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The Early Evolution of the Domestic Dog
Author(s): Darcy F. Morey
Source:
American Scientist,
Vol. 82, No. 4 (JULY-AUGUST 1994), pp. 336-347
Published by: Sigma Xi, The Scientific Research Society
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The Early Evolution of the Domestic Dog
Animal domestication, commonly considered a human innovation,
can also be described as an evolutionary process
Darcy F. Morey
Sometime within the past 12,000 or
so years, most of humankind began
to experience a profound shift in life?
style. Stone Age hunters and gatherers
of wild foodstuffs started to cultivate
plants and raise animals for their own
use. A landscape full of wild grasses,
woolly mammoths and sabertooth cats
gave way to giant-eared corn, fat cattle,
toy poodles and many other new
species. For reasons that remain ob?
scure, the shift happened rapidly, by
evolutionary standards, and in the
mere space of a few thousand years,
different domestic animals and plants
appeared independently in several
parts of the world.
The archaeological record indicates
that humankind's best friend?the do?
mestic dog, Canis familiaris?was likely
also its first. Consequently, I think of
dogs as the pioneers of an evolution?
ary radiation that had radical effects on
the composition of the earth's biota
and on the way people live. As such,
dogs are an appropriate focal point for
an ongoing debate about the origins
and nature of animal domestication.
Central to this discussion is the issue
of intentionality?whether domestica?
tion must be understood as a human
decision, as is commonly thought, or,
rather, is best modeled strictly as an
evolutionary process.
Those who explain domestication as
a rational decision suggest that people
recognized the potential benefits of
bringing animals and plants under
control. The assumption is that people
intentionally sought to raise, cultivate
and manipulate organisms in ways
that enhanced their economically use?
ful properties. In contrast, in the evolu?
tionary view, the behavior, diets and,
later, the physiology and morphology
of certain animals changed from that
of their wild counterparts in response
to the selection pressures of a new eco?
logical niche?a domestic association
with human beings. This view holds,
first, that knowing the intentions of
prehistoric people is beyond the abili?
ties of modern science. Second, and of
greater importance, knowledge of peo?
ple's rational intentions would not pro?
vide a scientific explanation for the
process of domestication.
Domestication as Human Design
Given the pivotal role of domestication
in shaping our present life-style, it is no
surprise to find that prehistorians have
argued vigorously about what domes?
tication really is, how it originated, and
why. Many classic definitions of the
concept focus on human subjugation of
other organisms. In a commonly drawn
scenario, people isolated individuals of
a particular species from their wild
counterparts and then selectively bred
them to exaggerate desirable traits and
eliminate undesirable ones in a process
known as artificial selection.
Such a scenario grows out of a com?
bination of common-sense reflection
on the conditions under which many
modern domesticates live, with the
presumption that those ends were
sought, at least in rudimentary form,
by people of the past. According to this
view, people turned to the domestica?
tion of plants and animals when increas?
es in human population or environmen?
tal changes reduced the availability of
wild foods. Given these pressures, peo?
ple invented or otherwise made a deci?
sion to experiment with domestication,
though not necessarily as a well-orga?
nized plan.
Theories that assume intentionality,
however, may be rooted more in the bi?
ases of modern culture than in any ob?
jective measure. Life in the 20th centu?
ry without domesticates is virtually
unimaginable to us, so it is tempting to
presume that people who lived with?
out domesticates during the late Pleis?
tocene and early Holocene surely
wished to improve their lives.
Figure 1. Relationships between Stone Age
people and wolves set the stage for dog do?
mestication. People and members of the dog
family have had a long association, as these
ll,000-to-12,000-year-old remains attest. A
puppy skeleton from either a dog or a wolf
can be seen under the human skeleton's left
hand. These burials were discovered at Ein
Mallaha in northern Israel and were origi?
nally reported by zooarchaeologists Simon
Davis and Francois Valla. Early dog remains
have been found at sites in other parts of the
world, suggesting that dog domestication
may have taken place independently in dif?
ferent regions. Prehistorians have disagreed
about whether different animals were inten?
tionally domesticated by ancient people, or
whether domestication is another example of
evolution driven by natural selection. The
author argues the latter, and proposes that
dog evolution is best viewed as the product
of selection pressures in a new ecological
niche, in this case a domestic association
with human beings. (Photograph by Alain
Dagand.)
