| Main | Site library | Forum | Guestbook |
A poster child for extinction: the thylacine, a prehistoric doglike marsupial once found across much of Australia and Tasmania, was both actively hunted by humans and a victim of competition from human-introduced wild dogs. The last thylacine died in a zoo in 1936.
118
SEVEN
AFTER THE RECOVERY
A New Age?
Prophets who take themselves too seriously end up preaching
to an audience of one. |
There is an often-articulated notion that if there is any consolation in the prospect — or process — of mass extinction, it is that at the end of the tunnel a new fauna emerges. According to this line of reasoning, the great sacrifice of species is a cleansing of the planet, making way for a renewal. The hope is that after the mass extinction is over, a new Age of some sort will dawn — a better, more diverse Age. It is the parable of the Flood: let us call it “Noah and the Recovery Fauna”. After all, this seems to have been the pattern after the two greatest of all mass extinctions, when the dinosaurs took over from the mammal-like reptiles at the end of the Permian, and the mammals from the dinosaurs at the end of the Cretaceous. Could it be that after the current mass extinction, the one group of chordates still waiting for its own “Age” — the birds — will dominate? Will there now be an “Age of Birds”, a world of land-based bird herbivores and carnivores, burrowers and climbers, as well as the numerous (or even more numerous) flying forms that characterize this class today? Or might some completely unforeseen
119
group take over, such as giant insects (biomechanically impossible), or
something totally new? Unless some altogether new class of vertebrates suddenly
appears (highly unlikely), only the birds have yet to hold the honorific
of “ruling” the planet. Perhaps the best bet is indeed an Age of Birds. In
such a world mammals would still be present, even if they are no longer the
evolutionary dominants.
In discussions about the impending biodiversity crisis, this new fauna argument
is sometimes used as a rationalization, even a justification. The Age of
Mammals — and the Age (or even existence) of Humanity — would never have
occurred but for the extinction of the dinosaurs, and in like fashion — or
so the argument goes — the modern extinction will yield some new age of organisms,
perhaps with some new form of global intelligence.
What might this new evolutionary biota be like? Why not something entirely
new? Can we imagine an entirely new type of animal that could replace the
current evolutionary dominants, the large mammals? This new class would have
to have evolved from some currently existing creature, but it could have
characteristics and a body plan vastly different from those of the preceding
dominants. Such a new body type could exploit some entirely new food type
or habitat. Let us imagine such a breakthrough — the conquest of the lower
atmosphere by floating organisms called Zeppelinoids.
After the extinction of most mammals (and humanity), Zeppelinoids evolve
(let’s say from some species of toad, whose large gullet can swell outward
and become a large gasbag). The great breakthrough comes when the toad evolves
a biological mechanism inducing electrolysis of hydrogen from water. Gradually
the toad evolves a way to store this light gas in its gullet, thus producing
a gasbag. Sooner or later small toads are floating off into the sky for short
hops (but longer hops than their ancestors were used to). More refinement
and a set of wings give a modicum of directionality. Legs become tentacles,
trailing down from the now thoroughly flight-adapted creatures, which can
no longer be called toads: they have evolved a new body plan establishing
them as a new class of vertebrates, the Class Zeppelinoida. Like so many
newly evolving creatures, the Zeps rapidly increase in size: when small they
are sitting ducks (flying toads?) for faster-flying predatory birds. Because
their gasbag is not size-limiting, they soon become large. Eventually they
are the largest animals ever to have evolved on Earth, so large that terrestrial
and avian predators no longer threaten them, reaching dimensions greater
than the blue whale. Their only threat comes from lighting strikes, which
result in spectacular, fatal explosions visible for miles. The Zeps can never
get
120
around this inherent flaw, for there is no biological means of producing
the inflammable, inert gas helium and thus avoiding instant death from lightning.
But then, life is never perfect, and the Zeps still do well, especially in
areas with little lightning.
Now the dominant animals of the world, the Zeps float above the ground like
great overgrown jellyfish, snagging with their dragging tentacles the few
species of deer (and other herbivorous vertebrates) still extant and stuffing
them into a Jabba-the-Hutt-sized mouth. Because the Zeps evolved from amphibians
and are still cold-blooded, they have a very low metabolic rate, and thus
need to feed only sparingly. Their design is so successful that they quickly
diverge into many different types. Soon herbivorous forms are common, floating
above the forests, eating the tops of trees, while others evolve into zep-eating
Zeps. Still others become like whales, sieving insects out of the skies;
in so doing they soon drive many bird species to extinction. The world changes
as more and more Zeps prowl the air, floating serenely above the treetops,
filling the skies with their numbers, their shadows dominating the landscape.
It is the Age of Zeppelinoids.
