Why do we test drugs on mammals and what part does evolution play in the answer?

We test drugs on mammals because of two main reasons:

a) Mammals tend to be more expendable than people. It's not such a biggie if a rabbit falls victim to a drug candidate that turns out not to be so promising, and it's also quite a bit more ethical to kill them at the end of the test so you can take a closer look at what the drug's done.

b) Mammals tend to be pretty similar to people (and we've got a good idea about where they differ and how that could affect the results), but they're far easier to get a hold of; Clinical trials usually can only recruit maybe a few tens of participants, at the cost of a few hundred pounds each for their travel and time. A single rat can be bought and fed for far less than that, and if you keep them in food and bedding, they're unlikely to complain. It's also much easier to track what they get up to and make sure they comply with the trial protocol.

As far as evolution goes, it's mostly because we share common ancestors with many species of mammals, and many of the biochemical bits have changed little since then, that it's possible to give a rat a drug and know that the results you get have a strong similarity to the results you'd get testing on a person that make it all possible.

Now I suppose I'm going to attract the ire of animal rights activists the world over by saying "Hell yeah, Animal testing!".

Because I really enjoy things like injectable insulin, defibrillators, and heart transplants.