SCIENCE June 29 2007
COVER A laborer uses a sieve to separate wheat husks from the grain at a market
in Amritsar, India. Two Reviews on pages 1862 and 1866 and a News story on page
1830 discuss human domestication of plants. Photo: Narinder Nanu/AFP/Getty Images
Science 29 June 2007:
Vol. 316. no. 5833, p. 1809
THIS WEEK IN SCIENCE HTTP://WWW.SCIENCEMAG.ORG/CGI/CONTENT/SHORT/316/5833/1809F
The original wild ancestors of wheat would have been tough to farm and tough to eat.
However, domestication of wheat as a crop some 10,000 years ago captured
advantageous changes in grain size, threshability, and retention of grains on the plant
spike. Dubcovsky and Dvorak (p. 1862) review recent insights from molecular genetics
and genomics to understand how gene mutations and genome ploidy paved the way for
successful domestication of our modern cultivated wheat varieties. Kareiva et al. (p.
1866) review human influences on the global ecosystem and suggest humans are in the
process of domesticating the world. On balance, human modifications of the environment
have historically provided net benefits, but the point may have been reached such that
harmful impacts outweigh the benefits.
Genome Plasticity a Key Factor
in the Success of Polyploid
Wheat Under Domestication
Jorge Dubcovsky* and Jan Dvorak
Wheat was domesticated about 10,000 years ago and has since spread worldwide to
become one of the major crops. Its adaptability to diverse environments and end uses is surprising
given the diversity bottlenecks expected from recent domestication and polyploid speciation
events. Wheat compensates for these bottlenecks by capturing part of the genetic diversity of its
progenitors and by generating new diversity at a relatively fast pace. Frequent gene deletions and
disruptions generated by a fast replacement rate of repetitive sequences are buffered by the
polyploid nature of wheat, resulting in subtle dosage effects on which selection can operate.
humans (1). Roughly 95% of the wheat crop is
common wheat, used for making bread, cookies,
and pastries, whereas the remaining 5% is
durum wheat, used for making pasta and other
semolina products. Einkorn wheat and other
hulled wheats, namely emmer and spelt, are
today relic crops of minor economic impor-
tance (2, 3).
Einkorn is a diploid species, whereas durum
and common wheat are polyploid species that
originated by interspecific hybridization of two
and three different diploid species, respective-
ly (Fig. 1). The success of these domesticated
polyploid species parallels the success of natural
polyploid species, which represent more than
70% of plant species [reviewed in (4)] and tend
to have more extended geographic distribu-
tions than those of their close diploid relatives
(5). Consequently, recent advances in wheat
genomics may shed light on the genetic causes
of the broad adaptability of natural polyploid
plant species as well.
The transition from hunting and gathering to
agrarian lifestyles in western Asia was a thresh-
old in the evolution of human societies. Domes-
tication of three cereals—einkorn, emmer, and
barley—marked the beginning of that process
(6). Genetic relationships between wild and
domesticated einkorn and emmer suggest that
the region west of Diyarbakir in southeastern
Turkey is the most likely site of their domes-
tication (Fig. 2) (7–9). From this area, the ex-
pansion of agriculture lead to the dissemination
of domesticated einkorn (T. monococcum, ge-
ith 620 million tons produced annu-
ally worldwide, wheat provides about
one-fifth of the calories consumed by
nomes AmAm) and domesticated emmer [T.
turgidum subspecies (ssp.) dicoccon, genomes
BBAA] across Asia, Europe, and Africa. South-
western expansion of domesticated emmer cul-
tivation resulted in sympatry with the southern
subpopulation of wild emmer (T. turgidum
ssp. dicoccoides, genomes BBAA). Gene ex-
changes between the northern domesticated
emmer and the southern wild emmer popula-
tions or emmer domesticated in the southern
region resulted in the formation of a center of
domesticated emmer diversity in southern
Levant (Fig. 2) (9). The consequence was a
subdivision of domesticated emmer into
northern and southern subpopulations with
an increase in gene diversity in the latter (9).
