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The Earth's history has been punctuated by important events that determined the course of its climate and life. A handful of those stand out as being especially critical. For instance, the Extinction Event at the close of the Cretaceous Period forever changed the course of life on Earth. That event extinguished the dominant dinosaurs and gave the mammals their chance to evolve and proliferate. The Extinction Event at the close of the Permian was bigger, but after several million years life went on more-or-less as it had before. Many players were gone from the scene after the Permian, but the principals were still there; the only change was that the Syapsids (us mammals) had lost their dominance to the Diapsids (dinosaurs).
The chart below is an attempt to compile a list of the events that were most important to life at the present time. (There goes that anthropcentric bias again; the rodents might have a quite different view.) The chart should be read from the bottom up, just like the timelines. If there is one thought that you should be left with after browsing this chart, it is "Hurray for Evolution!"
It may be obvious that there is an imperfect correspondance between this chart and the previous one laying out the geological ages. Some events that we consider important just didn't leave important markers in the geological record.
| Time | Event | |
|---|---|---|
| 15 | Holocene 15 to 12 Ka |
Homo Sapiens Takes Over the Earth. Many geologists and climatologists have argued that the Holocene should not be considered a special epoch, but only a relatively warm interval in the glacial Pleistocene. But there is something that makes the Holocene special: the presence of man. Man is important because he has spread over a range only matched by his companion, the Dog; and because his impact on the environment is far greater than any other animal in the entire history of the Earth. Without Man, the Pleistocene might be be wrapping up another interglacial cycle and getting ready for another glacial episode. That may not happen, because Man's activities are causing a rapid warming of the global climate. If, as seems likely, the warming continues, the Earth may find itself back in a climate similar to the early Miocene—before the whole Ice Age fuss started. |
| 14 | Pleistocene 2.5Ma |
Closure of the Isthmus of Panama The last piece of the puzzle leading to the ice ages was the closure of the gap between the North American and South American continents. The oceanic circulation was radically altered when all the east-to-west circuits were closed, except for a narrow passage around Antarctica). The oceanic circulation thereby acquired a strong north-south component. The atmospheric circulation was also diverted by long chains of north-south oriented mountains. The result was that the Earth's climate became more seasonal, and it became possible for ice to accumulate in the polar regions during the cold winters. This was probably the principal fact that made the Pleistocene ice ages possible; with a frequency and duration regulated by variations in the Earth's orbit. There have been many ice ages in the past; but they have often been separated by warm periods lasting millions of years, such as the long Jurassic and Cretaceous periods. Ice ages have usually required a special configuration of the continents: usually a large continental mass near one of the poles and a large land mass to block the east-west circulation. |
| 13 | Miocene 15 Ma |
The Formation of Grasslands. Our bias toward animals may keep us from seeing that several
of the major revolutions of life on Earth were achieved by the
plants. The rise of flowering plants eventually (the dates
are very uncertain) produced the most remarkable, most useful
flowering plant of all: grass. Other plants develop protection
against being eaten; but grass loves to be eaten. Its mode of
growth may actually encourage growth when grass is cropped. This
resulted in a wonderful cooperation between grass and browsing animals,
who found easy food, and in exchange, spread the seeds to almost every
part of the Earth. The formation of grasslands was a result
of the changing climate, and the drying of large regions during the Miocene.
