Teeth are like tiny biological time capsules. They tell stories about ancient diets and environments long after their owners have died and landscapes have changed.

After bones break down, tooth enamel stays hard and unchanged, even in fossilized teeth that have been buried under sediment and rock for millions of years and are now being uncovered by erosion or excavation.

Tooth enamel forms when an animal is young, and it remains chemically stable for the rest of that animal’s life. The food an animal eats and the water it drinks during its youth leave chemical signals within the enamel.

Because of that, hidden within the enamel of fossilized teeth, scientists can find traces of extinct forests, expanding savanna grasslands, shifting climates and evolving animal communities.

Over the past 30 years, my colleagues and I have been analyzing chemical traces in fossil teeth from Ethiopia’s Afar region in the East African Rift Valley – often referred to as the cradle of humanity – to uncover what animals ate there millions of years ago, around the time early human ancestors were evolving, and what the world looked like around them.

These clues from ancient meals are enabling scientists to reconstruct pictures of entire ecosystems, including forests, wetlands and grasslands that existed at the time. It’s a reminder that in a very real sense, organisms are what they eat.

Traces of ancient diets in fossil teeth

To determine which plants ancient........

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