2- Even more damning is DNA- this is very similar among all species on Earth. If you sequence any animal species' DNA, the majority of it is the same. If you put human genes for proteins and so forth into bacteria, they tend to work quite well. Again, alien species would not be expected to do this; in fact, aliens would probably not have DNA at all, it would be some other molecule (even if it was similar, it would likely have at least some structural differences).
3- For aliens to actually get here would be a bit of a feat. Even if you could travel at the speed of light, it would take years to get from even the closest of star systems to Earth. Now keep in mind that chances are if there is other life out there, it is probably not from the nearest star to Earth unless literally every other star system has life (which seems unlikely, given we have not detected any evidence of life so far), it would take decades to get here at least- who would fly decades through space to dump a few random animals on our planet? There is no evidence of any alien technology or spaceships that has brought anything here either.
4- If we assume these animals weren't brought here, then they would have had to arrive here "naturally" on an asteroid or something. These things generally don't travel anything near lightspeed, so we are looking at something spending hundreds, thousands, or quite possibly millions of years traveling through space, through radiation, freezing temperatures and hard vacuum, and then getting through our atmosphere. Chances are not many life-forms are going to handle that very well.
Basically, any species known very closely resembles all other life on Earth, there is no evidence of anything being brought here and it seems very difficult to do so. As such, it is extremely unlikely anything we know of is an alien from another world.
-
Justin Thyme say: OMG!
-
Brigalow Bloke say: Because aliens from another planet on Earth are utter pants, only believed in by those of little brain.
-