Human evolution is the evolutionary process that led to the emergence of anatomically modern humans, beginning with the evolutionary history of primates - in particular, genus Homo - and leading to the emergence of Homo sapiens as a distinct species of the hominid family, which includes the great apes. Scientific evidence shows that the physical and behavioural traits shared by all people originated from apelike ancestors and evolved over approximately six million years. Lamarck proposed his idea of evolution based on his studies on the patterns of fossils and matching the organisms to their environment. It states that parts of the body that are used extensively become larger and stronger while those that are not used deteriorate. This theory is explained by taking an example of a giraffe. Lamarck suggested that the giraffe evolved from ground-feeding herbivores. But at a later stage, the area fell short of its ground-level vegetation, and thus the giraffe took to feeding on trees. The constant stretching of its neck and fore-limbs over many generations resulted in these structures becoming longer and stronger to reach higher foliage.