Chapter 19-1 and 19-2: How Did Life Begin and Fossils

Chapter 19-1 and 19-2: How Did Life Begin and Fossils

Bellringer

Draw a picture of what you think Earth’s first life form may have looked like and label the parts of the organism.

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 other scientists hypothesized that the early atmosphere was rich in hydrogen gas (H2) and hydrogen-containing gases, such as water vapor (H2O), ammonia (NH3), and methane (CH4).

•  They thought that if the atmosphere lacked oxygen, a variety of organic compounds may form spontaneously. Miller and Urey designed an experiment to test this hypothesis.

The Miller-Urey Experiment

•  Put hydrogen-containing gases into a device and simulated lightning with an electrical sparks.

•  After a few days, organic molecules were found, showing that organic molecules (amino acids, fatty acids) can form from inorganic molecules.

Life’s Building Blocks

•  Some problems exist with the Miller-Urey experiments. It has been modified to suggest that the molecules formed in the sea or close to hydrothermal vents, not the atmosphere.

•  The heat from hydrothermal vents could have provided energy for chemical reactions. Within the sea, biological molecules would have been protected from harmful solar radiation.

•  Alternatively, the first biological molecules may have come from meteorites.

The First Cells

Forming a Cell Membrane

•  Scientists have conducted research to determine how a cell membrane may form.

•  Scientists have observed that lipids tend to combine into droplets in water that has a surface that resembles a cell membrane.

•  Further research has shown that lipids form tiny spheres called microspheres that act like a membrane.

Origins of Heredity

•  Scientists have been able to form short chains of RNA spontaneously in water. RNA molecules may have been the first self-replicating molecule.

•  In the 1980s, American scientists showed an RNA-type molecule called a ribozyme, can act like an enzyme.

•  RNA may have been involved in protein synthesis as shown here. RNA may have evolved into DNA.

Summary- 19-1

•  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. RNA may have been the first heritable material.

Chapter 19-2: The Age of Earth

The Fossil Record

How Fossils Form

•  Many fossils are found in sedimentary rock. These fossils form when organisms or traces of organisms are rapidly buried in fine sediments that are deposited by water (lakes, streams), wind, or volcanic eruptions.

•  Mineralized fossils are the most common types of fossils. In some cases, an organism breaks down, leaving a hollow space or impression called a mold. This mold may fill with minerals, preserving the shape of the organism.

•  In rare cases, fossils are preserved in hardened plant sap, or amber. In these fossils, soft parts of tissue are preserved in detail.

•  In order to analyze fossil evidence, paleontologists use both relative and absolute dating methods to date fossils.

•  Relative methods analyze the layer of earth that contains the fossil and estimate the age of the fossil in relations to other fossils in the same layer.

Absolute Age

•  A method called radiometric dating estimates the age in years of an object by measuring certain radioactive isotopes that the object contains. An isotope is a form of an element whose atomic mass differs from that of other atoms of the same element.

•  The time required for half of an isotope to decay into a daughter isotope is the isotope’s half-life. Each radioactive isotope has a specific half-life.

•  The ratio of different isotopes of an element can be used to determine the age of rocks and fossils.

Analyzing Fossil Evidence, continued

•  One radioisotope that is widely used to date organic materials is carbon-14. The half-life of carbon-14 is relatively short—5,730 years.

•  Carbon-14 is used to measure the age of carbon-containing materials (fossils) that are younger than 75,000 years old.

•  To find the age of older materials (rocks), scientists must measure other radioisotopes.

Radioactive Decay

Summary

•  Both the geographical distribution of organisms and when organisms lived on Earth can be inferred by examining the fossil record.

•  In order to analyze fossil evidence, paleontologists use both relative and absolute dating methods to date fossils.

•  The geologic time scale is based on evidence in the fossil record and has been shaped by mass extinctions.