life11e_ch01

key concept1.1Living Organisms Share Similarities and a Common Origin

Intuitively we all know what life is, but try to define 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 diversity and the complex processes that make up life.

focus your learning

Major characteristics are shared among all living things.
Living organisms have influenced the history of the planet Earth.
Biological populations change over time.

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: mainly carbohydrates, fatty acids, nucleic acids, and amino acids.

3
are made up of cells.
use molecules obtained from the environment to synthesize new biological molecules.
extract energy from the environment and use it to do work.
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.
regulate their internal environments.
exist in populations that evolve over time.

Figure 1.1 The Many Faces of Life The processes of evolution have led to the millions of diverse organisms living on Earth today. Prokaryotic archaea (A) and bacteria (B) are all single-celled organisms, as described in Chapter 25. (C) Many protists are unicellular but, as discussed in Chapter 26, their cell structures are more complex than those of the prokaryotes. This protist has manufactured “plates” of calcium carbonate that surround and protect its single cell. (D–F) Most of the visible life on Earth is multicellular. Chapters 27 and 28 cover the green plants (D). The other broad groups of multicellular organisms are the fungi (E), discussed in Chapter 29, and the animals (F), covered in Chapters 30, 31 and 32.

How do you think all organisms came to have these similarities? 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 characteristics listed above 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 composed 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. Because organisms do not all survive and reproduce with equal success, you will see again and again that through differential survival and reproduction populations of organisms evolve and become adapted to Earth’s many environments. The processes of evolution have generated the enormous diversity of life on Earth, and evolution is a central theme of biology.