A long, long, time ago the big bang occurred not only on earth but on another planet as well...
Let's go there now...
WE ARE ALL ALIENS! But, we are all different.
Natural Selection helped develop this alien to have more ways to run from predators, and better hearing. This was because of the variation of his species over time, since many were dying due to predators.
I have one eye, but many legs and arms to help me move easily and run faster. I also have good ears and can hear from a distance.
I have horns at the top of my head and claws to protect and attack. I also have a strong body. I am a predator.
Natural Selection helped develop this animal to become stronger and have more sharp horns and claws. This is because of selective pressures, as it was competing for the top of the food chain with another species, and ultimately needed to develop.
Natural Selection helped this alien develop more legs and claws to attack. This was due to stabilizing selection. Two different traits on the same species morphed into the traits on this alien and now only this trait exists.
I have multiple legs to run faster, and horns and claws. This helps me hunt and run.