Such is the Essence of Life.

Insects are segmented, and individual segments exhibit bilateral symmetry and two or four limbs (four when the segment supports wings). Other animals, like centipedes, I think of as “serially bipedal” (google “human centipede” at your own peril; it’s even less appetizing than googling “santorum”). Bottom line: Insects aren’t really a different body plan, they just have a replicated body plan.

For that matter, we don’t even know if the rise of technical social sentience is a natural development, or just a fluke. Intelligence hasn’t been around for long enough to have proven itself as a survival advantage, and it has only ocurred in one species here, late in the planet’s history. That argues against the convergent evolution suggestion.

I suspect that intelligence and toolmaking may give a species an edge, but it may also be destabilizing to the biosphere as a whole. Intelligence without technology (as in whales) may, in the long run, be the optimum survival strategy.

Man’s impact on this planet was minimal until he invented fire. That technology opened the way for the deliberate alteration of brushlands through burnoff to increase biodiversity and new growth, as well as greatly increasing the chances of totally uncontrolled and destructive accidental fires.

Other technologies, like stone tools, basket weaving to carry produce, clothing and shelter making, made it easier for individuals or bands to live better, but burning off scrubland to produce grassland (as happened in Australia) not only produces a more hospitable habitat for Man, it totally alters the habitat for everything else.

We look for life on Earth-like planets. The assumption is that life would evolve much like ours. It would have to defend itself from predators and have a reason to develop large brains (computing and regulating power) and consciousness.

Communicating like jellyfish or whales, or bees and ants, would have to develop in a creature which had a use for it, an ability to do it, and had survived the evolutionary process with an ability to manipulate it’s environment.

There may be other ways for conscious beings to develop technology, I’ll agree. And it’s understandable that humanoid figures are used to denote aliens. And as a matter of fact, they often are depicted as having extra-human abilities such as you mention.

Tails aren’t there just for looks. They provide balance, stability, sometimes grip, and a fan for cooling, shooing bugs, and gathering scent (by moving air towards the face.)

There’s that other appendage too…

This hive mind could be a lot smarter than we are, yet think much slower. There’s still no reason they couldn’t build machines that operated much faster than they do. We certainly can: electronics, computers, even internal combustion engines operate at hundreds, thousands, even millions of cycles per second. We can’t even do ten.

Squid use flashing colors to transmit holographic-type skin patterns we can’t understand. Maybe we can’t understand whale songs because they are exchanging Fourier Transforms.

We need to think out of the box. We may have a very good design, but it need not be the only one.

And what about cephalopods? A small group, but highly advanced for invertebrates.

No, you can make some general statements, like it makes sense that all the senses will be concentrated at one end of the body, perhaps a head of sorts. Some kind of locomotion, some kind of manipulators, but after that all bets are off. Think of Pierson’s Puppeteers, from the Niven novels. They would work.

I;m betting on centaurs, two for walking, two for light work, two for heavy lifting and running.

I forget which of so many Richard Dawkins books I read in which he tackled the question of how many limbs is the “right” number. It’s not arbitrary, it turns out, not just a matter of random mutations that either work or don’t. Each limb an organism grows has a cost, in energy and material, to create and maintain it. And there’s increased complexity of its nervous system and CPU to coordinate multiple limbs.

Here on Earth, evolution has produced only two answers, four and five limbs (if you don’t count segmented organisms such as centipedes). Four limbs tends to come in a bilateral symmetry design, five limbs with radial symmetry. Horses and starfish.

Why not three? Dawkins thought it was suboptimum, because loss of one limb would likely lead to the death of an individual, while four limbs minus one is survivable, with a limp. (Bipedalism has to be learned over many generations; a three-legger can’t suddenly convert.)

Why not six or more? Diminishing returns.

These ideas about terrestrial evolution can, I think, be legitimately generalized to consider life elsewhere. If common physical factors drive a general trend toward a multicellular, chordate, design, then I’d expect that planet to have lots of four-legged critters. And if one evolves into a tool-user, it’s going to use the two spare limbs and get by with bipedalism.

I’d generalize, too, that the five-limbed design will be “popular” on a minority of life-bearing worlds. The problem with pentacularism when it comes to evolving a tool-user is that five minus two equals three, and tripedalism wouldn’t produce a smooth efficient gate, thus creating a gantlet a species would have to run to free up manipulators; but the more natural quadrupedalism leaves only one spare limb for tool-use.

I should reiterate that I’m not dubious about the non-quadripedal->bipedal scenarios because they’re “weird”, but because they seem to offer suboptimum energy and material use.

So maybe there’s a real reason for thinking that we might encounter sort-of-humanoid shaped aliens.

Any technologically proficient alien will need some basic abilities. First, mobility. It will need a way to get around to collect the various components of tools. The easiest way to imagine that is legs, although two is an arbitrary figure. In fact, humans started out with four.

Next, a way to collect and manipulate the material around them. Hands.

And third, a way to perceive the world around them. Someplace for eyes, which operate at a long distance, ears, a shorter distance, nose, even shorter, touch, at the surface, and taste, internal. A head is optional, but more efficient than moving the entire body to sense the world.

And a brain to process information. Having that close to the main sensory organs makes sense, and those need to be able to move around. A head.

It’s difficult to imagine an arrangement which would allow all of those things which isn’t similar to humans. Porpoises and dolphins and intelligent, yet haven’t developed technology because of these limitations. Gorillas and chimps have, by contributing to the evolution of humans.

At least that’s my take on it. It could be that a humanoid developed the technology and then evolved into another shape to use it. That’s entirely possible.

Second, it will need a way to manipulate