Me, nursing my two year old, with my friend Dana in the back, nursing her four year old. We're waiting with our homeschool group for a tour of the King Tut exhibit in Seattle, Washington.
"Infancy, as we noted, is the period of nursing, and it typically lasts about four years in humans. When we consider how unusual it is for a mother to nurse her child for even a year in the United States or Canada, this figure may surprise us. But considering that four or five years of nursing is the norm for the great apes and for women in foraging societies, most anthropologists conclude that four years was the norm for most humans in the evolutionary past (Eaton et al.,1988; Dettwyler, 1995).
...Human milk, like that of other primates, is extremely low in fats and protein. Such a low-nutrient content is typical for species in which mothers are seldom or never separated from their infants and nurse in short, frequent bouts.
Breast milk also provides important antibodies that contribute to infant survival. Throughout the world, breast-fed infants have far greater survival rates than those who aren’t breast-fed or who are weaned too early. The only exception is in societies where scientifically developed milk substitutes are readily available and appropriately used, and even then, infants don’t get several important antibodies and other immune factors. The importance of adequate nutrients during this period of rapid brain growth can’t be overestimated. Thus, it’s not surprising that there are many cultural practices designed to ensure successful nursing."
- Essentials of Physical Anthropology, Chapter 13: The Anthropological Perspective on the Human Life Course, page 348