Don't argue with me, Sugarfoot, argue with astrobiologists. That's where I got most of what I wrote above.

Animals of any size at all tend toward four limbs. This applies to land animals, although the same thing tends to apply to aquatic ones. Count functioning fins on fish, cetaceans, et al. Squid and octopi are also bilaterally symmetrical, as are most microbiological creatures larger than a few cells, even thought they are not skeletal. A skeleton (support frame) is pretty essential to a land animal over a given size. More limbs than four tend to get in the way. But this may very well be subject to variation.

Astrobiologists (exobiologists) are pretty much in agreement that, given conditions essentially analogous to those on earth (what we would consider a "Goldilocks planet" which is not too cold and not to hot, but 'just right'"—i.e., where liquid water can exist).

From New Scientist by Stephen Battersby

What might extraterrestrial life look like?

Arguably, quite familiar. Astrobiologists suspect that chemical life will be cellular - vital to maintain a balanced chemical environment - so alien microbes might look a lot like terrestrial microbes. Even complex alien life may be forced to use the same body designs to solve the same basic physical problems: bilateral symmetry (to avoid running in circles), round eyes, fins, wings, grasping digits.

Then again, some worlds may present environmental challenges that sculpt very alien bodies.

A briefly stated view of the vast majority of scientists in the field. And this is a field in which almost all scientific disciplines are represented.

The general consensus is that scientific laws are universal. Two plus two equals four, whether you are on Earth, Mars, the second planet orbiting the largest star in the trinary group Alpha Centauri, on some planet orbiting a star in the Andromeda galaxy, or some planet orbiting a star in a galaxy so far away that even the Hubble telescope can't detect it.

Two plus two still equals four.

Don Firth