Kree physiology originally developed under heavier gravity, higher nitrogen-to-oxygen air content, and a form of dependency centered around the Cotati.
Outwardly, the Kree resemble humans with the exception of their skin color. They have denser skin with increased muscle flexibility that gives them twice the level of endurance, durability, and stamina as humans. They also have multiples of several internal organs and usually require more nitrogen to breathe comfortably on Earth.
Kree women possess the potential for psychic abilities of various kinds, collectively referred to as a seventh sense. This psychic potential has manifested as clairvoyance, precognition, or even lethal attributes. Some Kree women can reach into the minds of males to manipulate their desires or drain life-forces completely
A million years ago, a race of humanoid reptiles called the Skrulls came upon prehistoric Hala. At that time, the Skrulls were a largely peaceful space-faring race, and they set about educating the natives to the point where they could join their trading empire. However, Hala was home to two equally intelligent races, the Kree and the Cotati.
The Skrulls proposed a test to determine the worthier race between the Kree and the Cotati. They took members of both races to a distant planetoid in another galaxy, the Blue Area of the Earth's Moon, the Skrulls created an arena and bid the two races to use the resources they had to create something of worth and then the Skrulls left. The Kree constructed a magnificent city, while the Cotati created a beautiful garden.
After a year had passed, the Skrulls returned to judge the two races and found the Cotati's accomplishment to be more worthy. Enraged, the Kree killed the Skrulls and the Cotati and seized the Skrull's starship. They used the ship to return to their native planet, then set about deciphering the technology of the starship. Within a hundred years of acquiring interstellar technology from the Skrulls, the Kree empimre began to spread throughout the Greater Magellanic Cloud.
The Kree launched an attack upon the Skrull Empire and the peaceful Skrulls were forced to become warlike to defend themselves. The Kree-Skrull War would continue for hundreds of thousands of years.
At the beginning of the war, the 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 sentient life on Earth that had genetic potential invested in it by the Celestials. Intrigued, the Kree began to experiment on Earth's earliest humans.
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
For more information on this experimental race, see here.