The savannahs of Africa and the Great Plains of North America were created by hunter gatherers burning forests to create grasslands that would support large numbers of large, tasty mammals they could hunt. So hunter gatherers like Native Americans ate meat all the time. It got them through the winter because in the temperate climate where most of them lived vegetative nutrition was only available part of the year. Finding wild food to eat was no easier then than it is now, especially when there was snow on the ground. They preserved meat by smoking and/or drying it and turning it into jerky, or pemmican which is meat and berries together. They also developed a tolerance for eating rancid meat and fat. So actually hunter gathers outside of the tropics ate prepared food most of the year because it was all they had.