The Imperial College London study showed humans have approximately 10 times the number as do fruit flies and 20 times more protein interactions than do simple, single-cell yeast organisms.

The interaction between different proteins is behind all physiological systems in the human body, the scientists said. When the body digests food, responds to a change in temperature or fights an infection, numerous combinations of protein interactions are involved. However, until now it has been impossible to calculate the numbers of interactions that take place within different organisms.

"Understanding the human genome definitely does not go far enough to explain what makes us different from more simple creatures," Professor Michael Stumpf said. "Our study indicates that protein interactions could hold one of the keys to unraveling how one organism is differentiated from another."

The research is reported in the British Journal of Cancer.