The Combine are humans from the future.

[-eSp-]

I was messing around with the no clip cheat in HL2, in one of the water hazard levels you come out of a tunnel and there is a helicopter waiting to kill you on the right hand side of the helicopter there is a barrier above the barrier is a a surface on one of the walls is some grafiti which shows an ape, a human and then a combine soldier. You can see the muscle tissue of the three organisms. The combine soldier seems to have mechanical parts aswell. I belive the inclusion of mechanical parts are related to the predicitions in the follwing article.
http://en.wikipedia.org/wiki/Future_evolution_of_humans

Below is the grafiti and the barrier (for anyone who thinks its a photoshop, they can go there themselves.)

The Grafiti
http://img368.imageshack.us/img368/3913/thefutureofhumanevolution0tn.jpg

The Barrier
http://img368.imageshack.us/img368/1717/thebarrier3rr.jpg

[Laguna Loire]

Whoa, that's one hell of a find eSp, nice one... :)

Wow, that's so weird...I was kinda wondering what the hell the Combine actually were, they sound like humans, speak English and everything, but the game refers to them as not being human...evolved humans...that would make sence, what with their super-advanced technology and such...

*checks guide*

"The Combine

Attracted to Earth by the dimensional rift caused by the Black Mesa Incident..."

Evolved humans from another dimension? Or did they just come back in time with their technology? Jesus...confusing now...

[cHilli]

They take humans / apes and turn them into that, yeah that was the plan. And yeah thats a genuine pic