Lizard or Spider?  Try bacteria.

Wasn't there something like that a long time ago people claimed was from an asteroid?  Guess that was debunked/unproveable.

The truth is no-one knows.

I would honestly imagine though that through the process of evolution, different life forms find evolutionary solutions based on their environment. So for life to bear resemblance to Earth then it would need to have a similar environment to Earth. As we know, there are no other planets similar to Earth within our solar system, so for extra terrestrial life to be similar to ours we would have to look outside of our solar system. Where we are looking for life within our solar system (Titan, Europa, etc...), they all have underground oceans and hydrocarbon lakes, etc... Environments which vary greatly from Earths.

But as I said, we may not find out for decades. That's just what I think.

Even so. Comparing the different enviroments on earth.  The animals on earth are all still very similar.

No matter what a lake is made out of... it is still a lake.  Fins will still be the most efficent way to traverse it mechanically.

How do you know that alien life wouldn't find a better or worse way? How about propelling themselves through water with a jet of air or an organ that works like a propeller? It's not something we can predict to be honest.

An analogy I would use is studying exoplanets. We thought that all star systems would bear resemblance to ours in planet distribution. We assumed it would be the same as ours because that was logical. But if anything, studying exoplanets has taught us that our solar system is the odd one out if anything. The point is we have to expect the unexpected. We can't predict something we have no real knowledge about by basing it on terrestrial examples.

Those two methods of sea travel I believe also exist on Earth.  In the Squid (Cephalopods in general) and... some other creature who's name i can't remember.

Humans build stuff often times without the inspiration of the natural world.  We've got a mechanical world down pretty well at this point.  I'd be surprised if there was much out there that wouldn't resemble what we've invented.

That's not to see it'll be like Star Trek where every intellgent lifeform or even most are going to be human life... but i'd guess most alien anatomy will have earthbound analogues that are fairly easy to point out.

Yeah, a lot of microscopic waterborne life forms use propellers. And a lot of single celled water borne organisms use flagellum to propel themselves (If anything I would say that that is more of a successful propulsion that fins). But the fact that those exist on Earth kind of supports my point, fins may not work as well in a subsurface lake as a propeller or something, so fins may be uncommon there. Many solution may arise, but the one best suited to the task will thrive.

They are all ones we have expierenced or created ourselves though.  I find it unlikely there will be something we couldn't forsee existing. 

Like say, an animal that flies due to it's body developing a chemical that negates gravity or something crazy like that.

I think it'll all be fairly similar to stuff we have expierence with... and with some similar traits.

For example Symmetry.  I don't we'll find many nonsymmetrical animals.