Exploring EvolutionMichael Alan Park

Chapter 6

What does the pageant of life

look like?A (very) brief history of life on Earth

About 5 billion years ago, when the expanding universe was two-thirds its present size, our solar system formed around a medium-sized star in the Milky Way galaxy. The third planet from that star formed about 500 million years later and, in less than a billion years —around 3.6 billion years ago (bya)—life showed up.

How this happened is still debated, but it probably wasn’t dif-ficult. A simple laboratory procedure, for example, can easily produce amino acids, the building blocks of proteins, from the

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inorganic compounds of the atmosphere of the early earth. Add almost a billion years and truly countless molecules and their interac-tions, and the odds are that anything that could happen did. The evi-dence for the earliest life is indirect but compelling: fossils of accumu-lations of photosynthesizing single-celled organisms from Greenland, southern Africa, and Australia. There are even fossils of single-celled bacteria-like organisms themselves.

At 2 bya, we find evidence of complex single-celled organisms (those with nuclei and other components), and simple multicellular organisms appear about 1.7 bya. By 1.2 bya, enough free oxygen had accumulated in the atmosphere in the form of ozone (O3) to block

previous page Thermophyllic (heat-loving) bacteria in Yellowstone National Park hotsprings. One hypothesis is that life may have begun in such conditions on the early Earth.

Living stromatolites from Australia, formed when mats of blue-green algae (photosynthesizing cyanobacteria) are covered with sand, silt, and mud which the algae cement down and then grow over.

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Fossil stromatolites from Australia, over 3 billion years old.

Fossil of a chain of cyanobacteria cells from 1 billion year old rock in Australia, photographed using electron microscopy.

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ultraviolet radiation and allow the first creatures, still bacteria-like, to live on land.

All these earliest organisms reproduced asexually, that is, by copying their DNA and splitting, making clones of themselves. As a result, variation was limited to mutations. But about 1 bya, some organisms began to reproduce sexually. Sexual reproduction—com-bining portions of two parental genomes in offspring—increases genetic variation, the raw material of natural selection, and evolution begins to speed up.

About 543 million years ago (mya), complex multicellular organ-isms seem to burst onto the scene. Many possessed shells and other hard exterior parts. We refer to this relatively sudden change as the Cambrian Explosion but there is no agreement as to its cause. We do know, however, that in just a few million years all the major body plans of multicellular animals had evolved, to include the ancestors of the vertebrates.

Artist’s reconstruction of some of the bizarre creatures of the Cambrian. The light-colored fishlike organism at top-left is pikaia, the first fossil chordate, the group that gave rise to the vertebrates, represented today by fishes, amphibians, reptiles, birds, and mammals.

By 470 mya plants and fungi colonized the land. Fish and land animals appear in the fossil record around 425 mya, and insects by 400 mya; by 350 mya some of these insects had evolved wings. About the same time as flying insects appear, so do reptiles. The reptilian forms thought to have given rise to mammals are from around 260 mya.

Dinosaurs, a specialized form of reptile, make their appearance about 235 mya and true mammals show up a little later, about 220 mya. Sometime around 160 mya a group of dinosaurs evolved feath-ers, marking the ancestors of the birds.

Artist’s reconstruction of Anteosaurus, a mammal-like reptile from 266–260 mya. There were many mammal-like reptiles some of which were the remote ancestors of today’s mammals. Many, however, were wiped out in the greatest of all mass extinctions, 250 mya.

Artist’s reconstruction of Anchiornis, a feathered dinosaur from China dated at 160 mya. Knowledge of the feather colors comes from the remains of pigment sacs in the preserved feathers. These could be compared to those of modern birds. The pigment is melanin, the same pigment that gives humans their skin color.

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Flowering plants are fairly late in the story; they appear about 100 mya. The possible precursors of the primates, the group to which we humans belong, may be as old as 80 mya. And with the extinction of all the dinosaurs, and many species of other groups 65 mya—when that asteroid slammed into the earth—the more immediate and recognizable ancestors of extant forms of life are found in the fossil record.

To put this all in perspective it’s useful to compare the history of life on earth to some more conceivable frame of reference. I like astronomer Carl Sagan’s “cosmic calendar” the best. Here’s a cal-endar year with major events—in this case, the history of the whole universe—placed in their relative positions.

The evolving Earth

We have noted direct environmental changes to which evolving species must adapt or perish. Most, of course, eventually perish. And then there are those great punctuations in uniformitarian evolution—many local events, and those five global catastrophes, that profoundly reorganized the earth’s life. But there is another source of environ-mental change, one that is both uniformitarian in its slow and imper-ceptible nature as well as global in its reach. This is continental drift, which operates through the process of plate tectonics.

Simply put, the crust of the earth is not a solid shell. It is divided into some sixteen plates of various sizes, which fit together as in a giant, spherical jigsaw puzzle. The molten rock below the crust is constantly in motion and interacts with the plates, pushing them apart in some locations, pushing them together in others, sliding one plate under another in still other places. As a result, the continents (the locations where the crust protrudes above sea level) change shape and posi-tion. The motion itself is slow—an average of 2 centimeters per year (about the speed with which your fingernails grow)—but imagine how, over tens or hundreds of millions of years, these changes can affect global and local environments and, thus, the evolutionary histories of groups of earth’s organisms.

