In much of science fiction, humanoid aliens are abundant. One explanation is that authors use the only example of intelligent life that they know, humans. In live-action television and films, using humanoid aliens makes it easier to cast human actors to portray alien characters. A study by the zoologist Sam Levin suggests that aliens might indeed resemble humans, given that they are presumably subject to natural selection. Levin argues that this can be expected to produce a hierarchy of cooperating systems that make up any organism. Luis Villazon points out that animals that move necessarily have a front and a back; as with animals on Earth, sense organs tend to gather at the front as they encounter stimuli there, forming a head. Legs reduce friction, and with legs, bilateral symmetry makes coordination easier. Sentient organisms will, Villazon argues, likely use tools, in which case they need hands and at least two other limbs to stand on. In short, a generally humanoid shape is likely, though octopus- or starfish-like bodies are also possible. An opposing view is given by Mike Wall, who argues that intelligent extraterrestrials able to contact Humans would most likely have reached a phase allowing them to develop themselves into machines.