A fairy tale — but there is a glimmer of reality in this fable. Evolution
in the past has produced vast numbers of new species following some new morphological
breakthrough that allows some lucky winner to colonize a previously unexploited
habitat. The first flying organisms, the first swimming organisms, the first
floating organisms, all followed these breakthroughs with huge numbers of
new species quickly radiating from the ancestral body type, all improving
some aspects of design or changing styles to allow variations on the original
theme.
But is the fundamental assumption underlying this scenario — a long period
of extinction followed by the emergence of a new class of evolutionary dominants
— at all likely? No. For just as humanity has changed the “rules” of evolution
that have operated on this planet for hundreds of millions of years, so too
has the usual sequence of events following mass extinction been modified.
Potential Winners of the Future
Picking the evolutionary winners of the future — those species that will evolve to take the place of the “losers” (those going extinct) — is something like trying to pick winners in the stock market, or forecasting the weather. There are some data available to help us make educated guesses, yet the system is so large, and subject to such a plethora of stochastic and chaotic effects, that prediction of specifics is
121
Whatever happens to life on Earth, one thing is certain: evolution will not stop. Here is one possible scenario for the evolution of the rat.
impossible. The colors, habits, and shapes of the newly evolved fauna can
only be guessed at. There is information available that might shed light
on the future winners, however, in the fossil record.
One of the interesting (and rather unexpected) findings of paleontological
research is that higher taxa (the taxonomic categories above genus and species,
such as families, orders, classes, and phyla) seem to show typical rates
of evolution. The rate of evolution for a taxon can be described in two ways:
as the rate at which some morphological character changes through time, or
as the longevity of an average species in geologic time. Related to the rate
of evolution are origination and extinction rates. Some groups of organisms
seem to produce many new species, others very few. And of the species produced,
those of some groups last for long periods of time, while those of other
groups die out more rapidly.
The importance of understanding evolutionary rates was first pointed out
by George Gaylord Simpson, a pioneering evolutionist. More recently, Steven
Stanley of Johns Hopkins University has taken up many of the themes of research
pioneered by Simpson and added fascinating new insights. Stanley’s landmark
122
1979 book Macroevolution explored these themes in detail. Paleontologists
know well the groups that show high origination and extinction rates, for
these are the most important fossils used in biostratigraphy, the science
of subdividing and dating sedimentary rocks using fossils. Good biostratigraphic
markers are those fossils that have a short temporal duration — and thus
occur in only a few strata — yet are at the same time widespread, common,
and have sufficiently distinct morphological attributes that evolutionary
change and new speciation events are immediately apparent. Examples include
trilobites, ammonites, and mammals, among others. Other groups — sometimes
called “living fossils” — show the opposite trends: they speciate slowly,
and once originated, rarely go extinct. They are thus useless for biostratigraphy,
but fascinating evolutionarily — what is it about such organisms that bestows
the equivalent of near-immortality?
A useful way of quantifying evolutionary rates is by arriving at an estimate
of doubling time, the average amount of time for a particular higher taxon
to double the number of species within it. Mammals, for instance, show a
doubling time of
123
3.15 million years. In contrast, bivalve mollusks take 11 million years
to double their number of species. Mammals show rapid evolution and bivalves
very slow evolution. Within both of these groups there is much variation,
including subgroups of rapidly evolving bivalves and slowly evolving mammals.
In general, however, it is clear that mammals evolve faster (and produce
more taxa in an equal amount of time) than do bivalve mollusks.
There is a third group of taxa that can also be recognized in the evolutionary
world. Stanley has proposed the term supertaxa for groups of organisms that
show both high origination rates (they produce many species) and low extinction
rates (their species last a long time). Such groups have a tendency to diversify
rapidly, and in so doing they become prime candidates for refilling the world
with new species following any mass extinction — including the current one.
The title of champion supertaxon in the world today belongs to the family
Colubridae: the snakes. Stanley has suggested that rather than being in an
Age of Mammals, we are really in an Age of Snakes! And as the future of evolution
unfolds, we may find ourselves in a world filled with many new species of
snakes. Another champion “evolver” is the group containing rats and mice,
which, perhaps not coincidentally, are one of the prime food sources of snakes.
This is probably not what most of us have in mind when contemplating some
future world: a world of snakes and rats in untold varieties of form, color,
and habit. Joining them will be other rapidly evolving species, many of which
can be classified as “weeds” in that they are capable of rapid and wide distribution
and are widely tolerant of harsh conditions. Many insect species both evolve
rapidly and are consummate weeds (look at all of the flies in the world).
Birds are also relatively fast evolvers. Each of these groups can be projected
to be very common and proliferate a diversity of new species. Other mammals
evolve slightly more slowly than these groups; in general, the larger the
animal, the slower its evolutionary rate or doubling time.
