Dating Uranium: How do scientists tell time using rocks?

What is it? 

Uranium-series dating is a method scientists use to figure out how old certain rocks and minerals are. It relies on the natural breakdown (or decay) of uranium into other elements, like thorium. When minerals such as calcite form from water that contain tiny amounts of uranium, almost no thorium is included because thorium does not dissolve easily in water. 

As time passes, uranium slowly changes into thorium at a known rate. By measuring how much uranium and thorium are in a sample, scientists can estimate how long ago the mineral formed—like reading a chemical clock. Because of its precision, uranium-series dating is especially useful for materials that are between about 1,000 and 100,000 years old, with an accuracy of roughly ±1%. 

In practice, this technique is most effective for cave formations such as stalactites and stalagmites (made of calcite), and it can also be applied to fossil bones that have absorbed uranium over time, though those results tend to be less reliable. Beyond caves, uranium-series dating has also helped confirm that the Miami Limestone in Florida formed during a period of high sea levels in the Pleistocene, between about 1.75 million and 11,000 years ago. Together, these examples show how the same dating method can reveal both local geological history and broader patterns of environmental change.

Why it matters: 

Understanding when and where modern humans evolved helps us piece together the story of our shared origins. Scientists use fossil evidence, genetics, and precise dating methods—like uranium-series dating—to test different ideas about human evolution. 

The Multiregional Continuity Hypothesis suggests modern humans slowly developed in several parts of the world, while the Out-of-Africa Hypothesis argues that we evolved in one place (most likely Africa) and then spread outward. To figure out which is true, researchers need accurate timelines showing when different human species lived. 

For example, new dating of fossils from caves in Israel suggests that early modern humans and Neanderthals lived around the same time instead of one directly evolving into the other. Similarly, studies in Indonesia and China show that Homo erectus may have survived longer than once thought, overlapping with early humans. 

These discoveries reveal that human evolution was not a straight line, but a complex web of connections between populations across regions. 

Thinking about the future 

Looking ahead, advances in dating technology and DNA analysis will keep reshaping our understanding of human origins. Every new discovery adds another piece to the puzzle of where we came from—and reminds us how connected we all are to each other. 

Dating uranium also connects us to the rest of our planet. As organisms die, their remains are often preserved as fossils, and the abundance of uranium allows us to date them. Using the timeline established by uranium dating and other clues about fossils, we can determine what the organisms looked like, the environment they live in, how they evolved, and how they went extinct. 

As science becomes more precise, we are not just rewriting the timeline of the past; we are learning lessons about cooperation, adaptation, and resilience that can guide us in the future. By understanding the deep roots of our shared environmental history, we can better appreciate the diversity and unity that define our world today. 

Information from the Florida Museum of Natural History, the Thompson Earth Systems Institute, American Scientist, ThoughtCo., ScienceDirect, Human Origins, GeoScienceWorld, and the National Park Service.