All living things today evolved from simple life forms over 3 billion years ago. Charles Darwin published his theory of natural selection in 1858, proposing that organisms better suited to their environment are more likely to survive and reproduce, passing on their genes. Over time, natural selection leads to organisms changing as traits that increase chances of survival and reproduction become more common. While controversial initially, natural selection explains how species evolve in response to their environment over many generations without need for divine intervention.