History of Life on Earth

Chapter 12

HOW DID LIFE BEGIN?Section 12-1

The Early Earth was Lifeless

When Earth formed about 4.3 billion years ago, it was a fiery ball of molten rock. It could not have supported the development of life. Eventually, the planet’s surface cooled and formed a rocky crust. Water vapor in the atmosphere condensed to form vast oceans. Many scientists think life first evolved in these oceans. Scientists think the evolution of life took hundreds of millions of years to occur. Evidence that Earth has existed long enough for this evolution to have taken place can be found by measuring the age of Earth.

Measuring Earth’s Age• Scientists are able to calculate the

age of Earth through radiometric dating.– Calculation of the age on an object

by measuring the proportions of the radioactive isotopes of certain elements

• Radioactive isotopes, or radioisotopes, are unstable elements that break up and give off energy in the form of charged particles (radiation).

• By measuring how many half-lives have passed since a rock was formed, – The time it takes for one-half of a

given amount of a radioisotope to change is called its half-life.

How the Basic Chemicals of Life were Formed

Most scientists think that life on Earth developed through natural chemical and physical processes. Spontaneous origin is the process through which life is thought to have developed when molecules of nonliving matter reacted chemically during the first billion years of Earth’s history. These molecules formed many different simple, organic molecules. Energized by the sun and volcanic heat, these molecules formed more complex molecules that eventually became the building blocks of the first cells. This hypothesis has been tested and confirmed using scientific methods in laboratory experiments.

The “Primordial Soup” Model

• In the 1920s, Russian scientist A. I. Oparin and British scientist J.B.S. Haldane each suggested that the early Earth’s oceans once contained large amounts of organic molecules.

• They both hypothesized that these molecules formed spontaneously in chemical reactions activated by energy from solar radiation, volcanic eruptions, and lightning.

• Oparin hypothesized the atmosphere lacked oxygen which allowed hydrogen to react with other molecules.

• In 1953, the primordial soup model was tested by Stanley Miller by placing the proposed gases into a device and stimulating lightning by providing electrical sparks.

• After a few days, Miller had a complex “chemical zoo” which included life’s basic building blocks: amino acids, fatty acids, and other hydrocarbons.

Reevaluating the Primordial Soup Model

• Recent discoveries of fossils indicate life started much earlier than first thought.

• Without the protective ozone layer, ultraviolet radiation would have destroyed any ammonia and methane present in the atmosphere.

• Without these molecules, the experiment is not successful in making key biological molecules.

The Bubble ModelIn 1986, geophysicist Louis Lerman suggested that the key processes that formed the chemicals needed for life took place within bubbles on the ocean’s surface.

Organic Chemicals Became Complex

Though many scientists disagree about the details of the process that led to the origin of life, most scientists accept that by adding energy, the basic molecules of life could have formed spontaneously through organic molecules and living cells. How did amino acids link to form proteins? How did nucleotides form the long chains of DNA that store the instructions for making proteins? In the laboratory scientists have not been able to make either of these macromolecules form spontaneously in water. However, short chains of RNA, the nucleic acid that helps carry out DNA’s instructions, have been made form on their own in water.

A Possible Role for Catalysts• 1980s, Cech and Altman found

that certain RNA molecules can act like enzymes.

• Its 3-d shape provides a surface on which chemical reactions can be catalyzed.

• Perhaps RNA was the first self-replicating information-storage molecule.

• After such a molecule had formed, it could also have catalyzed the assembly of the first proteins.

• Such a molecule would have been capable of changing from one generation to the next.

Microspheres May Have Led to Cells• Observations show that

lipids, which make up cell membranes, tend to gather together in water.

• Lab experiments have shown that short chains of amino acids tend to gather into tiny vesicles called microspheres.

• Scientists think that formation of microspheres might have been the first step toward cellular organization.

Origin of Heredity• Most scientists agree that

double-stranded DNA probably evolved after RNA and that RNA “enzymes” catalyzed the assembly of the earliest proteins.

• Because researchers do not yet understand how DNA, RNA, and hereditary mechanisms first developed, how life might have originated naturally and spontaneously remains a subject of intense interest, research, and discussion among scientists.

