Why are (most) aliens humanoid?

I read a book released by SETI when I was a kid. It had articles by Shostak and various scientists and philosophers. In amongst these chapters were short stories by Bova, Asimov and Clarke. A chapter described how dolphins have a similar language cortex (sylian?) to us. It fairly caught my imagination.

One chapter reasoned that the most likely form ETs would take would be upright, two legs, two arms, opposable thumbs and binocular stereo vision.

Opposable thumbs would be required for the manipulation of tools. Stereo vision to focus on intricate manipulation of material objects. It reasoned that nature is efficient. If we had three arms they would be useful, but two arms serves our needs adequately. One arm would fail through natural selection. The same sort of logic applies to two legs. The ability to remain upright whilst in motion would offer advantages towards survival when predators are a factor. Rodents can hold and manipulate objects (squirrel eating a nut), but they can't hold an object when moving.

I don't know how well those ideas would stand up to modern scrutiny, but they seemed fairly reasonable.

First before I go into this I wanted to say that yes I understand there is no way we could know how another planet's life would evolve. Furthermore I understand also that they may not even have the same atomic base or DNA structure as life on this planet.

With that said lets take a look at life here on Earth since it is the only reference we have.

What do we see in most non-invertebrate life here on earth (save for fish)? 4 appendages right? Take a small lizard. It has 2 front legs and two rear legs (tail doesn't count since some have tails some don't but other things remain constant).

Take a dog 2 front paws 2 rear paws. Same with every mammal even ones with a single tail like a manatee show signs of 4 appendages even if they are attached to each other forming a single tail to swim. The fact of the matter is most life on this earth have 4 appendages.

All it took for us was that one evolutionary kicker to make us stand up. Who is to say since having 4 appendages (ergo a solid foundation) is common on this world why would it not be common elsewhere? Logic would tell you if there are crawling critters out there in the universe at least some would have 4 appendages correct?

Now those 4 legged ET critters eventually evolve hand like structures with a thumb therefore giving them the ability to use tools and create technology. There is more than one line on this planet with a similar hand structure btw outside the simian realm of thought. (Example an eagle has hands/feet that can grasp).

The humanoid form could develop elsewhere just like that. This is a logical way of looking at it IMO.

reply to post by Kandinsky


Well that's the thing that bugs me. We are trying to answer these questions with our logic. Our logic is based on the things we know and understand.

Why should they have opposable thumbs? Nature and/or evolution is probably millions years ahead on their design. There can be something much more efficient instead of opposable thumbs.

Again using my logic to think about how we can't use our logic to answer these questions is quite the paradox