"There was a period of transition where I had all sorts of soreness in my feet and my calves," said Thompson, who does not believe people should run more than a few kilometres in bare feet. "But very quickly that disappeared and I got used to landing on my forefoot."
The knee pain that he had been struggling with eventually disappeared too.
"I was healed," he said. "I run without pain now and the crisis I was having as a runner before I started the project seems to have disappeared."
Early humans, the film explains, didn't have speed on their side but were able to survive thanks to their unmatched endurance that allowed them to outrun their prey.
The documentary says our ancestors became "persistence hunters" who could run down other animals to the point of exhaustion.
"Humans are built to run," said Thompson. "On a hot day, we can outrun all the animals in our environment."
Thompson also takes his cameras to Chukotka in northeastern Russia, where he spends time with a remote community of nomadic reindeer herders. For them, running with their animals is necessary for survival