Download complete text Chapter 1: Evolution: An Introduction
One of the best feelings paleontologists can ever have is to realize that they have just found a fossil that will fill an empty space in our understanding of the history of life. Hans Thewissen got to enjoy that feeling one day as he dug a 49-million-year-old fossil out of a hillside in Pakistan. As he picked away the rocks surrounding the bones of a strange mammal, he suddenly realized what he had found: a whale with legs.
There were no whales three billion years ago. There were no mushrooms or trees, either, not to mention people. Paleontologists have scoured ancient rocks for signs of life, and, as far as they can tell, the Earth three billion years ago was home only to single-celled microbes. Some of those tiny organisms were tossed by ocean waves. Others formed slimy films on the seafloor. Others thrived in undersea chambers of boiling water heated by volcanoes. The whales, the mushrooms, the trees, and all the rest came later. That transformation of life occurred through the process known as evolution.
This book is an introduction to evolution, both to the process by which life evolves and to the pattern evolution has produced over life’s history. It is also about how scientists study evolution. When Charles Darwin formulated the theory of evolution in the mid-1800s, the most sophisticated tool he could use was a crude light microscope. Today, scientists can study evolution by analyzing our DNA. They can probe the atoms of ancient rocks to determine the age of fossils. They can use powerful computers to apply new statistical equations to the diversity of life. They can observe evolution unfolding in their laboratories.
Evolution is the foundation of modern biology. But that doesn’t mean that only biologists should know about it. Evolution underlies many of the most important issues society faces. We are witnessing a wave of extinctions the likes of which the Earth may not have seen for millions of years. Doctors are battling rapidly evolving bacteria and viruses. Evolution is also part of the answer to some of the biggest questions we all ask. How did we get here? What does it mean to be human? This book is intended for those who are not planning to be biologists—in other words, most people. It does not deal at length with the mathematics and the advanced experimental techniques evolutionary biologists use. But it does describe the key concepts in evolution with the help of graphs, diagrams, and other illustrations.
What Is Evolution?
The short answer is descent with modification. The patterns of this modification, and the mechanisms by which it unfolds, are what evolutionary biologists study. They have much left to learn—but, at this point in the history of science, there is no doubt that life has evolved and is still evolving. The fundamental principles that make evolution possible are pretty straight-forward. Organisms inherit traits from their ancestors because they receive a molecule called DNA from them (page 87). Cells use DNA as a guide to building biological molecules, and, when organisms reproduce, they make new copies of DNA for their offspring. Living things do not replicate their DNA perfectly, sometimes introducing errors in its sequence. Such errors are referred to as mutations (Chapter 6). A mutation may be lethal; it may be harmless; or it may be beneficial in some way. A beneficial mutation may help an organism to fight off diseases, to thrive in its environment, or to improve its ability to find mates.
Evolution takes place because mutated genes become more or less commonover the course of generations. Many mutations eventually disappear, while others spread widely. Some mutations spread simply by chance. Others spread because they allow organisms to produce more offspring. This nonrandom spread of beneficial genes is known as natural selection. The effect of a mutation depends on more than just the mutation itself. It is also influenced by all the other genes an organism carries. The environment in which an organism lives can also have a huge effect. As a result, the same mutation to the same gene may be devastating in one individual and harmless in another. Depending on the particular circumstances, natural selection may favor a mutation or drive it to oblivion.
These processes are taking place all around us every day, and they have been transforming life ever since it began, some 3.5 billion years ago (Chapter 3). Charles Darwin argued that over such vast stretches of time, natural selection could have produced very complex organs, from the wings of birds to human eyes. Today, the weight of evidence overwhelmingly supports that conclusion (Chapter 8). Changes in organs are not the only adaptations that have emerged through evolution; behavior and even language have evolved (Chapter 14).
To trace the origin of these traits, evolutionary biologists reconstruct the tree of life (Chapter 4). Natural selection and other processes can make populations genetically distinct from one another. Over time, the populations become so different from one another they can be considered separate species (Chapter 9). One way to picture this process is to think of the populations as branches on a tree. When two populations diverge, a branch splits in two. As the branches diverged over and over again over billions of years (and sometimes joined back together), the tree of life emerged.
To reconstruct the tree, evolutionary biologists identify which species are closely related to each other. They do so by comparing anatomical traits and DNA among many different species. Close relatives share more traits inherited from their common ancestor. We humans have a bony skull, for example, as do all other mammals, as well as birds, reptiles, amphibians, and fishes (Chapter 4). We are more closely related to these animals than to animals without a skull, such as earthworms and ladybugs.
Biologists today not only understand what evolution is, but what it is not. Evolution is not a steady progress towards some particular goal. Our apelike ancestors did not evolve big brains because they “needed” them. The conditions in which they lived—searching for food on the African savanna in big social groups—favored genetic changes that led to bigger brains. The long-term process of evolution emerges from the way life works on a generation-to-generation scale.
Biologists also recognize that evolution does not make life perfect. All adaptations have shortcomings. Humans have evolved very large brains, which have allowed us to become nature’s great thinkers (Chapter 14), but those big brains also make childbirth much more dangerous for human mothers than for other female primates. Evolution is imperfect because it does not invent things from scratch: it only modifies what already exists. There are only a limited number of beneficial mutations any particular organism can acquire, and so evolution can only produce new forms under tight constraints. Because mutations can have several different effects at once, evolution also faces trade-offs. This means that evolution, unfortunately, has left us susceptible to many diseases. But it also means that studying evolution can help researchers better understand—and perhaps even treat—those disorders (Chapter 13).
It’s also a mistake to think that evolution produces a peaceful harmony in nature. There are many helpful partnerships in nature, such as the one between flowering plants and the insects that pollinate them (Chapter 11). But the same process by which species adapt to one another can also give rise to what looks to us like cruelty. Predators are exquisitely adapted to finding and killing prey. Parasites can devour their hosts from the inside out. Their adaptations are finely honed, allowing them to manipulate individual molecules within their hosts. Yet parasites and predators are not evil. They are just part of a dynamic balance that is constantly shifting, driving the diversification of life but also leading to extinctions.
The diversity of nature, in other words, is not eternally stable. More than 99 percent of all species that ever existed have become extinct, and, at some points in the Earth’s history, millions of species have disappeared over a relatively short span of time. We may be at the start of another period of mass extinctions, this time caused by our own actions (Chapter 10).