The absence of human placentophagy, the maternal consumption of the afterbirth, is puzzling given its ubiquity and probable adaptive value in other mammals. We propose that human fire use may have led to placentophagy avoidance in our species. In our environment of evolutionary adaptedness, gravid women would likely have been regularly exposed to smoke and ash, which is known to contain harmful substances. Because the placenta filters some toxicants which then accumulate there across pregnancy, maternal placentophagy may have had deleterious consequences for the overall fitness of mother, offspring, or both, leading to its elimination from our species’ behavioral repertoire.
The Conspicuous Absence of Placenta Consumption in Human Postpartum Females: The Fire Hypothesis - Ecology of Food and Nutrition - Volume 51, Issue 3. (Paid article but the abstract, reproduced above in its entirety, is free.)