My anthropologist colleague, Judith Scheele, tells me that in sudan it is still common to use wet nurses and that, furthermore, there is a custom forbidding wet-brothers and sisters from marrying. I find this intriguing. It might just be because wet-siblings tend to grow up alongside each other, making this an extension of the incest taboo. But could it also be a cultural symptom of the fact that people tend to feel sexually attracted to individuals whose immune system is defended against very different parasites to their own**. Wet siblings would gain the same antibodies from their nurse, making their immune footprint less different from people who drank different milk.
On a related note, it has been proposed that babies acquire key digestive bacteria during a vaginal birth***.
Without getting too gross (what me?), via "direct contact of the mouth of the newborn with the vaginal and intestinal microbiota".
No, I'm not going to suggest that we wipe neonates on the passages of multiple mothers in order to enhance their intestinal biodiversity (but I might try pitching the idea to Yakult). Instead, I wondered whether there might be benefits to the practice i've heard of where mothers chew food before passing it to their baby during weaning. 'Premastication', or 'baby bird feeding' put on my mental map by Alicia Silverstone****, is allegedly an ancient American Indian practice, thought to be a safe way of pre-digesting baby's first foods. To my knowledge, nobody has yet suggested that it might also be a way for the mother to help populate the baby's gut with the right bacteria, since many intestinal bacteria are also found in saliva. This could perhaps be of especial use to babies born by caesarian section, who missed out on all that passage-wiping, poor things : )
Breastfeeding Provides Passive and Likely Long-Lasting Active Immunity (1998) Lars A Hanson, Annals of Allergy, Asthma and Immunology, Vol. 81, Iss 6, p. 523.** http://www.jstor.org/stable/10.1086/303166 -The Evolution of Mating Preferences and Major Histocompatibility Complex Genes (1998) Dustin J. Penn and Wayne K. Potts, The American Naturalist Vol. 153, No. 2, pp. 145-164.
***http://www.ncbi.nlm.nih.gov/pubmed/18716189 - Cesarean delivery may affect the early biodiversity of intestinal bacteria. (2008) G. Biasucci, B. Benenati, L Morelli, E Bessi and G Boehm, Journal of Nutrition 138(9), pp. 1796-1800.