Inhumans

At the beginning of the Kree-Skrull War, millions of years ago in Earth time, the alien Kree established a station on the planet Uranus, a strategic position between the Kree and Skrull empires. Through their work at this station, they discovered that sentient life on nearby Earth had genetic potential invested in it by the alien Celestials. Intrigued, the Kree began to experiment on Earth's then-primitive homo sapiens by splicing Eternals DNA into Cro-Magnons. Their goal was apparently twofold — to investigate possible ways of circumventing their own evolutionary stagnation, and to create a powerful mutant race of soldiers for use against the Skrulls. However, although their experiments were successful in creating a strain of humanity with extraordinary abilities, the Kree abandoned their experiment for reasons which are not yet clear.

Their test subjects, the Inhumans, went on to form a society of their own, which thrived in seclusion from the rest of humanity and developed advanced technology. Experiments with the mutagenic Terrigen Mist gave them various powers, but also caused lasting genetic damage and deformities. This led to a long-term selective breeding program to try to mitigate the effects of these mutations.