The earliest known fossils of modern humans are about 200,000 years old. Hominid fossils, representing ancestral human species, are more than 4 billion years old.
Fossilization is a relatively rare process, and most organisms are not preserved in the fossil record. Because soft tissues, for example, usually don't form fossils, there can be "gaps" in the fossil record.
Fortunately for science, some exceptional deposits of fossils nevertheless provide a surprisingly detailed glimpse into our evolutionary past. With pieces of information from the fossil record, scientists are able to put together a relatively detailed picture of human history.
Even before the first human fossils were ever cataloged, scientists such as Carolus Linnaeus believed humans and great apes to be closely related based on our similar anatomies.
Lending support to theories of common ancestry between apes and humans, the first early hominid fossils were found in Africa in the 1920s. Ardipithecus ramidus is currently considered the earliest member of the hominid family, which includes both modern humans and extinct ancestral species. Seventeen fossil fragments representing the early hominid Ardipithecus ramidus were found in Ethiopia in 1992 and 1993, including two child-sized skulls, jaw and arm bones. These fossils are about 4.4 million years old.
The oldest known modern human fossils date back almost 200,000 years, and were found in Ethiopia by a team led by paleontologist Richard Leaky. Two skulls and associated bones were dated by both potassium-argon and genetic methodologies, which agreed upon the age of the fossils. These fossils lend support to the well-accepted theory that modern humans originated in Africa and later spread to colonize the rest of the world.
Many misconceptions surround the science behind dating fossils. Today, multiple methods are used to date fossils, with the results cross-checked against one another to verify fossil ages through agreement between methods. Radiometric dating methods are based on the fact that certain naturally occurring elements decay at predictable rates. Because a radioactive isotope of potassium decays into argon gas over time, scientists use potassium-argon ratios to date very old fossils.
Many people think carbon dating is the only method used to date fossils. Carbon dating is relatively precise but can only be used to date relatively "young" fossils since carbon's half-life is only 5,730 years. Fossils more than about 75,000 years old must be dated with other methods. That's why potassium-argon dating, and not carbon dating, was used to date the world's earliest human fossils.