The estimated percentage of left-handed people across the globe is 8-15%, with most figures of studies suggesting it is 10-12%. Taking an average of 11%, this would mean that 836,000,000 people are left-handed across the globe, out of a population of 7.6 billion. It has also been shown in a 2008 study that a left-handed person is 23% more likely to be male than female.
There is no definitive answer to this question, but the most accepted theory relates to which side of the brain controls the right hand. According to Jason Goldman for the BBC, the right side of the body is largely controlled by the left hemisphere on the brain, the same side of the brain that deals with language. It was about 1.5 million years ago that evidence of right-handed dominance became clear through tools found in Kenya, whereas there was little or no evidence that there was a preference before that. It is from that time on that language is believed to have developed in humans and with the left hemisphere processing the linguistic skills, right-handedness may well have occurred as a side effect. This is called the Homo loquens hypothesis.
It appears that some people's brains are organized differently from the majority, although this is not necessarily a negative thing. 'Left-handers are much more variable in the way that their brains are organized,' explained psychologist Chris McManus, from University College London to the BBC.