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The transformation of domestic plants and animals from age-old selective breeding to todays genetic engineering has had an enormous impact on all living things on the planet.
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One of the most remarkable features in our domesticated
races is that we see in them adaptation, not indeed to the animals
or plants own good, but to mans use or fancy. |
Sitting high atop a hill with an unbroken view to the west, Seattles Harborview
Hospital enjoys a commanding vista of the vast city sprawling along the shores
of the inland waterway known as Puget Sound. On clear days the rugged Olympic
Mountains float above the far western horizon, seemingly trying to snare
the setting sun with their jagged rocky tops, while to the south Mt. Rainier
looms like some upstart Baskin Robbins flavor of the month. Perhaps no other
public hospital in the world enjoys such a magnificent setting.
Such bucolic pleasures are largely lost on Harborviews clientele, however.
Most who come here are beyond caring about such things, for those passing
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through these doors usually do so only as a last resort. And they do not
come alone. The transients, drug addicts, gunshot victims, and uninsured
who make up a large percentage of Harborviews patients often bring with
them exotic new strains of microbes, organisms that are assuredly only
recently evolved.
Beginning in the middle part of the twentieth century, scientists and doctors
waged a campaign of eradication against bacterial illnesses, using the then
newly developed antibiotic drugs. The result was a mass extinction of bacteria
a concentrated interval of death resulting in the loss of uncountable individual
microbes, and for all intents and purposes, the extinction of whole species.
Smallpox, rabies, typhoid, rubella, cholera: the ancient microbial scourges
of humankind were wiped out. The bacteria causing these ancient plagues were
faced with two alternatives: evolve or die. Most died. But a few evolved
forms resistant to antibiotic drugs. Fifty years after their invention, these
miracle drugs have unleashed a diversity of new drug-resistant species
that would never have evolved but for the influence of humanity. These are
surely but the beginning of a host of new microbial species.
And so too with the biosphere, except that the antibiotic is us. As a result
of our antibiotic influence, and the current pulse of extinction it has engendered,
many currently living species will die. Some, however, will survive and thrive,
becoming the rootstock of a new biota. Some have already done so, for one
of the precepts of this book is that significant portions of the recovery
fauna that follows any mass extinction are already with us, and dominating
terrestrial habitats, in the form of domesticated plants and animals. Evolution
obviously continues, but much of it is now directed for human purposes,
or occurs as a by-product of human activities.
Charles Darwin began his On the Origin of Species with a chapter on domestication.
Before introducing any other data or argument, he pointed to the many varieties
of domesticated animals and plants as one of the clearest proofs of the
existence of organic evolution in this case, the evolution of new types
of animals
and plants bred to serve as food or as companions to humanity.
As with most of his conclusions, Darwin was right about this point. But
we can take it one step further. Domesticated animals and plants are the
dominant
members of what can be called the recovery fauna accruing, directly or
indirectly,
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from the extinction of megamammals (and that extinctions impetus toward the development of agriculture). That many of these animals are taking the functional place of extinct or endangered megamammal species is no accident. The cows, pigs, sheep, horses, and other familiar domesticated animals now covering the worlds grasslands rapidly replaced the many species of extinct or endangered large wild grazers. The stimulus to that evolutionary changeover has, of course, been the stern hand of humanity.
Although most animal species can be tamed, or to some extent habituated
to the presence of humans if raised by them from a young age, domestication
goes far beyond this simple behavior modification. Domesticating a species
requires not only a concerted effort over extended periods of time, but certain
pre-existing features of the species in question. In the past this effort
was made only for reasons such as enhanced food yield, transportation, or
protection from predators. Domesticated animals are the evolutionary results
of human-induced unnatural selection.
Very few large mammals have been domesticated. Biologist Jared Diamond of
UCLA showed that of the 150 species of terrestrial, noncarnivorous mammals
larger than about 30 kilograms, only 14 have been domesticated. All but one
of these originated in the Eurasian region; the only New World exception
to this rule is the llama. All of the domesticated species are derived from
wild species with similar characteristics: all seem to show rapid growth
to maturity, an ability to breed in captivity, little tendency to panic when
startled, an amenable and tractable disposition, and a social structure and
hierarchy that permits domestication. All of these characteristics were further
selected for by a brutal form of natural selection: those individuals that
showed the favored characters were allowed to breed; those that did not were
killed.