Northeast expansion of domesticated emmer
cultivation resulted in sympatry with Aegilops
tauschii (genomes DD) and the emergence of
hexaploid common wheat (T. aestivum, genomes
BBAADD) (10) within the corridor stretching
from Armenia to the southwestern coastal area
of the Caspian Sea (11) (Fig. 2).
The genetic changes responsible for the suite
of traits that differentiate domesticated plants
from their wild ancestors are referred to as the
domestication syndrome (12). In wheat, as in
other cereals, a primary component of this syn-
drome was the loss of spike shattering, pre-
venting the grains from scattering by wind and
facilitating harvesting (Fig. 1). Abscission scars
of einkorn remains from archeological sites in
northern Syria and southeastern Turkey revealed
a gradual increase in nonshattering einkorn spikes
from 9250 to 6500 years before the present (BP),
a discovery interpreted as evidence of a pro-
longed domestication period of cereals (13). The
chromosome locations of the genes controlling
shattering in einkorn are unknown, but in em-
mer wheat shattering is determined by the Br
(brittle rachis) loci on chromosomes 3A and 3B
(14) (Fig. 1).
Another important trait for wheat domes-
tication was the loss of tough glumes, con-
verting hulled wheat into free-threshing wheat
(Fig. 1). The primary genetic determinants of
the free-threshing habit are recessive muta-
tions at the Tg (tenacious glume) loci (15),
accompanied by modifying effects of the dom-
inant mutation at the Q locus and mutations
at several other loci (15). The recent cloning
of Q, which also controls the square spike
phenotype in common wheat, showed that it
encodes an AP2-like transcription factor. The
Department of Plant Sciences, University of California, One
Shields Avenue, Davis, CA 95616, USA.
*To whom correspondence should be addressed. E-mail:
Fig. 1. Wheat spikes showing (A) brittle rachis, (B to D) nonbrittle rachis, (A and B) hulled grain, and (C
and D) naked grain. (A) Wild emmer wheat (T. turgidum ssp. dicoccoides), (B) domesticated emmer (T.
turgidum ssp. dicoccon), (C) durum (T. turgidum ssp. durum), and (D) common wheat (T. aestivum). White
scale bars represent 1 cm. Letters at the lower right corner indicate the genome formula of each type of
wheat. Gene symbols: Br, brittle rachis; Tg, tenacious glumes; and Q, square head. [Photos by C. Uauy]
29 JUNE 2007 VOL 316
on June 28, 2007
mutation that gave rise to the Q allele is the
same in tetraploid and hexaploid free-threshing
wheats, suggesting that it occurred only once
Seeds of free-threshing wheat began to ap-
pear in archaeological sites about 8500 years BP
(2, 17). The tetraploid forms of these Neolithic
free-threshing wheats may be the ancestor of
the modern large-seeded, free-
threshing durum (Fig. 1), which
is genetically most closely re-
lated to the Mediterranean and
Ethiopian subpopulations of do-
mesticated emmer (Fig. 2) (9).
The first archaeological records
of durum appeared in Egypt dur-
ing the Greco-Roman times [re-
viewed in (2)].
Other traits of the wheat do-
mestication syndrome shared
by all domesticated wheats are
increased seed size (Fig. 1, A
and B), reduced number of tillers,
more erect growth, and reduced
seed dormancy. One gene af-
fecting seed size is GPC-B1, an
early regulator of senescence
with pleiotropic effects on grain
nutrient content (18). In some
genotypes and environments, the
accelerated grain maturity con-
ferred by the functional GPC-B1
allele is associated with smaller
seeds (19). Therefore, indirect
selection for large seeds may explain the fixation
of the nonfunctional GPC-B1 allele in both
durum and T. aestivum (18). Except for Q and
GPC-B1, no other genes relevant to the wheat
domestication syndrome have been isolated
so far, and a systematic effort to do so is long
overdue. Not only is this knowledge critical
for understanding the genetic and molecular
mechanisms of domestication, it is also possi-
ble that genetic variation at these same loci
plays an important role in the success of wheat
as a modern crop.