Darcy F. Morey received his Ph.D. in
anthropology in 1990 from the University of
Tennessee at Knoxville. His primary training is as
an archaeologist, with a specialization in
zooarchaeology. His research on early
domestication, the core of his doctoral project, grew
out of a desire to integrate technical training in
zooarchaeology with a long-standing interest in
evolutionary theory and its application to
sociocultural phenomena. He currently holds an
adjunct affiliation with the University of
Tennessee, where he teaches occasionally and does
research. Address: Department of Anthropology,
252 South Stadium Hall, University of Tennessee,
Knoxville, TN 37996-0720.
336 American Scientist, Volume 82
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1994 July-August 337
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Probable time of
earliest domestication
(years before present) Domestic species and probable place of domestication
A second theme, this one anthro
pocentric, also underlies theories that
assume intentionality?that people ex?
ercise rational control of their collective
destiny. This perspective is appealing,
for it places people at the evolutionary
helm, charting the course from the
start. To borrow anthropologist David
Rindos's apt term, a "paradigm of con?
sciousness" is our conceptual anchor,
and from it stems the discussion of do?
mestication as invention, decision, idea
and so on.
Maybe the shift to economic reliance
on domestic species was in some sense
necessary, given human population
growth and environmental changes in
the Holocene. Maybe domestication
was indeed a strategy that prehistoric
people intentionally implemented.
Both propositions are debatable, but
my immediate objection stems from a
problem more fundamental than the
need for better data.
The human beings who participated
in the earliest domestic relationships
thousands of years ago are all dead.
They cannot tell us what was in their
minds or what they sought to accom?
plish. For early domestication, the data
required to evaluate scenarios based on
human intention are, by definition, unat?
tainable. In other words, models that ex?
plain domestication this way cannot be
empirically challenged, and on this ba?
sis alone, they are not scientific models.
The real issue is whether it is neces?
sary to presume the intentions of pre?
historic people to make sense of early
domestication. Over the years, some
scholars have attempted to describe do?
mestication in more mechanistic terms,
focusing on the implications of organ?
isms sharing space and resources in
symbiotic relationships. This approach,
however, has not led to a uniform per?
spective. In 1959, for example, zoologist
Charles Reed characterized domestica?
tion as "beneficial mutualism." At
about the same time, in 1963, archaeol?
ogist F. E. Zeuner was using the term
"slavery" as a virtual synonym for
some cases of domestication. Neverthe?
less, such efforts can be viewed as the
foundation for more recent attempts to
model domestication as evolution.
Evolutionary perspectives differ from
anthropocentric approaches in several
Figure 2. Humankind's best friend was likely
also its first. This time line shows the esti?
mated times and probable places of origin for
several other important domesticates.
338 American Scientist, Volume 82
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ways. First, they do not restrict domes?
tic relationships to people. The complex
symbiosis between ants and aphids is a
handy example, and is even used in my
dictionary to illustrate use of the term
"domestication." Certain ants herd
aphids, providing protection in ex?
change for the sugary, honey-like liquid
they "milk" from the aphids. Second,
domestic relationships involve two
species. Focusing solely on the human
role in domestication ignores the evolu?
tionary stakes for participating animals
and plants. The ubiquity of dogs, for ex?
ample, suggests they have profited well
from the domestic arrangement. Their
wolf ancestors, on the other hand, have
been extirpated from most of their for?
merly vast range, and many subspecies
are now extinct. From a Darwinian per?
spective, wolves who took up residence
with people a few thousand years ago
made a smart move?at least from to?
day's vantage point.
Finally, an evolutionary perspective
discourages an assumption that changes
in an animal's size or shape during do?
mestication must be products of human
selection.
Ancient Associations
If one is to eliminate rational intention
as a scientific explanation for early ani?
mal domestication, one must conclude
that the process originated with a nat?
ural association between people and
the wild ancestors of dogs. Skeletal re?
mains of early dogs from various ar?
chaeological sites around the world
place the beginnings of their domesti?
cation in the late Pleistocene era, possi?
bly as far back as 14,000 years ago. The
data therefore indicate that canid do?
mestication took place among people
who still pursued a hunting-and-gath
ering way of life.
