A study has found that early Americans relied heavily on mammoths and giant mammals as a primary food source. The extent of this reliance varies, with some reports suggesting that these large mammals made up nearly all of their calories, while others indicate a shift in diet occurred when these animals became extinct. The role of these mammals in the early american diet is confirmed, but details such as the exact proportion of their diet and the impact on the mammals' extinction are still being reported with some variation.