How old is the oldest human skeleton? How do we date human bones? Find out.
A few days ago my three-year-old asked me, “How do humans get on our planet?” Preschool grammar aside, I was impressed by her question, and so, being an astronomer, I launched into an excited explanation of nucleosynthesis in the cores of stars and the creation of the heavier elements. As Carl Sagan reminds us, “We are made of star stuff.” This was not the answer she was looking for.
Luckily, before I had to attempt an explanation of the primordial soup fit for a three-year-old, she interrupted me to ask instead, “Can we learn about people from their bones like the dinosaurs?” I told her that not only is that the case, but we continue to learn about our origins as more bones are discovered and as our dating techniques improve. In fact, a study published just last month in the journal Nature challenges what we thought we knew about human origins and migration.
How do we date human bones?
The challenge in identifying the oldest human remains is two-fold: first, the bones need to be dated correctly. Scientists use several methods for determining the ages of human remains and those methods can often be checked against one another for consistency.
Age-dating via electron spin resonance, for example, measures how many electrons have been absorbed by the bone over time. Radioactive elements decay over time on a precisely-timed schedule, and so other methods like potassium-argon dating or Carbon-14 dating, measure the extent to which these elements have decayed to assess how long the bone has been waiting to be unearthed. A technique called paleomagnetism examines the direction of the magnetized particles in nearby rock and links it to known global shifts in our planet’s magnetic field.
Scientists also use context to help place age constraints – were any artifacts or tools found nearby? Where were the bones found? While context can be extremely helpful, particularly in cases where only a few bones are found, it also must be used with caution so as not to preclude new ways of thinking about our origins.
Another challenge in identifying the oldest human skeleton is determining whether or not new bone discoveries are in fact, human. Neanderthals, for example, who we once thought were our ancestors but now believe were a kind of distant cousin with a shared ancestor, have longer and flatter skulls. It is hypothesized that our facial features shortened as our brain organization and connectivity improved. So while there are key markers that distinguish a member of the homo sapien species from other hominin relatives, incomplete fossils and evolutionary changes that have led to what we recognize now as modern day humans can make the identification of these markers challenging.