Splash Contents Chapter Introduction The Origin of Earth 17.1The Big Bang 17.2 Early Earth Evolution of Life on Earth 17.3The Beginnings of Life 17.4Chemical

Chapter Introduction

The Origin of Earth

17.1 The Big Bang

17.2 Early Earth

Evolution of Life on Earth

17.3 The Beginnings of Life

17.4 Chemical Evolution

17.5 Biological Evolution

The Record of the Rocks

17.6 Microfossils and Prokaryotes

17.7 Eukaryotes

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A Describe the origin of the universe and probable conditions on early Earth.

Learning Outcomes

By the end of this chapter you will be able to:

B Evaluate hypotheses about the origin of life and identify the probable characteristics of early life-forms.

C Distinguish between chemical and biological evolution.

D Describe the fossil record for prokaryotes and for eukaryotes.


How did life arise on Earth?

The Origin of Life What is required for

life to begin?

This photo shows stromatolite structures formed by cyanobacteria, in Shark Bay, Australia.


The Origin of Life• Of all the questions scientists investigate, the origin of

life on Earth probably evokes the most public curiosity

and controversy.

• Where did life come from?

• When did it begin?

• How did the first living things look?

This photo shows stromatolite structures formed by cyanobacteria, in Shark Bay, Australia.



• Measurements of light coming from deep space indicate that the stars and galaxies are rapidly moving away from each other.

The Origin of Earth

17.1 The Big Bang

• In the 1920s, Edwin Hubble studied light from distant galaxies to test the idea that the size of the universe is stable.


• The length of any wave (including light) changes if the object that is producing it moves toward or away from the object receiving the wave.

17.1 The Big Bang (cont.)

– When a source of light moves away from the observer, the wavelength of the light increases and the visible light is a bit redder—a redshift.

– When the source of light moves toward the observer, the wavelength of the wave decreases, and the visible light is a bit bluer—a blueshift.

The Origin of Earth


• Hubble developed his theory of the expanding universe after measuring red shifts in the light from distant galaxies.


The Origin of Earth

Photographed by the Hubble Telescope, the spiral galaxy NGC 4414 is 60 million light-years away. Measurements of light from such distant galaxies provide evidence that the universe is expanding.


• Evidence indicates that, about 15 billion years ago, the whole universe was concentrated in one superdense mass that exploded.

• That explosion, called the big bang, hurled matter and energy into space.

• Gravity pulled some of the matter together to form galaxies, stars, and planets.

• Clumps of matter around our own star, the Sun, became planets.


The Origin of Earth


• The best current estimate, based on the analysis of rocks and meteorites, is that Earth formed about 4.6 billion years ago.

• The Moon may have formed when a meteor collided with Earth, sending a large chunk of Earth into space.

• Meteors are thought to be bits of material left over from the formation of our solar system.


The Origin of Earth


Geologists have worked out a history of Earth from evidence in its rocks. This timeline relates several major biological events to the history of Earth.



The Origin of Earth

The history of Earth—and life—compressed into the distance between the tip of a person’s nose and the tip of the fingernail on the index finger. The history of our species, Homo sapiens, would disappear with one pass of a nail file.


• The decay of radioactive elements such as uranium (U), thorium (Th), and some isotopes of potassium-40 is the primary source of heat energy within Earth.

17.2 Early Earth

• The entire planet was probably hot when it first formed, and as the outer crust cooled, gases escaping from the planet’s hot interior formed a primitive atmosphere.

The Origin of Earth


• The early atmosphere likely originated from volcanic gases: mostly nitrogen (N2), carbon dioxide, water vapor, and hydrogen (H2), with small amounts of carbon monoxide (CO).

• All geologic evidence indicates that oxygen gas (O2) probably was not present in the early atmosphere.

17.2 Early Earth (cont.)

The Origin of Earth


• Oxygen began to accumulate in the atmosphere after the first photosynthetic organisms started to produce it about 2.1–2.4 billion years ago.

• Modern levels of oxygen in the atmosphere were probably reached about 360 million years ago as plants became abundant on land.

