Don't believe people when they say rubbish like this:
Because they are so complicated, they are unlikely
to have evolved twice.
It's not like that complexity has spontaneously come into existence, it's the result of normal evolutionary processes which give rise to complexity of all sorts all the time. If you ask biologists about this, most would agree there is nothing inherently improbable about the evolution of any known cell type.
Of course that doesn't mean those processes wouldn't produce a completely different organism on another world, but chances are this organism would have to solve similar problems as the ones here on Earth. The need to process information and react to it in some way is not unique to our world in any way.
> So if neurons did evolve twice
Looking at all the systems organisms are using to carry information (not just neurons, but all kinds of chemical signaling) suggests that these systems perform a very basic and ubiquitous role in all ecosystems. Even if the assertion about neurons was true (which it most certainly isn't), claiming all of our biological information-passing systems as special and "unlikely to evolve twice" is stretching an implausible proposition even further.
> it is unlikely they are the Great Filter
It's unlikely anything within biology is the Great Filter, but the thing at the beginning of biology might be a candidate: it's unclear how probable the formation of a simple cell from non-biological material is. My personal hunch is that the Great Filter might really be a combination of factors, both local to us and global, and abiogenesis _might_ be one of those that globally lower the odds for life significantly.
Extrapolating from other organisms' "great filter" here on Earth, although on a smaller scale, I think it's overconsumption of resources. That's how you let large populations wipe themselves out and it has happened numerous times here already. It's always a problem when a population does not have a natural enemy to keep themselves in balance. If it it happens, it'll happen on a more extreme scale for us, but only because we are so influental. While a smaller population will provide us resources once again, I'm not sure an advanced society will have an easy time getting up to speed again. We wouldn't be able to build the society of today all over again from scratch since oil and ores have become too scarce -- it's built on the foundation that is was once a highly available and cheap resource requiring low tech tools for extraction.
> We wouldn't be able to build the society of today all over again from scratch since oil and ores have become too scarce
Are you sure? We don't burn iron, we concentrate it on the surface. Ruined cities would be more concentrated ore sources than any early mines could be.
Oil is harder, but you can make biogas, so it will just mean higher fuel prices.
Good point about ores. I think it's a bit of give and take there. There are many unrecycable products trashed but there's also no doubt major advantages there.
Yes, as for oil I was thinking more along the lines of being essential to produce machinery, and the problems when you end up walking up that chain all the way to the lowest common denominator, so to speak, or conversely the added costs which get added on top of each other. It would be a quite different world. I wonder what it would look like.
I had never considered that we might actually be ahead of the Great Filter. From what I'd read about it I've always thought of it as something we are yet to run into.
