Evolution is the process of change over time. It is the process by which all living things have developed from primitive organisms to more complex ones over billions of years. It is the process responsible for the diversity of life on earth today. Evolution is driven by natural selection. This is the process through which organisms with beneficial traits are more likely to survive and reproduce, passing their beneficial traits to their offspring. Over time, these beneficial traits become more common in a population, and the population can change and adapt to its environment. For example, one human species, Homo habilis, evolved into Homo erectus and then Homo sapiens, better known as modern humans. This change was driven by natural selection, as certain beneficial traits allowed certain groups of humans to survive and reproduce more than others. The beneficial traits included increased brain size, the ability to stand upright and the ability to use tools. Fun Fact: Over 99% of all species that ever lived on Earth are now extinct.