Interestingly, neurologist Terry Deacon has noted a further characteristic:
all domesticated animals appear to have undergone a loss of intelligence
compared with their wild ancestors. This observation makes one wonder whether
humans, compared with their ancestors, have also been domesticated, and
undergone a similar reduction in intelligence.
The first domesticated species may well have been the dog. All modern dog
species seem to be derived from the Asiatic wolf. Although the first anatomically
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Foremost among the survivors of the next millennium will be those species that humans have had a hand in developing: domesticated plants and animals.
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modern dogs may date back as far as 12,000 years ago, bones of canids recognizable
as dogs first appear around human campsites between 7000 and 6000 B.C.
Even though wolves can still interbreed with dogs, implying that true biological
speciation has not occurred, behavioral differences make such interbreeding
rare; thus the modern dog varieties as we known them are functionally distinct
from wolves. The domestication of the dog also led to distinct anatomical
changes. Compared with wolves, dogs have smaller skulls, shorter jaws,
smaller teeth, and pronounced variation in coat color. Dogs also appear
to be less intelligent than wolves. Most dog varieties now recognizable
were produced in the eighteenth and nineteenth centuries; prior to that,
dogs generally were used for hunting (hounds) or tending flocks (sheepdogs).
The bones of domesticated livestock first appear in the fossil record slightly
later than those of dogs. Sheep and goats came first; the earliest evidence
for their domestication, dated at around 8000 B.C., comes from various sites
in southwestern Asia (the areas now composed of Israel, Iran, Jordan, and
Syria). Cattle were derived from an entirely extinct species of wild cowlike
creatures. Domesticated pigs also date back to about 8000 B.C. Four thousand
years later, the domesticated horse was developed from wild horses in Eastern
Europe. (The ancestor of the domestic horse, called Przewalskis horse, still
exists, in small numbers, in reserves in Poland.) Donkeys, water buffaloes,
and llamas were domesticated at about the same time, while chickens and camels
were not brought into the menagerie until about 2500 B.C. A wide variety
of smaller pets have been domesticated as well: house cats, guinea pigs,
rabbits, white rats, hamsters, and various birds. All are the result of human
effort.
In almost every case, the transformation of a wild species into a domestic
species involves substantial physical and behavioral modification. It has
long been thought that this process occurred in stages, beginning with taming
and progressing to gradual genetic transformation. How such great genetic
change was forced in such a short period of time has always been a puzzle.
New research by geneticists may have provided an answer. There appears to
be a master gene controlling a complex of genes that in turn affect tameness,
reaction to stress, coat color, facial morphology, and social interactions.
By changing a single gene rather than an entire complex, domestication could
proceed relatively quickly.
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While it may be argued that the near coincidence of the beginning of agriculture
and the end of the Age of Megamammals is just that coincidence a strong
argument also can be made for cause and effect. The extinction of many of
the larger animals upon which humans depended for food, coming as it did
with a major climate change (affecting plant and small animal resources also
used for food) may not have been mere chance.
While cereals such as wild wheat and barley were harvested as much as 12,000
years ago, it seems that the first domestication of plants took place about
10,000 years ago, at the time when the last mammoths, mastodons, and many
other larger animal species were dying out in North America and had just
disappeared in Europe and Asia. This was the time when food-gathering peoples
began to collect the seeds of wild plants and replant them in the ground.
The domestication process appears to have involved the natural hybridization
of several wild species, followed by selection by humans for desired characteristics.
Thus domestication of plants, like that of animals, involved the genetic
modification of the wild species through a very rough form of natural selection:
those plants with usable traits were kept; those without were killed. Since
the trend in plant modification has been toward an increase in the size of
the edible or usable parts, most plant species have lost the ability to disperse
widely, and protective mechanisms such as thorns have generally been lost
as well.
The number of domesticated plant species is relatively quite small. There
are more than two hundred thousand species of angiosperms, or flowering plants,
yet only ten of these provide the vast majority of human food. Among these
ten are grasses and cereals such as wheat, rice, and maize, which are all
characterized by seeds rich in starch and protein. Cereals are planted on
70% of the worlds cropland and produce about 50% of the calories consumed
by humanity. Other plants in the top ten include sugarcane, yams, potatoes,
bananas, soybeans, and manioc. Worldwide, about three thousand species of
plants are used as human food, but only about two hundred of these have become
domesticated.