Let us imagine some of these outcomes. Snakes could move into niches that
they are rare in, or do not completely occupy, today. Many new species of
marine snakes seem possible, as do snakes replacing the many small to medium-sized
mammalian carnivores now dwindling in numbers. As agricultural fields and
cities continue to enlarge in size over the millennia, and even tens of millennia,
rodents will proliferate in a variety of new species to take advantage of
these new feeding opportunities, and this too will provoke further evolution
of new snake species.
Birds and insects are also potential winners. The many species of birds doing
well now in urban and agricultural environments could become the rootstock
of
124
A possible future cladogram, or evolutionary family tree, for the dandelion (bottom to top): original dandelion, cactuslike, aquatic, arboreal, carnivorous, epyphite.
125
One future cladogram for the snake (bottom to top): timber rattler, walking, millipede, pygmy giant, flying, three swimming types.
126
One future cladogram for the crow (bottom to top): crow, vulture, shoe bill, raptor, honeyeating, wading, ratite crow.
127
many new species. Successful forms such as crows, pigeons, and sparrows
might undergo great evolutionary diversification. Of all the birds, crows
seem best adapted for coexisting with humanity, and might be among the most
successful at diversifying into new species — the dominant species of the
new recovery fauna.
The groups mentioned above are all familiar. What about totally new types
of creatures, like the fanciful Zeps — the kind that would make entertaining
television or great fantasy books? Can there be a reasonable expectation
of totally new types of creatures, with novel body plans?
The Cambrian Explosion and the Expectation of Novelty
The history of life, like any history, has occurred as a vector of time.
And as in any history, there is never any going back, at least in any meaningful
way. Events and their history create irrevocable changes that make each slice
of time unique as it passes through the sequence from future to present to
past. In the context of the future of evolution, it appears that there will
never again be an Age of Fishes, or Reptiles, or Mammals even approximately
similar to those that have occurred in our planet’s past. This is a point
that conservationists refuse to accept: the Age of Megamammals is over. There
will never again be an African veldt with the rich assemblage of mammals
now confined to Africa’s game parks, and soon enough there may be no game
parks at all in Africa. Even if we could somehow remove all humans from the
planet in an instant, it is doubtful if things would return to the state
they were in 50,000 years ago, at the onset of the end of the Age of Megamammals.
But leaving aside a return to any past era, if humanity suddenly were removed
from the planet, could we expect to see new body plans? The reality is that
there has been little true evolutionary novelty since the Cambrian Period,
500 million years ago. Although the conquest of land allowed vertebrates
and arthropods — the two most successful terrestrial phyla — to evolve and
explore new themes of shape, these were only modifications of existing body
plans, and even that evolutionary adventure seems far nearer its end than
its beginning. The birds are the last class of vertebrates to have evolved,
and they did so almost 200 million years ago. Yet there seems to be an expectation
that something altogether new will arise. Part of this expectation is raised
by what did happen long ago in the past, when evolutionary novelty was cheap,
during an event called the Cambrian Explosion.
128
One future cladogram for the pig (bottom to top): pig, genetically engineered, rhino pig, aquatic pig, pygmy, giraffelike, garbage-eating.
129
For the first 3.5 billion years of its existence, our planet was without
animal life, and it was without animals large enough to leave a visible fossil
record for another half billion years after that. But when, 550 million years
ago, animals finally burst onto the scene in the oceans, they did so with
a figurative bang in a relatively sudden event known as the Cambrian Explosion.
Over a relatively short time, all of the animal phyla (the large categories
of animal life characterized by unique body plans, such as arthropods, mollusks,
and chordates) that exist today either evolved or first appeared in the fossil
record. Uncontested fossils of animals have never been found in sedimentary
strata more than 600 million years old, no matter where on Earth we look.
Yet the fossils of animals are both diverse and abundant in 500-million-year-old
rocks, and they include representatives of the majority of the animal phyla
still found on Earth. It appears that in a time interval lasting perhaps
20 million years or less, our planet went from a place devoid of animals
that could be seen with the naked eye to a planet teeming with invertebrate
marine life rivaling almost any species on Earth today in size.
The rates of evolutionary innovation and new species formation during the
Cambrian Explosion have never been equaled. It produced both huge numbers
of new species and body plans of complete novelty. That all of the animal
phyla would appear in one single, short burst of diversification is not an
obviously predictable outcome of evolution. From this observation comes the
second finding concerning the Cambrian Explosion that is equally puzzling,
if far less well known: The Cambrian Explosion marked not only the start,
but also the end of evolutionary innovation at the phylum level. Since the
Cambrian, not a single new phylum has evolved. The extraordinary fact is
that the evolution of new animal body plans started and ended during the
Cambrian Period.
The lack of new phyla and the paucity of new classes after the end of the
Cambrian Explosion may simply be an artifact of the fossil record; perhaps
many new higher taxa did evolve, and subsequently went extinct. This seems
unlikely. It is far more likely that the great surge of innovation marking
the Cambrian came to an end as most ecological niches became occupied by
the legions of newly evolved marine invertebrates.
