[shiroiushi]

It's quite possible that the humanoid lifeform is optimal to have a technological species that can travel between stars. An aquatic species would have huge difficulties just building technology and civilization, because of the habitat. A species without arms and thumbs would have a hard time manipulating its environment (just look at all the 4-legged animals now). A species with more than 4 limbs would likely either be too small (insects) to accomplish much, or would need too much energy (and probably evolve to lose the extra limbs over time).

There's good reasons to think that alien species might not look all that different from animals on this planet, simply due to physics. Animals here didn't simply spring to life in their current form; they evolved from single-celled organisms to best suit their environments.

[throwanem]

The word 'parochialism' positively vaults to mind. Not to mention your flagrantly unjustifiable opening assumption, given the number of bipedal technological species known to us to travel among the stars is currently zero.