Dating Fossils

One of the most fascinating parts of studying anthropology is analysing fossils and determining how old they are. In Human Evolution: A Very Short Introduction, Wood explains that understanding the age of a fossil reveals details about how our ancestors lived and evolved.

One of the first methods to age fossils was looking at the surrounding rock and dirt. Generally, older sediment is found deeper in the earth and younger rocks are closer to the surface. This idea is called the law of superposition. If a fossil is found inside a certain layer, scientists can often assume it is around the same age as that layer of rock. However, this method is not foolproof as erosion and caves can move fossils or mix old and new sediments together.

As technology advanced, scientists discovered more absolute methods to date fossils. In East African fossil sites, many bones are found between layers of volcanic ash. These ash layers can be dated using potassium/argon or argon/argon dating because radioactive elements decay at a steady rate. Researchers also utilize palaeomagnetic dating, which analyzes changes in the earth’s magnetic field recorded in rocks. By comparing the magnetic pattern in sediments to known magnetic records, scientists can estimate the age of the fossils.

Other more specific methods help date fossils from different time periods. Ostrich eggshell dating uses changes in amino acids over time. Electron spin resonance and uranium series dating are often used for sites too old for radiocarbon dating but not old enough for potassium/argon dating. This plethora of techniques show that fossil dating is a careful process that combines geology, chemistry and biology to piece together the timeline of human evolution.

When I was younger, my family visited Salt Lake City in Utah and my brother and I had the opportunity to dig up fish fossils. Unfortunately, I wasn’t old enough to handle fossils so the most interesting specimen I dug up was fish poo. I remember being jealous of my brother, on the other side of the field, discovering fish bones while I uncovered their feces. Nonetheless, the trip inspired my fascination for archeology, and later anthropology.