[jennyp]

I think D is an ancient alien. Notice the eerie blue glowing, uh, stuff, packed inside its skull. In fact I'll bet the stuff pulsates & hums when you see it in person. Whatever you do, don't ever touch that skull!

[jennyp]

Note: Skull F is also an alien, but its blue substance has lost all its dangerous energy rays. It's safe to touch.

[VadeRetro]

The aliens no doubt took over the body of some ancient habiline hominid. In fact, this has been known for some time because the movie Horror Express, 1972, perhaps the only time Telly Savalas ever worked with Peter Cushing and Christopher Lee, explored exactly that possibility. The ancient hominid thing gets dug out of the ice in Siberia and comes back to life. Soon the invisible alien life force jumps out of it, however, and starts taking over modern humans.

"One of us, the monster? But we're British!"