Yet there remains a puzzling mystery: why is it that no new phyla evolved
after the two great mass extinctions, the Permo-Triassic and Cretaceous-Tertiary
disasters? While the Permian mass extinction may have caused the number of
species to plummet to levels as low as those found early in the Cambrian,
the subsequent diversification in the Mesozoic involved the formation of
many new species, but
130
 |
The Cambrian Era saw an astonishing explosion of diverse new body plans.
131
very few higher taxonomic categories. The evolutionary events of the Cambrian
and the Early Triassic were dramatically different: although both produced
a myriad of new species, the Cambrian event resulted in the formation of
many new body plans; the Triassic event resulted only in the formation of
new species that followed already well established body plans.
Two hypotheses have been proposed to explain this difference. The first supposes
that evolutionary novelty comes only when ecological opportunities are truly
large. During the Cambrian, for instance, there were many habitats and resources
that were not occupied or exploited by marine invertebrate animals, and the
great evolutionary burst of new body plans was a response to these opportunities.
This situation was not duplicated after the Permo-Triassic mass extinction.
Even though most species were exterminated, enough survived to fill most
ecological niches. Under this scenario, there was sufficient survival of
animals with various body forms to inhabit most of the various ecological
niches (even if at low diversity or abundance) and in the process block evolutionary
novelty.
The second possibility is that new phyla did not appear after the Permo-Triassic
extinction because the genomes of the survivors had changed sufficiently
since the early Cambrian to inhibit wholesale innovation. Under this scenario,
evolutionary opportunities were available, but evolution was unable to create
radically new designs from the available DNA. This is a sobering hypothesis,
and one not easily discredited, for we have no way of comparing the DNA we
find in living animals with the DNA from the long-extinct forms now preserved
only as rock (movies such as Jurassic Park notwithstanding). It could be
that genomes gradually become encumbered with ever more information — as
they gather more and more genes — and in the process became less susceptible
to critical mutations that could open the box of innovation.
One of the central — and currently controversial — aspects of the Cambrian
Explosion concerns diversity and disparity. Diversity is usually understood
as a measure of the number of species present. Disparity is a measure of
the number of body plans, types, or design forms among those species. The
controversy centers on the wondrous assemblage of fossils found at the Burgess
Shale localities in western Canada, where not only early animals with hard
parts, but also early forms without skeletons, are preserved as smears on
the rocks.
The Burgess Shale has had an enormous impact on our understanding of the
initial diversification of animal life. In large part, it is responsible
for showing us that most or all of the various animal phyla (the major body
plans) originated rela-
132
tively quickly during the Cambrian. But the Burgess Shale may also be telling
us that not only were the body plans found on Earth today around in the Cambrian,
but so too were other body plans that are now extinct. One of the central
messages of Stephen Jay Gould’s book Wonderful Life is that the Cambrian
was not only a time of great origination, but also a time of great extinction.
Gould (and others as well) asserts that there were far more phyla present
in the Cambrian than there are today. How many? Some paleontologists have
speculated that there may have been as many as a hundred different phyla
in the Cambrian, compared with the thirty-five still living today. In observing
this pattern, says Gould, “we may acknowledge a central and surprising fact
of life’s history — marked decrease in disparity followed by an outstanding
increase in diversity within the few surviving designs”.
This view — so forcefully and beautifully described in Gould’s Wonderful
Life — is vigorously disputed in the 1997 book The Crucible of Creation by
British paleontologist Simon Conway Morris, also about the Burgess Shale
and the Cambrian Explosion. Conway Morris is, ironically enough, a central
and sympathetic figure in Gould’s book, which portrays him as one of the
architects of our new understanding of the Cambrian Explosion. But he is
not so sympathetic to Gould. He disputes Gould’s assertion that disparity
has been decreasing since the Cambrian, citing several cases suggesting just
the opposite. Conway Morris also attacks another of Gould’s ideas, the metaphor
of “re-running the tape”. Conway Morris argues that convergent evolution
(in which distinct lineages evolve similarly in response to similar environmental
conditions) can produce the same types of body plans from quite unrelated
evolutionary lineages. He argues that even if the ancestor of the vertebrates
had gone extinct during or soon after the Cambrian, it is likely that some
other lineage would have then evolved a body plan with a backbone, since
this design is optimal for swimming in water.
Simon Conway Morris’s point is that convergent evolution will dominate evolutionary
processes. He even makes what might be the first academic reference to Dougal
Dixon’s book After Man, a semi-whimsical prediction of how animals might
look in the far future at a time when humankind has mysteriously gone extinct.
Conway Morris notes that the animals conjectured by Dixon all seem to resemble
animals living on Earth today, even though they are portrayed as evolving
from quite novel sources:
