None of the following assumes the theory of evolution (at the end I do discuss the relevance to this theory), and a lot of this was known before Darwin wrote On the Origin of Species.

In 1837, Reichart and Gaup showed that the mammalian ear is homologous with the reptilian jaw.  This means that they are related in physical terms.

How can we say that they are homologous?  By studying where they originate from in the embryo.  In the developing chordate and vertebrate embryo, some lumps appear early on.  These are known as the pharyngeal arches, or the gill arches in fish. 

The fact that we can see the same details in fish embryos is something else worth discussing.  These gill arches form the gills in fish; each of them forms a pair of gills with the muscles and nerves to supply them.

We should actually go further back, because only in jawless fish do they all become gills, but even furthest back, in the non-vertebrate chordates they don't form gills, they form slits for filter feeding.

Now we are at the start of the story.  There are some arches in the embryo of chordates and these form slits with nerves, muscles and blood supply to enable the chordates to filter feed.  Water is pumped through these slits and food particles are filtered out. 

In the embryos of jawless fish, these same arches are seen, and they also form slits, but in this case the slits are modified to take oxygen out of the water that is pumped through them. They also have the blood supply and nerves that would be expected.  Lamprey larvae actually use their gill slits for both oxygen intake and filter feeding.

In the human embryo, this same pattern is seen in the 26 day old embryo (and is seen in all vertebrate embryos).  The details in the 26 day human embryo are identical to the fish embryo.  In effect, at that point, humans have what amount to developing gills with identical circulation, muscles and nerves.  It is only after they have formed in the same way as fish to this point that they then change their development into other things.  This is seen most clearly in a letter sent to Charles Darwin by Ernst Von Baer, where he said he hadn't labelled two vertebrate embryos and that this made it impossible to tell them apart.

With the incredible similarity, we can therefore hypothesise that whatever happens in the first arch in humans is related to what happens in the first arch in fish, and the same can be said for each of the arches.  Of course, this we must then test this hypothesis using closer examination.  I am jumping backwards and forwards a lot but I will summarise at the end.

Let us look at what happens to the first arch in all the different vertebrates.  This is known as the mandibular arch as it forms the jaw in all jawed vertebrates.  This is one detail that helps us show that the hypothesis above is a valid one.  As part of the process of jaw formation, a length of cartilage is formed in the lower jaw known as Meckel's Cartilage.  This forms nearly all, or part of, the lower jaw in all jawed vertebrates.  In mammalian embryos, this piece of the lower jaw forms for a while but then disappears except for part of it which forms two of the bones in the inner ear.

What is the connection between the jaw bone of most vertebrates and the ear bones of mammals?  Let us examine what happens in reptiles.  The reptilian lower jaw, which comes from the mandibular arch, is composed of three bones, and in the ear of the reptile there is only one bone.  The process is reversed in mammals, with the jaw consisting of one bone and the inner ear of three.

In summary, we can see a definite progression of uses for the arches in general and the mandibular arch in particular from the simple use to the more complex:

1) Arches used as filter feeding slits in chordates,
2) Arches used as filter feeding slits and gills in lamprey larvae and gills in jawless fish
3) Mandibular arch used as a jaw in jawed fish with all associated blood vessels and muscles.
4) Mandibular arch used as a jaw in the majority of tetrapods, forming a jaw with three bones with all associated blood vessels and muscles.
5) Mandibular arch used as a single jaw and two ear bones in mammals with all associated blood vessels and muscles.
As stated, none of the above assumes evolution.  The above are facts that in Victorian times were waiting for an explanation.

What can explain these facts?  There are two options in the normal way of these debates; evolution and creation.

Let us look at how creationism can explain these facts.  We can say that the creator chose to create in this way, but that fails to actually explain anything.  If we ask why the mandibular arch becomes the jaw, then our answer is the creator chose that.  It does not answer the specific question.

Now the theory of evolution;  why did the mandibular arch become the jaw? Evolution is descent with modification.  Jaws cannot be created from nothing.  Jaws need to start with something.  The gill arches are hinged to allow more water through the gills by acting like a pump.  The mandibular arch just hinged more and more until it could be called a jaw.

Now, you may not agree with the details that I have described, but it is a possible, testable, explanation.  In this way, this is evidence for evolution.