The genetic engineering our ancestors used to introduce new characters into their agricultural crops and domestic animals was crude but effective: save the favored
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varieties and let them breed; kill off the others. But in the twentieth century
a new type of genetic engineering appeared one that alters the genomes
themselves. This new way of introducing novelty is sweeping the agricultural
regions of the Earth, and will surely have unintended consequences. It
may be that the transgenic revolution will bring novelty into the biotic
world in ways almost unimaginable and not all of them desirable. It appears,
for example, to be on the verge of creating superweeds.
Modern genetic technology allows the transfer of genetic material from one
species to another. This new genetic information is permanently integrated
into the genome of the second species, conferring new traits upon it. For
all intents and purposes, a new type of organism is let loose into the biosphere
each time this is done. The organism thus transformed is called a transgenic plant, animal, or microbe. These transgenic creatures have not arisen through
the natural processes of evolution, but they are among the most portentous
developments for the future of evolution on this planet.
Transgenic organisms are possible because of the existence of certain genes
capable of jumping from one chromosome to another. The first discovery
of jumping genes was made by American geneticist Barbara McClintock in the
1940s. McClintock was studying the genetics of maize (corn), and observed
that certain genes, such as those responsible for seed color, were capable
of moving from one chromosome to another. The significance of this discovery
was largely overlooked until the 1970s, when it was independently rediscovered
by other researchers examining the ways in which certain bacteria develop
resistance to antibiotics. The genes, or sections of DNA, coding for these
specific characters in bacteria do not actually jump; instead, they produce
copies of themselves, which are inserted at other points either on chromosomes
or in genetic code-carrying organelles called plasmids.
The discovery of these jumping genes, technically called transposons, unleashed
a torrent of research in the 1980s and into the 1990s. These peculiar strings
of DNA have the ability to repeatedly cut and paste themselves into different
parts of an organisms genetic code. What made them famous and may eventually
make them infamous is that the transposons of one organism can be used
to paste new genetic information into the DNA of entirely unrelated organisms.
Much of the research using transposons was conducted on fruit flies. The
fruit fly Drosophila is one of the stalwarts of experimental genetics, since
it breeds
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quickly and its genetic code is well known. In the early 1980s Gerald Rubin
and Alan Spradling of the Carnegie Institute discovered a transposon in
Drosophila that could be used to incorporate new genetic information into
these flies. They succeeded in changing the genetic code of the transposon
and re-inserting it into the fly. With this operation, they had succeeded
in creating a fly with a new genetic code that could be passed on to a
succeeding generation. They had created a transgenic species one entirely
new to the world, a species with a genetic code created not by nature,
but by science.
These early experiments altered very little in the new fly. The majority
of its genetic code was the same as that of the unaltered species. But certain
characters, such as color or eye type, could be changed. Further work showed
that certain fruit fly transposons were not only useful in changing the genetic
code in fruit flies, but could be placed into entirely unrelated species.
A method had been developed that allowed true genetic engineering of insects.
The goals of genetic alteration of insects are laudable. Insects create havoc
in human society in two ways: they serve as vectors of diseases (e.g., malaria,
yellow fever, and some types of sleeping sickness), and they consume a large
proportion of human crops. Genetic engineering is attempting to mitigate
both of these problems. Nevertheless, the results of this work have been
slow. In certain disease-spreading mosquitoes, geneticists have so far been
able to change eye color, but they have yet to alter the structures involved
in spreading disease-causing microbes. To further aid this process, geneticists
have infected the target insects with viruses that act like transposons.
The virus, once in the body of the insect host, can alter the way a disease
is passed on. Some crop pests, such as the Mediterranean fruit fly and the
screwworm, have been successfully targeted using transgenic or other genetic
techniques (e.g., producing sterile members of a species that spread among
the viable members of the population).
While transgenic techniques are just coming into use in controlling insect
pests, such tools are already used widely in crop plants. Genetic engineering
has succeeded in adding new genes to the DNA of various crops, whereas conventional
breeding only adds variants to an already existing genetic complex. So, while
domestication has enhanced the valued characteristics of many plant species,
transgenic research has added entirely new characteristics, such as greater
tolerance of heat and drought, greater resistance to insect predation and
disease, and greater yield.