Success of Wheat as a Crop
Domesticated wheat exemplifies the positive
correlation between ploidy and success as a
crop. In almost all areas where domesticated
einkorn and domesticated emmer were cultivated
together, it was domesticated emmer that be-
came the primary cereal (2). Emmer remained
the most important crop in the Fertile Crescent
until the early Bronze Age, when it was re-
placed by free-threshing wheat (2). Although a
free-threshing form of einkorn has been identi-
fied, it is not widely cultivated because of the
association between soft glumes and reduced ear
length in this diploid species (17).
The story repeated itself, with hexaploid
T. aestivum expanding further than durum.
Today, hexaploid T. aestivum accounts for most
of the global wheat crop and is grown from
Norway and Russia at 65°N to Argentina at
45°S (Fig. 2) (20). However, in tropical and
subtropical regions wheat is restricted to higher
elevations. Although the dominance of tetra-
ploid wheat over diploid wheat potentially could
be attributed to the greater robustness of tetra-
ploid wheat, this does not explain the domi-
nance of T. aestivum over durum. Durum often
has larger seeds than hexaploid wheat (Fig. 1, C
and D) and similar yield potential as that of
hexaploid wheat under optimum growth condi-
tions (table S1).
The vast majority of polyploid plants,
including wheat, originated by hybridization
between different species (allopolyploidy). Al-
lopolyploidy results in the convergence in a
single organism of genomes previously adapted
to different environments, thus creating the po-
tential for the adaptation of the new allopoly-
ploid species to a wider range of environmental
conditions. This has clearly been the case for
hexaploid wheat, which combines the D ge-
nome from Ae. tauschii with the AB genomes
from tetraploid wheat. Compared with tetra-
ploid wheat, hexaploid T. aestivum has broader
adaptability to different photoperiod and ver-
nalization requirements; improved tolerance to
salt, low pH, aluminum, and frost; better re-
sistance to several pests and diseases; and ex-
tended potential to make different food products
This does not mean, however, that gene
expression in an allopolyploid is the summa-
tion of gene expression in its diploid ancestors.
Nonadditive gene expression has been reported
in numerous artificial allopolyploids [reviewed
in (4, 21)]. Rapid and stochastic processes of
differential gene expression (22) provide an ad-
ditional source of genetic variation that could be
important for the successful adaptation of new
There are detrimental aspects to polyploidy
as well. Polyploid speciation is accompanied
by a polyploidy bottleneck (5), in which the
small number of plants contributing to the
formation of a new polyploid species con-
strains its initial gene diversity. Because only
a few Ae. tauschii genotypes participated in the
origin of T. aestivum (23, 24), its D-genome di-
versity is expected to be limited.
Recent advances in the understanding of the
dynamics of gene diversity during domestication
and the subsequent evolution of polyploid wheat
are reviewed in the following sections to
reconcile these opposing effects of polyploidy
and to shed light on the mechanisms by which T.
aestivum has come to be one of humankind’s
most important crops (Fig. 2).
The Capture of Preexisting Diversity
Domestication is accompanied by domestica-
tion bottlenecks, resulting in reduced gene di-
versity [reviewed by (25)]. A study using 131
restriction fragment length polymorphism (RFLP)
loci showed that gene diversity values in cul-
tivated emmer were 58% of those observed in
wild emmer across its entire geographic distri-
bution (9). A similar estimate (51%) was obtained
for nucleotide diversity (26). For comparison,
nucleotide gene diversity values in domesti-
cated maize and pearl millet are 57% (27) and
67% (28), respectively, of those present in their
wild progenitors. That self-pollinating emmer
has an approximately equivalent proportion of
the genetic diversity of its wild ancestor as do
cross-pollinating maize and pearl millet is sur-
prising. Several lines of evidence indicate that
gene flow between wild and domesticated
Fig. 2. The origin and current distribution of wheat. The wheat production map was provided by Dave Hodson, CIMMYT
dotted red lineindicatesa southerncenterofdomesticated emmer diversity.Theapproximatedistributionsofwildemmer
Cafer Höyük; 4, Jericho; 5, Cayönü; 6, Nahal Hemar; and 7, Nevali Cori [from (2)].
