March 2011, Week 2


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Fri, 11 Mar 2011 22:19:00 -0500
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New View of How Humans Moved Away From Apes

By Nicholas Wade
March 10, 2011

Anthropologists studying living hunter-gatherers have
radically revised their view of how early human
societies were structured, a shift that yields new
insights into how humans evolved away from apes.
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Early human groups, according to the new view, would
have been more cooperative and willing to learn from
one another than the chimpanzees from which human
ancestors split about five million years ago. The
advantages of cooperation and social learning then
propelled the incipient human groups along a different
evolutionary path.

Anthropologists have assumed until now that hunter-
gatherer bands consist of people fairly closely related
to one another, much as chimpanzee groups do, and that
kinship is a main motive for cooperation within the
group. Natural selection, which usually promotes only
selfish behavior, can reward this kind of cooperative
behavior, called kin selection, because relatives
contain many of the same genes.

A team of anthropologists led by Kim R. Hill of Arizona
State University and Robert S. Walker of the University
of Missouri analyzed data from 32 living hunter-
gatherer peoples and found that the members of a band
are not highly related. Fewer than 10 percent of people
in a typical band are close relatives, meaning parents,
children or siblings, they report in Friday's issue of

Michael Tomasello, a psychologist at the Max Planck
Institute for Evolutionary Anthropology in Germany,
said the survey provided a strong foundation for the
view that cooperative behavior, as distinct from the
fierce aggression between chimp groups, was the turning
point that shaped human evolution. If kin selection was
much weaker than thought, Dr. Tomasello said, "then
other factors like reciprocity and safeguarding one's
reputation have to be stronger to make cooperation

The finding corroborates an influential new view of
early human origins advanced by Bernard Chapais, a
primatologist at the University of Montreal, in his
book "Primeval Kinship" (2008). Dr. Chapais showed how
a simple development, the emergence of a pair bond
between male and female, would have allowed people to
recognize their relatives, something chimps can do only
to a limited extent. When family members dispersed to
other bands, they would be recognized and neighboring
bands would cooperate instead of fighting to the death
as chimp groups do.

In chimpanzee societies, males stay where they are born
and females disperse at puberty to neighboring groups,
thus avoiding incest. The males, with many male
relatives in their group, have a strong interest in
cooperating within the group because they are defending
both their own children and those of their brothers and
other relatives.

Human hunter-gatherer societies have been assumed to
follow much the same pattern, with female dispersal
being the general, though not universal, rule and with
members of bands therefore being closely related to one
another. But Dr. Hill and Dr. Walker find that though
it is the daughters who move in many hunter-gatherer
societies, the sons leave the home community in many
others. In fact, the human pattern of residency is so
variable that it counts as a pattern in itself, one
that the researchers say is not known for any species
of ape or monkey. Dr. Chapais calls this social pattern

Modern humans have lived as hunter-gatherers for more
than 90 percent of their existence as a species. If
living hunter-gatherers are typical of ancient ones,
the new data about their social pattern has
considerable bearing on early human evolution.

On a genetic level, the finding that members of a band
are not highly interrelated means that "inclusive
fitness cannot explain extensive cooperation in hunter-
gatherer bands," the researchers write. Some
evolutionary biologists believe that natural selection
can favor groups of people, not just individuals, but
the idea is hotly disputed.

Dr. Hill said group selection, too, could not operate
on hunter-gatherer bands because individuals move too
often between them, which undoes any selective effect.
But hunter-gatherers probably lived as tribes split
into many small bands of 30 or so people. Group
selection could possibly act at the level of the tribe,
Dr. Hill said, meaning that tribes with highly
cooperative members would prevail over those that were
less cohesive, thus promoting genes for cooperation.

The new data on early human social structure furnishes
the context in which two distinctive human behaviors
emerged, those of cooperation and social learning, Dr.
Hill said. A male chimp may know in his lifetime just
12 other males, all from his own group. But a hunter-
gatherer, because of cooperation between bands, may
interact with a thousand individuals in his tribe.
Because humans are unusually adept at social learning,
including copying useful activities from others, a
large social network is particularly effective at
spreading and accumulating knowledge.

Knowledge can in fact be lost by hunter-gatherers if a
social network gets too small. One group of the Ache
people of Paraguay, cut off from its home territory,
had lost use of fire when first contacted. Tasmanians
apparently forgot various fishing techniques after
rising sea levels broke their contact with the
Australian mainland 10,000 years ago.

Dr. Chapais said that the new findings "validate and
enrich" the model of human social evolution proposed in
his book. "If you take the promiscuity that is the main
feature of chimp society, and replace it with pair
bonding, you get many of the most important features of
human society," he said.

Recognition of relatives promoted cooperation between
neighboring bands, in his view, allowing people to move
freely from one to another. Both sons and daughters
could disperse from the home group, unlike chimp
society, where only females can disperse. But this
cooperation did not mean that everything was peaceful.
The bands were just components of tribes, between which
warfare may have been intense. "Males could remain as
competitive and xenophobic as before at the between-
tribe level," Dr. Chapais writes.


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