17.2 Early Earth (cont.)

The Origin of Earth



• The surface of Earth 4.6 billion years ago would have been hostile to modern life.


17.3 The Beginnings of Life

– Organic compounds do not form easily in an atmosphere rich in nitrogen and carbon dioxide.

– Ultraviolet radiation from the Sun bathed the surface of Earth.

– There were extreme temperature variations.

– The was a scarce supply of oxygen gas.


Earth’s atmosphere


• Popular scientific explanations for the beginning of life include the following:

17.3 The Beginnings of Life (cont.)

1. Life originated on some planet of another star and traveled to Earth through space.

2. Life originated by unknown means on Earth.

3. Life evolved from nonliving substances through interaction with the environment.



• Many people believe that a supernatural force or deity created life.

• That explanation is not within the scope of science, therefore, such beliefs are not part of scientific debates about the origin of life.

17.3 The Beginnings of Life (cont.)



• Most research on the origin of life focuses on the idea that life evolved from nonliving substances by interacting with the natural environment.

17.4 Chemical Evolution

• In the 1920s, the Soviet scientist Alexander Oparin and the British scientist J. B. S. Haldane separately described this hypothesis in detail.



• Oparin and Haldane proposed:

– The early atmosphere consisted of methane, ammonia, hydrogen, and water vapor.

17.4 Chemical Evolution (cont.)


– Energy sources such as radioactivity, lightning, cosmic radiation from space, and heat energy from volcanoes caused gases in the atmosphere to react, forming organic compounds.

– Those compounds then accumulated in the soup-like hot oceans.


– They hypothesized that life evolved by further chemical reactions and transformations in the complex organic soup.

– Both thought the first life-forms were heterotrophs that fed on the organic compounds in the soup.



– These ideas are the basis of the heterotroph hypothesis for the origin of life.


• The Oparin-Haldane version of the heterotroph hypothesis requires three major steps for the origin of life:

1. There had to be a supply of organic molecules, produced by nonbiological processes.

2. Some processes had to assemble those small molecules into polymers such as nucleic acids and proteins.

3. Other processes had to organize the polymers into a system that could replicate itself, using the organic molecules produced in step 1.




• In the 1950s, Harold Urey and Stanley Miller worked on the problem of how organic compounds could form in an inorganic environment.

• They were able to create organic compounds and amino acids in an airtight apparatus under conditions that might have existed on Earth 4.6 billion years ago.




A drawing of Miller’s apparatus


• More recent experiments have produced at least 13 of the 20 common amino acids found in proteins.

• Those experiments also have produced all the bases found in DNA and RNA and even small amounts of ribose, the sugar in RNA.




• Evidence from meteorites indicates that organic compounds also form in space.



Instruments flown through the comet’s tail of Comet Halley detected organic compounds.

(a), Cyril Ponnamperuma and his associates were the first to detect amino acids in a meteorite when they analyzed a sample of the Murchison meteorite (b).


• Some organic molecules also may have formed at volcanic vents deep in the ocean.

• Regardless of origin, there is a plausible explanation for step 1 of the Oparin-Haldane hypothesis.




• For the formation of complex molecules to occur, smaller molecules must been concentrated enough to make them collide and react with each other.

• In 1985, A. G. Cairns-Smith suggested that clay particles might have helped to form the first organic polymers by catalyzing the bonding together of these small molecules to form proteins and other polymers.



Repeating crystal structures in clay particles (scanning electron micrograph) may have served as a blueprint for assembling the first biochemical polymers.


• The work by Cairns-Smith and others since allows us to infer that polymers could have formed from simple organic compounds in the prebiological soup.

• Self-replication, a central feature of living systems, is the third and final step for the origin of life.

• Research into self-replication has led to the hypothesis that life began in an “RNA world.”




• According to the RNA-world hypothesis:

– RNA served as both information molecule and catalyst at first.

– DNA later became the main information molecule and protein enzymes the primary biological catalysts.



• So far, scientists have found no RNA that can replicate itself completely.


• RNA molecules can undergo simulated Darwinian evolution in the laboratory.



Beginning with a mixture of RNA molecules with various sequences, directed evolution allows a scientist to develop a ribozyme (RNA enzyme) that can catalyze its own replication or another desired reaction.


