Recently a biological anthropologist who studied breastfeeding and immunity among a community in Kenya came to speak at my university. After her presentation I searched for her research online and found her dissertation. I found her literature review section on breastfeeding and human evolution to be interesting and useful for my own research, so I thought I'd share this short section here.
Breastfeeding and Human Evolution
by Elizabeth Miller
Breastfeeding is one of the defining characteristics of the class Mammalia – so named because all female members develop mammary glands that feed their offspring. The evolutionary origins of the mammary gland is lost in time, but it may have evolved from a sweat gland that was used to keep egg shells moist (Oftedal 2002). The length of time mammals breastfeed their infants depends on each species’ life history. The life history of an organism is the collection of phenotypes that impact the survival and reproduction of an organism (Stearns 1992). Life history characteristics are driven by variation in extrinsic mortality and tend to correlate highly with one another (Charnov 1993). Life history characteristics include adult body and brain size, age at weaning, puberty, and first birth, age specific mortality and fertility, interbirth interval, litter size, and duration of gestation. In general, small-bodied mammals have shorter lifespans and therefore earlier occurrence of life stages. Members of the order Primates have longer lifespans relative to their body size compared to other orders of placental mammals, which extends many life stages, including breastfeeding duration.
Within the order Primates, every segment of the lifespan increases with increasing body size. Large-bodied primates such as the great apes generally tend to wean their offspring later than do smaller primates such as strepsirrhines. Chimpanzees wean around 5 years of age (Watts and Pusey 1993), gorillas between 3 to 4 years (Watts and Pusey 1993), and orangutans between 5-7 years (van Noordwijk et al. 2009). Dettwyler (2004) used several lines of evidence from primate life history characteristics to predict the expected age at complete weaning for humans. She used five primate traits that have been proposed to predict age at weaning: 1) tripling or quadrupling birth weight, 2) reaching one-third of adult body weight, 3) adult female body weight, 4) gestation length, and 5) age at 1st molar eruption. Based on the allometric relationships between these characteristics and age at weaning, she predicted that the “natural” age at weaning for human infants is between 2.5 and 7 years of age. Most values appear to center around 6 years of age (Dettwyler 1995).
However, cross-cultural evidence indicates that most non-industrial societies have an average age of complete weaning of less than 2.5 years (Sellen and Smay 2001). There are few, difficult-to-test hypotheses that explain this shorter breastfeeding period. Kennedy (2005) proposed that when early members of the genus Homo shifted to consumption of energy-dense meat, infants were given this food at younger ages to facilitate brain growth. However, this hypothesis does not consider that a long weaning process, in which infants are breastfed for long periods of time while eating high-calorie food, would provide more calories while offering protection from pathogens that may be introduced through food. It is also possible that human evolution selected for shorter interbirth intervals, requiring earlier weaning times (Wells and Stock 2007). Human infants have a higher percentage of fat than
other mammals, helping buffer infants through malnutrition and poor health associated with early weaning and helping alleviate the heavy energetic demands of brain growth (Kuzawa 1998). In contrast to adaptive hypotheses, Dettwyler posits that shorter breastfeeding duration is a maladaptive artifact of cultural expectations (Dettwyler 1995). The cultural ecologies that contribute to individual and population variation will be discussed further below.
Although the evolution of a shorter human breastfeeding period is unclear, patterns of breastfeeding in living human populations can also be examined from an evolutionary perspective. Life history theory generates predictions that can also be used to examine physiological and reproductive phenotypes at the population and individual level. Variations in life history characteristics are based on the allocation of energy to different bodily functions. Adaptive life history strategies seek to allocate limited energy to growth, maintenance and reproductive efforts in a way that maximizes individual fitness (Stearns 1992). Immune function, along with cellular repair and organ function, is generally considered as part of the maintenance effort of an individual. Immune function, particularly adaptive immune function, is thought to compete with growth and reproductive efforts for available energy. There is evidence in the animal kingdom that this is the case (reviews in Sheldon and Verhulst 1996; Lochmiller and Deerenberg 2000). It is reasonable to assume that humans allocate energy in similar ways. In one of the few examples of trade-offs between growth and immunity, McDade et al. (McDade et al. 2008) found that acute inflammation predicted smaller gains in height after three months. In addition, I found that pregnant women have different immune profiles than non-pregnant women, down-regulating more energetically costly adaptive immunity while up-regulating less costly innate immunity (Miller 2009). Both results hint at energetic or immune life history trade-offs in humans.
Because breastfeeding is so energetically costly, it is an ideal system to investigate life history trade-offs in humans. Parent-offspring conflict, a hypothesis driven by competing reproductive, growth, and maintenance strategies, has been used to explain patterns of behavior in breastfeeding mammals. Trivers (1974) made several general predictions about the evolutionary patterning of transfer of resources between parents and children. Parent offspring conflict theory predicts that mothers will decrease their investment in offspring relative to trade-offs they make between current offspring and future reproduction. This resource patterning is time-sensitive relative to the reproductive interests of the mother and the age-specific mortality probabilities of the offspring (Hill and Kaplan 1999). Often the maternal decrease in investment occurs before it is in the infant’s best reproductive interests, leading to conflict between the two. This conflict can be evident in behavioral observations (Fouts et al. 2005) or allocation of resources during fetal growth (Haig 1993).