Human Hair Distribution, Immuno-resistance and Juvenile Adaptations to the First Baby Slings

This controversial paper, which was disseminated to advance readers in the UK, United States, South Africa and Australia in August 2008, contains two related, testable hypotheses that, taken together, describe how the earliest baby-carrying devices - or “slings” - could have driven human evolution by contributing to or even triggering the speciation and brain expansion of either Homo habilis or Homo erectus - and, most likely, both. The first hypothesis is that the first baby slings formed microenvironments that subjected infants, who have weaker immunological systems than adults, to sustained contact with a distinct set of pathogens and parasites, probably associated with ungulate hides. The paper postulates that these would have unleashed selection pressures that affected immuno-resistance and the reduction of juvenile body hair by the neotenic prolongation of fetal body baldness – which would have permitted easier cleansing of babies.

Its second hypothesis is that the practice of carrying infants in slings ended the need for newborns to have brains that were mature enough soon after birth to provide them with strong clinging reflexes and quick mobility – and that infants gradually became more altricial once the constraint was lifted. Existing pressures for achieving larger brains through postnatal brain maturation and growth, which had been blocked by the combination of a need for a certain maturity at birth and an upward limit on the girth of birth canals - given the engineering constraints of bipedalism - would have been unbridled. Any new pressures for encephalization would have increased selection for postnatal brain development still further, leading once again to less mature brains among newborns.

The hypothesis postulates that having such babies, whose brains reached previously neonate levels of maturity later, gave parents a greater appetite for the nutrients required by altricial brain development. As is shown in studies of passerine birds, for example, this would have increased their appetite for such animal products as meat, encouraging adults to scavenge and hunt even more avidly, which would have selected, in turn, for new thermoregulatory solutions to the heat stress induced by the kinds of strenuous activities involved in meat acquisition. The evolution of a full-body cooling system based on sweating would have built on body baldness, which had first appeared in juveniles as a neotenic adaptation to infectious slings, extending it for entirely different reasons to adults.

Most importantly, the paper suggests ways of testing these hypotheses while arguing that convergent fossil and genetic clues already indicate that babies in our lineage had begun to adapt to such infested microenvironments by 1.2 million years ago, and perhaps as far back as the late Pliocene. For example, the coalescence dates of candidate genes that confer increased immuno-resistance could provide an especially robust test of the body baldness-sling hypothesis, even if the coalescence is more recent than the CMAH deactivation, explored in the paper, which may have reduced infant susceptibility to diarrhea caused by ungulate hides.

Even though the paper has been well received by some prehistorians and paleoanthropologists*, the most extreme variant of its hypotheses, which basically states that the speciation at the origin of our genus could have resulted from juvenile adaptations against dysentery and other debilitating conditions caused by the earliest sacks, is bound to remain controversial for a long time to come.

© 2008 Duncan Caldwell on text

 

 

1. *Comments by advance readers include:
   

•    Robert Bednarik, the editor of Rock Art Research: “I think that your paper is very well argued, but will be hard to ‘sell’, because of its sophistication” (email, 10 Dec. 2008).

•    Francesco d'Errico, Director of Research CNRS, University of Bordeaux, France & Honorary Professor, Institute for Human Evolution, University of the Witwatersrand, South Africa: “... I found the idea and integrated interdisciplinary way you deal with the question fascinating. It is certainly worth publishing” (email, 12 Jan. 2010).

© 2009 Duncan Caldwell on photos