I love radial symmetry.

On the side of alien creation, Poul Anderson was the best, in my opinion. He created a constellation of vivid, believable aliens in a cycle of short stories linked to his rogue hero, Nicholas van Rijn, The Earth Book of Stormgate (the most memorable is The Man Who Counts, in which he tackles the issue of speciation) — plus, of course, his flying Ythrians and Adzel and Chee Lan, the two marvelously drawn alien partners of David Falkayn.

In line with Bart’s point, the dilemmas and conflicts that arise from these interactions are similar to those between different terrestrial cultures, particularly first-contact situations, with occasional added biological complications.

I have a suspicion I will really like Sheila Finch…

The bilateral versus radial symmetry body plan was indeed coincidence and then inertia, as the instructions got embedded into the developmental program (the famous Hox genes and their relatives).

There is no question that people would have to evolve if they lived on other earth-like planets, unless they were content to live forever under domes, a scenario that would resemble a prison or an ICU unit, rather than a new world. Evolution of this type would lead to speciation, which I suspect will be the topic of your “divergent evolution” post.

To enlarge on the excellent point of the strictly-by-function scoring system of evolution: we won’t know which attributes of living organisms are universal and which are parochial until we have an independent life sample. The gamut of terrestrial lifeforms alone and their divergent solutions to the same problem already show that only the outcome matters. The ability to fly, rather than exactly how to do so (insect, bird and bat flight is based on very different principles). To ability to see well enough to survive, thrive and propagate; and so forth.

Also, there are other senses that we don’t possess that are useful to other terrestrial organisms — sensing EM, for example, or being able to smell pheromones (something that humans cannot do no matter what the tabloids say, because our vomeronasal organ is not functional). So one interesting question of diverging humans beyond earth is what other senses may become important in the new context, and what accommodations our body and brain can/will make to develop them. We’ve done it before: we became able to read, for example. Making chlorophyll, on the other hand, may require a bit more effort!

Which bits are functional, which bits are coincidence?

It’s useful to have your eyes up off the substrate, and it’s useful to have more than one so you get depth perception. But is there any reason to have two or would seven work just as well?

It’s useful to have your brain really close to your major sensory organs (which may not be eyes or ears) because your nerve pathways only work so quickly. Unless you have more efficient nerves.

It’s useful to have opposable manipulators that can grasp at a distance, but there’s definitely no reason that four fingers and a thumb is the correct combination. Arms seem plausible, but again why two?

The bilateral symmetry vs radial symmetry split in terrestrial organisms goes way back. Is there any reason that one should be favored over another? And are there other arrangements that would work equally well?

Convergent evolution is all about function – some shapes work better for particular tasks. But the underlying body plan constrains everything: not all options are even available.

There’s constraint, function and coincidence, and they all go together to make an organism.