The ancestor of these early dogs can
be identified with confidence as the
wolf Canis lupus. This assertion rests on
a growing body of molecular data and
is buttressed by the striking physiologi?
cal and behavioral similarities between
the species. It is not currently possible to
identify which subspecies of wolves
gave rise to domestic dogs (although
new advances in comparative DNA
analyses to establish relatedness be?
tween species may soon change that).
For now, scholars simply recognize the
wolf as the dog's ancestral progenitor,
and many people suspect that canid do?
mestication involved several wolf sub?
species in different parts of the world.
Figure 3. Dogs served a number of economic
purposes in past human societies, the variety
of which makes it difficult to glean a primary
benefit that people derived from the animals
during early domestication. This dog bone
from Qeqertasussuk, a small island off of the
west coast of Greenland, for example, was dis?
carded by people along with large quantities
of food debris almost 4,000 years ago. A series
of cut marks on the bone indicate that the ani?
mal from which it came was skinned or
butchered. Another dog bone from the site
had been fashioned into what Danish archae?
ologist Bjarne Gronnow describes as a needle
case. Later arctic peoples used dogs to pull
sleds, and some skulls from later sites in
Greenland and elsewhere in the Arctic bear
marks that indicated blows to the head. (Pho?
tograph by Geert Brovad.)
Figure 4. Dog effigy vessel was made by a Col
ima artist The Colima, who inhabited western
Mexico about 2,000 years ago, and some other
Precolumbian groups in Mesoamerica appar?
ently used dogs as dietary fare, as did later
groups, such as the Aztec. According to a Span?
ish observer at one Aztec market, 400 dogs were
sold on a slow day. (Photograph used with per?
mission from the Appleton Art Museum,
Ocala, Florida.)
Wolves and late-Pleistocene hunters
and gatherers undoubtedly came into
contact regularly, since both were so?
cial species who hunted many of the
same prey items. Wolves are also op?
portunistic scavengers; they were like?
ly to have been familiar with human
hunting practices and to have hung
around human settlements regularly.
Let us then assume that the road to?
ward domestication began when some
wolf pups became incorporated into a
human social and residential setting.
One could speculate endlessly about
the conscious motivation people had
for taking on wolf pups. It seems suffi?
cient, however, to note that different
people often kept wild animals for a
variety of reasons without attempting
to achieve long-term domestication.
Somewhere, at some time, one or
more adopted pups managed to sur?
vive to adulthood in the new setting.
To have a chance in human society, the
animals minimally had to adjust to
new social rules and to an altered diet.
Socialization, according to studies
conducted by J. P. Scott during the
1950s and 1960s, is oest achieved early
in a dog's life. Scott and his colleagues
at the Jackson Laboratory in Bar Har?
bor, Maine, conducted long-term stud?
ies of behavior and socialization in
dogs and found that the first few
weeks of a puppy's life are crucial for
forming primary social bonds with
both people and other dogs. Not sur?
prisingly, wolves are similar. Several
other studies have shown that young
wolf pups also form lasting bonds with
people, a process that becomes more
difficult for the animals as they mature.
Bonding between people and wolves
is facilitated by similarities in social
structure and in nonverbal modes of
communication. Wolves are organized
hierarchically and they communicate
status through vocal, facial and postur?
al displays of dominance or submis?
sion. These displays involve many cues
that are recognizable to people. Dogs
use much the same repertoire of cues.
Wolves and dogs can also respond ap?
propriately to many human signals.
It is clear that animals living within
human settlements had to learn that
subordinate status to dominant hu?
mans was an inviolable rule. Some
wolves were undoubtedly more adapt?
able than others to human dominance,
and those that did not follow the rules
were likely either killed or driven
away. Some of those that adjusted be
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came tolerated in human society?a
phenomenon that must have occurred
within many human settlements. Se?
lection for behavioral compatibility in a
setting with new social boundaries was
a strong force among founding domes?
tic populations.
From the beginning of their domestic
life, wolf pups would also have had to
adjust to a different diet. Wild wolves
take almost all their nutrition from meat.
