I found this article quite interesting, especially in light of the debates over with who and for how long a child should spend his or her time (i.e. the sah/wah debate). One researcher's take on human development, of course, but thought-provoking nonetheless.
"In a Helpless Baby, the Roots of Our Social Glue"
From the article:
As Dr. Hrdy argues in her latest book, “Mothers and Others: The Evolutionary Origins of Mutual Understanding,” which will be published by Harvard University  Press in April, human babies are so outrageously dependent on their elders for such a long time that humanity would never have made it without a break from the great ape model of child-rearing. Chimpanzee and gorilla mothers are capable of rearing their offspring pretty much through their own powers, but human mothers are not.
Human beings evolved as cooperative breeders, says Dr. Hrdy, a reproductive strategy in which mothers are assisted by as-if mothers, or “allomothers,” individuals of either sex who help care for and feed the young. Most biologists would concur that humans have evolved the need for shared child care, but Dr. Hrdy takes it a step further, arguing that our status as cooperative breeders, rather than our exceptionally complex brains, helps explain many aspects of our temperament.
By contrast , human mothers in virtually every culture studied allow others to hold their babies from birth onward, to a greater or lesser extent depending on tradition. Among the !Kung foragers of the Kalahari, babies are held by a father, grandmother, older sibling or some other allomother maybe 25 percent of the time. Among the Efe foragers of Central Africa, babies spend 60 percent of their daylight hours being toted around by somebody other than their mother. In 87 percent of foraging societies, mothers sometimes suckle each other’s children, another remarkable display of social trust.