Humans, like the other Great Apes, are physiologically riders; breast milk composition is not suited to parking infants for long periods between feeding (Ross). For this reason among many others, our species has had to carry our babies with us wherever we go. Unfortunately, humans find carrying infants more difficult than our evolutionary cousins for three reasons: lack non-grasping feet and body hair, physically helpless infants and, most importantly for this post, the relative size of our infants.
Non-human apes in the taxonomic family Hominidae (literally means “Great Ape” and makes me think of Charlotte’s Web’s “Some Pig” but I digress) 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 human newborns. Chimpanzees, which are our closest extant evolutionary relative and have a similar adult body weight to us, give birth to newborns that are around 3% of their adult size, while humans have newborns that are around 6% of adult size (DeSilva). 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, often referred to as “Lucy” for the famous specimen discovered by Dart.
“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, 2011).
Once larger infants emerged the trait remained through to modern humans. Information gained from the remains of an 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). While brain and body size increased with the genus Homo, the IMMR remained stable at 5-6% of adult size. DeSilva’s hypothesis is that these large neonates drove our ancestors from the trees as they could not safely carry such large infants at altitude.
Rock-a-bye baby in the tree top…
“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)
For our modern cousins, a number of factors allow them to carry their infants without the use of a tool: grasping feet, adult body hair, precocious neonatal physical development, and relatively small size. If any one aspect changes, adaptations must be made in order to safely transport their young– and 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). We know that these animals use tools in the wild and adapt human-made objects into tools to solve problems in captivity.
Australopithecus Afarensis, with their combination of chimpanzee and bipedal morphology, would have built on similar postural adaptations for carrying large infants who could not cling. However, tripedalism and bipedalism as a carrying strategy would not have allowed our ancestors to climb through trees while carrying a baby, yet many researchers argue that our bipedal ancestors would still have needed to climb trees for resources and to escape predation.
Natural selection should have selected for smaller infants that were easier to carry, yet not only did the traits that made it more dangerous for mother and infant emerge but they have survived through to our own species. This is a prime example of survival of the cleverest, not the fittest. Some kind of carrying technology must have been invented during the transition to large bipedal infants of the earliest bipedal hominins to allow big babies who could not cling to be carried safely both on the ground and in the trees.
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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.