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CHAPTER 25

THE HISTORY OF LIFE ON EARTH

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Artist’s Conception of Earth 3 billion years ago

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I. Concept 25.1: The Origin of LifeA. Introduction

1. Conditions at this time on Earth were such that life could have arisen by “spontaneous generation” or abiogenesis (life developing from nonliving materials)2. Louis Pasteur disproved spontaneous generation in

1862.3. Today we accept the theory of “biogenesis” (life comes

only from preexisting life).4. It is possible that chemical and physical processes on

early Earth produced the first cells.B. Earth and other planets in the solar system formed about 4.6 billion years ago.

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Louis Pasteur

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C. Current theory about how life arose consists of four main stages:1. Abiotic synthesis of small organic molecules (monomers)2. Joining of monomers to form macromolecules3. Packaging of macromolecules into “protobionts,” droplets with membranes that maintained a distinct internal chemistry4. Origin of self-replicating molecules that eventually made inheritance possible.

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D. Closer Look at these Four Main Stages 1. Abiotic synthesis of small organic molecules (monomers)

• In the 1920’s, A. I Oparin (Russian) and J. B. S. Haldane (British) independently postulated that the conditions on early Earth (reducing environment) favored the synthesis of organic compounds from simple molecules.

–Energy possibly from lightning and UV radiation• In 1953, Stanley Miller and Harold Urey tested the

Oparin-Haldane hypothesis

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•They conducted lab experiments that showed that the abiotic synthesis of organic molecules in a reducing atmosphere is possible

•Atmosphere of primitive Earth:water vapor

hydrogen gasmethaneammonia

•Used electrical sparks as source of energy

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Miller and Urey’s Experiment

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2. Joining of monomers to form macromolecules•Monomers could link to form macromolecules

without enzymes and other cellular equipment.•Macromolecules, including polypeptides, have been

produced in the lab by dripping solutions of monomers onto hot sand, clay, or rock.

•These conditions are thought to be similar to early Earth at deep-sea vents or when dilute solutions of monomers splashed onto fresh lava.

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3. Packaging of macromolecules into “protobionts,” droplets with membranes that maintained a distinct internal chemistry•Two properties that defined life: accurate replication

and metabolism•Protobionts—aggregates of abiotically produced

molecules surrounded by a membrane or membrane-like structure

--they exhibit simple reproduction and metabolism and maintain an internal chemical environment

•Can be formed spontaneously from abiotically produced organic compounds (liposomes)

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Protobionts (Liposome)

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4. Origin of self-replicating molecules that eventually made inheritance possible. •First genetic material was probably RNA, not DNA •RNA molecules called ribozymes can catalyze many

different reactions•Has been demonstrated that RNA sequences can

evolve under abiotic condition

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Clock Analogy of Earth’s History

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II. Concept 25.2: Fossil Record of Life on EarthA. Sedimentary rocks are deposited into layers called

strata and are the riches source of fossils. B. Methods used to date fossils and rocks:

1. Radiometric dating•Can determine absolute ages of fossils• Involves half-life of isotopes (time required for

half the parent isotope to decay)2. Radiocarbon dating (carbon-14)

•Used to date fossils up to 75, 000 years old

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III. Concept 25.3: Major Lineages of LifeA. Oldest know fossils are stromatolites, rock-like

structures composed of many layers of bacteria and sediment•Date back 3.5 billion years ago

B. Prokaryotes were Earth’s sole inhabitants from 3.5 to about 2.1 billion years ago

C. About 2.7 billion years ago, O2 (probably from bacteria similar to modern cyanobacteria) began accumulating in the atmosphere•Many prokaryotes were doomed by the increase in

atmospheric oxygen

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Stromatolites•

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D. Oldest fossils of eukaryotic cells date back to 2.1 billion years

E. First Eukaryotes1. Endosymbiosis

• Proposes that mitochondria and plastids (chloroplasts and related organelles) were formerly small prokaryotes living within larger host cells

• Endosymbiont—a cell that lives within a host cell• Prokaryotic ancestors of mitochondria and

plastids probably gained entry to the host cell as undigested prey or internal parasites

• Eventually host and endosymbiots would have become a single organism

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• Serial endosymbiosis supposes that mitochondria evolved before plastids through a sequence of endosymbiotic events

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2. Evidence to support an endosymbiotic origin of mitochondria and plastids:a. Similarities in inner membrane structures

and functionsb. Division is similar in those organelles and

some prokaryotesc. Their ribosomes are more similar to

prokaryotic than eukaryotic ribosomesd. Circular DNA

3. Earliest Multicellular Eukaryotes•Oldest known fossils of multicellular eukaryotes are of small algae that lived about 1.2 billion years ago

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IV. Concept 25.4: Continental DriftA. Defined as the process of the Earth’s continents moving slowly over the underlying hot mantleB. Interactions between plates cause the formation of mountains and islands, and earthquakesC. Pangaea—supercontinent

• Break-up lead to allopatric speciation• Current distribution of fossils reflects the movement

of continental drift-Ex: the similarity of fossils in parts of South

America and Africa is consistent with the idea that these continents were formerly attached

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V. Concept 25.5: Evolutionary Effects of Development Genes

A. Genes that program development control the rate, timing, and spatial pattern of changes in an organism’s form as it develops into an adult

B. Changes in Rate and Timing1. Heterochrony

•Defined as an evolutionary change in the rate or timing of developmental events

•Can have a significant impact on body shape•Contrasting shapes of human and chimpanzee

skulls are the result of small changes in relative growth rates

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• Can alter the timing of reproductive development relative to the development of nonreproductive organs

• In paedomorphosis, the rate of reproductive development accelerates compared with somatic development

-Sexually mature species may retain body features that were juvenile structures in an ancestral species

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Paedomorphosis

C. Changes in Spatial Pattern•Substantial evolutionary change can result from alterations in genes that control the placement and organization of body parts

•Homeotic genes determine such basic features as where wings and leg will develop on a bird or how a flower’s parts are arranged

•Hox genes are a class of homeotic genes that provide positional information during development

-If expressed in the wrong location, body parts can be produced in the wrong location

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VI. Concept 25.6: Evolution is Not Goal Oriented (not on exam)A. Evolutionary Novelties

• Evolve in many stages from previously existing structures

• Complex eyes evolved from simple photosensitive cells independently many times

• Exaptations are structures that evolve in one context but become co-opted for a different function

• Natural selection can only improve a structure in the context of its current utility

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You should now be able to:

1. Define radiometric dating, serial endosymbiosis, Pangaea, snowball Earth, exaptation, heterochrony, and paedomorphosis

2. Describe the contributions made by Oparin, Haldane, Miller, and Urey toward understanding the origin of organic molecules

3. List and discuss the 4 main stages of how life arose4. Explain why RNA, not DNA, was likely the first genetic

material

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5. Describe and suggest evidence for the major events in the history of life on Earth from Earth’s origin to 2 billion years ago

6. Explain the function of Hox genes