That's the premise behind the Paleo diet, a health and weight-loss trend that encourages people to eat modern-day versions of Paleolithic food.
"A lot of people at our gym were getting good workout results, but I knew they weren't supporting it with their diets," Larson said. Because other gyms in the CrossFit family have had success with the Paleo diet, Larson decided to test it at his gym. Fifteen people took the bait.
Like any diet, the hardest thing about the Paleo diet is what you can't eat.
Out is anything that humans began eating after the agriculture and animal husbandry revolutions, meaning no dairy, beans, grains or starches and absolutely nothing processed.
"If you can't eat it raw, then you shouldn't consume it," Larson said. (Although, since our Paleolithic ancestors did have fire, cooking food is permissible.)
The idea of the Paleo diet has been around since the 1980s, but it was popularized in the 2002 book "The Paleo Diet: Lose Weight and Get Healthy by Eating the Food You Were Designed to Eat" by Loren Cordain, a professor at Colorado State University.
Twenty years ago, Cordain read about the diet in a scientific journal and decided to try it.
"For the first time in my life, I started to feel quite healthy, I didn't get any respiratory problems, my arthritis problems went away, and I felt like I gained more muscle mass," he said.
He's been eating like a Neanderthal ever since.
"We are genetically made up with a Stone Age disposition, but we are living in a world of Space Agers," Cordain said.
The Paleo diet even clears up acne, he said.
Larson, the West Sacramento, Calif., gym owner, put his own twist on the Paleo diet. He combined it with the Zone diet, which centers on setting calorie intake parameters but doesn't forbid any kind of food. In Larson's Paleo/Zone diet, the Paleo diet determines what kind of food to eat and the Zone diet guides the quantity.
Following the diet is tough; Larson holds weekly social support sessions with participants.
On a recent Tuesday, Larson told the group they are allowed to have two vices: tea and coffee.
"But not sweetened, and with no dairy," he said.
He points to himself. He's been doing the Paleo/Zone diet for 11 weeks, and his body fat percentage is 2.7, he said.
"Come on! That's how much fat is in my finger," said Santinia Pasquini, 33. Pasquini said she has tried everything from Weight Watchers to diet pills, and in the week she's been doing the Paleo/ Zone diet, she has lost 8 pounds.