[Captain Kwok]

Originally posted by Randallw:
Anyway in the example I mentioned there were 20 "forms" of animal found. Thats the 5 current ones, and 15 "types" that are extinct.

It's actually a bit different than that...

In reference to animals, there are actually 34 basic forms "phyla" currently classified. Although they've developed at different times in history, I don't believe any have gone extinct. Mostly they've just gotten more complex (but not always sort of)!

From an evolutionary and multicellular viewpoint, things have progressed like this:

Sponges "Porifera" - No tissues; like a colony of single-celled organisms

Jellyfish, Corals "Cnidaria" - True Tissues (two); radial symmetry; partial digestive cavity; nerve net

Flatforms "Platyhelminthes" - no body cavity; bilateral symmetry; full digestive cavity; 3 tissues; head ganglia (nerve center)

Rotifers "Rotifera" / Roundworms "Nematoda" - pseudocoelom (sort of have a body cavity); 3 tissues; have "butts" and mouths ; head ganglia with primitive nerve cord

Segmented Worms "Annelida" - body cavities; 3 tissues; exoskeleton; circulatory and digestive system; head ganglia, simple nerve cord

Insects, Crustaceans "Artropoda" - body cavities; 3 tissues; exoskeleton; circulatory, digestive, and respiratory systems; head ganglia, increased nerve network

Octopus, Snails "Mollusca" - body cavities; 3 tissues; exoskeleton; circulatory, digestive, and respiratory (gills, lungs) systems; brain; nerve network

Sea Stars, Urchins "Echinodermata" - bilateral symmetry (but radial adults!), 3 tissues, body cavity, endoskeleton, no circulatory or excretion system, no brain, nerve network in skin

Vertebrates (Us!) "Chrodata" - 3 tissues; bilateral symmetry; developed digestive, circulatory, respiratory systems; developed brain; nerve cord; endoskelton

I'd imagine alien life given similar conditions as Earth (O2 + water) would progress in a similar fashion. I don't think its nearly as random as you'd think. If you want to have complex organs like eyes and ears, you're going to require some sort of sophisticated nerve system and brain to process and interpret that info. Big brains require lots of energy, so you'll need to be mobile and have sophisticated respiratory, circulatory, and digestive/excretion systems for that to be possible, and so on.

Of course, other factors like the planet's gravity or surface conditions may dictate physical appearance or types of sensory organs (i.e. sonar, electromagnetic, etc), but the internal mechanisms would be probably be quite similar. After all, evolution is just a process of selection - and given the same sort of conditions you'd expect given enough time, to see the same end results.

Originally posted by Randallw:
Basically 1 billion years ago there were 20 possible templates for the creatures that would occupy the earth but 15 got covered with a landslide and only the other 5 survived (I am simplifying this alot, and my numbers might be wrong, but I am trying to point out something). So what if instead of the 5 that survived, 5 others survived. The earth might be populated by animals with no backbone and 3 legs and a head with an extendable mouth.

I think your end conclusion is a little far-fetched, but your general idea holds true on a smaller scale, i.e., Dinosaurs vs. Mammals sort of scale. If the Dinosaurs had been able to hang around longer, then perhaps we'd be more of a Reptillian-humanoid...however, we'd still have the same body systems and macroorgans that we do now. It's just a good setup for evolutionary success.

Now, talking about other types of planets and possible life or intelligent Alien behaviour - that's just another serious can of worms.

[ March 25, 2004, 19:43: Message edited by: Captain Kwok ]