Adults often hunt cooperatively for large
prey items, which for modern wolves in?
cludes deer, caribou or moose. Young
wolves often accompany the adults and
learn hunting skills, but that opportunity
would be lost to wolf pups living in the
domestic setting. Instead, they would
have needed to rely more on people to
share scraps of their own meals, a mix?
ture of meat and plants. Adeptness at so?
liciting food from people was surely a
valuable skill. To supplement this diet,
wolf pups would have had to learn to
scavenge competitively and to hunt
small animals.
To maintain their toehold in the do?
mestic niche, the domesticated wolves
had to succeed in reproducing. One
could assume that a male might leave
the human setting and mate with wild
animals. If he were successful, the
progeny would be wild and would
therefore not help perpetuate domes?
tic populations.
Alternatively, the domestic setting
might have included a male and fe?
male whose progeny remained in the
human settlement. Although this sce?
nario successfully creates more domes?
tic animals, it also creates a genetically
inbred population, which, in the long
run, weakens the gene pool of the do?
mestic population.
But a female surely had other op?
tions. A wild male that was unsuccess?
ful in breeding within a wild pack
might have found a domestic female an
easier target. The female would most
likely raise her offspring within the do
Figure 5. DNA sequence comparisons and
other lines of evidence allow scientists to es?
tablish the evolutionary relationships be?
tween members of the dog family. This
analysis suggests that the gray wolf was the
immediate ancestor of the domestic dog. The
two species share so much genetic material
in common that some scientists have de?
scribed dogs as gray wolves with a few ge?
netic alterations. The images do not depict
true size relations between the species. Time
is shown in millions of years ago. (Adapted
from Wayne, 1993.)
340 American Scientist, Volume 82
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mestic setting, although a few might at?
tempt to return to the wild with their
pups. The continuation of the domestic
line requires that only some females
raise offspring in a domestic setting.
An irony here is that canid domesti?
cation might have foundered if not for
the role of wild males finding alterna?
tive reproductive opportunities. Still,
their strategy ensured that the domes?
tic population was not isolated geneti?
cally from wild populations. Genetic
input from wild wolves was probably
strong for many generations. Even to?
day dogs and wolves are capable of
mating and producing fertile offspring.
Evolution in a Domestic Setting
The new population of domestic
wolves undoubtedly continued to ex?
pand. But at some point the animals be?
gan to change physically and behav
iorally, evolving toward the form we
recognize today as the dog. Early dogs
conveniently exhibit consistent mor?
phological changes when compared
with wolves. Briefly (and not exhaus?
tively), dogs became smaller overall,
and the length of the snout became pro?
portionally reduced. The result was a
smaller animal with a shorter face, a
steeply rising forehead and proportion?
ally wider cranial dimensions. This
general pattern suggests that adult ani?
mals retained juvenile characteristics, a
phenomenon known as paedomorpho
sis. Paedomorphic dogs have a some?
what puppylike cranial morphology
when compared with adult wolves.
In seeking to explain this pattern,
many discussions presume that domes?
tic animals must change in ways that
serve people. For example, some dis?
cussions suggest that people involved
in early canid domestication may have
found paedomorphic features endear?
ing and favored animals that retained
them. Similarly, it has been suggested
that people found smaller animals
more manageable and favored them as
well. Such suggestions appear reason?
able, especially because they reflect
common biases in people's present-day
choices for good household pets. But
these changes were taking place ubiq?
uitously some 10,000 years ago, despite
tremendous variability in cultural and
geographic settings. It seems unlikely
that all these human groups would
have selected for exactly the same traits
in dogs. Surely, the consistent appear?
ance of these traits in animals living
within so many different cultures raises
Figure 6. Gray wolf was almost certainly the ancestor of the domestic dog.
the possibility that some selection pres?
sure other than human preference
brought about the changes.
Specialists in life-history studies
have developed some tools for probing
this issue. The life-history analyst fo?
cuses on the entire life cycle of an ani?
mal, especially how changes in timing
of developmental processes and impor?
tant life events can have consequences
that impact reproductive success. Life
history analysts might consider when
and how often an animal should repro?
duce, or how big and how fast it
should grow, depending on its situa
tion. Different ecological circumstances
pose different selection pressures, and
the answers to these kinds of questions
depend on the specific conditions faced
by the animals.
In addition to selection for social
compatibility, I propose that the condi?
tions faced by early domestic canids
led to strong selection on reproductive
timing and body size. These selection
pressures ultimately produced the
smaller, paedomorphic animal known
as the dog.