COMPLEX ORGANISMS DEVELOPEDSection 12-2

Prokaryotes Are the Oldest Group of Organisms

When did the first organisms form? To find out, scientists study the best evidence of early life that we have, fossils. A fossil is the preserved or mineralized remains or imprint of an organism that lived long ago. The oldest fossils, which are microscopic fossils of prokaryotes, come from 3.5-billion-year-old rock deposits found in western Australia.

Recall that prokaryotes are single-celled organisms that lack internal membrane-bound organelles.

Prokaryotes are also known as bacteria. Among the first of the bacteria to appear were cyanobacteria. Cyanobacteria are photosynthetic bacteria. Before cyanobacteria appeared, oxygen gas was rare on Earth. But as ancient cyanobacteria carried out

photosynthesis, they released oxygen gas into Earth’s oceans. After hundreds of millions of years, the

oxygen produced by cyanobacteria began to escape into the air. Over the billions of years that followed, more oxygen was added to the air. Today oxygen gas

makes up 21 percent of the Earth’s atmosphere.

Two Groups of BacteriaEubacteria

• Prokaryotes that contain a chemical called peptidoglycan in their cell walls

• Have the same type of lipids in their cell membranes as eukaryotes do

• Include many bacteria that cause disease and decay

• E. coli

Archaebacteria• Prokaryotes that lack

peptidoglycan in their cell walls

• Have unique lipids in their cell membranes

• Thought to be closely related to the first bacteria to have existed on Earth

• Sulfolobus

The First Eukaryotes Evolved

About 1.5 billion years ago, the first eukaryotes appeared. Recall that a eukaryotic cell has a complex system of internal membranes and has DNA that is enclosed within a nucleus. A third characteristic of almost all eukaryotes is the presence within the cells of complex bacteria-sized mitochondria and chloroplasts. These organelles contain their own DNA. Except for one rare primitive form, all eukaryotes have mitochondria. Chloroplasts, which carry out photosynthesis, are found only in protists and plants.

The Evolution of Mitochondria and Chloroplasts

• Endosymbiosis was first presented in 1966 by Lynn Margulis

• It proposes that mitochondria are the descendants of symbiotic, aerobic eubacteria

• A second successful invasion of by photosynthetic bacteria gave rise to chloroplasts

Multicellularity Evolved Many Times

The unicellular body plan has been tremendously successful, with unicellular organisms today constitution about half the biomass on Earth. But a single cell has limits. Distinct types of cells in one body allow an organism to specialize in different functions including protection from diseases, predators, and environment or movement to find prey or mates. With all these advantages, it is not surprising that multicellularity has arisen independently many times.

Mass Extinctions Have a Major ImpactThe fossil record indicates a sudden change occurred at

the end of Ordovician period about 440 million years ago. A large percentage of the organisms on Earth

suddenly became extinct. This was the first of five major mass extinctions that have occurred on Earth. A mass

extinction is the death of all members of many different species, usually caused by a large ecological disaster.

• 1st Extinction- 440 million years ago• 2nd Extinction- 360 million years ago• 3rd Extinction- 245 million years ago; worst of all

mass extinctions• 4th Extinction- 210 million years ago• 5th Extinction- 65 million years ago; bye bye

dinosaurs• Is a 6th happening right now? Many scientists think

one is taking place right now with the destruction of many of Earth’s ecosystems.

LIFE INVADED THE LANDSection 12-3

Ozone Enables Survival on Land

The sun provides both life-giving light and dangerous ultraviolet radiation. Early in Earth’s history, life formed in the seas, where early organisms were protected from UV radiation. These organisms could not leave the water because the UV radiation made life on dry ground unsafe. What enabled life-forms to leave the protection of the seas and live on the land?

Formation of the Ozone Shield• About 2.5 billion years ago,

photosynthesis by cyanobacteria began adding oxygen to the atmosphere.

• As oxygen reached the upper atmosphere, the sun’s rays caused O2 to bond and from molecules of O3.

• In the upper atmosphere, ozone blocks the UV rays of the sun.

• After millions of years, the Earth’s land became a safe place to live.

• Plants and fungi began living on the land together• Arthropods crawled out of the sea• Vertebrates followed onto land–Fishes evolved–Amphibians were the first vertebrates

on land–Reptiles arose from amphibians–Evolution of mammals and birds