Human fossil isotopic signatures exhibit elevated nitrogen-15 levels compared to those of other apex predators, indicating a diet primarily composed of animal tissue.

Study: Interpretation of the Analysis of Stable Nitrogen and Carbon Isotopes in Human and Animal Bone Tissue (Selivanovsky II Burial Ground in the Southern Trans-Urals)

Scientists checked the bones of ancient people and found they had much higher levels of a special nitrogen marker than animals like cows or sheep. This means those people ate mostly meat and dairy, not plants.

Study: First data on diet of the medieval population of Belarus (based on carbon and nitrogen stable isotope analysys in bone collagen)

Scientists looked at ancient human bones and found high levels of a special nitrogen type that only builds up when you eat a lot of meat or dairy — just like the claim says. This means ancient humans were eating mostly animals, not plants.

Study: A new era of isotope ecology: Nitrogen isotope ratio of amino acids as an approach for unraveling modern and ancient food web

Scientists found a better way to read the 'chemical signature' in ancient bones, and it confirms that early humans ate mostly meat, not plants, because their bones had more of a special nitrogen type that builds up when you eat animals.