Baby Feet

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 will focus on how the differences between primate and hominin feet, how feet affect carrying, and when bipedalism emerged.

“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)

Humans are altricial, our infants are relatively helpless. Nonhuman primates are precocious, meaning that they have accelerated physical development. Both human and ape newborns instinctively grasp with hands and feet. When a chimpanzee is born, they are able to grasp their mother’s fur with their hands and feet and cling on effectively. Whereas a human newborn is not as strong or directed in their efforts though they can strongly grasp with their hands (palmar grasp reflex), they can only flex their toes (plantar reflex) due to the morphology, or shape, of their feet which lack an opposable hallux (a big toe that looks more like a thumb). In humans, this grasping instinct is considered part of the primitive reflexes, which fade as humans develop.

One of the first morphological changes that came with bipedalism was in the foot. The hallux, or big toe, moved up the foot to be in line with the rest of the toes. An arch was formed to create a spring, helping bipedal species’ walk and run more efficiently, but not to climb or cling. In the following video, “One Foot in the Past”, Dr. Bruce Latimer and Dr. Beverly Saylor discuss the changes in foot morphology from grasping to walking upright (the relevant material begins at 2:45 but I recommend the whole video).

laetolifoot1 by john readerlaetolifoot2 by john readerAs far back as four million years ago, and likely prior to Lucy’s and Selam’s species, Australopithecus afarensis, our evolutionary ancestors had to adapt their carrying strategies to cope with large infants who lacked grasping feet. We know that Australopithecus afarensis was bipedal based on the fossil record and the Laetoli Tracks in Tanzania. They show a foot with a non-opposable hallux and an arch (Reader).

 “The cost of carrying an infant in one’s arms would have been meaningful enough to reward the development of carrying tools rapidly following the advent of bipedalism,” (Wall-Scheffler, et al, 845)

Lucy walked upright but still had arms and hands adapted for climbing, her offspring would not have been able to cling to her body hair with their feet, especially as her body hair may have been too thin to support the weight of an infant (DeSilva). In her infancy, Selam would have been proportionally large and immobile until aged six or seven months (DeSilva), she would require support from her mother’s arms or postural changes to be carried safely. Lucy possessed wider hips, shorter legs and longer arms (similar to a chimpanzee) than modern humans. Due to these differences Australopithecus afarensis may have been able to carry their offspring in-arms without a caloric deficit (Harcourt-Smith; Wang) but it would have left their mother’s only one grasping limb for climbing trees to get to safety or food sources– and made for a much more dangerous fall if the infant was dropped (Wang; DeSilva).

“Chimpanzees are skilled and frequent tree climbers. The females have little difficulty ascending and vertical substrate even while carrying an infant because of the relatively small size of the infant, and because of the grasping halluces keeping both the infant attached to the mother and the mother to the tree. The postcranial anatomy of A. afarensis and A. africanus is largely inconsistent with frequent and skilled tree climbing, although these hominids may have occasionally taken refuge in trees and there may be more locomotor diversity in the genus. The results of this study suggest that females of these two Australopithecus species were transporting proportionately large infants, a situation that would have rendered arboreality a more dangerous activity. This is further exacerbated by the absences of a grasping toe in A. afarensis, the elimination of dorsal riding as an option for infant hominids, and the possibility that body hair was thinning by 3.3. Myr ago. With a limited capacity to grasp, Australopithecus infants may have been parked or actively carried by their bipedal mothers, at times leaving these females with only a single arm free for climbing” (DeSilva).

Considering that natural selection weeds out inefficiencies (i.e. heavy, poorly clinging infants) a tool to help support a poorly clinging infant must have been invented soon after the development of bipedalism, in order for the traits to be conserved into our own species. Infant carrying technology not only kept early bipedal infants alive, it helped supersede natural selection: larger, helpless bipedal infants survived to pass their genes on– survival of the weakest.

“Technology can and does supersede biology and lead us into a new form of life, one not primarily governed by Darwinian process.” (Taylor)

Without the invention of the infant carrier to support large, poorly grasping infants, bipedalism would have been another evolutionary dead end. Fortunately for our ancestor and ourselves, the ingenuity of an ancient mother provided a technological adaptation that is still used around the world millions of years later by billions of her descendants.

Sources
Alemseged, Zeresenay, et. al. “A juvenile early hominin skeleton from Dikika, Ethiopia.” Nature 443, 296-301 (21 September 2006).

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.

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.

Taylor, Timothy. The artificial ape: how technology changed the course of human evolution. Basingstoke: Palgrave Macmillan, 2010.

Wall-Scheffler, C.m., K. Geiger, and K.l. Steudel-Numbers. “Infant Carrying: The Role of Increased Locomotory Costs in Early Tool Development.” American Journal of Physical Anthropology 133.2 (2007): 841-46. Web.

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.

Media
Nature video. “One Foot in the Past.” Youtube video, 6:48. Posted March 28, 2012. https://www.youtube.com/watch?v=mZ6v5ilOqec

Reader, John. Laetoli Footprints. Digital image. Modern Human Origins. N.p., n.d. Web. 25 Jan. 2015.

Walker, Susan. “Plantar Reflex.” Youtube, 0:33. Feb 23, 2016

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