The theory of evolution, or descent with modification by natural selection was first proposed by Charles Darwin in his book "The Origin of the Species" published in 1859. He came up with this theory after his voyage around the world on the H.M.S. Beagle. At first he was ridiculed for his idea, especially by the religious people, but the scientists were soon convinced. His theory was that every individual organism is different, and some of those variations are inheritable. Animals produce more offspring than can survive. Therefore there is a fight for survival amongst the offspring, and only the best suited to their environment survive. Since each organism is different, each has slight variations that make it better suited to a certain environment. The organisms with the most favourable traits survive and pass those traits on to future generations. The organisms with the less favourable traits don't survive as well and have fewer offspring. So eventually the favourable trait is present in all organisms of a population, due to natural selection. Descent with modification is the theory that all life descended from one organism. The first bacteria were present on Earth around 3.5 billion years ago. And from that single bacterium, all living things evolved. Fish, reptiles, flowers, grasses, mammals, humans, everything that is living on this Earth descended from one single living cell.