Not to mention how it affects our task of piecing together those histories. There are dinosaur fossils found in what is now the cold desert of Antarctica. As I write this from my office in Connecticut,

opposite Carl Sagan’s “cosmic calendar” the best. Here’s a calendar year with major events—in this case, the history of the whole universe—placed in their relative positions.

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some of the rock strata under my feet end just to my east and pick up again in what is now Morocco.

So, we must see the pageant of life in a multidimensional way, with lines of influence running in every direction spatially and temporally, a complex and asymmetrical cobweb of relationships, with generaliza-tions that define our theory of evolution but with details idiosyncratic to each case. Let me leave you with three more things to ponder.

Philosophical considerations

The pageant of life is not linear or progressive. Many visual depic-tions of evolution, and many accounts of it (mine above being an example) are necessarily narrative in their format, telling a story of change through time as if there were a relatively simple and linear progression. We used to, in fact, divide life’s history into “ages:” The Age of Reptiles, The Age of Mammals, The Age of Man.

But these ages, in fact, overlap. Mammals have been around almost as long as dinosaurs and didn’t just suddenly appear when the dino-saurs met their demise. Furthermore, all forms of life are always evolving. The bacteria around today are not all just relicts of the past. They have evolved too, as have geraniums and jellyfish and everything else. As we noted above, the overall tree of life is one of increasing diversity, since diversity gives rise to further diversity, with the oldest forms of life still represented in today’s biota. Indeed, if there is any “age” of some form of life on earth, it is the “Age of Bacteria,” and in a way it’s the only “age,” because all life relies on bacteria which are the most common life form on this planet.

“There is no new thing under the sun.” That’s Ecclesiastes 1:9. We speak of origins in evolution: the origin of the earth, the origin of life, the origin of humans. But the only real origin is that of the universe (and that we don’t fully understand). But since the universe began—as best we know, with the famous Big Bang some 13.7 billion years ago—all subsequent events have been rearrangements of what already existed: matter condensed from energy as it cooled; large atomic par-ticles formed from smaller ones; stars came together from cosmic dust; heavy elements were created from lighter ones in the nuclear furnaces of stars; inorganic molecules were rearranged to form the

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organic molecules of life; and the genetic code constantly shuffled to produce the extraordinary multitude of living things on this planet.

History is contingent. Nothing in history—your personal history, the culture history of humans, the evolutionary history of life—is inevita-ble. Each event is contingent upon— dependent upon—all the events that came before. If one event had not happened as it had, the course of subsequent history would be changed. So, history makes sense when we work backward but it can’t be predicted looking forward. Imagine if that asteroid had missed the earth 65 million years ago; or if the continents had drifted in other directions; or if some other chemistry had been behind the self-replicating system we call life. Things would have been different. But how, we can’t know.

A narrative visual of the history of life. Such depictions note the first appearances of new forms of life but don’t—almost necessarily because of the complexity—indicate that the history of life is an incredibly complex tree with many twigs and branches, not all becoming extinct when a new form evolves.

Exploring Evolution is an illustrated introduction to the fascinating and controversial topic of evolutionary theory and its impact on science and the larger society.

Aimed at an educated reader who may know little about the topic and written in a clear, straightforward way, this book will explain the basic prin-ciples of evolutionary theory, the role it has played in the history of science and the reaction of society to the concept of evolution. In the public mind, human evolution and the controversies that surrounded it are seen as almost synonymous with the concept of evolutionary theory, but in fact the basic elements of evolution apply equally to humans, sponges and geraniums.

The author, an anthropologist with a specialty in evolutionary theory, discusses how evolution fits into the way we view the world and our fellow creatures. The book helps to answer questions such as “do all living things continue to evolve?” and “what does it mean when they say that we share 99% of our genes with chimpanzees?”. Michael Park seeks to demystify this topic and to show how the thinking behind evolutionary theory has also been influential in a broader cultural context.

Illustrated with numerous photographs, line drawings and charts, this book will serve as a primer for students or for anyone interested in knowing more about this important area of science

The authorMichael Park is a professor of anthropology at Central Connecticut State University. He lectures widely and has written numerous articles and books for both students and a popular readership including Anthropology: An Intro-duction and Human Antiquity: An Introduction to Physical Anthropology.

ContentsWhat is the Scientific Method?How old is the Earth?What do we need to know about genetics?How do species change through time?How do new species evolve?What does the pageant of life look like?What do we know about human evolution?What about creationism and intelligent design?How has evolution influenced modern society?

Specifications245 x 190 mm (7½ x 9¾ in)192 pages with 120 illustrationsHardbackRecommended retail price: £ 19.95 | € 24.95 | US$ 35.00ISBN 978-1-908126-25-235,000 wordsSeptember 2012

Key features◾ Clear explanations of a complex

subject◾ Beautiful illustrations illuminate the

discussion◾ Engagingly written for all ages

www.vivays-publishing.com

Exploring EvolutionMichael Alan Park