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Sci-fi shows like STAR TREK are dominated by aliens that are
essentially humans with superficial makeup, and even productions with
larger budgets, like STAR WARS, are predominantly inhabited by
humanoid beings. Obviously the real reasons for this are budget and
familiarity, but could it also be argued that intelligent life might
convergently evolve similar humanoid forms? That is what I will try to
justify here. I have no particular background to draw on; so the
following is more or less uninformed musing based on first principles.
As a starting point, I will assume that evolutionary pressure guided
by the survival of the fittest is what decides which shape will be
taken by life forms. This implies that intelligent life must evolve
from non-intelligent life. I will also assume that evolution will
tend to produce the simplest solution to problems, including
minimising the amount of genetic information that must be maintained.
Life on Earth is fundamentally carbon-based; our genetic information
is encoded via nucleic acids, and our proteins are chains of amino
acids. I don't know my chemistry well enough to say whether life
based on another fundamental element would be possible (silicon is
sometimes mentioned), or how that would affect the macroscopic
structure of life.
Life can exist without free oxygen (it still does, in the form of
anaerobic bacteria), but oxygenation seems like a rather efficient way
of releasing energy. Oxygen may not be the only element that can
perform oxygenation, but it may be the most prevalent one in the
universe.
If life is based on carbon and water, then it is likely to prefer
roughly the same temperature range as us. Much lower, and water will
freeze. Much higher, and it will boil. Similarly for pressure. This
argues for a planet similar to Earth, at a similar distance to its Sun
(depending on its energy output), which will probably imply it has a
similar amount of gravity, give or take an order of magnitude. This
will affect the maximum size of any life that evolves - while it may
be smaller or larger than humans, it is unlikely to be enormously so.
Although life is most likely to arise in water, because it acts as a
solvent and thus makes a rich variety of chemical compounds easily
available, it is plausible that intelligent tool-making life will
benefit from dry land. This is simply because setting things on fire
is such an effective and simple way of accessing more energy than can
be provided by an organism's own body (interestingly, still through
oxygenation). The evolution of intelligence is inherently about
harnessing ever increasing amounts of the energy in the environment.
Now we're at carbon- and oxygen-based tool-making life on dry land.
This still leaves a lot of design freedom for what it might look like.
First, in order to gain the maximum amount of body parts relative to
the amount of genetic information that has to be maintained,
well-optimised life exploits symmetry. Most terrestrial animal life is
laterally symmetric (your left and right sides look the same), but
radial symmetry is also found to some extent (e.g. octopuses), and of
course many plants appear to be radially symmetric as well. I have no
very strong argument for why intelligent life would prefer lateral
symmetry, but I wonder if having a "front" where energy-costly sensors
(e.g. eyes) can be focused is efficient if you have enough
intelligence to ensure they're always pointed in the most profitable
direction. Obviously, radially symmetric intelligent life will look
*very* different from humans.
Beyond lateral symmetry, the humanoid body plan has a few more
identifying characteristics: two legs, two arms, and a head on top.
The purpose of arms is to be able to handle tools, which is definitely
crucial for intelligent life. From an evolutionary perspective, it
makes sense for arms to be an adaptation of what might once have been
another pair of legs.
While you need two legs for efficient locomotion, one arm might have
been sufficient, but lateral symmetry essentially gives you two of
each limb for free, so there's no great cost here. Two eyes are
significantly better than a single one, because you gain depth
perception. But why have two complex eyes rather than a multitude of
simpler but specialised eyes? Perhaps it is simpler to have only two
instances of the supporting neural and muscular machinery. It is my
understanding that most of the life on earth with more than two eyes
doesn't quite have the ability to focus them the way two-eyed
organisms do.
Any organism will want an orifice for ingesting food and an orifice
for expelling waste, and putting the waste orifice on the other end of
the body means you don't have to worry about the waste containing
unhealthy things. You'll definitely want to put various sensor organs
very close to the food ingestion orifice, to determine whether
whatever you're considering eating is going to be good for you. The
humanoid face does this pretty well: a nose just above the mouth to
maximise olfactory impressions, eyes just above that, and the tongue
as a final line of detection. These sensors provide a lot of
information, and moving information is expensive and slow, so you want
to minimise the distance to the decision-making centre (the brain),
which is therefore right next to them. This is also why the ears are
located here. It fundamentally makes sense for the most complex sense
organs to be very close to the brain and thus each other. To provide
3D hearing, and due to the lateral body layout, the two ears should be
located opposite each other.
So now we have a head, two arms, and two legs. We can discuss the
fine details: a neck is useful so you can move the sensor cluster (the
head) around without having to move the entire body. Something
finger-like on the arms is definitely practical in order to allow tool
manipulation, but I have no argument for why it should be much like
the human hand. Two fingers might be sufficient, but you probably
need either that or something with similar functionality, in order to
be able to grip tools. The human foot is pretty bad and mostly a
relic of the fact that, from an evolutionary perspective, we are new
to bipedalism. You definitely don't need that many bones or toes;
something like an ostrich's foot is much more well-designed. But in
any case, STAR TREK aliens tend to wear shoes, so who knows what they
look like down there.