Lucy has two ages: one that refers to how old she was when she died, her biological age, and the other that refers to how long ago she lived, her geological age.
Lucy was an adult when she died. Scientists are able to determine this age because Lucy had fully erupted wisdom teeth (or third molars) and her bones appear fully fused. Bones form in separate pieces and fuse (connect) together at different times in life, just as teeth erupt at different ages as an animal grows. By the age of 35, most humans have fully erupted third molars and completely fused bones.
Lucy’s geologic age dates to 3.2 Ma3. After Lucy died, her bones settled into dirt and rocks, called sediments. She then was buried by many more layers of sediments. The weight of all these sediments crushed some of her bones. After a long time, the calcium in her bone was replaced with minerals from the surrounding soils. This process is called mineralization. It turns the bone into rock which help preserve it for a long time. Scientists can determine when Lucy lived by testing chemicals, called Potassium-Argon and Argon-Argon, in layers of volcanic ash above and below the rock layer where Lucy was found. Lucy was found in sandstone sediments associated with a paleo-stream. In this context, it is possible that a stream carried Lucy’s body some distance away from where she died4.
