Eric Michael Johnson has a master’s degree in evolutionary anthropology focusing on great ape behavioral ecology. He is currently a doctoral student in the history of science at University of British Columbia looking at the interplay between evolutionary biology and politics. He blogs at The Primate Diaries at Scientific American, where this post originally appeared.
“Attachment (with respect to Martin Schoeller),” by Nathaniel Gold
My son will be 3 years old next month and is still breastfeeding. In other words, he is a typical primate. However, when I tell most people about this the reactions I receive run the gamut from mild confusion to serious discomfort. Their concerns are usually that extended breastfeeding could be stunting his independence and emotional development–the “Linus Blanket Syndrome” in the words of Michael Zollicoffer, a pediatrician at the Herman & Walter Samuelson Children’s Hospital at Sinai Hospital in Baltimore. Worse yet, they hint that it might even cause“destructive” psychosexual problems that he will be burdened with throughout his adult life. Could they be right? Was our choice “a prescription for psychological disaster” as Fox News psychiatrist Keith Ablow wrote in response to TIME magazine’s provocative cover article on attachment parenting? Just when is the natural age to stop breastfeeding?
One thing I’ve learned in my research on human evolution is that people are quick to assume that what they do is “natural” simply because they don’t know of other examples where things are done differently. The primate brain is a pattern recognition machine and is adapted to quickly identify regularities in our environment. But when we are presented with the same pattern over and over again it is easy to fall victim to what is known as confirmation bias, or coming to false conclusions because the evidence we use does not come from a broad enough sample. In order to avoid falling for this bias on the question of extended breastfeeding the best way forward would be to draw from the largest sample possible: the entire primate lineage.
In their classic paper, “Life History Variation in Primates” published in the premier scientific journal Evolution, the British zoologists Paul H. Harvey at Oxford and Tim Clutton-Brock at Cambridge published the most comprehensive data then available on the world’s primates. The variables they measured included everything from litter size and age at weaning to adult female body weight and length of the estrous cycle among 135 primate species (including humans). By analyzing the relationships between these variables, using a statistical approach known as a regression analysis, they identified striking patterns that held across primate taxa.
One especially strong correlation was that adult female body weight was closely tied to their offspring’s weaning age, so much so that knowing the first would allow you to predict the second with a 91% success rate. As a result, as anthropologist Katherine A. Dettwyler has shown in her book Breastfeeding: Biocultural Perspectives (co-edited with Patricia Stuart-Macadam), it can be calculated that a young primate’s weaning age in days is equal to 2.71 times their mother’s body weight in grams to the 0.56 power. This calculation predicts, given the range of female body sizes around the world from the !Kung-San of South Africa to the Arctic Inuit, that humans should have an average weaning age of between 2.8 and 3.7 years old.
Mark Changizi is an evolutionary neurobiologist and director of human cognition at 2AI Labs. He is the author of The Brain from 25000 Feet, The Vision Revolution, and his newest book, Harnessed: How Language and Music Mimicked Nature and Transformed Ape to Man.”
I’m king of the world! You are too. We humans—all of us—get props for being the smartest Earthlings. And we’re not merely the smartest. No, we’re the only species worth writing home about; we’re the only truly worth building artificial intelligence to mimic. We’re the smart ones. The rest of the diversity of life may be rich in clever design, like well-engineered tools and gadgets, but they’re not designed to be intelligent. That’s for humans. Rationality and intelligence is something natural selection granted us.
But…what if our Homo sapiens intelligence is radically overrated? What if we’re smarter, but only quantitatively so, not qualitatively? What if many of our Earthly cousins are respectably intelligent after all? More intriguingly, what if there are systematic barriers that lead us to overestimate our true level of intelligence relative to that of others? And, although I won’t get into this here, what are the implications for the rights of chimpanzees, if the chasm between us and them is, instead, a slender fault line? That question has led to a recent movement to ban invasive research on chimpanzees in the U.S., a measure that the EU has already adopted.
Here I’ll discuss just two barriers, a little one and a big one, that conceal how smart we really are—or are not.
Individuality: The Little Bubble