VOL 31629 JUNE 2007
on June 28, 2007
emmer occurred in all places where the two were
sympatric (9). Additionally, if the emmer domes-
tication process took as long as that of einkorn
would probably be sufficient for domesticated
emmer to capture a significant proportion of the
genetic diversity of its wild relative.
Additional diversity bottlenecks occurred dur-
ing the transition from hulled to free-threshing
wheat (Fig. 1) and during the polyploid spe-
genome are less than 15% of those present in
populations of Ae. tauschii from Transcaucasia,
reflecting the severity of the initial polyploidy
bottleneck (11). A similar estimate (7%) was
obtained for nucleotide diversity (26). Howev-
er, in the A and B genomes of T. aestivum, the
average diversity at the nucleotide level was
found to be 30% of that present in wild emmer
(26, 29). This result suggests that difference in
ploidy has presented only a weak barrier to
gene flow from tetraploid wheat, including
wild emmer, to hexaploid wheat (30), a result
also supported by the discovery of hybrid
swarms between wild emmer and common
wheat (31). In summary, hexaploid wheat
captured a larger portion of the natural gene
diversity present in its tetraploid ancestor
than of the diversity present in Ae. tauschii.
The proportion of diversity captured by T.
aestivum from both ancestors is likely to in-
crease in the future, because modern wheat
breeders, realizing the importance of expanding
diversity for successful crop improvement, are
starting to use synthetic wheats in their breed-
ing programs (32). Synthetic wheats are
produced by hybridizing different tetraploid
wheats and Ae. tauschii genotypes and then
inducing doubling of the genomes through
colchicine treatment (32).
New Sources of Diversity
None of the plant genes that contributed to the
domestication of diploid and ancient polyploid
species (e.g., maize) discovered so far are null
alleles (33), consistent with the view that do-
mestication was achieved mostly through “tinker-
ing” rather than “disassembling” or “crippling”
key genes from wild relatives (33). In a young
polyploid species like wheat, however, null
mutations of one of the duplicate or triplicate
homologous gene copies may have only subtle
dosage effects and thus may appear as “tinkering”
mutations with a potential to generate adaptive
A null mutation of the GPC-B1 gene in the
B genome of polyploid wheat illustrates this
point. In tetraploid wheat, the GPC-B1 mutation
caused a few days’ difference in maturity, whereas
in diploid rice RNA interference (RNAi) of the
rice GPC gene brings about almost complete
seed sterility [Supporting Online
Material (SOM) text]. Mutations
in one of the three functional
copies of a gene in hexaploid wheat
are expected to have more subtle
effects than in tetraploid wheat.
This fact is illustrated by the higher
tolerance to induced mutations of
hexaploid wheat compared with
tetraploid wheat (34). The fact that
most of the 21 T. aestivum chromo-
somes can be removed to produce
nullisomic plants exhibiting only
minor phenotypic effects leaves
no doubt of the buffering effect
of polyploidy on gene deletions.
This buffering effect is eroded in
ancient polyploid species (SOM
The abundance of repetitive ele-
ments in the wheat genomes (about
83% repetitive) (35) greatly facili-
tates the generation of null muta-
tions, either by insertion of repetitive
elements into genes (36) or by gene
deletions (37, 38). As in maize, genes
in wheat are embedded within
long stretches of nested retroele-
ments and other mobile sequences
(Fig. 3). Studies of microsynteny
among orthologous chromosomal regions across
the tribe Triticeae showed that the intergenic
space is subject to an exceedingly high rate of
turnover (39). For example, 69% of the
intergenic space within orthologous VRN2 re-
gions from T. monococcum and the A genome
of tetraploid wheat (Fig. 3) has been replaced
over the course of the past 1.1 million years
(My) (SOM text).
