If your mirror has a monster in it do not shout
This kind of situation does not call for freaking out
And do nothing that you would not like to see him do
'Cause that monster in the mirror, he just might be you.

Lycanthropus cryptonecans, or James Lazović as his friends call him. This is the best depiction I've been able to come up with for 'my' werewolves.

They actually don't have anything lupine about them - in the story, werewolves (males only: sorry, massive levels of androgenic hormones are required for the shapeshift) inherit a pseudomitochondrion called a 'zygozoan' from their mothers which triggers the expression of certain recessive genes inherited from a short-lived species of carnivorous apeman that interbred with protohumans.

First off, there was at one point at least five and possibly many more species of homonids coexisting in Africa. They seem to fall into two basic types, the robust type which were basically upright gorillas, with huge jaws to grind up low qualityplant food, and the gracile types, which used tools and gathered, scavened, and did a little small game hunting. L. cryptonecans took a different route, evolving sharp fangs and claws to compete with lions, leopards, hyenas, hunting dogs and other African canrivoria on their own terms. They didn't make it, but before the species died out it interbred with ancestral humans.

Actual werewolves - or vargr, as they prefer to be called - date back to a genetic bottleneck caused by the Toba Catastrophe, a massive volcanic explosion about 70,000 years ago that almost decimated the human population. The zygozoan was originally an independant archean that lived in the steam vents of the dormant volcano. When the eruption kicked it up, it invaded the cells of some human survivors much like ancient bacteria had taken up residence in cells to become the mitochondria of eykaryotic cells.

The genes inherited from the carnivorous ancestors were concentrated by the genetic bottleneck, and combine that with the much more powerful cellular energy plant of the zygozoan and boom, you've got werewolves. Human are neotenous apes, that is, adult humans retain the characteristics of juvenile apes. This is what a neotenous human descended from those hybrids would look like if it finished maturing.

Vargr had a distinct survival advantage in small tribes of nomadic hunters. Who wouldn't love having a hunting buddy who could shapeshift into a 250lb carnivore? The overclocked werewolf metabolism demands so much food, though, that one tribe could only afford to have one vargr. Modern werewolves have inherited an overpowering territorial instinct and ususally try to kill each other on sight - unlike the pack-dwelling true wolves - a strategy to spread them out and ensure they didn't decimate the game population.

As humans shifted to agriculture, werewolves that had previously seen as avatars of the gods were now a threat. As vargr, they couldn't distinguish between game and livestock, and as humans began to aggregate in cities, they found themselves living so close to one another that they couldn't avoid bloody fights. Over the centuries the percentage of werewolves in the general breeding population was greatly reduced.

They might have been driven extinct by witch-hunters if it weren't for the general lack of systemized knowledge of genetics until recently. Women can't shapeshift, but they can pass on the zygozoan; a male werewolf cannot pass on the zygozoan, just as he doesn't pass on the normal mitochondrion. A few generations of carrier mothers and daughters, and a werewolf son would appear as a total unexpected surprise.