Krogan

The Krogan are a species of large reptilian bipeds native to Tuchanka, a world known for its harsh environments, scarce resources and over-abundance of vicious predators. Once hailed as the saviors of the galaxy, the Krogan are now a shadow of their former prominence. Due to the hostile environment of their home world, natural selection has played a large role in the evolution of the species. Krogan naturally reproduce and mature at an extremely high rate, but due to a biological weapon unleashed on them called the genophage, only 1 in 1000 Krogan births is successful. Because of this, the Krogan are a dying species; faced with this slow extinction, many Krogan have become very self-centered. The large shoulder humps on a Krogan individual store fluids and nutrients, which enables them to go for long periods without food or water. Krogan also posses multiple instances of major organs, in which secondary organs serve as backups should a main organ be damaged. A Krogan individual posses a thick hide, which is extremely hardy and very resistant to cuts, scrapes, and contusions. Krogan are also highly resistant to radiation, poisons, and extreme temperatures. Biotic individuals are rare, though those who do possess the talent are typically quite strong in their abilities.

History
Four thousand years ago, at the dawn of the Krogan nuclear age, battles for the small pockets of habitable land on Tuchanka broke out and escalated into a global conflict. Nuclear weapons were deployed in such numbers that the Krogan created a nuclear winter and transformed their planet into a radioactive wasteland. The Krogan were reduced to primitive, warring clans. Two thousand years later, the Salarians "culturally uplifted" the Krogan by giving them advanced technology and transplanting them to a new (and non-radioactive) world. This was in an effort to use the Krogan as soldiers to combat the Rachni, who were winning a war against the Council at the time. Within two Krogan generations, the Krogan had the numbers to not only defeat the Rachni for the Council, but also push them back to their home world and perform a genocide on the species.

However, the Salarians' gamble backfired when the Krogan population, uncontrolled by natural predators or harsh environments, exploded. Facing a population crisis, the Krogan expanded to other planets; previous inhabitants were removed by force. These so-called Krogan Rebellions continued for nearly three centuries, and it seemed that despite massive casualties, the Krogan birth-rate would allow them to prevail. The Council reacted by turning to the recently encountered Turian Empire for aid. The Turians unleashed the genophage on the Krogan, a biological weapon which caused near-total infant Krogan mortality. This forced the Krogan to accept the Turian terms of surrender.