The abberations observed in the Private's skeletal structure and reorganization of his internal organs show the unmistakable similarities to the specimens we fished out of the ice. The same force that twisted his corpses into a deranged killing machine is intrinsic to this planet's evolution.
Furthermore, everything appears to be linked to the signal carried by all Markers. Even now the signal can be traced through Freeman's body, making minute changes to his structure and causing his cells to continue living.
This may explain how the specimens we've found have remained in such excellent condition after two million years in the ice.