These data, along with a comparison of
orthologous regions in T. urartu and the A ge-
nome of tetraploid wheat (30), yield an average
replacement of 62% ± 3% (SEM) of the intergenic
regions during the first million years of diver-
gence (Fig. 4 and SOM text). The model in Fig. 4
predicts correctly the very proportion of sequence
Fig. 3. DNA insertions and deletions in orthologous VRN2 regions from
the Amgenome of T. monococcum (AY485644) and the A genome of
durum wheat variety Langdon (new sequence EF540321). These regions
diverged 1.1 ± 0.1 My ago. The red lines connect orthologous regions
(>96% identical). Arrows represent genes: red, orthologous; blue, ortholog
absent; and violet, pseudogene. Rectangles represent repetitive elements in
their actual nested structure: red, orthologous; blue, insertions after
divergence; green, deletion in the opposite genome (yellow region); and
black, not determined. Only 31% of the orthologous intergenic regions have
not been replaced. [See SOM text for details.]
Decay of synteny in intergenic regions
Million years since divergence
Fig. 4. Decay of the proportion of conserved sequences [C(t)] in
orthologous intergenic regions with divergence time. The upper
and lower red curves were calculated with two independent
decay rate constants (K1and K2), and the blue curve with the
average rate constant. The circle labeled A represents identical
sequences at the initial time of divergence. The comparison
between T. urartu and durum A genome PSR920 regions (circle
B) was used to estimate K1 (upper red curve) (30). The
comparison between einkorn and durum A genome VRN2
regions (circle C) was used to estimate K2(lower red curve).
Comparison of orthologous intergenic regions between wheat B
genome (AY368673) and D genome (AF497474) GLU1 regions
(circle D) (59). Comparison of orthologous intergenic regions
between wheat (AF459639) and barley (AY013246) VRN1
regions (circle E) (41, 42). [See SOM text for details.]
29 JUNE 2007 VOL 316
on June 28, 2007
conservation observed among orthologous inter-
genic regions in the A, B, and D genomes of
wheat (30, 40) and the complete divergence
observed in comparisons of orthologous regions
between wheat and barley (41, 42) (Fig. 4). To
put the magnitude of this rate into perspective,
indel polymorphisms from both chimpanzee and
human genomes (6- to 7-My divergence time)
equal less than 4% of the intergenic regions
from these genomes (43, 44).
Studies documenting the impact of this re-
markably high rate of DNA replacement on
wheat genes are starting to accumulate. Inser-
tions of repetitive elements within regulatory
regions of the wheat VRN1 and VRN3 vernaliza-
tion genes, as well as four large independent de-
letions within the VRN1 first intron, have been
associated with the elimination of the vernal-
ization requirement (45–48). A deletion upstream
of the PPD-D1 photoperiod gene is associated
with the widely distributed photoperiod in-
sensitive allele (49). Such diversity in genes
regulating flowering time is particularly rele-
vant because of its large impact on wheat adapt-
ability to different environments. Deletions have
also provided increased diversity in wheat
products. Puroindoline A and B gene deletions,
which have become fixed in the A and B ge-
nomes, are responsible for the hard grain
texture of pasta wheat. A polymorphism for a
Puroindoline A deletion (or for a point muta-
tion in Puroindoline B) in the hexaploid wheat
D genome dramatically affects grain hardness,
dividing wheat cultivars into those used for
bread (hard texture) and those used for cookies
and pastries (soft texture) (50). The Puroindo-
line genes code for proteins located in the
surface of the starch grains that facilitate the
separation of intact starch grains during milling
The example in Fig. 3 shows two genes af-
fected by deletions within a small genomic re-
gion, providing an additional example of the
high frequency of gene deletions. Such deletions
are fixed in polyploid wheat with an initial rate
of 1.8 × 10−2locus−1My−1, 10 times faster than
the rate in wheat’s diploid ancestors (51). How-
ever, most deletions are still polymorphic and
represent, together with point mutations, an im-
portant component of genetic diversity in poly-
ploid wheat (52).
