The Costs of Carrying

laetolifoot2 by john reader
The Laetoli Tracks in Tanzania, photographs by John Reader.
laetolifoot1 by john reader
The Laetoli Tracks in Tanzania, photographs by John Reader.

Our early bipedal ancestors were “riders”, meaning that they carried their young with them, that they had highly altricial, or dependent infants which cannot cling on, and which extremely large relative to adult size, compared with the infant size of quadrupedal primates. This creates some issues for carrying, not just logistically but energetically.

Based on the fossil record, including the Laetoli Tracks in Tanzania, bipedalism was established at least by the time of A. afarensis, also known as Lucy, around 4.2 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).

An experiment was done at the University of Wisconsin to determine the energetic costs (literally, the calories required to the do the job) of carrying an infant in-arms vs. carrying in a sling. They discovered that carrying in arms was 16% more costly than carrying in a sling, or to put it another way: carrying in-arms was more costly than lactation. It had to do with kinematic changes, the difference in the way the participants moved, whether walking unencumbered, carrying a baby in-arms or when using a sling. In the following video, my friend Leslie Kung demonstrates the kinematic changes discussed in the research.

lucy and modern human pelvis compare
Screen shot from the ELucy interactive site, the corresponding red lines are the same length. On the left is Lucy and on the right is a modern human. Lucy’s hips and pelvis would have been larger in proportion to her body than a modern human.

Using a sling, freed the arms, improved posture compared carrying in-arms and allowed for more hip movement. The movements of walking using a sling were more similar to the movement of an unencumbered walker. Carrying in-arms forced the walker to take smaller, uneven strides, by reducing the movement in the hips. This is why the researchers hypothesized that wider hips, such as the type Lucy, or A. afarensis, had allowed for larger strides even when carrying an infant in-arms. The differences in hip morphology may have made up for the energetic costs of carrying in-arms. For Lucy, the transition between an infant who could cling independently and development of the baby sling was eased by her hip morphology.

 “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).

A. africanus would not have had the benefit of his ancestor’s wide hips to reduce the energetic costs of carrying offspring– furthermore, A. africanus was a tool user, based on the fossil record. It is here that I hypothesize that the infant carrier was first invented. The invention of the infant sling would have support other tool production, for example transportation of raw material for the Oldowan and Acheulean stone tool industries which existed during the time of late australopithecus and early homo. The fossil record shows that the range of the stone tool industries widely expanded in range, from less than a mile to an average 12-24 miles, with one example of obsidian being transported over 62 miles around 1.5 mya (Wang and Crompton).

All of this points to some form of infant carrying technology having been developed long before humans and possibly before the genus homo– mostly like during the time of A. africanus, around 3 million years ago.

One Foot in the Past. Dir. Charlotte Stoddart. Prod. Nature Video. Perf. Bruce Latimer, Beverly Saylor. One Foot in the Past. Macmillan Publishers LTD, 2012. Web. 25 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.

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