It's funny how much optimism is part of the equation. For me it's the same thing. I usually think that the chances of life evolving must be relatively high. But then I start to think: Then why didn't life evolve  multiple times on earth? Every living thing (as far as we know) shares the same chemistry and can be traced back to the last universal common ancestor. <br>Or maybe it did evolve multiple times but RNA based life simply outcompeted it. But then again if it is chemically different enough, wouldn't you expect them to fill different ecological niches?<br><br>And it just keeps going like that.,(reply>), @Mountain Fisher of course because as far as we know it doesn't exist. However scientists know that there are certain elements with compareable properties to carbon, like silicium for example. That could in theory be the basis of non-carbon based life. So yes. As far as we know all life is carbon based but as you know, new discoveries are made all the time and there might very well come a day that we discover or are able to prove that life can be based on silicium or another element. They don't mean to say they know it exist, they mean that it very well might exist.<br><br>"Chemicaly different from us" isn't limited to non-carbon based life either. Even life that is carbon based but uses drastically different chemical process from life as we know and understand it could be possible.,(reply>), Biologists almost all state that life is carbon based and those that don't say it has to be carbon based do not have not even one example of non-carbon based life, not one.