He was alive on Earth when his people held dominance over the planet, when humankind were simple apes that were his race's primary livestock and food source. Tulok used genetic engineering to improve the creatures' flavour by increasing their breeding cycle and intelligence. This resulted in the creation of modern humans. In doing so, Tulok committed one of the greatest crimes of his people, tampering with nature. He was brought before the Silurian Justice Chamber for genetic crimes. Tulok was exiled along with his creatures to die in the wilderness at the time when the Moon was approaching Earth, which would have left the surface uninhabited for a period of ten years.
Tulok survived this sentence and wreaked his revenge by sabotaging the hibernation chambers of his people, which destroyed all but a dozen or so of them.
He later was revived in the 19th century, and discovered how much his creations had evolved and adapted to their environment. He hated how, in his mind, terrible they became, he planed to wipe out all the adults and leave the children alive so that he could manipulate them into becoming his slaves again. The Sixth Doctor and Evelyn Smythe along with Charles Darwin managed to stop him. (AUDIO: Bloodtide)