Neanderthals were apparently too busy hunting and scavenging to pay much attention to Michael Pollan's dietary advice: eat mostly plants.

New isotopic analysis suggests prehistoric humans ate mostly meat. As detailed in a new study published in the journal Quaternary International, the Neanderthal diet consisted of 80 percent meat, 20 percent vegetables.

Researchers in Germany measured isotope concentrations of collagen in Neanderthal fossils and compared them to the isotopic signatures of animal bones found nearby. In doing so, scientists were able to compare and contrast the diets of early humans and their mammalian neighbors, including mammoths, horses, reindeer, bison, hyenas, bears, lions and others.

"Previously, it was assumed that the Neanderthals utilized the same food sources as their animal neighbors," lead researcher Herve Bocherens, a professor at the University of Tubingen's Senckenberg Center for Human Evolution and Palaeoenvironment, said in a news release.

"However, our results show that all predators occupy a very specific niche, preferring smaller prey as a rule, such as reindeer, wild horses or steppe bison, while the Neanderthals primarily specialized on the large plant-eaters such as mammoths and woolly rhinoceroses," Bocherens explained.

All of the Neanderthal and animal bones, dated between 45,000 and 40,000 years old, were collected from two excavation sites in Belgium.

Researchers have long debated the precise diet of early humans, but the latest study is the first to nail down precise percentages.

Bocherens and his colleagues are hopeful their research will shed light on the Neanderthals' extinction some 40,000 years ago.

"We are accumulating more and more evidence that diet was not a decisive factor in why the Neanderthals had to make room for modern humans," he said.