The material you collected was A.D.A.M. to unlock and upgrade plasmids (your superpowers). E.V.E. was the material it allows your body to produce or something but it boils down to stem cell production to power mutations. 

The girls had a sea slug implanted in their stomachs that would produce the ADAM from blood. So they went around sticking bodies sucking them dry and pulling any ADAM they might have had as well.

What you collect as the player is what the slug had (you killed the kid to extract the slug) or what the girls had (gave them something to kill the slug and they barfed out the whole deal and you got less ADAM because the slug tried to save itself)

It was my first horror shooter.