William Birkin vs. Dante

Birkin took over Marcus' work in the late 70's. James Marcus founded the Umbrella Corporation with Spencer in the 1950's and created the Progenitor Virus, which became the T-Virus after he mixed it with leech DNA.

Birkin advanced the T-Virus, but he absolutely did not create it. Excerpts from Resident Evil Zero as prooof:

Marcus' Diary 1


December 4th
We finally did it....the new virus! We have called it the "Progenitor". I want to carry it back and start detailed invesgations immediately.

September 19th
At last...I've discovered a way to build a new virus type with "Progenitor" as a base. Mixing it with leech DNA was a breakthrough I needed.. I call this new virus "T", for "Tyrant".