How can you measure how long ago they had a common ancestor as you don’t measure time but generations? Good question. This is the problem of calibrating the “biological clock”. The oldest DNA found on Earth is about 0.5 billion years old. On Mars it would probably be older. It would not be easy to estimate the time of the earliest common ancestor.

Also the time difference between a hospitable Mars and Earth isn’t that large Perhaps not measured in absolute time, but maybe in relative time. The geological record shows that relatively advanced life forms appeared very shortly after Earth became hospitable to life. It seems to be down to the order of a 100 million years. Since we are ignorant about how DNA based life is created, it is difficult to say if this is surprising or not. It seems though, that life evolved at a more sedate speed later on. Compared to this brief window of evolution, Mars enjoyed a 600 million year long, early period, where it was hospitable to life.

Also the time difference between a hospitable Mars and Earth isn’t that large.

It is considered unlikely that life can survive on comets, but it is considered equally unlikely that advanced life forms could have developed spontaneously on Earth in a less than a couple of hundred million years, which is now the time window of the archeological record.

ps. 4.25 billion leads for enough time for Earthly life to be transported by asteroids to a wet Mars. In other words: “there should have been life on Mars”.

This has created a problem. How do we explain that life could develop so fast, when we don’t see any evidence of such processes happening now. We know that comets contain Amino Acids, the building blocks of life, and that they bombarded the early Earth, probably creating the oceans. Still, there is a big step from Amino Acids to life itself.

This is where speculations, that we are all “little green men”, come into the picture. Mars would probably have been hospitable to life long time before Earth. Alternatively, some people speculate that comets not only contain Amino Acids but also primitive life forms. I don’t think there is much support for this theory.

As a matter of fact, it is now a commonly hold opinion, that the early ( not late as Brostrom says) occurrence of life on Earth, can best be explained if it was seeded from Mars, which was presumably hospitable to life at an earlier time than Earth.

Not in the scientific community – hell, I’ve never heard this mentioned as anything other than one possible (if unproveable) way of getting life started on Earth.

For that matter, we don’t know what the “hospitable conditions” were for the early formation of life on Earth – all we know is that is emerged at some point.

They don’t need to be completely independent, self-regenerating and general like Bostrom suggests. Instead most probably they will be designed to perform very specific tasks (build a space base, or build a radar for example etc. etc.).

There’s still always the possibility for corruption and/or tampering when you create a program that has extensive learning capabilities.

For signs of extraterrestrial intelligent life, we would look for neutrinos with an energy of 6.3 petaelectron-volts , the so called “Glashow ressonance”. They are not produced by any natural processes. See this
link

In the short term (100-200 years) it would probably be economic to extract He-3 from the moon to use for fusion energy. In the long term (200-400 years) it could be cheaper to implement technologies which use less energy. The problem is not that such technologies don’t exist at the moment, but that they are too expensive. To take an example, we know very well how to construct a “zero-energy” house. However such a house is unaffordable to most people, and anyways, the typical lifetime of a house in most western countries is about one century.