A three-legged tripod-shaped biologist might argue that a bipedal lifeform is inherently unlikely, because it would be unstable in the direction perpendicular to the legs and would have a tendency to topple over without constant readjustments of its balance. It might argue that while having two eyes facing forward would provide stereoscopic vision in that direction, it would leave the organism blind from behind. So it would conclude that three eyes, located 120 degrees apart around the circumference of the head would be much more likely, because it would allow two of the eyes to come to bear on all parts of a 360 degree field of view. It might argue that bilateral symmetry with a distinct front and back would leave organisms only facing in one direction, obviously less efficient that their own trilateral symmetry that allows them to face in all directions at once. If we went back to the origin of life on Earth and replayed the whole history of evolution over again, we almost certainly wouldn't end up with human beings like ourselves a second time. I don't think that most people realize quite how contingent the history of evolution has been. Imagined abstractly, life expands like a phylogenetic tree outward into an unbounded and almost infinite possibility-space, where every point, the tip of every branch, is a different result, a different kind of organism, a different manner of life. The precise shape of the tree is the result of all kinds of conditions, environmental, ecological, competition between different species, the developments of diseases and disease resistance and countless other variables. Each branch of the tree consists of a collection of evolutionary variations on common developmental themes inherited from further back on that particular branch. Run evolution a second time, and we almost certainly would end up with a different evolutionary tree. We might end up with different kinds of cells, with different chemical pathways, membranes, genetic code and so on. If whatever killed off the dinosaurs didn't happen, mammals might not have enjoyed their subsequent success. The dominant form of dry land animal life might not even be tetrapod chordates if things with different body plans emerged from the oceans long ago and established themselves on land. (Imagine radially symmetrical creatures like starfish, or just look at the hugely successful arthropods, including the ever-present insects.) And that's right here on Earth. Imagine another planet where conditions are significantly different. My belief is that our human anatomy and biochemistry (both cellular and organismic) is probably just one of an almost infinite number of possibilities.