Dr. Birkin's transformation saw him go through a number of mutant forms. There were five distinct stages of his evolution, though these included intermediate stages as well. They are dubbed "G1" through "G5".
Dr. Birkin's initial mutation kept much of his body unchanged, with the mutations only being seen on the right side of the body, with the arm, torso and face being affected as it was on this side of the body he was shot. The left side of the body is more recognisably human. The heavily mutated right arm was noted as exhibiting superhuman muscular strength, allowing him to tear off iron beams from railings. A side-effect of maintaining a human brain was occasional bouts of dizziness.[excerpt 1] The G1 form appeared in two stages, distinguished by the appearance of claws on the mutated right hand, which were used on 23 September to kill a number of USS operators. By the night of 29 September Dr. Birkin had evolved to no longer require them, instead having an enlarged but otherwise human hand capable of handling weapons.
The G2 form experienced the growth of a new head and brain to replace the original, the face of which descended to the torso's left side. The torso in general sees an increase in size, becoming wider as not only the skeleton and muscles are changed but two new arms begin growing asymmetrically under both shoulders. The legs also become thicker, with the tibula and fibia increasing in length. The mutant right arm also went through further changes, regrowing the claws it once had early in the G1 stage.[excerpt 2]
The G3 form saw the increased symetricalisation of the body. The legs became black and dark grey coloured, with the growth of an eye on the left thigh. The two new arms grow further and became the same size, and grew their own shoulders to descend from. The original two arms were pushed backwards and kept their own shoulders, which allowed for the proper use of four arms. The vestigial human face shrunk in size and positioned itself against the front left shoulder. The G3 form also continued the changing movement of the heart; positioned near the shoulder in the G2 form, it moved to the middle of the upper torso. This form has been noted for its demonic appearance.[excerpt 3]
Following damage sustained in his G3 form, Dr. Birkin mutated to the G4 form. This form saw a radical change in gait, going from bipedal to quadrupedal as the two largest arms became legs. G4 saw an enhanced mobility surpassing the G3 form in spite of losing its high-jumping skills. A large mouth has formed out from his chest out of ribs that previously protected the heart. The need for such a large mouth was so the creature could satisfy its increasing demands for energy.[excerpt 4]
The G5 form was a further degradation of the G4 form. The mouth and digestive system of G4 remained to support the massive nutrient requirements. Whole bodies were consumed by G5, which could eat even Zombies. However, these were digested over time, and partially consumed and recognisably-human biomass covered its surface. The body itself became more mollusk-like in appearance, though propelling by using four tentacles to drag itself along.[excerpt 5]