J. P. Scott, whose experimental work
with dogs has already been noted, point
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ed out that canid domestication may be
regarded as ecological colonization of a
new niche. Population models view the
hallmark of colonization as rapid popu?
lation growth. One reason for this is that
mortality becomes less dependent on
population density compared with more
^ stable conditions. Under these circum?
stances, a classical prediction of life-his?
tory models is that selection should fa?
vor lowered age at first reproduction.
Increased fertility is at a premium then,
and precocious maturation is a remark?
ably efficient way to achieve this. Evolu?
tionary theory predicts that this change
should result in size reduction and pae
domorphosis in a descendant species,
owing to a truncation of the growth peri?
od. In such a case, both consequences are
only by-products of selection on repro?
ductive timing. It is tantalizing to note
that wild wolves reach sexual maturity
at about the age of 2 years, whereas most
modern dog breeds achieve maturity be?
tween 6 and 12 months. Unfortunately, it
is difficult to know when in their history
dogs started to reach sexual maturity
earlier, and the current observed ages
might just be an artifact of modern selec?
tive breeding programs.
A consideration of life-history stud?
ies also suggests that body size itself
was a likely target of selection. An ani?
mal's body size plays a crucial role in
defining its niche, and studies have
shown that adult size is correlated with
most life-history traits. Unfortunately,
causes can be difficult to disentangle
from effects. With early dogs, dietary
change had to be pronounced, and I
believe this placed smaller animals at a
distinct advantage, because of their
lower nutritional requirement. Admit?
tedly this idea is difficult to test, and
Figure 7. Natural selection may have brought
about many changes in the physiology and
overall body size of domesticated wolves and
led them eventually to form a separate
species?the domestic dog. Skeletal remains
show that early dogs were smaller and that
adult dogs appeared juvenile in relation to
their wolf ancestors. Here a prehistoric adult
dog skull (center) is compared with an adult
wolf skull (top) and a juvenile wolf skull.
The dog skull bears a striking similarity to
the juvenile wolf skull and is much less sim?
ilar to the skull of the adult wolf. These
changes suggest that the developmental pro?
gram of the dog was altered in such a way
that it would reach sexual maturity earlier
than its wolf ancestors, while other aspects of
its physical development were slowed down.
(The juvenile wolf skull is enlarged here for
the sake of comparison.)
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other factors were probably involved.
Different lines of evidence at least sug?
gest that dogs took a very direct route,
genetically speaking, to get to smaller
sizes. Zoologist Robert K. Wayne of the
Zoological Society of London studied
DNA sequences in modern canids and
concluded that dogs basically are
wolves, altered only by simple changes
in developmental timing and growth
rates. In related studies, Wayne also sug?
gested that reduced fetal growth rates
may be an important determinant of
adult size in small dogs. Simple changes
led to rapid size reduction in early dogs,
probably at the cost of problems in the
integration of different developmental
processes during growth. For example,
it is frequently observed that earliest
dogs often have crowded teeth, some?
times even overlapping each other in
jaws that are not really big enough to ac?
commodate them efficiently. Overall,
rapid size reduction with minimal genet?
ic change suggests strong selection for
smaller size among early dogs.
Consistent size reduction clearly took
place in the early evolution of the dog,
although causes are difficult to pinpoint.
But evolutionary theory also predicts
that the proposed developmental alter?
ations should produce paedomorphic
animals, and this requires a close look.
Evolutionary Paedomorphs
It is one thing to note that the cranial
morphology of early dogs appears pae?
domorphic. It is quite another to argue
that this pattern sets them apart from
other canids or reveals something im?
portant about evolution under domesti?
cation. Other wild canids might also ap?
pear paedomorphic when compared
with wolves. Dogs are frequently de?
scribed as paedomorphic because mod?
ern small breeds resemble juvenile
forms of larger breeds. But to have evo?
lutionary significance, it is important to
determine whether prehistoric dogs
were paedomorphic relative to their an?
cestral species, the wolves.
To tackle these problems, I armed
myself with calipers and a notebook
and visited several American and Eu?
ropean museums to measure canid cra?
nia. First, I took measurements from 65
adult prehistoric dog specimens from
archaeological sites, the vast majority
dated from between 3,000 and 7,000
years ago. Three-quarters of the speci?
mens are from the United States, and
the rest come from northern Europe.
