Alien life forms. Artwork of two alien life forms. These organisms resemble earth-based invertebrates, featuring a hard exoskeleton, antennae and several pairs of limbs. They are adapted to survive on a planet with a high surface gravity and dry conditions. On Earth, physical characteristics, such as eyes, wings and limbs, have been acquired independently by unrelated organisms at several points during evolution. This is known as convergent evolution. It is thought that on an Earth-like alien planet, convergent evolution may result in organisms with similar characteristics to life on Earth.