• The boundary between chemical evolution and biological evolution may be the formation of self-replicating polymers.

17.5 Biological Evolution

• Life could be described as “a self-sustained chemical system that is capable of undergoing Darwinian, or biological, evolution.”



• Biological evolution consists of three processes:

1. self-reproduction

2. mutation that can be inherited

3. natural selection

17.5 Biological Evolution (cont.)



• Cell theory holds that all life is made of cells and that all cells come from preexisting cells.

• The origin of cells and membranes is still not clearly understood.

• In the early 1980s, Carl Woese suggested that life began on Earth before the planet was fully formed as water droplets that functioned as primitive cells.




• Other possibilities for the first life-forms include proteins, DNA, and RNA.

• Amino acids in the oceans may have joined spontaneously to form proteins.

• Many proteins are enzymes that catalyze the synthesis of other substances, but there is no known way for proteins to replicate themselves without the help of RNA.




• Short strands of RNA can self-replicate.

• RNA also stores information, directs the synthesis of proteins, and has limited ability as a catalyst.

• In certain viruses, such as the virus that causes AIDS, RNA also directs the synthesis of DNA.




• A few scientists maintain that the first life-forms could have functioned only if they contained both proteins and nucleic acids.

• Viruses consist of DNA or RNA surrounded by a protein coat.

• Viruses are so dependent on their host cells that they probably originated after their hosts and their role in evolution is unclear.




• All the ideas about the origin of life and the nature of the first life-forms have supporters and opponents.





• Some scientists have tried to investigate early life-forms by searching for fossils.


17.6 Microfossils and Prokaryotes

• What may be the oldest known microfossils, or fossil microorganisms, were discovered in 1993 in northwestern Australia.


• The fossils are of tiny, single-celled filaments that resemble certain modern bacteria capable of photosynthesis.

17.6 Microfossils and Prokaryotes (cont.)


A 3.5-billion-year-old prokaryote fossil in a stromatolite from western Australia. This fossil contains the oldest known cells.


• The microscopic fossils were embedded in mineral grains encased in a type of rock that formed almost 3.5 billion years ago.

• This discovery showed that life appeared on Earth much earlier than previously thought.




• The Australian fossils were found with domelike structures called stromatolites that are composed of many wafer-thin layers of rock.

• Today, limestone-secreting bacteria form similar structures at Shark Bay, Australia.






(a), Approximately 530 million-year-old fossil stromatolites from Wyoming. Note the characteristic layered structure. The image is approximately 5 cm across. (b), Modern living stromatolites in Shark Bay, Australia, built by cyanobacteria. Each dome is about 30–100 cm in diameter.


• Older microfossils will be very difficult to find as they have melted, eroded, or otherwise changed until they are no longer recognizable.

• These fossils are thought to be prokaryotes, but their structures were probably less organized than modern prokaryotes.




• Carl Woese suggests that the first organisms may have been methanogens—anaerobic bacteria that obtain energy by using carbon dioxide to oxidize hydrogen.

• Methanogens and related bacteria live today in conditions thought to be like those of early Earth such as near hydrothermal volcanic vents on the seafloor.




• The first microfossils that may have been eukaryotes are about 2.1 billion years old.

• Fossil evidence indicates that eukaryotic cells became common by 750 million years ago.

• Eukaryotic cells contain membrane-enclosed organelles such as a nucleus, mitochondria, and chloroplasts.

17.7 Eukaryotes



• Margulis proposes that eukaryotes originated from a symbiosis between large anaerobic prokaryotes and smaller aerobic or photosynthetic prokaryotes.

• Eventually, the partners, or endosymbionts, lost the ability to live independently.

17.7 Eukaryotes (cont.)


• A large body of evidence supports Margulis’s endosymbiont hypothesis that mitochondria and plastids were once free-living prokaryotes.


The endosymbiont hypothesis


• Some single-celled organisms that do not have all the organelles usually found in eukaryotes may be survivors of a transitional phase to eukaryotic life.