My choice of samples emphasized sites
shorter -? longer
growth period
Figure 8. Size change is often a consequence of changes in growth rates and timing. This
schematic illustrates a hypothetical model to account for differences in size and morphology
between the wolf and the dog. This model postulates that the descendant species grows more
slowly very early in life and finishes growing sooner than the ancestral species. In this way the
dog comes to resemble the juvenile form of its wolf ancestor when it reaches its full size.
where people were still making their
living primarily through hunting and
gathering at the time corresponding to
the age of the sample, and therefore
where I had little reason to suspect sys?
tematic selective breeding.
Next, I measured crania from 222
modern wild canids representing four
species. These are, in descending order
of average size, the gray wolf, the red
wolf, the coyote and the golden jackal.
The wolves and coyotes are all North
American, from the continental United
States or southern Canada. Based on
cranial measurements, I determined
that most of the prehistoric dogs in my
sample were roughly the size of golden
jackals or the smaller coyotes. I did not
have prehistoric samples of wild
canids and must assume that modern
samples provide a generally valid ap?
proximation of morphological varia?
tions in these species.
I was particularly interested in learn?
ing how several snout-length and cra?
nial-width dimensions change in rela?
tion to the overall length of the skull as
one moves from large to small animals.
Size changes in animals are almost in?
evitably accompanied by patterned
changes in proportions, a phenomenon
known as allometry. Some allometric
patterns stem from basic laws of bio
mechanics. For example, an elephant's
mass could not be supported on geo?
metrically scaled-up mouse bones. To
begin with, an elephant has to have
proportionally thicker leg bones. Shape
changes shown by dogs could reflect
only this kind of allometry.
My analysis revealed some interest?
ing results. First, it turns out that most
dogs share snout-length proportions
with comparably sized wild canids.
What sets dogs apart is not changes in
the length of their snouts, but the
width of their palates and cranial
vaults. The cranial morphology of dogs
is unique and does not conform to allo
metric patterns among wild canids.
The issue then is whether this cra?
nial morphology reflects evolutionary
paedomorphosis. To determine this, I
compared dog morphology with the
morphology of its ancestral species,
the wolf, as it grows. If the dog is in
fact a paedomorphic wolf, I would ex?
pect to see the greatest similarities be?
tween dogs and juvenile wolves and
less similarity between prehistoric
dogs and adult wolves. Ideally, data
for answering this question would in?
clude cranial measurements from ju?
veniles of both species. Unfortunately,
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the archaeological record is not that
cooperative, and skulls of juveniles are
usually nothing more than a pile of
fragile, fingernail-sized pieces. With?
out data from juvenile dogs, it be?
comes imperative to have data from
juvenile wolves, and this cause is not
as hopeless. I measured skulls of 64
modern juvenile wolves ranging in
age from a few weeks to several
months, a sample that includes several
North American subspecies.
Using the allometric approach
again, I compared juvenile wolf pro?
portions with those of the adult wild
canids and prehistoric dogs. By plot?
ting snout-length measures against to?
tal skull length, I found that as a wolf
grows, its snout gets longer at a rate
that mirrors increasing snout length in
the adult wild canids. These plots
showed that all adult canid species, in?
cluding adult dogs, look something
like scaled-down adult wolves, if one
Figure 9. Behavioral alterations seem to accompany physical changes, so that dogs not only look
more juvenile than wolves; they also act more juvenile. Behavior and physiology were shown to
be linked by breeding experiments conducted by Russian geneticist D. K. Belyaev. Belyaev and
his colleagues interbred foxes that responded well to people. After about 20 generations, foxes
from this lineage actively sought contact with people, whined and wagged their tails. Like
many dogs, some tame foxes had drooping ears and erect tails, features that were decidedly ab?
sent from the control fox population in this study. (Adapted from Belyaev 1979.)
considers only the ratio of snout
length to total skull length.
When I plotted width-to-total-skull
length proportions, I saw some inter?
esting differences. Adult dogs are dis?
tinct in these dimensions from all the
adult wild canids. But adult dogs do re?
semble one wild canid group: juvenile
wolves. Of all wild canid species, in?
cluding adult wolves, the shape of
adult dog crania most closely resembles
that of juvenile wolves. The issue is not
closed, but these data do support the
hypothesis that dogs represent a pae
domorphic form of their wolf ancestors.
