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Dating Fossils
Paleontologists of the 19th century recognized that fossils are generally ordered by apparent age, with older fossils appearing in deeper strata than newer fossils. This technique of ordering can determine the relative age of fossils but cannot give an exact date. Modern paleontology uses the decay of radioactive isotopes within rock compounds to give a more precise date to fossils. Fossils older than 100,000 years are often dated using the Potassium-Argon technique, which measures the decay of radioactive Potassium isotopes into inert Argon. This technique is useful for dating fossils in the early human date range.
The Oldest Hominins
Humans belong to a sub-group of the great apes called hominins. Hominins diverged from the ape lineage that we share with chimpanzees approximately 8 million years ago. Modern humans are the only living hominin species. Thus, the earliest hominins are the first fossils that are unique to human lineage.
There are several candidates for the earliest hominin. The earliest possible candidate is Sahelanthropus tchadensis, a 7 million-year-old fossil known only from partial skull and jaw fossils. Much more evidence exists for Orrorin tugenensis, which dates to around 6 million years ago. Numerous Orrorin specimens have been discovered at four different sites in Kenya. Orrorin was an ape-like hominid with skull and tooth characteristics that place it within the hominin lineage that would eventually lead to humans.
The Oldest Humans
Several distinct lineages of extinct hominins, including Orrorin and Australopithecus -- a group that includes the famous "Lucy" fossil -- predate the existence of actual humans. These early hominins eventually gave rise to the genus Homo (H). Numerous species, including modern humans, belong to this genus; all members of Homo are considered to be humans.
The first human fossils originate just over two million years ago. There is some debate about which human species came first. These early humans are generally intermediate between Australopithecines and later humans and are best characterized by their increased brain size relative to earlier hominins.
Homo habilis, which was first discovered in 1962, is the best-characterized of these early humans, but several human species probably predate it. One such species is H. gautengensis, an early human distinct from H. habilis that lived over 2 million years ago. Fossils of these early humans are invariably found in eastern and southern Africa.
The Oldest Modern Humans
Modern humans belong to the species Homo sapiens and are often referred to as anatomically modern. Several poorly-classified subspecies of archaic H. sapiens predate anatomically modern humans, dating back to 400,000 years ago. These differ from modern humans in aspects such as the shape of the face and skull. The first definitively modern humans date to around 195,000 years ago. The oldest known specimens as of March, 2011 are a collection of fossils found near the Omo river in Ethiopia.