Soft body tissues do not fossilise, it is very likely that any organism on Earth will be either eaten by scavengers or decomposed by microorganisms after it dies. Internal organs decompose more quickly when they are in contact with oxygen. Most environments exposed to the open air and are in contact with plenty of oxygen, so the soft tissues of dead organisms, whether plants or animals, decay quickly. Many, if not most, underwater environments also have a lot of oxygen, since water can dissolve oxygen from the atmosphere. The hard parts of organisms, such as bones, shells, and teeth have a better chance of becoming fossils than do softer parts. One reason for this is that scavengers generally do not eat these parts. Hard parts also decay more slowly than soft parts, giving more time for them to be buried.
Traditionally paleoanthropologists used relative methods of dating fossils by studying what layers of strata, rocks and soil that the fossil was found in. In the late 1940s and early 1950s radiocarbon followed by the latest methods of Time Population Structure (TPS), that uses mutations to predict time in order to date the ancient DNA have shown that the traditional methods were remarkably correct. However TPS can calculate the mixtures of DNA deriving from different time periods to estimate its definitive age.
Some of the earliest hominins known are the Orrorin Tugenensis found in the Tugen Hills in Kenya, south of Lake Turkana and dated to be between 6.2 to 5.8 mya. Knowing that these fossils were of human origin is by a reasoned logic by paleianthropologists studying the shapes and lengths of bones. The leg bones Femur, Fibula and Tibia were much longer than those in apes, but the most revealing is the Formen Magnum, the hole in the skull of humans and apes that functions as a passage for the central nervous system through the skull connecting the brain with the spinal cord.
Formen Magnum on apes are much higher up the skull than on humans, the Formen Magnum on humans is under the skull, which combined with leg, arm and feet bones prove that this creature walked upright on two legs, they were bipedal, the same as Australopithecus, Denisovans, Neanderthals and all the other known and unknown hominins.
The Formen Magnum, the curvature of the spine, the length of leg and arm bones all indicate that humans and our ancestors walked on two legs as a normality and not occasionally as our cousins the primates. So where did our bipedalism come from? Another way of establishing if an animal was bipedal is the shape and length of leg and arm bones. Estimates vary for when humans and chimpanzees split but up until 2022 the earliest fossil of a hominid was believed to be Orrorin Tugenensis at about 5 to 6 million years old, but in 2022 Franck Guy of the University of Poitiers re-examined femur and ulnae bones first discovered 2001 in the Djurab desert in Chad and named Sahelanthropus tchadensis. By using computed tomography scans to see the internal structures the bones Guy and his team assess that the bones were from a biped hominid and were 7.5 million years old.