Having grasping hands (and having them coordinate with the eyes) is one of the important things that distinguishes primates from other mammals. And a special version of bipedalism, which allows hands to specialize for manipulation and feet for locomotion, is one of the first things that distinguishes hominins from other primates, even before hominin brains get big.
You find the same arrangement — a pair of arms with hands and a pair of legs with feet – with most science fiction aliens. (For TV and movie science fiction this just reflects the fact that aliens, pre-CGI, were mostly played by actors made up with pointy ears or fur suits or whatever.) But there are wilder possibilities, with no Earth analog. One of the most imaginative is the Pierson’s Puppeteers invented by Larry Niven:
“…. I was fed up with humanoids. Chad Oliver in particular, an anthropologist, wrote story after story claiming that this is the only workable shape for an intelligent being. The puppeteers were my first attempt to show him a shape that could evolve to intelligence. …”
The Puppeteers’ brains are safely tucked away inside their bodies, but they have two “necks” ending in “heads” each including one eye, one mouth, and a set of “fingers” around the lips. And the body has three legs. Decapitation is bad news for a Puppeteer, like having a limb amputated, but not a death sentence.
Even more exotic are Vernor Vinge’s “Tines.” These are dog-like aliens who have evolved a short-range ultrasonic communication system that transmits information at such a high baud rate that a pack of half a dozen separate organisms is integrated into an enduring single individual with a shared consciousness. Losing one member of the pack is more like losing a limb, or having a stroke, than like the death of an individual. The mouths of the pack act together, as coordinated as the fingers on a hand, allowing the Tines to build up a medieval level civilization. (Vinge is a computer scientist, not an evolutionary biologist, however, and he glosses over some potential problems in Tine sociobiology: “all for one and one for all” is all very well, but which member of the pack actually gets to pass on their genes when it’s time to mate?)
But we don’t have to travel to other planets to find alternatives to two hands / two feet. Elephant trunks, for example, let elephants browse while avoiding the need for a giraffe/diplodocus-style long neck. The trunks even have “fingers” (2 for African elephants, 1 for Asian elephants) that are sensitive enough to pick up a single piece of straw.
We’ll spend a lot of time on Logarithmic History asking how human beings got to be such an extraordinary species. Hands are an important part of the story, although the elephant case suggests that hands (or their near-equivalent) are merely unusual, not absolutely unique to humans and near relations.
Logarithmic History 