Evidence is accumulating that the creation
of artificial allopolyploids can be immediately
followed by reactivation of mobile elements
(53, 54). In one Arabidopsis allotetraploid, these
changes were associated with genomic rearrange-
ments, chromosomal abnormalities, DNA de-
letions (1% of the genome), and pollen sterility
(53). A higher proportion of DNA deletions
(12 to 14%) was found in two wheat artificial
allotetraploids involving different diploid spe-
cies than the ones that produced tetraploid
wheat (55). An association of these deletions
with chromosomal abnormalities would limit
the chances of these diploid combinations to
generate new successful allopolyploid species.
Examination of polymorphisms for gene de-
letions in the D genome of T. aestivum showed
that only 0.17% of the D genome has been
deleted during the past 8500 years and that
deletions are present at low frequencies, suggest-
ing a gradual accumulation of gene deletions
rather than a burst of deletions immediately
after the hexaploid wheat polyploidization
Repetitive DNA can also facilitate gene du-
plication. A study tracing the evolution of a dis-
persed multigene family in wheat showed that
duplication of a gene into the intergenic space
accelerated its subsequent duplication rate 20-
fold (56). Additionally, a promoter supplied by
a neighboring mobile sequence facilitated the
expression of one of the duplicated gene copies
as well as the generation of a new gene (56).
This study suggests that wheat intergenic DNA
facilitates both gene duplication and novel ex-
pression of duplicated genes. Studies in rice and
maize provide extreme examples of mobile re-
petitive elements duplicating gene fragments and,
occasionally, complete genes across the genome
[reviewed by (57)]. The importance of gene du-
plication in wheat is exemplified by the re-
cently isolated wheat VRN2 and GPC1 genes,
both of which likely originated as dispersed
duplications after the wheat-rice divergence
Although more research is needed to refine
our understanding of the specific mechanisms
by which repetitive sequences affect gene con-
tent in wheat, evidence already available indi-
cates that the dynamic nature of wheat repetitive
sequences readily generates new genetic varia-
tion, which may facilitate the success of poly-
ploid wheat as a crop.
Polyploid wheat has been able to compensate
for diversity bottlenecks caused by domestica-
tion and polyploidy by capturing a relatively
large proportion of the variability of its tetra-
ploid wild progenitor. In addition, new variation
is rapidly generated in the dynamic wheat ge-
nomes through gene deletions and insertions
of repetitive elements into coding and regu-
latory gene regions. These mutations can then
be expressed as quantitative gene dosage dif-
ferences because of the polyploid nature of
wheat. Synergy between the high mutation
rates and the buffering effects of polyploidy
makes it possible for polyploid wheat to capi-
talize on the diversity generated by its dynamic
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Supporting Online Material
Materials and Methods
Tables S1 and S2
Domesticated Nature: Shaping
Landscapes and Ecosystems
for Human Welfare
Peter Kareiva,1,2* Sean Watts,2Robert McDonald,3Tim Boucher1
Like all species, humans have exercised their impulse to perpetuate and propagate themselves. In doing
so, we have domesticated landscapes and ecosystems in ways that enhance our food supplies, reduce
exposure to predators and natural dangers, and promote commerce. On average, the net benefits to
humankind of domesticated nature have been positive. We have, of course, made mistakes, causing
unforeseen changes in ecosystem attributes, while leaving few, if any, truly wild places on Earth. Going
into the future, scientists can help humanity to domesticate nature more wisely by quantifying the
tradeoffs among ecosystem services, such as how increasing the provision of one service may decrease
ecosystem resilience and the provision of other services.
