I read the blood clotting thing, from "Darwin's Black Box". This is such a complex system.
Yeah, the thing is, Behe more or less ignores/was ignorant of the fact that we find simpler versions of the clotting cascade in ancient organisms like horseshoe crabs. So it's not really "irreducibly complex" after all. That's the pattern with the IC argument, after all - time after time, a system creationists thought was IC turns out to have a "reduced" form after all.
But with the coelacanth fish, it was said to be 30 m. years old, but when they found it near Madagascar, it was mostly the same.
It's not the same fish. It's a different species. "Coelacanth" refers to an order of fish, and, as such, is no more significant in it's continued existence than it's significant to say that there are "still" mammals.
So-called "living fossils" just aren't the stumbling block you seem to think they are. The Theory of Evolution doesn't say that all organisms must evolve. It simply says that, over time, organisms either adapt to their environment or they die out. What happens when an organism gets adapted to an environment that doesn't change? The organism doesn't substantially change, either. It's pretty simple, really.
I would like scientists to explain how species vary.
Random mutation. What exactly do you need to have explained?
But let's suppose we have now the first living organism, why would it evolve, unless it's written in its DNA then there's no way he can gain new functions.
Remember, evolution happens because of two things: random, heritable mutation; and natural selection. Anything that reproduces imperfectly - with the occasional, inheritable error - and doesn't always survive long enough to reproduce, evolves. Evolving is as simple as reproducing with some mistakes.
Why can't we inter-cross each others (animals and humans), if we really evolved from one single ancestor?
What, like transgenic genetic engineering? We do that all the time.
The reason you can't viably mate with a chimpanzee is because there's mechanisms that separate species. Differing chromosome counts, for instance. But no mechanism prevents the insertion of genes from one animal to another (or even animal to plant!). This basic compatability of DNA is a very compelling argument for common ancestry.
A very common argument is the eye. Why would Nature gives us "tools" to apprehend the world?
Because organisms that have eyes do way better than those that don't? Remember the expression "In the land of the blind, the man with one eye is king"?
Nature doesn't "give" us things. We have the things we do because the organisms that had those things left way more offspring than those that didn't.