Ancient 'Paleo' diet largely consisted of plants for some hunter-gatherers, study finds

Prehistoric humans are often believed to have largely eaten meat — to the extent that proponents of many modern low-carb diets portray them as closely resembling humans' “primordial” eating style.

But a recent excavation at a cave in Taforalt, Morocco, complicates this picture, according to findings published on Monday in Nature Ecology & Evolution.

These findings challenge “the prevailing notion” that prehistoric people had a “high reliance on animal proteins,” the authors wrote.

Their findings also suggested a strong advantage to a plant-based diet: that it makes it easier to wean young children. This factor offers a strong additional benefit for the transition to settled agriculture.

By analyzing the teeth of 13,000-plus-year-old bodies recovered from a cave in Morocco, researchers at the Max Planck Institute found evidence of “substantial plant-based” elements in their diet.

The researchers figured this out by analyzing the respective levels of the element zinc in the teeth of the bodies found at the site. 

Zinc has many isotopes — or forms with the same number of protons but different numbers of neutrons, and therefore different weights. The body’s breakdown of plant fibers tends to leave it with a higher ratio of the heavier zinc isotopes than the processing of meat.

The levels of zinc in the human teeth recovered from Tarofalt were far closer to those seen in Barbary sheep recovered from the site — that is, pure herbivores — than they were to those of local canids, like wolves, jackals and dogs.

While the canid teeth found at the site showed that they weren’t pure carnivores either, they seem to have largely supplemented the meat in their diets with fruit.

The humans also weren’t pure herbivores — the sheep bones recovered from the site had cut marks, which suggested they were hunted, butchered and processed. 

But the isotopic analysis suggests that humans at the site only got about 50 percent of their diet from meat — like gazelles, wild cattle and hartebeest.

They supplemented this with sweet acorns, pine nuts and legumes — which they seem to have ground into flour with grinding stones found on site. (The starches from these plants left cavities in their teeth.)

They appear to have gathered plants in periods of seasonal abundance and stored them on site to eat throughout the year — a step somewhere between foraging and agriculture — likely during periods when animal protein was less available. 

The isotope analysis also allowed the scientists to determine at what age those buried in the cave had been weaned — or eased off breastmilk and onto solid foods — and what the diet was that they transitioned onto.

They found something else surprising when doing this: The evidence from the teeth showed that infants had been weaned early — as in before 1 year of age — and possibly “with plant-based foods.” 

The researchers acknowledged that it was difficult to draw firm conclusions in this regard, given that they had such a small sample size to work with. But their findings may point to the use of starchy cereals as a means to ease children off breast milk — a practice that would contrast starkly with those of many hunter-gatherer societies, “where extended breastfeeding periods are the norm due to the limited availability of suitable weaning foods,” the authors noted.

The bodies of infants in the cave suggest this transition offered a harsh trade-off: Early weaning in the ancient past could mean “increased stress and mortality for infants,” the researchers wrote. 

But by drastically reducing the time needed for breastfeeding, the study indicates the practice could also starkly reduce the time in between births — allowing plant-eating populations to grow more quickly, and ultimately push hunters to the margins.

In past decades, research has found less of a hard distinction between farming and foraging than a more diverse spectrum of feeding behaviors: Ancient peoples gathered, nurtured, processed and stored plant-based foods long before the development of captive, human-engineered plant species that we formally call agriculture.

Where that formal transition happened, recent research suggests it was a result of an ecological crisis. In Southwest Asia, thought to be the heartland of agriculture in the West, declining populations of large and medium-sized game animals led hunter-gatherers to diversify their diets.

One explanation for the rise of farming in this region is that it arose from the so-called Broad Spectrum Revolution — as rising populations and a changing climate put pressure on human populations to expand the number of prey and plant species they ate far more widely than their ancestors might have.

But in Northwest Africa, despite the reliance on wild plants and close genetic links to the proto-farmers of Southwest Asia, prehistoric peoples didn’t develop farming. The first evidence of farming in the region — a single domesticate lentil seed — doesn’t appear for another 7,500 years after the cave sites where Monday's study was based were abandoned.

The Planck scientists can only point to the “dissimilarity” between these two regions — they can’t explain it. They note that as the climate continued to cool under the protracted glacial expansion called the Younger Dryas, plant resources became less available, and the cave sites where they conducted their research increasingly emptied out.