Our evolutionary cousins, non-human primates, 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 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.
We understand that the infant sling would have been among the first tools our ancestors invented after bipedalism. Early infant carriers would have required fastening technology of some sort: knotting, weaving, or other means. Early infant slings, the kind A. africanus may have used, would most likely have been simple slings of animal tissue or plant material (Russell, 47). They may have resembled the ayĩ strap of the Amazon Kayapo or the forehead strap of the Jawara people.
These straps simply provided a place to balance the weight of the baby, requiring most support from the body of the adult. There is little protection from the elements for the baby with this kind of carrier: they would have full sun, rain, wind, and possibly branches or leaves of plants brushing against them.
When H. erectus left Africa (or, for some populations, evolved out of Africa), they expanded across Asia, into Indonesia. They may have even populated portions of Europe (fossil evidence is disputed as to species on this point). H. erectus used complex tools, used fire and cooked food, and living in small hunter-gatherer groups. They even created abstract art. What is significant is that they remained in warmer regions, environments that suited tropically adapted bodies.
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).
Caroline Ross describes species that leave their young in a nest or burrow while they forage or hunt as “parkers”, while those species that carry their young (or have their young cling to them) as “riders”. Her research suggests that “riding” is a trait that evolved prior to the emergence of primates, meaning that primates, including humans, are preadapted* be carried as infants.
This is significant, because carrying young seems to be illogical in an evolutionary sense: the infants are exposed to the environment, and any danger the caregiver may face while searching for food. Carrying also requires more energetic costs for the adult, which limits reproduction. Species that park infants tend to have liters, larger groups of offspring, while riders tend to have singleton pregnancies. Reproductive success depends on having as many offspring reach sexual maturity as possible. Therefore carrying seems counter adaptive, yet it persists and based on the prevalence of humans, it is a successful strategy.
In altricial mammals, those born helpless, a phenomena called transport response has been observed in cats, dogs, mice. Transport response “induces a passive and compact posture with hind legs drawn up in carried infants” (Esposito, et al., 744). You may have observed this for yourself– when a mother cat or dog picks up their young by grasping the skin on the infant’s neck with their mouths, the infant becomes quiet, appears to calm, and curls up into a fetal shape.
In a study from Japan, a group of researchers compared infant calming in humans and mice, to see if human infants demonstrate transport response. After all, numerous studies on infant crying demonstrated that infants who are carried more, cry less. In this study they monitored the infants’ heart-rate and breathing, in each situation, crib, holding without moving and carrying, half of the infants began crying, while the others were not.