Life: The Science of Biology (12th edition)

Book Preface

Corals in Hot Water
Coral reefs support the largest diversity of life in the oceans. They provide fisheries and storm protection for about a billion people and are a magnificent source of natural beauty. But coral reefs are endangered. Over the past 25 years about half of the world’s reef-building corals have been destroyed by rising water temperature and other factors. High temperatures disrupt a fascinating aspect of coral biology. Corals are animals, yet most corals have algae (dinoflagellates) growing within their cells. Dino-flagellates use the energy of sunlight to produce carbohydrates. Corals provide a home for the dinoflagellates, which in turn pro-vide nutrients for the corals. When high temperature impairs the dinoflagellates, the corals eject them—a process called bleach-ing. Then, without nutrients from the dinoflagellates, the corals die, unless they can take in new dinoflagellates that are more resistant to the higher temperatures.
Understanding the effects of heat on corals was the motivation of graduate student Rachael Bay and her colleagues working with Professor Steve Palumbi at Stanford University’s Hopkins Marine Station. While studying corals in small back-reef pools in American Samoa, they observed that during low tides some pools reached higher temperatures than others. The research-ers predicted that corals in the warmer pools had mechanisms enabling them to resist bleaching. To test their prediction, the researchers brought corals into the laboratory, subjected them to temperature fluctuations, and showed that the corals from the warm pools were more resistant to bleaching. They also transplanted corals between the different temperature environ-ments in nature. As a result of these experiments, Bay and her colleagues proposed that two different processes contribute to the different abilities of the corals to survive heat stress. First, corals growing in the warmer pools could have genetic traits contributing to heat resistance, and second, individual corals may have the same genetic makeup but differ in their ability to adjust to their environment by changing the expression levels of certain genes. More knowledge of the mechanisms of heat stress and heat resistance in corals could lead to new strategies to decrease their losses as their environments change.

You likely have a sense of what we mean by “life,” but try to define •• are made up of cells. it; it isn’t easy. You can easily designate the things around you as  living or nonliving, but what are the essential differences? We call the living things organisms. In contrast to nonliving things, organisms sustain and renew themselves. The loss of the ability to sustain and renew means the loss of life, and organisms that die become part of the nonliving world. Biology is the scientific study of organisms, both living and after death (e.g., the study of fossils), with the goal of discovering and understanding the underlying unity as well as the amazing diversity of the complex processes that make up life.
Life on our planet is quite diverse (Figure 1.1), yet its many diverse forms share common features. What characteristics do organisms share that distinguish them from the nonliving world? Most organisms:

• are composed of a common set of chemical compounds: main-ly carbohydrates, fatty acids, nucleic acids, and amino acids. use molecules obtained from the environment to synthesize new biological molecules.

• extract energy from the environment and use it to do work.
• regulate their internal environments.
• contain genetic information—genomes—that enables them to develop, maintain themselves, function, and reproduce.

• use a universal molecular code to build proteins from their genomic information.

• exist in populations that evolve over time.
How do you think all organisms came to have these simi-larities? If life had multiple origins, we would not expect to see such striking similarities in chemical composition, cell structure, cell functions, and genetic codes across the living world. Instead, these common characteristics logically lead to the conclusion that all life has a common ancestry, and that the diverse organisms alive today all originated from one life form. Organisms from a separate origin of life—say, on another planet—might be similar in superficial ways to life on Earth, but they would not have the same genetic code, chemical composition, or cellular structures and functions that we see widely shared among living organisms on Earth. All evidence points to a common origin of life on our planet about 4 billion years ago.

Some forms of life may not display all the characteristics we have just listed all of the time. For example, the seed of a desert plant may go for many years without extracting energy from the environment, converting molecules, regulating its internal environment, or reproducing; yet the seed is alive. Viruses present a special case as well. Viruses are not com-posed of cells and cannot carry out physiological functions on their own. Viruses depend on the cells of host organisms to carry out these functions for them. Yet viruses contain genetic information, and their populations evolve over time, as we know from witnessing changes in the flu viruses each flu season. Even though viruses are not independent cellular organisms, their existence depends on cells, and it is highly probable that viruses evolved from cellular life forms. Thus most biologists consider viruses to be a part of life.

As you go through this book, you will explore details of the common characteristics of life, how these characteristics arose, and how they work together so that organisms survive and reproduce. You will also explore the amazing diversity of life on Earth and how that diversity has been produced through the pro-cesses of evolution—changes in the genetic makeup of populations through time. Because organisms do not all survive and reproduce with equal success, advantageous variations spread in a popula-tion from generation to generation and result in adaptations that better suit individuals to specific environments. The processes of evolution have generated the enormous diversity of life on Earth, and evolution is a central theme of biology.
Life arose from non-life via chemical evolution
Earth formed between 4.6 and 4.5 billion years ago, but at first it was not a place hospitable to life. The cooling of Earth, the forma-tion of surface water, and the evolution of the first life forms took about 600 million years. If we picture the 4.6-billion-year history of Earth as a 30-day month, life first appeared some time around the end of the first week (Figure 1.2).
The composition of Earth in terms of the chemical elements has not changed over its history, but random physical and chemi-cal interactions of those atoms produced an enormous variety of molecules. The young Earth’s atmosphere, oceans, and climate were very different than they are today, influencing how those molecules interacted. Experiments simulating those conditions have confirmed that the generation of complex molecules was inevitable. The critical step for the evolution of life was the ap-pearance of nucleic acids—molecules that could reproduce them-selves and serve as templates for the synthesis of proteins, large molecules with complex but stable shapes. The variation in the shapes of these proteins enabled them to participate in increasing numbers and kinds of chemical reactions with other molecules. These subjects are covered in Part One of this book.