Our common ancestor 'is' what?

We are separated from mice by 80million years of evolution -- 40million from the common ancestor, and 40million to the other modern species.

Or simply 40million years of divergent evolution.

Technically yes, but practically the mouse reached a more or less standard form millions of years ago, in a local maximum, and we've been developing into all that we are mostly far more recently and represent a far more advanced form with lots of metrics (fitness to local environment is not the be all end all metric -- just the one evolution uses).

This isn't true for all species, it's (in a way) different depending on species. Your statement assumes that evolution is a constant, time-dependent process; it isn't.

For an example, I point to the horseshoe crab. We're separated by x million years of evolution from the horseshoe crab, because at some point there's a common ancestor. Its ancestor evolved to the horseshoe crab, millions of years ago, and then stopped. Meanwhile, our ancestor continued to evolve into us. The horseshoe crab is a prehistoric animal, a "living fossil". It hasn't evolved significantly in millions of years. This is quite different from mice.

Another example is the coelacanth fish; it was thought to be long-extinct as we have prehistoric fossils of it dating to the dinosaur days. Turns out it's still alive. Another less-extreme example is the shark: it's a little different than its prehistoric ancestors, but not much. I'm sure a biologist could point to a bunch of other species that haven't changed significantly since the days of the dinosaurs.

Mammals like mice have indeed been evolving just as long as we have, but many non-mammalian species have not.

Hello - I said "somewhat similar animals" by which I meant very early primates who were a bit like squirrels and mice. I should have made it a bit clearer!

I did get the date wrong - I should have said 65my.

Here is a link

http://www.smithsonianmag.com/science-nature/five-early-prim...

Good point. Separation can be even greater since each is 40M apart from a common ancestor.