Those ancestors also started living longer than ever before - an unexpected evolutionary twist.
The research by USC professors Caleb Finch and Craig Stanford appears in Wednesday's Quarterly Review of Biology.
"At some point - probably about 2 1/2 million years ago - meat eating became important to humans," said Stanford, chair of the anthropology department in the USC College of Letters, Arts and Sciences, "and when that happened, everything changed."
"Meat contains cholesterol and fat, not to mention potential parasites and diseases like Mad Cow," he said. "We believe humans evolved to resist these kinds of things. Mad Cow disease - which probably goes back millions of years - would have wiped out the species if we hadn't developed meat-tolerant genes."
Finch, the paper's lead author, and Stanford found unexpected treasure troves in research ranging from chronic disease in great apes to the evolution of the human diet. They also focused on several genes, including apolipoprotein E (apoE), which decreases the risk of Alzheimer's and vascular disease in aging human adults.
Chimpanzees - who eat more meat than any other great ape, but are still largely vegetarian - served as an ideal comparison because they carry a different variation of the apoE gene, yet lack human ancestors' resistance to diseases associated with a meat-rich diet.
While chimpanzees have a shorter life span compared to humans, they demonstrate accelerated physical and cerebral development, remain fertile into old age and experience few brain-aging changes relative to the devastation of Alzheimer's seen in humans today. Finch and Stanford argued that the new human apoE variants protected the chimpanzees.
In a series of "evolutionary tradeoffs," the researchers said, humans lost some advantages over those primates, but gained a higher tolerance to meat, slower aging and longer lifespan.
Still, if humans developed genes to compensate for a meat-rich diet, why do so many now suffer from high cholesterol and vascular disease?
The answer is a lack of exercise and moderation, according to the researchers.
"This shift to a diet rich in meat and fat occurred at a time when the population was dominated by hunters and gatherers," said Finch, a USC University Professor and holder of the ARCO-William F. Kieschnick Chair in the Neurobiology of Aging.
"The level of physical activity among these human ancestors was much higher than most of us have ever known," he said. "Whether humans today, with our sedentary lifestyle, remain highly tolerant to meat eating remains an open question researchers are looking into."
Stanford, co-director of the university's Goodall Research Center, said that modern-day humans "tend to gorge ourselves with meat and fat."
"For example, our ancestors only ate bird eggs in the spring when they were available," he said. "Now we eat them year-round. They may have hunted one deer a season and eaten it over several months. We can go to the supermarket and buy as much meat as we want."
"I think we can learn a lesson from this," Stanford said. "Eating meat is fine, but in moderation and with a lot of exercise."