If that is true, it is possible that the dogs
evolved as the evolutionary model
would predict. Developmental changes
in these animals might have come
about as a response to selection pres?
sures in a new niche, and these changes
ultimately gave rise to a paedomorphic
form of the ancestral species.
Behavioral Paedomorphosis
Many adult dogs not only appear ju?
venile, they also act juvenile. They dis?
play a sort of behavioral paedomor?
phosis. Dogs routinely solicit attention,
play, grovel, whine, bark profusely and
otherwise exhibit behavior that wolves
more or less outgrow as they mature.
Biologist Raymond Coppinger and lin?
guist Mark Feinstein describe dogs as
"stuck in adolescence." They also make
the important point that the essence of
tameness is the submissive, solicitous
behavior style of juveniles. This leads
to the question of whether physiologi?
cal and behavioral paedomorphosis
are interrelated.
Experiments directed by Russian ge?
neticist D. K. Belyaev cause one to sus?
pect that the answer is yes. Belyaev's
group implemented a strict selective
breeding program with silver foxes
from a commercial fur farm. Their work
sprung from the observation that al?
though a majority of captive foxes were
aggressive or fearful around people, a
small number, about 10 percent, were
less so. More than 30 years ago Belyaev
began selectively breeding these calmer
individuals only with other such indi?
viduals, through successive generations.
Selection was for what Belyaev de?
scribed as domesticated behavior.
The results after only about 20 gen?
erations were fascinating. Many foxes
in the selected population now actively
sought contact with people. The foxes
would lick people's hands and faces,
whine and wag their tails. Whereas
344 American Scientist, Volume 82
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^^1^ Borzoi ' lf\sL
9?nd0?S [^^^ Salukl ^^^^^:
bulldog \ V/^^ -^^_^y ^SSv^c ?^^^Ei^ "
^^bKII^^ ^^^^^^^^^ ; ??' ^M^^ ^^^V ^hantoqi]ri;
7^ herding dofi^^j^ \. f ' ' "
I mastiffs ^^^|^^^^^HH^^^^^^^B ?^^?^ -^
Great Dane <^[^ ^^^^^^^^^
European spitz '?.??J:^/lhy ^M^^' ^ ; . " V^ r
?^^^ .. vv^-v;^;,'
Figure 10. Different geographic subspecies of wolves may have given rise to different dog-breed groups. This model shows one interpretation
of the ancestry of modern dog breeds. Modern dog breeds are the result of at least 2,000 years of breeding under human control, and no breed
is derived solely from one geographic origin. Ultimately, DNA studies may help assess the accuracy of models like this one. The images do not
depict the true size relations between breeds. (Adapted from Clutton-Brock and Jewell 1993.)
wild foxes, like wild wolves, breed
only once each year, females in the se?
lected population began a shift to?
wards more frequent receptivity, with
some later-generation females capable
of breeding twice each year. Domestic
dogs regularly breed more than once
each year. Other changes in the select?
ed population included a much longer
moulting time, drooping ears and erect
tails. These remarkably dog-like
changes were absent in the unselected
fox population and are absent in wild
wolves as well.
The experiments do not replicate,
even roughly, the conditions of early
domestication, but they show how
strongly behavior is linked to physiolo?
gy. Strict selection for certain behavioral
traits can disrupt previously stable pat?
terns of physiological development.
Oddities of domestication, such as erect
tails and drooping ears, make more
sense in light of this work.
Scientists are still far from under?
standing precisely how different fac?
tors combined to produce the changed
animal whose bones begin to turn up
in late Pleistocene archaeological sites.
Several important factors, however, at
least seem to point in the same direc?
tion. Whether focusing on social be?
havior, diet or reproductive tactics, one
should find that the evolution of a
smaller, paedomorphic canid during
domestication presents no surprise.
1994 July-August 345
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^^^^ j^^^^^^^^^^^^^S^j^^^k ^^^^^^
Figure 11. Canid domestication was undoubtedly helped along by the ability of the animals to
form strong social attachments to people. For many modern dogs, social bonding is vital to
their individual well-being. This scene, familiar to dog enthusiasts, emphasizes that bonding
is a two-way street.
