Humans, like all primates, are physiologically riders. Riders carry their young because their breast milk composition is not suited to parking infants for long periods (Ross). Nonhuman primates can carry their babies without the aid of a tool because their infants are small and able to cling to their mothers from birth. Humans find carrying infants more difficult for three reasons: the relative size of our infants, non-grasping feet, and lack of adult body hair. In this post, we’re going to focus on why human babies are so large and helpless compared to other primates and when in our evolution the trend for large infants began.
Our evolutionary cousins, non-human primates in the Family Hominidae, have smaller, more precocious babies than humans. Adult gorillas, for example, are significantly larger than adult humans, yet their newborns are about half the size of our newborns. Chimpanzees, which are our closest extant evolutionary relative and have a similar adult body weight to adult humans, have newborns that are around 3% of adult size, while humans have newborns that are around 6% of adult size (DeSilva).
Humans are an altricial species, meaning that our babies are totally helpless at birth, like kittens, they cannot cling to their mothers to assist with carrying. Whereas non-human primate babies are more physically developed, or precocious, at birth, they can cling from birth. This clinging is part of the attachment process, in Mother Nurture, Sarah Blaffer Hrdy describes the low incidence of primates’ maternal failure to attach to their offspring, when compared to other mammals:
“In fairness, however, it must be noted that primate neonates themselves deserve some credit for this, since they often clinch the deal by grasping hold of the mother’s fur right after birth. The neonate quite literally attaches to his mother.” (Hrdy, 178).
Human neonates have the same clinging reflexes but due to their non-grasping feet, adult lack of body hair, and, of course, their altricity the best they can do is grasp a finger– though they are capable of supporting their own weight momentarily.
In order for a human newborn to be similarly developed to a chimpanzee newborn, a human baby would require 18-21 months of gestation (Dunsworth), but this is not due to human infants being born prematurely compared to other primates. In fact, human gestation is longer than expected for our body mass when compared to similarly sized primates. In the past, it was believed that our species traded well-developed newborns for bipedalism (and the small pelvis that went with it), resulting in developmentally premature newborns. This theory was called the Obstetrical Dilemma, which posited that human females simply do not have a big enough pelvis to properly gestate and vaginally birth our big-headed offspring. This theory does not take into consideration what we know about the gestation length of other primates.
The Maternal Metabolic Hypothesis, or Energetics of Gestation and Growth (EGG), hypothesis uses data from humans and primates to reveal that humans gestate longer and give birth to larger babies than expected for our adult body weight (Dunsworth). The human pelvis is perfectly adapted to birth human babies. We are simply an altricial species so our babies are born extremely dependent due in part to our metabolism.
At what point since our common ancestor with chimpanzees, did hominins start having bigger babies? The evolution of large neonates, when compared with adult body size had begun by the time of A. afarensis, also known as Lucy. “By 3.2 myr and perhaps earliers, females of the genus Australopithecus were giving birth to relatively large infants, approximately 5% to 6% of their own body mass…” (DeSilva). Once larger infants emerged the trend remained through to modern humans. While brain and body size increased with the genus Homo, the IMMR remained stable. DeSilva’s hypothesis is that these large neonates drove our ancestors from the trees as they could not safely carry such large infants safely (DeSilva). Information gained from the remains of earlier A. afarensis remains, Selam, who died aged 2-3 years and whose brain was still growing, may indicate that her species had similarly altricial infants as modern humans do (Alemseged).
For our modern cousins, a number of factors allow them to carry their infants without the use of a tool: small IMMR (infant:mother mass ratio), opposable halluces (grasping feet), adult body hair strength and density to support the infant’s weight, and precocious neonatal physical development. If any one aspect changes, adaptations must be made in order to safely transport their young– such adaptations have been observed in the wild– adaptations made for dead or disabled infants, including postural changes, tripedalism, and carrying in-arm(s), (Macaskill) (Viegas, Biro).
“Birthing larger infants… also introduces the energetic and biomechanical challenge of transporting a relatively large, helpless newborn. This is particularly the case for pretechnological, upright walking hominids, some of which had reduced pedal grasping abilities.” (DeSilva)
Australopithecus Afarensis, with their combination of chimpanzee and bipedal morphology, would have built on similar adaptations for carrying large infants who could not cling. Tripedalism and bipedalism as a carrying strategy would not have allowed our ancestors to climb through trees (Harmon Courage) and life outside of the trees, carrying a large infant, would have favored those with bipedal morphology. Even with bipedal morphology, carrying a large infant in-arms would have been too energetically costly to be practical. Natural selection should have selected for smaller infants that were easier to carry. Yet hominin infants continued to have high IMMR, 5-6%, through to modern humans (DeSilva). Therefore, some kind of carrying technology must have been invented as big babies evolved, allowing big babies who could not cling to be carried safely without creating a caloric deficit in their mothers or caregivers.
Alemseged, Zeresenay, et. al. “A juvenile early hominin skeleton from Dikika, Ethiopia.” Nature 443, 296-301 (21 September 2006).
Amaral, Lia Q. “Mechanical Analysis of Infant Carrying in Hominoids.” Naturwissenschaften 95.4 (2008): 281-92. Web. 15 Jan. 2015.
Blaffer Hrdy, Sarah. Mother Nature: Maternal Instincts and How They Shape the Human Species. New York: Ballantine Books, 1999.
DeSilva, Jeremy M. “A Shift toward Birthing Relatively Large Infants Early in Human Evolution.” Ed. C. Owen Lovejoy. Proceedings of the National Academy of Sciences of the United States of America 108.3 (2011): 1022-027. PNAS. Web. 19 Jan. 2015.
Dunsworth, H. M., A. G. Warrener, T. Deacon, P. T. Ellison, and H. Pontzer. “Metabolic Hypothesis for Human Altriciality.” Proceedings of the National Academy of Sciences 109.38 (2012): 15212-5216. Web. 19 Jan. 2015.
Harcourt-Smith, W. E. H., and L. C. Aiello. “Fossils, Feet and the Evolution of Human Bipedal Locomotion.” Journal of Anatomy 204.5 (2004): 403-16. Web. 24 Jan. 2015.
Harmon Courage, Katherine. “Did Big Babies Help Bring Human Ancestors down from the Trees? | Observations, Scientific American Blog Network.” Observations. Scientific American Global, (3 Jan. 2011). Web. 19 Jan. 2015.
Taylor, Timothy. The artificial ape: how technology changed the course of human evolution. Basingstoke: Palgrave Macmillan, 2010.
Wang, W.-J., and R. H. Crompton. “The Role of Load-carrying in the Evolution of Modern Body Proportions.” Journal of Anatomy 204.5 (2004): 417-30. NCBI. Web. 25 Jan. 2015.
Betapicts. “Palmar – palm of the hand grasp reflex / reaction 2.” Youtube video, 0:25. Posted June 18, 2013.