When a chimpanzee is born, they are able to grasp their mother’s fur with their hands and feet and cling on effectively within weeks. A human newborn is not as strong or directed in their efforts though they still retain some of the so-called primitive reflexes from our common primate ancestor which fade as a human baby gets older. Newborn humans can strongly grasp with their hands (palmar grasp reflex) and they can flex their toes (plantar reflex) but due to their foot morphology, which lacks an opposable hallux (a big toe that looks more like a thumb) they cannot grasp with their feet as they do with their hands.
This is a problem for a species of “riders“, mammals whose breastmilk composition requires them to carry their baby with them for frequent feedings, unlike “parkers” whose offspring can be left for long stretches while their mother hunts or forages (or goes to the bathroom by herself for crying out loud!) If a baby cannot cling to it’s unlikely to survive, most rider mothers cannot afford the extra energy (literally in the form of calories) to carry their infant and it would certainly slow her down in the face of danger. Even if the mother did everything in her power to carry and protect her baby, the odds are against their survival and so that babies trait, the trait for not being able to cling on, wouldn’t get passed on. Unless of course, the mother was clever enough to make a technological adaptation for easier carrying… (you see where I am going here).
Based on the fossil record, including the Laetoli Tracks in Tanzania, bipedalism was established at least by the time of Australopithecus afarensis, one of our hominin ancestors that lived 4.2-2.2 million years ago. They show a non-opposable hallux, or big toe and a high arch: something only seen in bipedal species. For more about early bipedalism, I recommend the following video, “One Foot in the Past”, (2:45 is the start of the most relevant material, though the whole video is very interesting).
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 (Almaral, 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. Unlike modern humans, Lucy had wider hips, shorter legs, and longer arms (similar to a modern chimpanzee), therefore she may have been able to carry their offspring in-arms without a caloric deficit (Harcourt-Smith; Wang). But it would have left only one grasping limb for climbing trees to get to safety or food source, making a fall or death from predation much more like for infant and/or adult (Wang; DeSilva). Even in the case of cooperative breeding and alloparenting, in which other members of the group help care for the baby, a poorly clinging baby is a problem for anyone that needs to climb a tree or flee on foot.
We know that evolution doesn’t happen quickly, it takes generations (and generations) of tiny changes to add up to a new species. This is true for the emergence of bipedalism. A random bipedal infant wasn’t randomly born to parents with grasping feet. Foot morphology would have been gradually getting selected for over many generations. The only way for that to happen considering the cost (less effective clinging) would have been adaptations made by its mother to help it cling or to carry it without overburdening herself or putting both of them in danger of predation or falls. That adaptation may have started with postural changes, the kind we see today in primate mothers trying to carry a disabled, injured, or dead infant. But at some point, a tool became necessary: the infant carrier. It allowed for the poorly clinging predecessors of Selam’s and Lucy’s species to thrive and for other traits like higher proportional birth weight and lack of body hair to get passed on and on for millions of years.
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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.
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.
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