I am Steve Ember. And I am Barbara Klein. This month is the two hundredth anniversary of the birth of one of the most influential thinkers in science history. Charles Darwin developed the theory of how living things develop from simpler organisms over long periods of time. That theory is known as evolution through natural selection. How do new kinds of life come into existence?For much of recorded history, people have believed that organisms were created. Few people believed that living things changed. What process could make such change possible?These were some of the questions Charles Darwin asked himself over years of research in botany, zoology and geology. He was not the first person to ask them. His own grandfather, Erasmus Darwin, believed that species evolved. And others, like the French naturalist Jean-Baptiste Lamark, had proposed ways this could happen. But it was Darwin who identified and explained the process, natural selection, that causes life to evolve.