What Is Life?

• Life can be defined as an organized genetic unit capable of metabolism, reproduction, and evolution.
• Metabolism, the total chemical activity of a living organism, is controlled by genes.
• Biological evolution is a change in the genetic composition of a population of organisms over time.

Biological Evolution: Changes over Billions of Years

• Charles Darwin's theory of natural selection rests on three simple observations and one conclusion drawn from them: Any heritable traits that increase the probability that their bearers will survive and reproduce are passed on to their offspring. Review Figure 1.2

Major Events in the History of Life on Earth

• Life arose from nonlife about 4 billion years ago by means of chemical evolution. Review Figure 1.3
• Biological evolution began about 3.8 billion years ago when interacting systems of molecules became enclosed in membranes to form cells.
• Photosynthetic prokaryotes released large amounts of oxygen into Earth's atmosphere, making aerobic metabolism possible.
• Complex eukaryotic cells evolved by incorporation of smaller cells that survived being ingested.
• Multicellular organisms appeared when cells evolved the ability to transform themselves and to stick together and communicate after they divided. The individual cells of multicellular organisms became modified to carry out varied functions within the organism.
• The evolution of sex sped up rates of biological evolution.

Levels of Organization of Life

• Life is organized hierarchically, from molecules to the biosphere. Review Figure 1.6. See Web/CD Activity 1.1

The Evolutionary Tree of Life

• A major effort called Assembling the Tree of Life (ATOL) is underway to determine the evolutionary relationships among all species on Earth.
• The hierarchy of evolutionary relationships can be represented as an evolutionary tree. Review Figure 1.8. See Web/CD Activity 1.2
• Species are grouped into three domains: Archaea, Bacteria, and Eukarya. The domains Archaea and Bacteria consist of prokaryotic cells. The domain Eukarya contains the Protists, Plantae, Fungi, and Animalia.

Biology Is a Science

• Biologists use a variety of technical and conceptual tools to study living things.
• The hypothesis-prediction (H-P) approach is used in most biological investigations. Hypotheses are tentative answers to questions. Predictions are made on the basis of a hypothesis. The predictions are tested by experiments and comparative observations. Review Figures 1.9 and 1.10
• Science can tell us how the world works, but it does not form the basis for establishing meaning and values.
• Biologists are often called upon to advise governmental agencies on the solution of important problems that have a biological component.