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The genetic engineering techniques used in agriculture are widespread and
sophisticated. Genetic engineers can move genes from virtually any biological
source into crop species. Genes have been added to engineered crops from
organisms as diverse as chickens, hamsters, fireflies, and fish, as well
as from a slew of plant and microbial species. The new transgenic plants
are genuinely novel organisms, some of them containing the genetic codes
of plants, animals, and microbes in a single species.
The addition of new genes to various plant species has yielded spectacular
dividends in terms of crop yield. But this new technology also poses substantial
risks and clearly will affect the future of evolution on this planet. The
creation of new plant types could affect the biosphere in various ways. The
most important of these is the possibility that newly inserted genes will
jump to other, nonengineered species (such as weeds) or move out of the agricultural
fields altogether into wild native plant populations. It is this potential
intermixing of new genes with those of already established plant species
beyond the boundaries envisioned or designed by agricultural scientists that
could have the most interesting and potentially biosphere-altering effects.
Under rare circumstances, if new traits of transgenic species escape into
the wild, weeds adaptively superior to native plants could be created. Since
most of the traits being transferred into transgenic crops, such as hardiness,
resistance to pests, and growth rate, increase their fitness relative to
the original species, there is great potential for transgenics to become,
or to help produce, new weedy species.
There are several avenues by which the genes of transgenic crops can become
established in the wild. First and simplest, the transgenic crop itself can
escape and become a weed. Second, the transgenic crop can release pollen
into the wild, which can be incorporated into a wild relative of the original
transgenic host. The incorporation of the new genes into the wild plant creates
a new transgenic weed.
Weeds have many definitions, which are often colored by human values. In
agriculture, they are plants that occur in the wrong place at the wrong time
(some plants are weeds in certain situations and favored crops in others
lawn grass, for example). A more human-specific definition of a weed is
any plant that is objectionable to or interferes with the activities or welfare
of humans. Nevertheless, weedy species have a number of characteristics:
Their seeds germinate in many environments
Their seeds remain viable for long periods of time
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They grow rapidly
Their pollen is usually carried by nonspecialized pollinators or by the wind
They produce large numbers of seedsThey produce seeds under a wide range of environmental circumstances
They usually show vigorous vegetative reproduction or regeneration from fragmentsThey often compete either by choking out other plants or by producing toxic chemicals deleterious to other plants
From this list, we can see that the characters of weeds are also highly
desirable in crop plants. One goal of transgenic technology, therefore, has
been to confer characteristics of weeds on crop species. Transgenes spliced
into crops may alter traits such as seed germination ability, seed dormancy,
or tolerance of either biotic or abiotic factors such as pests, drought,
heat, or disease, creating a more persistent or resistant species in the
process. Such new traits may enhance the new crops ability to invade other
habitats. Genes affecting seedling growth rates, root growth rates, and drought
tolerance are currently being developed.
Jane Rissler and Margaret Mellon of the Union of Concerned Scientists have
studied the ecological risks of transgenic crops in thorough detail. One
of their most important concerns relates to the transformation of nonweedy
crop species into weeds through genetic engineering. They note that a widely
held notion is that changing a non-weed into a weed involves the conversion
of many genes, not just the two or three currently used in transgenic crops.
Changing corn from a crop to a weed, for example, would involve a number
of genetic changes, since corn is one of the most human-dependent (and thus
intolerant) plant species on Earth. Other crops, however, already possess
many weedy traits, and thus one to three new traits could indeed create a
new weedy species. Examples include alfalfa and other forages, barley, lettuce,
rice, blackberries, radishes, raspberries, and sunflowers.
Transgenic species may produce secondary effects as well. The invasion of
transgenic plants into new habitats affects not only the invaded plant populations,
but the entire ecosystem, including the suite of animals living within that
ecosystem. Perhaps more dangerous than the conversion of crop plants into
weeds is the escape and transference of new genes into already existing weeds,
making them superweeds. The transfer of disease resistance or pest resistance
to established
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weedy species may change a familiar weed into an even more formidable pest.
Weeds that have developed resistance to herbicides because of escaped or
transferred transgenes are already appearing in some parts of the world.
