Mars. Add to this the fact that certain bacterial spores from Earth are capable of surviving the trip through space, driven by the solar wind, to get to Mars. The asteroid impact that killed the dinosaurs would have tossed a lot of Earth rock into space, launching bacterial spores on a likely trip to Mars.

However, my prediction may well be wrong. I am totally aware of the limitations placed on my own ability to speculate, caused by lack of data.

I did not say that anyone could accurately predict where, if anywhere, extraterrestrial life could be found 🙂

Despite this, I think we can safely assume that, for example, life is more likely to originate on a planet that has liquid water and surface temperatures around zero degrees with a one hundred degree range or so, than it would be on an ice planet with liquid methane seas. Still, this does not mean that the latter could not have any kind of life. Also, I think we can similarly assume that a planet or red-hot rock and atmospheric temperatures of several hundreds of degrees is even much less likely to have any kind of life, but again we cannot be 100% sure.

There is a major fallacy in a lot of thinking on this subject. That is : to assume that, because life on Earth survives and even thrives in a wide range of environmental conditions, that it can do the same on other planets. The problem is that, while living things can adapt to these widely variable conditions, after 3 billion years of evolution, there is absolutely no way of knowing what conditions life needs to originate in the beginning.

It is the whole matter of the common and very basic experience. The life on the earth on the earth normal with the water and we normally thinks that the life is based on the carbon atoms.It might be possible to use genetic theory,to make DNA based life survive indefinitely, or at least for a hundred thousand years. But an easier way, which is almost within our capabilities already, would be to send machines. These could be designed to last long enough for interstellar travel.