Hairy humans hunting, preparing fire and meals in the cave, making clothes, tools and even art with their own hands: a very prominent image in our collective imagination. The seemingly unimportant feature of carrying objects in one's arms should not be taken for granted: we could have easily stayed on four, never throw a spear, turn pages of a book nor start colonizing other planets.
The why and how it happened that we have two out of four limbs free for other functions than climbing, walking and running has taken many scientists a lot of time and creative thinking.
As a result, there are several theories explaining how some quadrupedal primates became bipedal. All of them agree that this change had to be to the survival advantage of our kin and a breaking point in human evolution. It happened around four million years ago and coincided with other changes in our bodies - the bodies we have and keep reproducing today.
Some theories of the transition from four to two maintains that walking in water made our ancestors stand up, another say carrying meat was the reason, to pick a few. Nancy Tanner and Adrienne Zihlman in 1976 journal Signs maintain that it was carrying infants that drove the change. Being able to carry food, babies and whatever needed to be transported over long distances paid off and helped our predecessors succeed, survive and reproduce. Carrying might have contributed to the evolution of Hominidae and aid the phenomena key for advancing our kind such as dexterity, handedness, language acquisition, and social interactions.
For millions of years before, hominid offspring were carried on the backs of their caregivers in a horizontal position supported by gravity, as we can see in chimpanzees today, for instance. Gradually, human infants evolved a capacity to cling onto an upright caregiver whose body co-evolved to enable offspring carrying (Rose, 2005). As many scientists declare, infants are born with the expectation to be carried and for their primary caregiver to fulfill this role, calling human infants riders (Ross, 2001) and active clinging young (Kirkilionis, 1992). For millions of years, babies are born with motor skills and reflexes that make them capable of actively contributing to staying in physical contact with their caregiver right from birth.
As Tanner suggests, walking upright and freeing hands for carrying and gathering also brought a good opportunity to use carry containers for food first baby carriers, first in arms, later around the body, first ready-made by nature such as piece of bark or large leaves, later maybe inspired by nest weaving (as chimpanzees do), using made of natural materials.
A change in foot anatomy became one more reason to carry a baby in a carrier: our feet adapted to walking and lost the grasping ability they once had. To an upright and moving caregiver using her/his arms for gathering, the baby couldn’t cling to as tight any more. As the organic material of early slings and carriers do not preserve, we have no fossil remains to rely on and have to speculate on the exact nature of the first tools keeping human babies alive, safe and happy.
Keep on carrying ❤️