Section 1: How Did Life Begin? Chapter 19 Section 1: How Did Life Begin?

Key Ideas What did the Miller-Urey experiment show about the formation of the basic molecules of life? What are two theories that propose where the building blocks of life originated in early Earth? How could molecules have become packaged into cells that contain heritable cellular instructions?

The Basic Chemicals of Life Oparin and Haldane Suggested early oceans contained organic molecules 1920’s Molecules formed spontaneously Energy from sun, volcanoes, lightning Hypothesized early atmosphere contained H2, H2O, NH3, and CH4. The hypothesis was tested in the 1950s by American scientists Harold Urey and Stanley Miller.

The Basic Chemicals of Life, continued The Miller-Urey Experiment In this experiment, Miller placed hydrogen-containing gases into a device. He used electrical sparks to simulate lightning. Miller found organic molecules in his device, including some of life’s basic building blocks: amino acids, fatty acids, and other hydrocarbons

Miller-Urey Experiment

The Basic Chemicals of Life, continued The Miller-Urey experiment organic molecules could form from inorganic molecules. Some molecules used in Miller’s experiment could not have existed in abundance on early Earth. Early Earth did not have a protective ozone layer.

The Basic Chemicals of Life, continued UV radiation from the sun would have destroyed any methane and ammonia in the atmosphere. Lack of CH4 and NH3 Key organic compounds were not made However, the experiment shows that complex biological compounds can form from simple building blocks.

Life’s Building Blocks One hypothesis early biological molecules formed close to hydrothermal vents. protected from harmful solar radiation. Another hypothesis arrived on Earth in meteorites or comets. Some meteorites contain amino acids. could have arrived on early Earth, when frequent meteorite impacts were common.

The First Cells lipids tend to combine in water. Certain lipids can form a tiny droplet resembles a cell membrane. short chains of amino acids can form tiny spherical structures called microspheres.

The First Cells, continued Microspheres may have been the first step toward cellular organization. could not be considered cells unless they had the characteristic of heredity. In the laboratory, scientists have not been able to make proteins or DNA form spontaneously in water. They have been able to form short chains of RNA, the nucleic acid that helps to carry out the instructions of DNA. RNA molecules may have been the first self-replicating molecule.

The First Cells, continued Cech and Altman found a type of RNA molecule, called a ribozyme, that can act like an enzyme. They also showed that RNA can form spontaneously in water, without DNA.

The First Cells, continued Cech and Altman further hypothesized that RNA could have changed—evolved—from one generation to the next. Scientists hypothesize that DNA and proteins eventually took over these roles in the cell.

Summary The Miller-Urey experiment showed that, under certain circumstances, organic compounds could form from inorganic molecules. Among the scientific theories that address the origin of life, one suggests that life began close to hydrothermal vents, and another proposes that organic molecules arrived on early Earth from a meteorite. The formation of microspheres might have been the first step toward cellular organization.