Archaeologists have a group of unlikely allies: trees. Dendrochronology, the scientific method of studying tree rings, can pinpoint the age of archaeological sites using information stored inside old wood. Originally developed for climate science, the method is now an invaluable tool for archaeologists, who can track up to 13,000 years of history using tree ring chronologies for over 4,000 sites on six continents.

Trees don’t grow their trunk uniformly; though they add a new ring each growing season, trunk growth is closely linked to climate conditions. Under ideal conditions, trees grow quickly, leaving wide annual rings behind. During droughts, unseasonable cold, and other unusual conditions, growth slows, leaving behind narrow rings.

In the early 20th century, astronomer Andrew Ellicott Douglass