It has been suggested by several paleontologists that the development of large brains was encouraged by the new open environment, where good eyesight, quick reactions, and rapid flight became important defense and offense strategies. When your principal defense is to remain difficult to find in dense forests, you don't have to be very intelligent. Environmental factors must have been crucial to the evolution of large brains in the horses, which are completely adapted to a mobile life in open country. Moreover, grasslands did not require evolutionary extremes just to reach sources of food; grassland animals could devote more of their evolutionary energy to senses, brains, and agility. Grasslands today are the source of most animal protein; and support a greater density of large animals than any other environment—perhaps even greater than the vast Jurassic plains. The relative biomass of a shrubby plain filled with dinosaurs compared with a grassland filled with bison is a research problem that has not yet been satisfactorily answered. My personal guess is that the grass and bison win by a factor of 10. |
| 12 | Oligocene / Miocene 35 to 20 Ma |
The Formation of Modern Continents and Mountains. The Mesozoic was warm, apparently because the continents were assembled into a single Pangea, and the oceanic circulation did not mix polar and equatorial waters. The breakup of the continents opened new north-to-south seas, particularly the Atlantic, and caused the rise of high mountain ranges. The atmospheric circulation was strongly affected by the formation of the American Cordillera, which runs from the far north almost to Antarctica. The deflection of air by the mountains caused the weather to become more seasonal, bringing cold winters as the polar air was deflected far to the south through the action of the coriolus force. Though the summers probably remained warm, the year-around climate became gradually cooler throughout the Oligocene. At the same time Antarctica became covered with permanent ice. The changes in the atmospheric circulation caused some areas to beome much drier, especially in the lee of the mountains. The climate was ameliorated a bit when the Drake Passage opened between South America and Antartica, but the Earth would never again experience climates as warm as the Mesozoic and early Cenozoic. The stage was being set for the onset of ice ages. |
| 11 | Cretaceous 65 Ma |
The Extinction of the Dinosaurs. The Dinosaurs had been among the greatest success stories of
evolutionary history. But suddenly, at the end of the Cretaceous,
most of them disappeared, leaving only their smallest representatives:
the birds. It seems a bit strange that the asteroid collision
that wiped out the Dinosaurs should have extinguished all of them,
and left the land occupied only by a few lizards, turtles, crocodiles,
and birds. That may be indirect evidence that the dinosaurs had
evolved a warm-blooded way of life, and couldn't survive long without
food. In the absence of cold winters during the Jurassic and
Cretaceous, the dinosaurs apparently never had to learn to
hibernate. Whatever happened, the sudden disappearance of all
the large predators opened up evolutionary possibilities for the
mammals, who quickly increased in size and expanded into most
environmental niches.
It was once believed that the dinosaurs were headed definitely toward extinction as a result of their supposed stupidity. There is a growing body of evidence that some dinosaurs not only had well developed senses, such as sight, but also were becoming at least as clever as the furry creatures scurrying about at their feet. In fact, some modern dinosaurs, the birds, may be as intelligent as mammals of comparable size. For anyone who doubts the intelligence of birds, I suggest watching hummingbirds. Here are creatures with brains the size of a grain of rice, who have well developed personalities and an astounding reasoning ability. |
| 10 | Cretaceous 100 Ma |
The Perfection of Flowering Plants. The early forms of plants spent a lot of evolutionary energy developing defenses against being eaten. The animals at the same time developed ever better ways of reaching and ingesting the plants. This led to grotesque specializations to reach the tender tops of trees, like the tall Sauropod Dinosaurs. The flowering plants developed ways of actually encouraging cooperation with animals; at first by relying on insects and perhaps birds for pollination. The angioisperm reproductive apparatus of the flowering plants proved so efficient, that they quickly came to dominate the forests and shrubs of the late Cretaceous. Some of the adaptations of the late Dinosaurs may be related to the increasing availability of fast-growing flowering plants. |
| 9 | Permian 251 Ma |
The Mother of all Extinction Events.
The end of the Permian period was marked by an
extinction event that may have been the biggest of all. Most
of the creatures on Earth were wiped out, and the evolutionary
clock was reset. During the Permian the Synapsids, of which
we and other mammals are the living examples, attained a high
degree of development. They acquired features far in advance
of their more reptilian cousins, such as complex teeth to manage
all kinds of food and anatomies that permitted rapid movement.
The Permian was a time of unstable climate change, including
massive ice ages; so it has been speculated that the Synapsids
may have evolved mechansims for dealing with extreme climates,
such as warm blood and fur.
