Human Evolution

The history of how the human species, Homo sapiens sapiens, evolved is reconstructed by evidence gathered by paleontologists, anthropologists, archeologists, anatomists, biochemists, behavioral scientists, and many other professions. The evidence comes from the record left by fossils and by extrapolation from modern primates and human hunter-gatherer tribes. Fossils are evidence of past life. In practice, human fossils are mostly bones and teeth, which are the parts of the human body least likely to decompose. Most types of fossils are rare; it is extremely unusual for bodies to be subjected to all of the favorable conditions necessary for fossilization. Scientists date fossils by one of several techniques; carbon-14 dating, which measures the ratio of radioactive carbon to stable carbon, and potassium-argon dating, which measures the ratio of a radioactive form of the element potassium to its breakdown product, argon. Before these methods were available, index fossils of a particular geologic period were used to give an approximate date to other fossils. More recent dating methods include thermoluminescence, electron spin resonance, and fission track dating.

Paleontologists try to recreate the entire animal from sparse bone fragments by comparing the fossil fragments with similar animals, both now living or fossil, of which more information is known. Since complete fossils are rarely found, anatomists recreate the entire skeleton by comparing it with other individuals from the same species or with closely related species. Muscles are reconstructed over the skeleton based on a knowledge of anatomy, and the animal is positioned based upon how a similar living animal would move.

Studies of the DNA of humans and the great apes indicate that the closest living relatives of humans are chimpanzees and gorillas. Humans are not thought to be direct descendants of apes, rather we have descended from a common ancestor. Initial studies comparing chimpanzee and human DNA estimated that the similarity is 98.5%. However, recent studies showed that this similarity is more likely to be lower and is estimated at 95%. The final verdict will be delivered in a few years when the chimpanzee genome project undertaken by the Riken Institute, is finished. Despite being closely related and having some things in common (number of bones) there are distinct differences between humans and chimps. These include the human's larger brain, ability to speak due to a differently-built larynx, ability to walk upright on two legs instead of swinging or knuckle-walking, and greater manual dexterity, due to the opposable thumb that enables humans to manipulate small tools with precision. The faces of humans are flattened, or reduced compared to the apes. The human skeleton is similar to that of a chimp or a gorilla, but is modified for walking upright on two legs. At some point in our development, humans began to rely more on learned behavior (which creates culture) than on genetically fixed or instinctive behavior. This cultural development might be indicated by remains other than bones or teeth, including objects such as stone tools. The first appearance of those traits in the fossil record indicate that those animals were nearly as human as us, which makes them a possible ancestor.