Throughout most of human evolutionary history, the diet of early human ancestors consisted mainly of animal-based foods, with meat being the main source of nutrition.
Mechanism
Synthesis from 5 studies
Early humans evolved to thrive on meat by developing a super-acidic stomach to digest tough protein, a liver that makes sugar from meat and burns fat for energy, and gut bacteria that break down meat instead of plants. Their bodies also stopped making enzymes to digest starch because they didn’t...
Most probable mechanism
When early humans ate mostly meat, their bodies evolved to break down protein efficiently, use fat for energy instead of carbs, and protect themselves from germs in raw meat. Their stomachs became very acidic to digest tough meat and kill bacteria, their liver learned to make sugar from protein and produce alternative fuels from fat, and their gut bacteria changed to handle meat instead of plants. Their bodies also stopped making as many enzymes to break down starch because they didn’t need them, and their teeth and digestion slowed down to fully process large chunks of meat.
Consumption of animal tissue stimulates gastrin release, triggering parietal cells to secrete hydrochloric acid and lower gastric pH to 1–3, which denatures meat proteins and activates pepsin for proteolysis.
Low dietary carbohydrate intake reduces insulin signaling, leading to hepatic insulin resistance that promotes gluconeogenesis using amino acids from dietary protein to maintain blood glucose for the brain.
Reduced insulin and elevated free fatty acids activate hepatic beta-oxidation, producing excess acetyl-CoA that is diverted into ketogenesis, generating ketone bodies as alternative fuels for the brain and peripheral tissues.
Low insulin and high ketone levels suppress lipogenic enzymes and activate fatty acid oxidation pathways, shifting liver metabolism from fat storage to fat breakdown.
High protein intake stimulates release of cholecystokinin, triggering pancreatic secretion of trypsin, chymotrypsin, and carboxypeptidases to further hydrolyze peptides into absorbable amino acids.
Intact meat particles larger than 2 mm are retained in the stomach by the pyloric sphincter, prolonging gastric residence time to allow complete acid and enzymatic breakdown.
Reduced dietary fiber selects 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 meat are metabolized by gut microbes into trimethylamine, which is oxidized in the liver to trimethylamine-N-oxide, a systemic biomarker of animal protein intake.
Evolutionary reduction in salivary amylase gene copies limits pre-gastric starch digestion, reflecting reduced reliance on plant carbohydrates.
Zinc isotopes in tooth enamel become depleted in heavy isotopes due to consumption of muscle tissue, which has lower zinc isotope ratios than plant material, preserving a dietary signature of meat consumption.
Elevated beta-hydroxybutyrate enters the nucleus and inhibits histone deacetylases, increasing histone acetylation and activating genes that enhance cellular resistance to oxidative stress.
Less supported by current evidence, but not ruled out
When meat was scavenged and left to decompose, microbes broke down proteins and released lighter nitrogen isotopes into the air, leaving behind heavier isotopes in the remaining tissue. Animals or humans eating this decomposed meat absorbed the heavier isotopes, which showed up in their bones and teeth as a signal of meat consumption.
Microbial activity during putrefaction preferentially releases lighter nitrogen isotopes (14N) as volatile compounds, enriching the remaining tissue in heavier nitrogen isotopes (15N).
Consumption of 15N-enriched decomposing tissue leads to incorporation of heavy nitrogen isotopes into consumer tissues, elevating isotopic ratios in bone collagen.
Evidence from Studies
Supporting (4)
Community contributions welcome
Neanderthals, hypercarnivores, and maggots: Insights from stable nitrogen isotopes
Zinc isotopes in Late Pleistocene fossil teeth from a Southeast Asian cave setting preserve paleodietary information
Human Digestive Physiology and Evolutionary Diet: A Metabolomic Perspective on Carnivorous and Scavenger Adaptations
Contradicting (1)
Community contributions welcome
Australopithecus at Sterkfontein did not consume substantial mammalian meat.
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.