For a long time, early domestic dogs
were consistently smaller compared
with wolves. In contrast, modern dogs
include breeds, such as the Great Dane,
that are as large as or even larger than
wolves. Given the context of domesti?
cation, only one set of circumstances is
likely to account for large dogs or can
account for the size range of modern
breeds. That set of circumstances is se?
lective breeding under human control.
It is important to stress that a domes?
tic relationship does not mean that nat?
ural selection has become something
other than natural. It is not a process that
distinguishes human factors from oth?
ers in the environment. Natural selection
is simply the statistical summation of the
reproductive fates of organisms that use
their physical and behavioral equipment
to compete for genetic representation in
the next generation. Dogs are no excep?
tion. Tameness and other traits were the
currency of competition from the onset
of the domestic relationship, regardless
of whether people had goals for the ani?
mals or were even aware of what
changes were unfolding.
Beyond Dogs
A couple of years ago, a colleague com?
mented that I was fortunate to have cho?
sen dogs as my subject, because my per?
spective would not hold up for other
cases of domestication. Naturally, I asked
why not. He answered that dogs were
first, but after that, the idea of domestica?
tion was in place. People then had a
model, one they could apply to animals
of considerably greater economic impor?
tance, for example goats or cattle. The
domestication of such animals, my col?
league argued, would best be under?
stood as the product of people's pur?
poseful efforts to achieve that goal.
Applying the same logic to Belyaev's
experiments, we might just as well ex?
plain the evolution of modified, tame
foxes as a consequence of Belyaev set?
ting out to accomplish that. Such an ex?
planation is not scientifically meaning?
ful. The mechanistic explanation begins
with the observation that foxes with
certain heritable traits mated only with
foxes bearing similar traits through suc?
cessive generations.
The issue is not whether prehistoric
people engaged in behavior that led to
the domestication of goats or cattle.
They certainly must have. The issue lies
with the presumption that the eventual
result?highly modified animals under
conscious human subjugation?ex?
plains the process that started those an?
imals toward that end. Figuring out
what prehistoric people actually did
that contributed to the evolution of do?
mestic organisms is hard enough. To
presume their purposes, and then prof?
fer that as part of an explanation for
evolutionary change, is to flirt with
mysticism.
To be fair, my colleague's argument
reflects a broader tendency for schol?
ars to treat dogs as a special case be?
cause they are not perceived as eco?
nomically important and therefore
provided no compelling reason for
people to have sought to domesticate
them. Many societies, however, have
made regular use of dogs as dietary
fare. In addition, dog skins have served
as clothing, and bones as raw material
for tools, and the living animals have
often been used as beasts of burden or
as hunting aids.
Ultimately, the present exercise is
only a minor part of the much larger is?
sue of how to fit human cultural evolu?
tion into a scientific framework. Hu?
man culture, not being genetically
determined, is widely assumed to su
346 American Scientist, Volume 82
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persede the Darwinian processes that
explain how other organisms evolve.
By extension, domestication is also fre?
quently exempted from Darwinian
models of evolution for the simple rea?
son that it arises in a human sociocul
tural context. In a field hungry for gen?
uine theory, however, anthropologists
and archaeologists are currently debat?
ing the applicability of Darwinian theo?
ry to sociocultural evolution. Biologists
should be keenly interested in this de?
bate, for in the exclusion of cultural
evolution from the Darwinian model
makes it irrelevant to a good portion of
life on this planet.
More than a decade ago, archeologist
R. C. Dunnell suggested that if archae?
ology should achieve its widely pro?
fessed goal of becoming scientific, few
people would be pleased with the re?
sult. For one thing, there theories about
cultural evolution would not be
grounded in human intention. Even if
we could document people's goals and
intentions, they are phenomena to be
explained, not explanations in them
selves. Consider how difficult it is to
take even the seemingly small step of
bringing domestic organisms under the
Darwinian umbrella. We are a long way
from knowing whether Dunnell is right.
Acknowledgments
This research has been supported by
the Jacob K. Javits Program of the U. S.
Department of Education, the Smith?
sonian Institution, the American-Scan?
dinavian Foundation and the Wenner
Gren Foundation for Anthropological
Research. I thank Michael Logan for of?
fering suggestions and helping locate
sources for several illustrations in used
in this article.
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