It started simply enough when life was simpler. If a crime were committed in Saxon times, a group of 11 freeholders might be called to consider, witnesses called and law applied. Rome refined the system, introducing a magistrate and a citizen judex. Laws were codified.
Perhaps the only constant in the development of a jury system has been the number 12. The Scandinavians carried on with tribunals called Things who met in groups of 12 (or multiples of 12) to administer or invent the laws. The Magna Carta of 1215 guaranteed the right to jury trial (though juries of 12 were well established by then).
Today, finding a consensus is much more complex and difficult where the object is to find a group of strangers who know as little as possible about the case and parties involved. Enlightened spokesmen now argue for a "collection of wisdom" among people of diverse ages, classes and experience, thus fairly representing we, the people.