Radiometric dating or radioactive dating is a technique used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed.
The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, which form at a known constant rate of decay.
This distributes the weight of the head over the spine so that modern humans do not need huge necks muscles.
The rate of decay (half-life) is different for different radioisotopes.
C-14 and K-40 are the two radioisotopes most commonly used.
The discovery of small numbers of fossils has caused huge changes in theories of human evolution, perhaps indicating that too much has been constructed on too little.
Conversely, discoveries such as those made in Dmanisi, Georgia provide examples of falsification of earlier held positions, indicating why paleoanthropology can be considered a science.