Palaeontology was once limited to digging up fossils and trying to deduce their age with inaccurate methods. However, fossil analysis improved dramatically in the 1960s, with the advent and refinement of two techniques: radiometric dating and stratigraphy. The first radiometric method was also known as carbon-14 dating, and it was usable for specimens younger than 50,000 years. Later, potassium-argon dating revolutionized the field by enabling scientists to detect the radioactive decay of elements found naturally in rocks and soil surrounding much older fossils. Stratigraphy, which is the study of rock layering, actually was developed well before the 1960s, but that was the decade scientists began to better understand how geological conditions, earthen layers, and fossil records all relate. The resulting refinement of biostratigraphy, i.e., the study of the complete life of a stratum of earth, allowed scientists to determine the environment and lifestyle of human ancestors based on fossilized flora and fauna found within the same layer as the hominine fossils. Since the 1960s, DNA testing has come to be used widely. As all living organisms have the same genetic code, scientists can use DNA variations as a molecular clock. After splitting with a common ancestor, each generation develops a constant rate of genetic mutations. The molecular clock allows scientists to calculate how long ago the split occurred based on the number of differences between species. The method is now helping scientists map the routes that humans took out of Africa.