Convergence generally happens as a solution to a general technical problem which can be arrived at through a series of small adjustments.
For instance if the species moves in a fluid streamlining will develop by itself because those who are more streamlined move faster.
If you have an advantage from eyes, a hemisphere of light sensors behind a lens which can change shape is a solution that comes about naturally (there are other solutions to the problem).

If you have a predator/prey pair where the prey is armored against most things and the predator develops weapons to take it down, you'll get the escalating arms race of prey getting ever more armor and predators getting ever bigger and more specialized weaponry until the prey becomes too slow to survive at which point both species go extinct, this has happened at least 6 times on earth.

Lots of things like those are bound to be repeated, but a lot of traits are incidental, results of the first random differences.

Legs seems to be a general solution to motion as they have been developed independently many times, how they are put together and how many to have is not.

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Taking fluid streamlining for example, all vertebrate swimming creatures look similar because of the requirement to reduce drag in water. However they also look similar because they share many structures in common- anterior skulls, dorsal vertebral columns and paired ventral limbs.

To see how a different type of creature would evolve fluid streamlining, we have to look to invertebrates; squid for example, resemble fish in general outline, but their lack of skulls, backbones or ventral limbs makes them look very different in detail.

Some, many or most alien fish-like species will be at least as different from Earth fishes as squid are.

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That was a good example. Never thought that way before...

That is one of the interesting questions of course. Would convergence with terrestrial forms happen with a "second genesis" or would have have their own set of convergences? I think Morris shows well the number of times different characteristics, ranging from body form to organs to biochemical pathways have evolved independenty.

Basic physics will force some convergence, for example the shapeof aquatic creatues, eyes, etc., I suspect. How big a range of biochemistries is actually possible will control others. If nuclei acid-protien-lipid-sugar based life is the only type possible, then considerable conmveregence is possible at a biochemical level. If other biochemistries exist then convergence may be much less.

Jon

I expect that if there is extra terrestrial life in our solar system, it will be in the form of microbes. If there are more complex forms in other star systems, we will never know what they look like. Not for hundreds of years, anyway.

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This Wiki article is a fine answer to the OP's question:

http://en.wikipedia.org/wiki/Alternative_biochemistry

Don't expect yourself to fully understand everything written in there, though. But I'm sure you'll get the jest of it anyhow.

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First, I'm not a chemist so I will defer to those here with more knowledge.

Is it true that, where formation of life is concerned, we have discovered all elements in the universe and how they may combine? Is the evolution of our understanding of the Periodic Table complete and set in stone?

Or have we been limited by what we observe in our own star system, and to some extent outside it through astronomical observations of chemical signatures? Could there be more chemically that is still undiscovered?

Here are a couple of links detailing the Periodic Table's evolution:

http://www.wou.edu/las/physci/ch412/perhist.htm
http://www.ausetute.com.au/pthistor.html

Mike

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We have discovered all elements and it has nothing to do with 'formation of life' - elements are elements. We've even created exotic elements in the lab (from theory of atomic stability) which do not occur naturally. These tend to be very heavy and unstable and decay in fractions of a second. So if there are other elements to be discovered, they would occur in extremely exotic environments and be highly unstable, which makes them irrelevant to the formation of life.

As to how they combine chemically, that's different. I'm sure there many possibilities that we haven't considered, however, as DaveC426913 pointed out, carbon based chemistry provide the most possibilities by far.

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I think the chances would or at least should be very high theres no way we are the only ones that have such a wonderful..well use to be wonderful planet.

Everything can only be speculative since the only planet that we know of that harbors life is our own, but I think that any other living things on any other planet would be very different from us. I feel that the laws of natural selection may be familiar on other planets with life, however, with different starting conditions, and possibly different basic compounds (who is to say that a silicon based life form is not possible, etc.), the results would be drastically different. Things as trivial as music (sound for pleasure and not function) and art may be lonely things that we alone posses in the entire universe.

I'm going to disagree with Conway Morris' premise, in that even among distantly related forms, convergence is only skin deep. Bats, birds, pterosaurs, and flying insects all have wings, but the details of their constructions are significantly different. Convergence is forced because a wing -- or a complex eye -- has to do similar things, with constraints by physical law. I do not believe that biophysics would favor an upright featherless biped with two distinct sexes, a mass of about 70kg, and some rather poor areas of detail design over significantly different configurations.

What constraints do I see on extraterrestrial life?
It's biochemistry is most likely to be carbon-based. The other possible chemistries don't seem to combine sufficient stability and mutability to form a basis for biology.
It will be based on a liquid (or supercritical) solvent. I suspect that the solvent will more likely be polar, like water, than not, but my very limited chemistry knowledge doesn't permit me to do more than speculate without data.

Its evolution -- once the big hurdle of life actually starting is overcome -- is more likely to be Darwinian than LaMarckian, mostly as I suspect that the sort of genetic-environmental feedback implied by LaMarckian evolution would be unstable.

More fun that speculating about what the life will actually look like is how the sentients form a society.

I agree. Convergence seems to be an obvious explanation when its resemblance is observed, but its absence is not so easily explained in many cases. Morris points out sabre-tooth tiger convergent on marsupial lion, but why did no furry four-legged sabre-tooth dinosaurs appear if that was such a compelling morphology. Or, Dinosaurs seemed to produce a lot of large cursorial bipeds, why have such animals not appearred among mammals. The answer is the contingent nature of evolution, of course, not any a priori drive towards a form.

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What about convergent design rather than convergent evolution? This (pretty old) thread has been discussing naturally evolved life forms, but there's also the possibility of artificial life forms.

If you were to look at signs of life in our Solar System outside of Earth orbit, you'd see artificial robots which are silicon based, not carbon-based. Might aliens also design artificial robots of silicon/steel/aluminum for similar reasons we build artificial robots out of such materials?

I would expect any large-scale space-faring civilization to rely at least in part on artificially designed (or enhanced) entities.
Observing any convergence in those designs would indeed be fascinating.

Actually, convergence in solutions for space exploration/colonization might be much more likely
than for biological solutions evolved on different planets (unless conditions on those planets are very similar)

Why not artificially designed organic life? The ability to grow and repair is an invaluable aspect of life to survive and regenerate. Genetically extend that life expectancy and improve its durability to harsh environments and you have an advanced versatile life form capable of long distance space travel.

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One might indeed think that for your average technologically highly advanced sapients, pheno- and genotyping would be rather the norm and not necessarily done only when situation warrants it. And for life forms of the type one would most expect to encounter, space travel would most likely be a situation that does warrant it.

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