Agribusiness, which counts transgenic technology as the jewel in its technological
crown, is in the business of feeding the worlds humans and making profits
in doing so, of course. One of the great fears of those producing transgenic
crops is that farmers will simply take the seeds from the first crop and
use them ever after, and will not need to buy new seeds from the corporation
that produced them in the first place. Like the software industry, which
fears the copying of its products above all else, the major biotechnology
companies dealing in transgenic crops have been searching for some way to
stem the illegal use of their products after the first purchase. The solution
is something known as the terminator gene.
The first terminator gene was produced by the large American biotechnology
firm Monsanto, and was engineered to protect Monsantos patent rights on
several types of transgenic crops. It is a genetic modification that prevents
seeds from germinating after the season in which they were sold. In the works
are genes that will allow but a single crop and not produce future seeds
a bit like the seedless watermelon, but more efficient.
The great fear is that such terminator genes will jump to unmodified varieties
of crop plants. If the terminator gene in a tomato plant jumped to other
varieties of tomatoes, there a is real potential of plants never producing
the crop they were intended to produce.
Our species has learned how to circumvent the normal rules of evolutionary change: we have learned how to build new species. Have we also achieved the ability to alter those rules? Norman Myers of Oxford asks this question in his prescient and disturbing 1998 paper, The Biodiversity Crisis and the Future of Evolution. Myers makes a subtle but important point: humans pose pronounced threats to certain basic processes of evolution such as natural selection, speciation, and origination. Myers has cried wolf before, but the wolf was always there, dining happily on flocks of the worlds species. Is he being alarmist in this case? Although many have warned of a biodiversity crisis, Myers alone warns of an evolution crisis. He bases his conclusion on two perceptions: first, that we have entered a new phase of mass extinction, and second, that the normal recovery
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period from mass extinction will not pertain to this one; in fact, the recovery
will be considerably delayed.
Myers cites three aspects of this particular mass extinction that will affect
its evolutionary outcome (and which make it different from any mass extinction
of the past):
Its onset was extremely fast (compared with those of the past), within a single century or two, and thus there will be scant opportunity for ecosystem reorganization and evolutionary response.
There is currently a higher biodiversity on the planet than at any time in the geologic past, so that if 50% of species are lost, the total number going extinct will be higher than in any mass extinction of the past.
During past mass extinctions, plant species have been largely spared, but that may not be the case in the current mass extinction.
The current mass extinction may be unique not only in what it kills, but
in how its recovery proceeds. In past times the tropical regions of the world
have served as storehouses for recovery. Because they have always held the
greatest diversity of species on the planet, they have long served as evolutionary
powerhouses areas that seem to spawn new species and new types of species
at a higher rate than other parts of the world. Paleontologist David Jablonski
of the University of Chicago has shown that innovation can be related to
geography. Innovation in evolution is the appearance of evolutionary novelty,
and the tropical regions seem to be home to more innovation than other regions.
Yet the tropics are now the sites of the highest densities of humans and,
the greatest human population increases. This pattern may curtail the evolution
not only of new species, but also of new types of species.
The current crisis in biodiversity may also substantially reduce the number
of new species evolving a large body size. Megamammals need very large habitat
areas to survive; it may also be true that they need equally large areas
to speciate. With the reduction of wild habitat, and especially free rangeland,
virtually everywhere on Earth, there may be no way for large mammals and
other vertebrates to produce new species. Therefore, a consequence of human
population growth and habitat disturbance may be not only the extinction
of large mammals, reptiles, and birds, but the inability of new large species
to take their place, simply because the mechanism of speciation for large
body sizes has been derailed by environmental fragmentation.
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A large and vibrant community of conservationists, scientists, politicians, and laypeople are actively engaged in intensive efforts to preserve biodiversity. One of the most important such efforts is habitat preservation. Yet even the most Herculean of efforts will save only patches of habitat in a sea of agricultural fields and spreading human landscapes. As long as humanity rules, it is doubtful that hundreds of thousands of miles of unfenced, unimpeded native habitat will be available to replace the species already lost since the end of the Ice Age. This fact has led Norman Myers to pose the following questions:
Is it satisfactory to safeguard as much of the planetary stock of species as possible, or should greater attention be paid to safeguarding evolutionary processes at risk? This is an entirely new way of looking at the world not in terms of losing species, but in terms of losing pathways of speciation. Perhaps the motto should be save speciation rather then save species.