The advanced Synapsids were almost totally wiped out at the end of the Permian. A few small forms managed to escape, to spend the next 186 million years scurring about, hiding from larger reptiles and evolving into mammals. A bunch of Diapsids quickly became dominant, and evolved into dinosaurs within the next 20 million years. The first true mammals appeared about the same time as the first dinosaurs; but they and other Synapsids had lost their dominance. |
| 8 | Carboniferous About 300 Ma |
Radiation of the Forms of Reptiles. The late Carboniferous was a time of rapid changes in the Earth and its climate, which drove the evolution of several types of land animals. The reptiles diverged into the Anapsids—which are represented today by the turtles, the Diapsids—which include the Dinosaurs and all living reptiles, and the Syanpsids—of which we, the mammals, are the sole survivors. The distinctions have to do mainly with the arrangement of the bones of the skull. The Diapsid skull permits strong jaws and lots of undifferentiated teeth; so they concentrated on evolving efficient locomotion and efficient ways of shovelling down huge amounts of unchewed food. The Synapsids altered their jaws in ways that allowed them to develop specialized teeth; so they, and their descendents, concentrated on developing especially efficient chewing apparatus to cope with almost any kind of food. |
| 7 | Devonian Sometime after 450Ma |
Animals Move Out of the Sea, onto the Land. The move from sea to land was not especially difficult for plants, which only had to slightly modify their ways of breathing and getting moisture and nutrients. For animals that had to find ways of moving about, the move to the land presented more perplexing challenges. Their external skeletons were an advantage for arthropods, which quickly found ways to brace their bodies against gravity. For vertebrates gravity provided a greater challenge; and the first of them had little support between the rear and fore limbs. But the vertebrates eventually made the transition, and found many new potential habitats on the land. The arthropods quickly filled the niches that are available to them today; while the vertebrates spent the next 50 million years becomming bigger and faster. |
| 6 | Ordovician About 500 Ma |
The First Fishes. Many
animals have developed skeletons of some form. Some of these
are so rigid they leave their owners unable to move easily from one
place to another. The Crustaceans have external skeletons
that are very effective for small creatures; but severely limit the
range of possible sizes and habitats. The early fishes did not
have a bony skeleton, but they had a notochord and primitive skeleton
that permitted them to grow relatively large, and to invent very
efficient propulsion mechanisms. It was the ability to go
almost anywhere and adapt to almost every ecological niche that
led to the successive developments of vertebrate animals.
Evolution may have been given a boost during the Ordovician by several major extinction events. |
| 5 | Cambrian 550 Ma |
The Cambrian Explosion. Something happened just before the Cambrian—perhaps it
was the "Snowball Earth"—to drive a rapid proliferation of new
species of complex plants and animals. Almost all of the modern
phyla, plus some others, appeared in the early Cambrian. The seas
rapidly filled with life, all trying to eat each other.
Eighteenth and nineteenth century geologists had an easy time distinguishing the Cambrian from everything that preceded it because animals (and some plants) developed hard protective structures that are easily preserved as fossils. In the late Cambrian we find all kinds of shells and hard external and internal skeletons. We now know that there was a rapid "explosion" of complex life forms in the early Cambrian; and many of those life forms did not leave many traces for early geologists. |
| 4 | Mesoproterozoic Before 1000 Ma |
The First Metazoans. All sorts of possibilities are opened with the first organisms made up of multiple, differentiated cells. This allows some cells to specialize in, for instance, energy production; others to specialize in locomotion or defense. Evidence for colonies of undifferentiated cells goes back almost to the first signs of life; but all complex organisms are Metazoans. One advantage that Metazoans have is in being able to eat other Metazoans, rather than looking for other, more specialized sources of energy. |
| 3 | Paleoroterozoic About 2.1 Ga |
The First Eukaryotes. This is the first appearance of Us; we are the result of a pairing of several kinds of cells—each with its own DNA—to form a complex new kind of cell. The Eukaryotic cell had many advantages, both in the way their DNA is organized and in the ways they use energy. Sexual reproduction became possible with the Eukaryotes, and probably arose about the same time. It was sex that made possible the "scrambling" of chromosomes and rapid evolution. |
| 2 | Archaean About 3.5 Ga |
The First Life. Give or take a few hundred million years, the first undeniable signs of life on Earth appeared sometime about 3.5 Billion years ago. These were probably the most primitive forms of Archaeobacteria or Eubacteria. |
| 1 | Archaean About 4.5 Ga |
Formation of the Earth. The Earth, the Sun, and the rest of the Solar System formed about the same time—almost ten billion years after the formation of the Universe. The early Earth remained quite inhospitable: hot, dry, and bombarded by other large objects for at many millions of years. Eventually on the larger planets, the lighter water and gases separated from the rocks to form oceans and atmospheres. Some of the planets lost their atmospheres and oceans; only Earth retained an atmosphere and ocean barely hospitable for life. |
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