What are alien lifeforms really like?

[Moontanman]

Originally Posted by Galapagos

For more complex life, what about two or more prehensile appendages? This would seem like an obvious basic, just because it happens to be very useful to be able to move one object relative to another in our universe.

Being able to manipulate the environment is important to all complex life forms. I would expect life on other planets to have some means to change the environment to their benefit. Arms, hands, tentacles, tails, they might not be like ours but they would be there.

[Boerseun]

I would think that at least two eyes are required, close to a nerve center for quick response.

This is almost a universal feature of Life on Earth, so I guess we could make a handy extrapolation (however dangerous extrapolations might be) from it.

Obviously, there are spiders and such with more than two eyes, but from a point of view of "evolutionary economics", two is the most economic for stereoscopic vision.

So, whether your aliens has aerials, eyeballs, a nose, a mouth, a tongue, ears etc., they should be close to a "brain" or whatever will pass as the information processing nexus, and all of it for quick response. In other words, as a bare minimum, an alien should have a head with two eyes.

[mynah]

Perhaps some of our features evolved the way they did because it was the only logical way they really could - but it is likely that at least some are widespread simply because our common ancestor had them, they served their purpose well enough, and changing them would have demanded too much disruption of the ancient body plan.

One or two extra eyes behind the head might, for instance, have been useful if you had to look out for predators, but slotting an extra pair into the operating system would demand some serious reorganisation of brain circuitry. Alien brains, however, are likely to differ somewhat from vertebrate brains, and may have been sent along very different developmental paths early in their evolution.

[Moontanman]

Originally Posted by mynah

Perhaps some of our features evolved the way they did because it was the only logical way they really could - but it is likely that at least some are widespread simply because our common ancestor had them, they served their purpose well enough, and changing them would have demanded too much disruption of the ancient body plan.

One or two extra eyes behind the head might, for instance, have been useful if you had to look out for predators, but slotting an extra pair into the operating system would demand some serious reorganisation of brain circuitry. Alien brains, however, are likely to differ somewhat from vertebrate brains, and may have been sent along very different developmental paths early in their evolution.

Actually at least some vertebrates did have at least one extra eye originally, the tuatara lizard has a third eye between the other two, it is almost blind and covered by scales but it can detect ligth and dark. In most other vertebrates the location of the third eye is occupied by a gland that might be the rements of the third eye