Of prime importance is the question of biodisparity the number of body types. There could be many species on Earth, but few body types. Is it enough to save a large number of species if we fail to save biodisparity as well?
Should the evolutionary status quo (the current makeup of the Earths biota) be maintained by preserving precise phenotypes of particular species that will enable evolutionary adaptations to persist, thereby leading to new species? For example, should two elephant species be maintained, or should we keep the option of elephant-like species in the distant future?
Is there some minimum number of individuals necessary not just for the survival of a species, but the survival of the potential for future evolution in that species? Should the slow breeders (the megamammals) be given greater attention than, say, the rapidly breeding insects? Are we in a triage situation?
How do we assess the relative importance of endemic taxa as compared with evolutionary fronts such as origination centers and radiation lineages? Myers thinks it far more appropriate to safeguard the potential for origination and radiation than any individual species. Let endemic taxa go.
This last recommendation is heresy by the rules of modern conservation. It has long been argued that endemic centers those regions that contain species found
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nowhere else are among the most important places to save. But Myerss point is that endemic centers exist because they have not produced large numbers of successful species. Endemic centers are often living museums of ancient species that do not have much potential for future evolution.
The vast human enterprise has created a new recovery fauna, and will continue
to provide opportunities for new types of species that possess weedy qualities
and have the ability to exploit the new anthropogenic world. Chief among
these will be those species best preadapted for dealing with humanity: flies,
rats, raccoons, house cats, coyotes, fleas, ticks, crows, pigeons, starlings,
English sparrows, and intestinal parasites, among others. These and our domesticated
vassals will dominate the recovery fauna. Among plants, the equivalents will
be the weeds. According to many seers, this group of new flora and fauna
will be with us for an extended period of time a time span measured in
the millions of years. And if humanity continues to exist and thrive (as
I believe it will), this recovery biota may dominate any new age of organisms
on Earth.
A sense of how long the recovery fauna may last was estimated in a disturbing
paper published in the prestigious journal Nature in the spring of 2000.
The authors, James Kirchner and Ann Weil, posed a question: how quickly does
biodiversity rebound after a mass extinction? How long will the world exist
at a very low biodiversity? The answer, it turned out, was far longer than
anyone had heretofore estimated. By analyzing the fossil record of all recoverable
organisms (compiled by the late Jack Sepkoski of the University of Chicago),
Kirchner and Weil found that fully 10 million years passed by, on average,
before the biodiversity of the world recovered to its pre-extinction values.
Even more surprising than this long lag period between extinction and full
recovery was their finding that it occurred whether the extinction was small
or large. We paleontologists had assumed that the time to recovery would
somehow correlate with the magnitude of the extinction that after a small
extinction, the biosphere would recover quickly, and that it was only after
the greatest of the mass extinctions that a long recovery period was necessary.
But to the surprise of us all, Kirchner and Weil found this not to be the
case 10 million years was necessary even after the smaller extinctions.
They concluded their paper with the following passage:
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Our results suggest that there are intrinsic speed limits that regulate recovery from small extinctions as well as large ones. Thus, todays anthropogenic extinctions are likely to have long lasting effects, whether or not they are comparable in scope to the major mass extinctions. Even if Homo sapiens survives several million more years, it is unlikely that any of our species will see biodiversity recover from todays extinctions.
It appears that our return to a new biota will take a long time after the mass extinction is finished. And what might that new fauna and flora look like? Some predictions can be made and such predictions are the subject of the next chapter.
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vii | ||
Biological Futures Niles Eldredge |
ix | |
PREFACE | xiii | |
INTRODUCTION | The Chronic Argonauts | 1 |
ONE | The Deep Past: A Tale of Two Extinctions |
13 |
TWO | The Near Past: The Beginning of the End of the Age of Megamammals | 37 |
THREE | Into the Present | 47 |
FOUR | Reuniting Gondwanaland |
63 |
FIVE | The Near Future: A New World |
79 |
SIX | The First Ten Million Years: The Recovery Fauna |
103 |
SEVEN | After the Recovery: A New Age? |
119 |
EIGHT | The Future Evolution of Humans |
139 |
NINE | Scenarios of Human Extinction: Will There Be an After Man? |
155 |
TEN | Deep Time, Far Future |
169 |
BIBLIOGRAPHY |
|
177 |
INDEX | 183 |
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