Evolution doesn't discriminate between introverts and extroverts, which is why both kinds have survived and continue to thrive. Now you may be in the happy middle space of having qualities of both kinds, which makes you an ambivert – and also means you'll be among the most productive employees at a call centre. But most people tend to be one or the other.
The question is why? The video above offers some answers.
There is something called “baseline arousal”, which indicates what a person's mental energy is in a state of inactivity. This is lower for extroverts, which means they need more activities, more people, more excitement, to feel content. An introvert gets the same kind of satisfaction from a solitary walk in the park or a quiet weekend spent reading a book.
Social and institutional structures around the world seem to prefer extroverts, rewarding as they do group activities more than solitary ones. A slam poetry session got quite some attention recently as it looked at the world from an introvert’s point of view, listing the problems with being unable to “put yourself out there”.