Hunters all over the world, for uncounted thousands of years, would eat the raw livers of the animals they killed. (ancestralsupplements.com)
The reason for this seems to be insulin - like growth factor (IGF), a protein that is released by the liver of all animals (humans included) in response to growth hormone. (pregnancy-info.net)
After observing the transformation in a petri dish, they injected these cells into the livers of animal models to see if the transformation could become complete. (gladstone.org)