Believe it or not, humans weren't always at the top of the animal kingdom. Our ancestors were able to survive great threats, such as larger predators and starvation, by collaborating within a group. Group members could alert others when a threat was near or help to fight it off. This group mentality created the social beings we are today, but it also instilled a fear of ostracism. We've evolved to protect our reputations because banishment from a group had fatal consequences. Kip Williams, a professor of psychological studies at Purdue University, explains this notion to Psychology Today: "Ostracism is not only a form of social death, it also results in death. The animal is unable to protect itself against predators, cannot garner enough food, etc., and usually dies within a short period of time."
While this is untrue for humans today, the survival instincts in your brain still process public speaking as a dangerous situation. Your innate fight-or-flight response is what activates the hypothalamus in your brain, triggering hormones that stimulate the adrenal glands in your kidneys, thus releasing adrenaline into your bloodstream. That's when your palms sweat uncontrollably, your heart races, and you can't manage to squeak out an intelligible sentence.
Learn more about the science of stage fright and how to overcome fears like public speaking in the videos below.