IntroductionBiological evolution is descent with modification. Theories of evolution have their roots in early Greek and Roman cultures. Before the 7th century BCE, people believed that supernatural forces were responsible for life. The philosophers Plato and Aristotle opposed this doctrine. They used logic to explain the origins of life. It was commonly accepted that the Earth was only a few thousand years old, that species were immutable, and that life was organized in a linear fashion. In the 7th century, the naturalist John Ray used the term "species" as the basic unit of classification. In the 18th century, Carolus Linnaeus created a hierarchical classification system to organize species based on resemblance using phylogenetic trees. In 1809, the French theorist Jean-Baptiste Lamarck published his theory of the inheritance of acquired traits. In 1830, Charles Lyell introduced uniformitarianism as a mechanism for evolution; the theory that geological change is relatively constant. In the mid-19th century, Alfred Wallace and Charles Darwin each individually considered natural selection to be the mechanism of evolution. Darwin received credit for the theory of natural selection because he published his book, On the Origin of Species, before Wallace. In 1865, Gregor Mendel was the founder of modern genetics thanks to his work on crossing pea plants. To date, the work of Mendel and Darwin is widely accepted. Important Players in the History of Evolutionary Thought Aristotle In Greece, in 300 BC, Aristotle made his contribution to evolutionary thought. Aristotle organized living organisms hierarchically. This concept was known as scala naturae, or the Great Chain of Being. With plants at the bottom, Aristotle's scala nat...... middle of paper ......They discovered that when the climate was dry, there were no major changes in the brain , but that when the climate changed, there were changes in brain size. As soon as brain size increased, stone tools appeared. Homo erectus was the first to leave Africa, hunt, make fire and create societies. Homo erectus lived 1.3 million years ago in the Great Rift Valley. Homo erectus evolved to have thinner arms, longer legs, and larger brains. This is considered a huge step forward in the evolution of ape bodies to more human bodies. In northern Kenya, the skeleton of Turkana Boy was found. At 5 feet 3 inches tall, Turkana Boy had a build closer to that of a human than an ape. Turkana Boy represents the first real human. Although he was one of the first humans, he was still very primitive. Turkana Boy had a lower forehead and smaller brain capacity than a human.