In the nichd early child care research network study researchers studied the impact of parenting on children's cognitive abilities, while taking into account the family income, mother's intelligence, and other variables. in this study, the researchers followed the same group of 700 children from age 6 months to first grade. this study relied upon a research design.

The controlled use of fire by hominids: (a) limited the expansion of hominids into certain environments, such as dry grasslands. (b) decreased the amount of food energy available in the hominid diet. (c) contributed to geographical expansion and food production techniques in positive ways. (d) has been shown to contribute little to the digestive process of hominids.