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No Turning Back
Nearly every species that has lived on earth is extinct. The last of the dinosaurs was wiped out after a Mount Everest-sized meteorite slammed into the earth 65 million years ago. The great flying and marine reptiles are no more. Before humans crossed the Bering Land Bridge some 15,000 years ago, North America was populated by mastodons, mammoths, saber-toothed tigers, and cave bears. They too are MIA. The passenger pigeon, once the most numerous bird in North America, is gone forever.
In No Turning Back, renowned naturalist Richard Ellis explores the life and death of animal species, immortalizing creatures that were driven to extinction thousands of years ago and those more recently. He documents those that were brought back from the brink, and most surprisingly, he reveals animals not known to exist until the twentieth century -- an antidote to extinction.
Title: No Turning Back
Author: Richard Ellis
Extinction (Sort of) Explained
Standard textbooks on evolutionary biology and paleontology
hardly mention extinction. Much is said about the origin of
species and the evolution of species once they are formed, but
discussions of extinction are usually limited to casual references
and the enigma of the great mass extinctions. On causes of extinction
we are apt to read, "Species become extinct when population
sizes drop to zero," or "Species die out if they are unable to
adapt to changing conditions." These statements are true, of
course, but are virtually devoid of content.
-- David Raup, 1991
Everybody knows what extinction is. The dictionary defines it as "the act
of extinguishing, or, the fact of becoming extinguished or extinct." (Extinguish
is in turn defined as "to put out a fire, a light, or to bring an end to.")
More to our point, Thain and Hickman's Penguin Dictionary of Biology
(1996) defines extinction as "Termination of a genealogical lineage. Used
most frequently in the context of a species, but applicable also to populations
and to taxa higher than species." Thus the fundamental precept of
extinction is self-evident: a species (or population, genus, or family) is extinct
when its last member has died.
Yet extinction theory is greatly complicated by a number of factors,
among them the inability of biologists and paleontologists to agree on exactly
what a species is. "Alarmingly," noted Purvis, Jones, and Mace (2000),
"there are over 20 species concepts now in common use." For living animals,
we recognize as separate species those that are morphologically
similar but cannot interbreed. The same criterion obviously cannot be applied
to fossils, so paleontologists have to make use of anatomical differences
and similarities -- when there is enough fossil material to make a
In recent times, however, with the introduction of DNA analysis,
what was long believed to be a single species can now be fragmented into two or more. Killer whales, the most widely distributed of all cetaceans,
found from southern polar waters to the Arctic and many places in between,
were once thought to be a single worldwide species -- Orcinus orca.
New observations and analyses have shown that there may be an Antarctic
species that is quite different from its northern counterparts in coloration
and behavior; it has been provisionally named O. glacialis. Where
there were once believed to be six species of balaenopterid whales (blue,
fin, sei, Bryde's, and two species of minke), two more were added as of
2003 (see pp. 25455). The two distinct species of elephant -- African and
Asian -- have been subdivided into three with the recent addition of the
genetically distinct Bornean elephant; there may actually be as many as
six different subspecies. The gorilla may indeed be not one (Gorilla gorilla)
but two closely related species. In a New Scientist article dated November
22, 2003, Bob Holmes and Jeff Hecht wrote, "A new trend is to delineate
species as evolutionarily separate lineages, including separated populations
that are evolving in divergent ways. This has already happened for
albatrosses. There are 13 recognized species, but the IUCN* lists 21 threatened
lineages." On the other hand, it has recently been shown that the animal
known as the red wolf, which was once awarded the species name of
Canis rufus, is not a separate species at all, but a hybrid of the gray wolf
and the coyote.
Even if we are able to plug in an acceptable definition of a species,
however, identifying the moment that it became extinct is much more
problematic. For example, rumored sightings of such animals as the ivorybilled
woodpecker and the Tasmanian tiger continue to circulate, and
while these animals are generally considered extinct, it is impossible to
state unequivocally that a few stragglers may not be found in their often inaccessible
habitats. The 1938 discovery of the coelacanth, thought to have
been extinct for 75 million years, is the paradigmatic case of a rediscovered
"lost" animal, and the unexpected appearance of the previously unknown
megamouth shark in Hawaiian waters in 1975 indicates how difficult it is to
make categorical statements about existence or nonexistence. If it is so dif-
ficult with modern animals, imagine how hard it is to decide from fossil evidence
alone that a particular species became extinct at a particular moment. There are no more trilobites, pterosaurs, or ichthyosaurs, but
when did the last one die?
Extinction is one of the most powerful forces on earth, and one of the
most enigmatic. It affects every species that has ever lived, and has eliminated
most of them. In time, it will eliminate us too. Despite its tremendous
importance, however, nobody is quite sure what it is or how it works.
We know that there have been countless numbers of living things that
have walked, run, crawled, flown, swam, or just remained stationary over
the past 3 billion years, and that the great majority of them are gone; but
beyond that, we know very little. The fossil record not only supports the
all-encompassing theory of evolution, demonstrating conclusively that life
changes over time, but it is also our primary evidence for extinction. Because
so many creatures are no longer viable, extinction can be clearly read
in the fossil record, although the actual evidence of evolution -- "change
over time" -- is only infrequently revealed.
Until the nineteenth century, almost everybodyscientists included -- accepted the traditional Christian view that the Bible was to be taken literally,
and that God had made the sun, the moon, the earth, and the oceans.
He also made all the mammals, birds, alligators, snakes, fishes, and insects,
but his crowning achievement was "to make man in our own image, after
our likeness; and let them have dominion over the fish of the sea, and over
the fowl of the air, and over cattle, and over every creeping thing that
creepeth upon the earth." Even Aristotle believed that the animals had
been divinely arranged in a ladder, with humans confidently perched on
the top rung, the epitome of life.