Between 1.2 million and a few hundred thousand years ago, early human ancestors like Homo and Neanderthals relied mainly on meat and other animal foods for nutrition, while earlier ancestors like...
Why refined: The high-quality contradicting evidence from Australopithecus at Sterkfontein (score=60.0) demonstrates that early hominins (~2–3 million years ago) did not rely on meat as a primary nutrient source, directly challenging the original claim's temporal generality. To preserve consistency with all evidence, the refined claim narrows the scope to later hominins (post-1.2 million years ago), where multiple high-relevance studies (Simas del Elefante, zinc isotopes, nitrogen isotopes, and metabolic adaptations) consistently support meat dominance. This adjustment accommodates the early plant-based diet of Australopithecus without invalidating the strong evidence for meat centrality in later human evolution.
Mechanism
Synthesis from 10 studies
When early humans started eating mostly meat, their bodies changed to digest it better: their stomachs got more acidic, their liver learned to make energy from fat instead of sugar, and their gut bacteria shifted to thrive on meat. Their teeth also wore down in ways that show they chewed tough...
Most probable mechanism
When early humans ate mostly meat, their bodies changed to digest it better: their stomachs became more acidic to break down tough meat and kill germs, their liver started making alternative fuels from fat when there wasn't enough sugar from plants, and their gut bacteria shifted to thrive on meat instead of plants. Their teeth and jaws also adapted to chew hard, gritty foods like meat and bone, and their bodies stopped making as much enzyme to digest starch because they didn't need it. These changes made it possible to get enough energy and nutrients from meat, which became the main food source.
High intake of animal protein and fat triggers increased secretion of hydrochloric acid and pepsinogen in the stomach, lowering gastric pH to 1–3 to denature proteins and activate digestive enzymes.
Low dietary carbohydrate availability reduces insulin signaling, leading to hepatic insulin resistance that promotes gluconeogenesis using amino acids from dietary protein to maintain blood glucose for the brain.
Excess acetyl-CoA from fatty acid oxidation in the liver exceeds tricarboxylic acid cycle capacity, diverting metabolism toward ketogenesis to produce ketone bodies as an alternative fuel for the brain and peripheral tissues.
Reduced dietary fiber and increased protein intake select for proteolytic gut bacteria that ferment amino acids into branched-chain fatty acids and nitrogenous metabolites, replacing fiber-fermenting taxa.
Dietary carnitine and choline from animal tissues are metabolized by gut microbes into trimethylamine, which is oxidized in the liver to trimethylamine-N-oxide, a systemic metabolite marker of animal protein consumption.
Consumption of hard, abrasive animal tissues and embedded grit generates microscratches on buccal enamel surfaces through mechanical shearing during mastication, creating a durable record of long-term dietary texture.
Evolutionary reduction in salivary amylase gene copies limits pre-gastric starch digestion, reflecting reduced reliance on plant carbohydrates in the diet.
Intact meat particles larger than 2 mm are retained in the stomach by the pyloric sphincter, prolonging gastric residence time to allow complete proteolytic breakdown before passage to the small intestine.
Zinc isotopes in tooth enamel become progressively lighter with increasing trophic level due to preferential retention of heavier isotopes in plant tissues and depletion in animal muscle, enabling dietary reconstruction.
Nitrogen isotopes in bone collagen become enriched in heavier isotopes due to microbial deamination during putrefaction and assimilation by fly larvae, which are then consumed and incorporated into hominin tissue.
Less supported by current evidence, but not ruled out
In some regions, Neanderthals consumed large amounts of plants and used certain plants with natural antimicrobial properties to treat infections, suggesting dietary flexibility and local adaptation to plant-rich environments.
Ingestion of antimicrobial plant compounds such as salicylic acid from poplar bark and fungal metabolites from Penicillium reduces oral and gastrointestinal pathogen load.
Chronic colonization by oral and gut microbes such as Methanobrevibacter oralis and Enterocytozoon bieneusi persists over generations, indicating host-microbe co-adaptation in plant-consuming populations.
Dietary shift to plant-based foods selects for fermentative and cellulolytic oral microbiota, altering local microbial ecology and reducing proteolytic bacterial dominance.
Evidence from Studies
Supporting (8)
Community contributions welcome
Australopithecus at Sterkfontein did not consume substantial mammalian meat.
Neanderthals, hypercarnivores, and maggots: Insights from stable nitrogen isotopes
The diet of the first Europeans from Atapuerca
Neanderthal behaviour, diet, and disease inferred from ancient DNA in dental calculus
Zinc isotopes in Late Pleistocene fossil teeth from a Southeast Asian cave setting preserve paleodietary information
Archaic humans in the Middle Palaeolithic Levant conducted planned and selective intercepts of aurochs, but not mass hunting
Human Digestive Physiology and Evolutionary Diet: A Metabolomic Perspective on Carnivorous and Scavenger Adaptations
Contradicting (1)
Community contributions welcome
Dental calculus indicates widespread plant use within the stable Neanderthal dietary niche.
Gold Standard Evidence Needed
According to GRADE and EBM methodology, here is what ideal scientific evidence would look like to definitively prove or disprove this specific claim, ordered from strongest to weakest evidence.