Roughly 2000 years ago the Krogan were a primitive tribal species trapped on a world suffering through a nuclear winter of their own making. They were liberated from this state by the Salarians, who "culturally uplifted" the Krogan by giving them advanced technology and relocating them to a planet not cursed with lethal levels of radiation, toxins or deadly predators.
The Turians unleashed the genophage on the Krogan home worlds after the Krogan Rebellions: a terrifying bio-weapon engineered by the Salarians. The genophage caused near total infant mortality in the Krogan species, with only 1 birth in every 1000 producing live offspring.