17.7 Eukaryotes (cont.)


(a) Giardia lamblia, x16,000, has two haploid nuclei and no mitochondria. (b) Paramecium, x160, has one large nucleus (stained brown here) and several small nuclei (stained green here). The first eukaryotes may have resembled these single-celled organisms.



Summary

• The big bang probably occurred about 15 billion years ago.

• Planets, including Earth, formed around the Sun about 4.6 billion years ago. Gases escaping from within Earth formed a primitive atmosphere.

• Scientists hypothesize that chemical evolution, driven by a variety of energy sources, led to the origin of life on Earth.

• Catalytic RNA may have served as both information molecule (before DNA) and functional molecule (before proteins).

• According to one definition, living systems must be able to reproduce, must be subject to mutations that can be passed on to offspring, and must be subject to natural selection.

• It is not possible to repeat historical events, such as the origin of life, but inference helps scientists study such events and draw plausible conclusions about how they occurred.


Summary (cont.)

• The first life-forms were probably heterotrophs that lived on organic compounds in Earth’s oceans.

• The oldest fossils appear similar to modern bacteria.

• Photosynthesis probably evolved very early, but significant levels of oxygen did not accumulate in the atmosphere until about 1 billion years ago.

• The oldest known fossils of eukaryotes are about 2.1 billion years old and they became abundant by 750 million years ago.

• Mitochondria and plastids may have arisen as free-living prokaryotes that occupied host cells and increased their energy yield.

• There are more questions than data relating to the origin of life on Earth.

• The first living thing may have been a “naked” information molecule such as RNA, DNA, or protein. It may also have been a cell-like structure.


Reviewing Key TermsMatch the term on the left with the correct description.

___ stromatolites

___ methanogens

___ symbiosis

___ endosymbiont

___ hydrothermal

a. fossils containing some of the earliest known prokaryotes

b. an ecological relationship between organisms of two species that live together in direct contact

c. referring to hot water vents on the ocean floor

d. anaerobic bacteria that produce methane as a by-product of their metabolic processes

e. an organism having a mutually beneficial relationship with a host organism while living in the host’s body

a

d

b

e

c


Reviewing Ideas1. What are the three popular scientific

explanations of how life started on Earth?

The three popular scientific explanations of the origin of life are: (1) Life originated on some planet of another star and traveled to Earth through space. (2) Life originated by unknown means on Earth. (3) Life evolved from nonliving substances through interaction with the environment.


Reviewing Ideas2. What is the endosymbiont hypothesis?

Lynn Margulis’ endosymbiont hypothesis states that mitochondria and plastids (such as chloroplasts) originated as free-living prokaryotes. Eukaryotes originated from a symbiosis between large anaerobic prokaryotes and smaller aerobic or photosynthetic prokaryotes. The large cells absorbed the smaller ones (or alternatively, small parasitic cells bored into larger ones). Eventually, the internal partners, or endosymbionts, lost the ability to live independently.


Using Concepts3. What is the significance of determining the rate

of expansion of the universe?

By determining the rate of the universe’s expansion, scientists can work backward to determine the time at which the universe was much smaller. Evidence indicates that, about 15 billion years ago, the whole universe was concentrated in one superdense mass that exploded in an event called the big bang.


Using Concepts4. When did photosynthesis began on Earth?

How do we know this?

Oxygen began to accumulate in the atmosphere only after the first photosynthetic organisms started to produce it, about 2.1–2.4 billion years ago. At first, the oxygen released by photosynthesis would have combined with other elements, such as iron, instead of remaining free in the atmosphere. By dating the oxygen-containing compounds such as iron oxides that are present in the oldest rocks, we can estimate the approximate beginning of photosynthesis.


Synthesize5. In our search for the origin of life, what

evidence may support the theory that life is not unique to Earth?

Amino acids have been found in meteorites and in the tail of Comet Halley. This proves that these compounds are not unique to Earth. If organic compounds originally were the basis of life on Earth, they could very well have performed the same role elsewhere in the universe.



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Splash Contents Chapter Introduction The Origin of Earth 17.1The Big Bang 17.2 Early Earth Evolution of Life on Earth 17.3The Beginnings of Life 17.4Chemical