Why do we assume that life on other planets would be a carbon-based humanoid like us?

I completely agree with you, John. The point is that here on Earth we see organisms the breathe methane. We see organisms that breathe and produce hydrogen as a waste product, and we see organisms that produce hydrogen sulphide as a waste product. In fact some of the first organisms on Earth were methanogenic bacteria. They produced methane by taking simple carbon building blocks from the environment, jamming them together with four hydrogens and making methane. Some scientists from Japan found some of the world's oldest bacteria locked away inside tiny bubbles inside quartz. Life can come in all shapes and sizes, and evolution can pattern us and puts pressure on us to become adapted to our environment. On a different planet, things could be completely different and we have no reason to assume that alien life would look or even metabolise anything like we do.