Big Babies, Baby Feet, and Body Hair

Our evolutionary cousins, non-human primates, have smaller, more precocious babies than humans. Adult gorillas, for example, are significantly larger than adult humans, yenewborn and adult weightst their newborns are about half the size of our newborns.  Non-human primate babies are more physically developed (precocity) at birth than human babies. Human newborns are both huge and totally helpless (altricial). In order for a human newborn to be similarly developed to a chimpanzee newborn, our babies would require 18-21 months of gestation. Humans, like that of all primates, are riders, we carry our young because our breast milk is not suited to parking infants for long periods. We must carry our infants with us. Yet for humans, infant carrying is more energetically costly than for our cousins: our babies are relatively large, they lack grasping feet, and they are born to comparatively hairless parents. Our evolutionary ancestors faced similar challenges with changes to infant and adult morphology and the result was the invention of a tool to help carry large, altricial infants with less energetic expenditure than carrying in arms.

Note: It is important to understand that non-human primates, in the Family Hominidae, are not our ancestors. They are our cousinfamily hominidae diagrams. We did not evolve from them but rather from a common ancestor. They retain traits of our common ancestor, including body hair, foot morphology, and infant size, which is why I compare humans with their species’ to help better understand human evolution with regards to infant care strategies.

Big Babies

The evolution of large neonates, when compared with adult body size had begun by the time of A. afarensis, also known as Lucy about 4.2 mya. DeSilva’s hypothesis is that these large neonates drove our ancestors from the trees. His study of the post-cranial (back of the skull) anatomy of A. afarensis (Lucy) and A. africanus demonstrates that neither were consistent with tree-dwelling primates, which suggest that they were transporting proportionally large infants (DeSilva, 1025).

Precocity, or accelerated physical development, in non-human primate newborns is not due to human infants being born prematurely compared to other primates. In fact, humans gestation is longer than expected for our body mass when compared to similarly sized primatehominidae weight and gestations. 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 fDunsworth, maternal investment, p. 15213emales 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– our babies are born extremely dependent due in part to our metabolism.  This may mean that our ancestors, A. afarensis may have had similarly altricial infants as modern humans do.

obstetrical dilemma vs maternal metabolicEGG

For our modern cousins, a number of factors have to come together to allow them to carry their infants and have their infants cling to them, without the use of a tool: foot morphology, body hair strength and thickness, physical development. If any one aspect of morphology  or development changes, there would have to be adaptations made in order to 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).

The following videos depict tripedalism, the mother holds the infant in one arm while walking using the other three limbs. In the first video, the mother is holding her dead infant; in the second a disabled infant.

In the following clip, the adult carries the infant (who seems to be trying to escape) in-arms, while running a short distance on two legs.

Our ancestors would have built on similar adaptations for carrying 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 would have favored those with bipedal morphology. Gradual changes in infant morphology are exactly what our ancestors experienced and they would have had to adapt carrying strategies in order for their offspring to survive.

Baby Feet

Human neonates still maintain some of the instinctive reflexes of their evolutionary cousins: they grasp strongly with their hands and flex their toes. However, human newborns are not as strong or directed in their efforts as a gorilla neonate. Whereas humans only have two grasping limbs (our hands) and very little body hair, the gorilla is helped by having four grasping limbs and adults with long, thick body hair.

One of the first morphological changes that came with bipedalism was in the foot. The hallux (or big toe) lost its opposable (grasping) ability and moved up the foot to be in line with the rest of the toes. An arch was formed to created a spring, helping bipedal species walk and run more efficiently, but not to climb or cling. As far back as Lucy (A. afarensis), our ancestors had to adapt infant carrying strategies to cope with infants who did not have grasping feet. Lucy walked upright, for the most part, but still had arms and hands adapted for climbing. She had fairly short legs and very wide hips– similar to that of a quadrupedal primate and less like a human. However, we know that her species was functionally bipedal based on the fossil record.  The Laetoli Tracks (photos by John Reader) in laetolifoot1 by john readerlaetolifoot2 by john readerTanzania show a foot with a non-opposable hallux and an arch. Therefore, Lucy’s offspring would not have been able to cling to her body hair with their feet. Lucy’s newborns would have required support from her arms or postural changes them to be carried safely.

