Quitting the hominid fight club: The evidence is flimsy for innate chimpanzee--let alone human--warfare
By John Horgan
Extraordinary claims, Carl Sagan
liked to say, require extraordinary evidence. Here is an extraordinary
claim: "Chimpanzeelike violence preceded and paved the way for human
war, making modern humans the dazed survivors of a continuous,
five-million-year habit of lethal aggression."
The anthropologist Richard Wrangham of Harvard University made this
statement in his 1996 book Demonic Males: Apes and the Origins of
Human Violence (Houghton Mifflin, co-written with journalist Dale
Peterson) and has reiterated it ever since. He asserts that both male
humans and chimpanzees, our closest genetic relatives, are "natural
warriors" with an innate predisposition toward "coalitionary killing,"
which dates back to our common ancestor.
The theory has been touted by such influential intellectuals as the
philosopher Francis
Fukuyama of Johns Hopkins University and the psychologist Steven
Pinker of Harvard. "Chimpicide," Pinker wrote in his 2002 bestseller The
Blank Slate the Modern Denial of Human Nature, "raises the
possibility that the forces of evolution, not just the idiosyncrasies of
a particular culture, prepared us for violence."
A new
study seems to corroborate Wrangham's demonic-males thesis. A group
led by John Mitani of the University of Michigan at Ann Arbor observed a
troop of chimpanzees in Uganda's Kibale National Park killing chimps
from neighboring troops during a 10-year period of "territorial
expansion". This "warfare," Nicholas
Wade reported in The New York Times, suggests that "both
humans and chimps inherited an instinct for aggressive territoriality
from their joint ancestor."
I've been reading and talking to anthropologists about the demonic-males
theory for years, and I've turned from a believer to a skeptic. Here
are some reasons why:
Wrangham and other chimpanzee researchers often present the rate of
"intercommunity killing" in terms of annual deaths per 100,000
population. Mitani, for example, estimates the mortality rate from
coalitionary attacks in Kibale to be as high as "2,790 per 100,000
individuals per year." But the researchers witnessed only 18
coalitionary killings. All told, since Jane Goodall began observing
chimpanzees in Tanzania's Gombe National Park in 1960, researchers have
directly observed 31 intergroup killings, of which 17 were infants.
I obtained these figures by adding numbers from a 2006
paper by Wrangham and two colleagues and from the new report by
Mitani's group. Researchers have "suspected" or "inferred" a few dozen
more lethal attacks, in which a chimpanzee is found dead or simply
disappears. All these violence statistics, according
to an analysis published this year by the anthropologists Robert
Sussman and Joshua Marshack of Washington University in Saint Louis
(W.U.), are based on 215 total years of observations at nine different
sites in Africa. In other words, researchers at a typical site directly
observe one killing every seven years.
Wrangham conceded in
a response to Sussman and Marshack, published in the same volume as
their analysis, that chimpanzee coalitionary killings are "certainly
rare." He also acknowledged that "there are various sites where
scientists have studied chimpanzees without any record of coalitionary
killing or other kinds of violence." He suggests that these nonviolent
chimpanzees are not "habituated" to their human observers or are
isolated from other communities. But that raises another question: Could
unusual environmental conditions be triggering intergroup chimpanzee
killing?
The first lethal gang attack was witnessed in 1974 at Gombe, after
Goodall and her co-workers had spent 14 years closely observing
chimpanzees. Goodall, who began supplying bananas to chimpanzees in
1965, once expressed concern that the feeding "was having a marked
effect on the behavior of the chimps. They were beginning to move about
in large groups more often than they had ever done in the old days.
Worst of all, the adult males were becoming increasingly aggressive.
When we first offered the chimps bananas the males seldom fought over
their food; ÖnowÖthere was a great deal more fighting than ever before."
(This quote appears in Sussman and Marshack's paper.)
Chimpanzees throughout Africa are also increasingly threatened by
poachers, farmers and other humans. Ian Tattersall, an anthropologist at
the American Museum of Natural History, told me that chimpanzee
violence is "plausibly related to population stress occasioned by human
encroachment." In other words, outbreaks of lethal violence among
chimpanzees may stem primarily from environmental and even cultural
factors. Wrangham himself has emphasized that chimpanzees display "significant
cultural variation" in tool use, courtship and other behaviors.
Another challenge to Wrangham's theory is Pan paniscus,
otherwise known as the pygmy chimp, or bonobo. Bonobos are
darker-skinned and slimmer than Pan troglodytes, the more
common chimpanzee species, and much less aggressive. Researchers have
never observed coalitionary killing among bonobos. Noting that bonobos
are just as genetically related to us as chimpanzees, Frans de Waal, a
primatologist at Emory University, suggested last year in The Wall
Street Journal that bonobos may be "more representative of our
primate background" than are chimpanzees.
As evidence, de Waal cited new studies of Ardipithecus ramidus,
or "Ardi," who roamed Ethiopia 4.4 million years ago and is the oldest
known human ancestor. Although the species was first discovered in the
early 1990s, its evolutionary significance was only spelled out in
detail last fall in a
multipaper report in Science. One author, anthropologist
Owen Lovejoy of Kent State University, told me that Ardi has triggered a
"tectonic shift" in views of human evolution. "We now know, especially
in light of Ardipithecus, that hominids have always been a far
less aggressive clade than are chimpanzees or even bonobos."
Male and female Ardipithecenes were closer in size than male and
