This is the fourth in a five-part series written by experts featured in the Smithsonian's new Hall of Fossils—Deep Time exhibition, now on view at the National Museum of Natural History. The full series can be found by visiting our Deep Time Special Report

“No fossil is buried with its birth certificate,” wrote the renowned science editor Henry Gee in his 2000 treatise, In Search of Deep Time. While true, fossils are buried with plenty of clues that allow us to reconstruct their history. An array of absolute dating techniques has made it possible to establish the timescale of Earth’s history, including the age and origin of life, the timing of mass extinctions and the record of human evolution.

In 2013, in Ethiopia's Afar region, our research team discovered a rare fossil jawbone belonging to our genus, Homo. To solve the mystery of when this human ancestor lived on Earth, we looked to nearby volcanic ash layers for answers. Using a geologist’s magnifying glass, we can carefully scan the ash in search of tiny minerals that are smaller than a single sprinkle on a sugar cookie and that hold the key to determining the age of a fossil.

Working in this part of Ethiopia is quite the adventure. It is a region where 90 degrees Fahrenheit seems cool, dust is a given, water is not, and a normal daily commute includes racing ostriches and braking for camels as we forge paths through the desert. But, this barren and hostile landscape is one of the most important locations in the world for studying when and how early humans began walking upright, using tools and adapting to their changing environments.

image: https://thumbs-prod.si-cdn.com/JNR276vAfNW2poEDUe08C6CX6MM=/1024x596/https://public-media.si-cdn.com/filer/bc/83/bc839a3d-868c-4397-b2b7-0b5a315cb847/dscn9667a.jpg
The Afar region has layered sedimentary rocks, containing thin white volcanic ash that we could use to date the fossil jawbone.
The Afar region has layered sedimentary rocks, containing thin white volcanic ash that we could use to date the fossil jawbone. (Erin DiMaggio, Penn State University)
Early on, before we had more precise means to date fossils, geologists and paleontologists relied on relative dating methods. They looked at the position of sedimentary rocks to determine order.

Imagine your laundry basket—the dirty clothes you wore last weekend sit at the bottom, but today's rest on top of the pile. The concept for sedimentary rocks is the same. Older rocks are on the bottom, younger ones are on top. Researchers also used biostratigraphy, which is the study of how fossils appear, proliferate and disappear throughout the rock record, to establish relative ages. We still use these relative dating methods today as a first approach for dating fossils prior to assigning a numerical, or absolute, age.