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Comment by A_D_E_P_T | Hacker News Reader

On timescales of billions of years, we don't know if any of those steps are "hard," "easy," or "inevitable." As the first commenter to that article notes at the link, we have only a single datapoint.

It's certainly unclear that photosynthesis is an ironclad requirement for life, and, in any case, it evolved multiple times in parallel on Earth. (Anoxygenic photosynthesis in fact evolved, de novo, more than once.)

I'd add that it's also by no means a given that life, broadly defined, can only evolve around a sunlike star.

Speculation around "hard steps" is pseud naval gazing until we gather more datapoints. It's not even valuable as speculation -- except perhaps as an exercise in countering sloppy thinking.

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> I'd add that it's also by no means a given that life, broadly defined, can only evolve around a sunlike star.

Not a given, but not unreasonable either. Larger stars die faster so they probably won't be around long enough for life. Smaller stars last longer, but they also emit less energy and so there is less likely to be higher life forms just because there isn't enough energy to support life.

Of course life might not be like we know it, but carbon and water are very common in the universe compared to most of the other options and so they are more likely.

> Smaller stars last longer, but they also emit less energy and so there is less likely to be higher life forms just because there isn't enough energy to support life.

This isn’t really true. The energy density at a planet’s surface is a function of both the star’s luminosity and the planet’s distance. Most star’s (at least in the Milky Way) are small, and most stars have planets in orbits closer than is Earth’s. The so called “Goldilocks zone” is expected to have at least one planet in it for a very large number of stars (again, at least in our galaxy).

> most stars have planets in orbits closer than is Earth’s.

I don't think that this is true. IIRC it's an artifact of the techniques we use to detect extrasolar planets -- our techniques are biased to detect planets that are closer to their stars than further away,

Yes, we have several techniques for detecting exoplanets, and at least two of them have that bias. However, what I said still holds - our best estimates are that most stars with planets have (at least some) planets in orbits closer than is Earth’s, and therefore asserting that “planets with sufficient energy density at their surface to support complex life are rare” is unwarranted

I guess the important bit that always gets left out is that these things revolve around life as we know it. Of course you could have a completely different biochemistry resemble the processes that we generally associate with life. But until we either replicate that in a lab or find signs of life on other planets (earth-like or otherwise), the best speculation we can do is based on what we see here. And we see that certain steps on the path towards complex multicellular life took very long compared to others.

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