It is unclear if Lucy’s species used tools, that is, there are not verified stone tool remains associated with A. afarensis. Therefore it is unlikely, but not improbable, that Lucy’s species invented infant carrying tools. As mentioned in “Costs of Carrying”, due to Lucy’s hip morphology, the costs of carrying her offspring in-arms would have been less than it is for modern humans. This morphological difference may have helped bridge the gap between the evolution of poorly-clinging, proportionally large, altricial infants and the development of the infant carrier by the later and less hairy, A. africanus or early members of Homo.

Body Hair

Lia Amaral studied the hair qualities of a number of non-human primates to determine its properties for carrying infants. She discovered that, for gorillas, an infant weighing six pounds needs to be able to grasp groups of 100 hairs per limb in order to cling on safely. This requires a certain level of hair density per the width of the infant’s hand or foot. A gorilla with thinning or missing hair will have difficulty carrying their infant, or rather, their infant will have difficulty clinging on.

Around 2.2 mya the Genus Homo emerged and the great denuding occurred. Our ancestors lost their body hair and their pink skin was replaced with dark pigment.  Early Homo infants would have been completely unable to cling to the body of an adult due to foot morphology, altricial development, and lack of caregiver body hair. Transporting infants (or stones) in-arms would have been too energetically costly to allow long distance travel.

Yet, we know that by the time of late Australopithecus, the stone tool industry was underway and stones were being transported many miles for processing. The invention of a rudimentary infant carrier likely preceded the stone tool industry during the time of Austrolopitcheus, ~3 mya, before the emergence of the Genus Homo. Morphological changes to body hair in the time of early Homo would have been easier to deal with in the presence of such technology and may have been an impetus for the invention of sewing prior to the evolution of modern humans.


Amaral, Lia Q. “Mechanical Analysis of Infant Carrying in Hominoids.” Naturwissenschaften 95.4 (2008): 281-92. Web. 15 Jan. 2015.

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.

Macaskill, Scotch. “Baboon Mother Carrying Dead Baby.Wildlife Pictures Online. N.p., n.d. Web. 19 Jan. 2015.

O’Neil, Dennis. “Early Human Evolution:  Homo Ergaster and Erectus.Early Human Evolution:  Homo Ergaster and Erectus. Anthropology Palomar College, 2013. Web. 22 Jan. 2015.

Ruxton, G. D., and D. M. Wilkinson. “Avoidance of Overheating and Selection for Both Hair Loss and Bipedality in Hominins.Proceedings of the National Academy of Sciences 108.52 (2011): 20965-0969. Web. 24 Jan. 2015.

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.

Viegas, Jennifer, and Dora Biro. “Chimpanzee Mothers Carry Their Mummified Dead Infants.Juesatta Blogs. CJ Photography, 29 Apr. 2010. Web. 19 Jan. 2015.


Earth Touch. “Graphic Content Warning: Mother Carries Dead Baby.” Youtube video, 1:56. Posted Dec. 5, 2012.

Nature video. “One Foot in the Past.” Youtube video, 6:48. Posted March 28, 2012.

The Incredible Human Journey. Dir. Charles Colville, Peter Oxley, Philip Smith, and David Stewart. Perf. Dr. Alice Roberts. BBC Worldwide, 2009. DVD.

Wall Street Journal. “Chimpanzees Cared For Disabled Infant in the Wild.” Youtube video, 1:10. Posted Nov 11, 2015.

Macaskill, Scotch. “Baboon Mother Carrying Dead Baby.Wildlife Pictures Online. N.p., n.d. Web. 19 Jan. 2015. (not featured)

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



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