Domestication involves the selection of traits
that fundamentally alter wild species to become
more useful to us. For example, wheat has been
selected for larger and more seeds per plant,
hatchery-raised trout are selected for rapid
growth, and dogs have been selected for an abili-
Humans did not, however, stop with simply
domesticating a few chosen species; we have
domesticated vast landscapes and entire ecosys-
tems. Moreover, just as domesticated plants and
animals have predictable and repeatable traits
among different species, domesticated ecosys-
tems also reveal common traits. In particular,
when humans tame nature they seek enhanced
productivity, convenient commerce, and protec-
tion from predators and storms. However, along
with domestication, there is often concurrent
and inadvertent selection for maladaptive fea-
tures in either species or ecosystems. For exam-
ple, selecting for rapid growth in crop plants
may result in plants with reduced investment in
structural and chemical defenses (2). Similarly,
hatchery trout that are selected for rapid growth
often have smaller brains (3). Whereas plant and
animal breeders are well aware that domestica-
omestication of plants and animals may
be the single most important feature of
the human domination of our planet.
tion involves tradeoffs in vigor, the notion of
tradeoffs resulting from the domestication of en-
tire landscapes has only recently received seri-
ous scientific attention.
Conservation has often been framed as the
protecting nature from people. We restate here
what others have already emphasized: There
really is no such thing as nature untainted by
people (4). Instead, ours is a world of nature
domesticated, albeit to varying degrees, from
national parks to high-rise megalopolises. Facing
this reality should change the scientific focus of
environmental science. Instead of recounting
doom-and-gloom statistics, it would be more
fruitful to consider the domestication of nature as
the selection of certain desirable ecosystem
attributes, such as increased food production,
with consequent alteration to other ecosystem
attributes that may not be desirable. Under this
paradigm, our challenge is to understand and
thoughtfully manage the tradeoffs among eco-
system services that result from the inescapable
domestication of nature.
The Global Footprint of Humans
Domesticated nature in its simplest form means
nature exploited and controlled. To that end,
converted to grazed land or cultivated crops (5).
More than half of the world’s forests have been
lost in that land conversion (5). Thewhole notion
of a “virgin rainforest” may be erroneous, with
extensive prehistoric human activity evident in
harvest or elimination. On every continent,
humans have eliminated the largest mammals,
leaving behind a fauna of smaller species (7).
Nature can be dangerous. To protect them-
selves and their domesticated animals, humans
humans suppress wildfires (9). To reduce storm
surges, humans fortify marine shorelines with
jetties and sea walls. In Europe alone, 22,000
km2of the coastline are artificially covered with
concrete or asphalt, and where the coasts are
severely retreating or eroding, over half are
artificially stabilized by jetties or other structures
(10). To control rivers for irrigation, hydropower,
and flood mitigation, humans have built so many
storage as occurs in free-flowing rivers (5).
Humans have so tamed nature that few loca-
tions in the world remain without human influ-
escaped direct influence by humans (4), as indi-
cated by one of the following: human population
density greater than one person/km2; agricultural
land use; towns or cities; access within 15 km of
a road, river, or coastline; or nighttime light de-
tectable by satellite (Fig. 1). The huge magnitude
of human impacts is recent, but the presence of
impacts such as purposeful wildfires goes back
thousands of years (9). The reality of the human
footprint renders discussions about what areas of
the world to set aside as wild and protected areas
as somewhat irrelevant; more germane is a dis-
as a result of the domestication of nature.
The Tradeoffs of Domestication
There is no question that humans have been suc-
cessful in their efforts to avoid predators, produce
food, and create trade, thereby enhancing their
production has kept up with, and even outpaced,
human population growth (11). In South America,
rangelands maintain 10 times as much herbivore
biomass as natural ecosystems (12). This massive
increase in food supply has been achieved by
variety of plants. As of 1999, barley, maize, rice,
mouth lifestyle of preagriculturalhumanshas been
1The Nature Conservancy, 4245 North Fairfax Drive, Suite
100, Arlington, VA 22203, USA.2Environmental Studies
Institute, Santa Clara University, Santa Clara, CA 95053,
Cambridge, MA 02138, USA.
*To whom correspondence should be addressed. E-mail:
3Graduate School of Design, Harvard University,
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