Humans started eating meat because climate change shrunk the forests and fruit supplies our ancestors depended on, pushing them toward animal foods as a survival strategy. This dietary shift began roughly 2.4 million years ago in eastern and southern Africa, and it set off a cascade of physical and social changes that shaped nearly every aspect of human evolution.
Climate Change Forced the First Shift
For millions of years, early hominins thrived on a plant-heavy diet in forested environments. But during the late Pliocene and early Pleistocene, eastern and southern Africa underwent dramatic environmental upheaval. Tectonic movements, long-term drying, and extreme wet-dry cycles shrank forested areas and spread open grasslands across the landscape. The fruits, leaves, and other plant foods our ancestors preferred became harder to find.
After about 2.4 million years ago, increasingly severe drought cycles forced at least one group of hominins, early Homo, to broaden a primarily plant-based diet to include vertebrates and invertebrates. This wasn’t a sudden lifestyle choice. It was a response to scarcity. When familiar, preferred foods ran low, our ancestors turned to resources that had fewer competitors: animals, insects, and fish found in the wetland-savanna mosaics they inhabited. Meat wasn’t the first choice. It was the available one.
Scavenging Came Before Hunting
The earliest meat eating almost certainly involved scavenging rather than hunting. One of the most accessible animal foods didn’t require bringing down prey at all: bone marrow. After predators like lions or hyenas finished with a carcass, the marrow locked inside long bones remained untouched and rich in fat and calories. Early hominins used stones to crack these bones open, and the percussion marks they left behind are some of the clearest evidence we have of early meat processing.
Analysis of a well-known archaeological site called FLK Zinj in Tanzania shows patterns of percussion marks, bone breakage, and cut marks consistent with hominins smashing bones for marrow extraction. These same bones also show cut marks in locations that indicate hominins had access to carcasses while significant flesh was still attached, not just bare skeletons. Whether they were chasing off smaller predators, arriving at kills before competitors, or occasionally hunting on their own, early Homo was getting meaningful amounts of animal tissue by around two million years ago.
Some researchers initially claimed that cut marks on bones from the Dikika site in Ethiopia pushed butchery evidence back to 3.4 million years ago. However, experimental studies comparing those marks to damage caused by natural trampling in sediment found that the Dikika marks were indistinguishable from non-human processes. The scientific consensus holds that convincing butchery evidence begins closer to 2.6 million years ago at sites in Gona, Ethiopia, where marked bones appear alongside stone tools.
Meat Fueled Bigger Brains
The shift toward animal foods coincided with one of the most striking changes in human evolution: rapidly expanding brain size. Brains are extraordinarily expensive organs to run. The human brain accounts for about 2% of body weight but consumes roughly 20% of the body’s energy at rest. A diet of raw leaves and tough plant fibers simply couldn’t supply that kind of fuel efficiently.
The Expensive Tissue Hypothesis, proposed in the 1990s, offers an explanation. Both the brain and the digestive tract are metabolically costly organs, and in primates, there appears to be a trade-off between the two. A shorter, simpler gut requires less energy to maintain, freeing up calories for the brain. But a shorter gut only works if the food coming in is already dense in calories and easy to digest. Meat fits that description perfectly, especially cooked meat. Experimental evidence supports this trade-off: when researchers selectively bred fish for larger brains over several generations, the larger-brained fish consistently developed shorter guts.
Around two million years ago, the fossil record shows exactly this pattern in hominins: brains getting bigger, teeth getting smaller, guts getting shorter, and bodies becoming more modern in proportion. These changes unfolded alongside increasing evidence of meat consumption and stone tool use.
Cooking Made Meat Even More Valuable
Raw meat provides more calories than raw plants, but cooked meat is in another category entirely. Cooking breaks down tough proteins and connective tissue, dramatically reducing the energy your body spends on digestion and increasing the nutrients you absorb. The question is when our ancestors figured this out.
The oldest confirmed evidence of controlled fire for cooking comes from an archaeological site in Israel called Gesher Benot Ya’aqov. Researchers analyzing the remains of a carp-like fish found there determined the fish had been cooked roughly 780,000 years ago. That pushed the cooking timeline back by about 600,000 years from the previous oldest evidence of around 170,000 years ago. The transition from raw to cooked food had enormous implications: with less energy diverted to digestion, more could be invested in brain development, physical growth, and activity.
Fish may have played a particularly important role. The fats found in fish, including omega-3 fatty acids along with zinc and iodine, are critical building blocks for brain tissue. Some researchers argue that regular fish consumption was a central catalyst in the cognitive leap that distinguishes humans from other primates.
Meat Shaped Social Behavior
Getting meat, whether through scavenging or hunting, rewarded cooperation in ways that gathering plants did not. A single forager can pick berries alone, but driving a predator off a kill or bringing down a large animal is far more effective in a group. Group hunting increases capture success, opens access to larger prey, and improves a group’s ability to defend a kill against competitors trying to steal it.
This cooperation wasn’t automatic or equal. In many social species, dominant individuals use aggression to claim the largest share of a kill, leaving subordinates with little reward. For group hunting to evolve and persist, lower-ranking individuals needed enough of a payoff to make participation worthwhile. Research on social hunting dynamics shows that tolerance during feeding, particularly higher-ranking individuals allowing others to eat, is one of the key factors that promotes cooperation. Kin relationships help too: individuals hunting alongside relatives gain indirect benefits even when their personal share is small, because helping kin survive passes along shared genes.
Over time, the social skills required to coordinate hunts, share food, and manage conflicts over meat likely reinforced the development of communication, trust, and reciprocity. These are the same foundations that underlie human social life today. Meat didn’t just change what our ancestors ate. It changed how they related to each other.
Genetic Traces of a Meat-Eating Past
Millions of years of meat consumption left marks in the human genome. One example involves a gene called APOE, which produces a protein responsible for transporting cholesterol and other fats through the bloodstream. The ε4 variant of this gene, which is the ancestral version carried by our earliest meat-eating ancestors, makes carriers more sensitive to dietary fat and cholesterol. People with this variant show faster changes in blood lipid levels in response to diet compared to those without it.
This heightened sensitivity likely helped early humans thrive on a diet that fluctuated between feast and famine, allowing their bodies to quickly process and store fats from a successful hunt. In the modern world, where high-fat food is constantly available rather than occasional, that same sensitivity is associated with higher cholesterol levels and increased cardiovascular risk. It’s a reminder that human biology was shaped by an environment very different from the one most people live in now.
Even parasites tell the story. Genomic analysis of a human-adapted tapeworm suggests that the infection pathway from animals to humans was established by 1.7 million years ago, indicating that hominins were eating enough raw or undercooked meat by that point to sustain parasite transmission across generations.