68 Scientific American, October 2020
HARALD SUNDGetty Imagesanthropologist Emma Cohen of the University of Oxford.
“If something is emerging really early in life, then it’s
likely fairly automatic and really salient for us as
humans,” she says. It may even have played an impor-
tant role in our evolution.EVOLVING IN UNISON
psychologIst robIn dunbar of Oxford believes that by
facilitating prosocial behaviors and cooperation, syn-
chrony could have encouraged bonding in groups of ear-
ly humans as their populations grew. He has been
researching synchrony for years, a fascination that start-
ed at a conference on the archaeology of music. One of
the evening sessions was unusual. A musician from
South Africa invited Dunbar and other attendants to
participate in something resembling a traditional Zulu
dance. He told them to stand in a circle, handed them
plastic pipes cut to different lengths and instructed them
to blow across the top of the pipes, making random
noise, and to start walking around the circle. At first,
Dunbar says, the noise was horrible, but after a few min-
utes the sounds and movements changed without par-
ticular effort, the scientists became synchronized, play-
ing music in a consistent tune with one another. “Every-
body felt this sense of belonging, being part of the group.
I realized this was an amazing effect,” he says.
Dunbar now theorizes that in human evolution syn-
chrony might have enhanced grooming as an impor-
tant bonding mechanism. Nonhuman primates groom
one another to remove fleas and other parasites, and
the time spent doing this promotes group cohesion. The
activity does take a lot of time and effort as the number
of groomed individuals increases, and Dunbar argues
that sets an upper limit on the size of a close-knit group.When he plotted the time that various primate species
spend grooming one another against their typical group
size, it appeared that the two were directly correlated.
The upper limit corresponded to a group size of 50 pri-
mates. No monkey or ape species forms groups that are,
on average, larger, but humans do. Dunbar calculated
that a natural community for us stands at about 150
people. He worked out this number based on the size
of the human neocortex compared with those of other
primates, as well as the populations of villages in small-
scale societies and the number of friends and family
that people in larger-scale societies tend to have. That
figure is borne out in early historical records, too: it was
the average village size in England in a.d. 1086 when
William the Conqueror surveyed his new kingdom. (Not
everyone agrees 150 is a solid number; some scientists
have argued it is based on overly selective data.)
One reason early humans may have been able to sus-
tain a group size that is triple that of the average ape,
Dunbar suggests, is they came up with a way to “groom”
several people at once—using voices or body move-
ments instead of picking parasites with fingers. The
larger size offered groups protection against raids by
other humans, increasing their ability to survive and
reproduce, which in turn allowed for the natural selec-
tion of a propensity for synchrony in future generations,
Dunbar contends.
The adoption of such behavior often has biological
underpinnings. In nonhuman primates grooming trig-
gers the release of neurochemicals called endorphins,
which seems to enhance good feelings, Dunbar says. And
research suggests that endorphins, which the body pro-
duces to reinforce pleasure and relieve pain, might be
among the mechanisms that allowed singing and danc-ROWERS
who train
in synchrony
have higher
levels of endor-
phins, brain
chemicals that
blunt pain.
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