For the sake of argument, let's say that
evolution has never been
observed. Of course, neither have
black holes, or
quarks. yet we still
know that they are there. This is because instead of
direct evidence,
we use
indirect evidence to know.
One way of indirectly inferring something has happened is by predicting
consequences and then seeing if they apply. For example, if there is a
black hole in one section of space, then stars would be orbitting
around a void. And we see stars orbitting around a void, so therefore
there is a black hole.
Now you may be thinking, "But there could be another cause!" If you
are, good! That shows you're starting to think rationally. In fact, the
example I gave is an often-used logical fallacy called affirmation of
the consequent, which takes the form of "If A then B, B, therefore A".
If you don't see why this is a fallacy, let
me give an extreme example: if magical anti-flight demons existed, then
objects dropped will fall to the ground. Objects we drop fall to the
ground, so magical anti-flight demons exist.
There are two ways to circumvent this. The first is an "if and only if"
statement. That means that if one side of the equation is true, then
the other side *must* be true. The first example can be rephased like
this, but the second example can't.
Evolution does have a line of evidence like this. If and only if all
species rose from a common ancestor, then we would see big genetic
similarities between us and other creatures of our genus, and also
genetic similarities between us organisms in our kingdom. This is true;
we share around 98% of our DNA with certain primates, and about 85% of
our DNA with mice (http://tinyurl.com/nenmm).
The other way to circumvent the affirmation of the consequent problem
is multiple lines of indirect proof. To illustrate this, pretend we
find a dead man on the floor. If taken individually, his broken neck
could be attributed to tripping, his cuts could be attributed to his
line of work, and his rumpled clothes could mean he put his clothes on
in a hurry. All taken together, however, we can assume he was killed
during a fight.
Evolution also has evidence like this, too. In addition to genetic
similarity, other proofs include vestigal organs, transitional fossils,
and the age of the earth in correllation with the timespan evolution
would take. All of these have a large pool of evidence backing them up.
If taken individually, each could be chalked up to a competing theory
of origins, but when taken together it points towards an evolutionary
theory.