Albia Dugger • Miami Dade College

Chapter 19Life’s Origin and Early Evolution

19.1 Looking for Life

• Astrobiology is the study of life’s origins and distribution – astrobiologists study Earth’s extreme habitats to determine the range of conditions living things can tolerate

• Life on Earth is protected by the ozone layer, which serves as a natural sunscreen, preventing most UV radiation from reaching the planet’s surface

• Life can adapt to nearly any environment with sources of carbon and energy – including extreme temperatures, pH, salinity, or pressure

Lessons from Chile’s Atacama Desert

19.2 The Early Earth

• Knowledge of modern chemistry and physics are the basis for scientific hypotheses about early events in Earth’s history

Origin of the Universe and Our Solar System

• Big bang theory• The universe began in an instant, 13-5 billion years ago• All existing matter and energy suddenly appeared and

exploded outward from a single point• The universe is still expanding

• Earth formed from dust and debris orbiting the sun, about 4.6 billion years ago

Formation of the Earth

Conditions on the Early Earth

• Earth’s early atmosphere came from gas released by volcanoes, and was low in oxygen

• Rain washed minerals and salts out of rocks to form early seas

ANIMATION: Origin of organelles

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Early Earth

Take-Home Message: What were conditions like on the early Earth?

• Earth’s early atmosphere had little or no oxygen

• Meteorites pummeled the planet’s surface, and volcanic activity was more common than it is today

19.3 Formation of Organic Monomers

• All living things are made from the same organic subunits: amino acids, fatty acids, nucleotides, and simple sugars

• Small organic molecules that serve as building blocks of life can be formed by nonliving mechanisms

Possible Sources of Life’s First Building Blocks

1. Stanley Miller showed that amino acids form in conditions that simulate lightning in the atmosphere of early Earth

2. Wächtershäuser and Huber synthesized amino acids in a simulated hydrothermal vent environment

3. Amino acids, sugars, and nucleotide bases may have formed in interstellar clouds and been carried to Earth on meteorites

boiling water

gases

water in

spark discharge

electrodes

water droplets

water containing organic compounds

liquid water in trap

CH4

NH3

H2O H2

to vacuum pump

condenser

water out

Figure 19-4 p311

A Hydrothermal Vent on the Seafloor

Take-Home Message: What was the source of organic molecules to build the first life?

• Small organic molecules that serve as the building blocks for living things can be formed by nonliving mechanisms.

• For example, amino acids form in reaction chambers that simulate conditions on the early Earth, and are present in some meteorites

ANIMATED FIGURE: Miller's reaction chamber experiment

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19.4 From Polymers to Protocells

• We will never know for sure how the first cells came to be, but we can investigate the possible steps on the road to life

Properties of Cells

• All living cells carry out metabolic reactions, are enclosed within a plasma membrane, and can replicate themselves

• Cells have a genome of DNA that enzymes transcribe into RNA, and ribosomes that translate RNA into proteins

• Studies support the hypothesis that cells arose from a stepwise process that began with inorganic materials

inorganic molecules

…self-assemble on Earth and in space

organic monomers

…self-assemble in aquatic environments on Earth

organic polymers

…interact in early metabolism

…self-assemble as vesicles

…become the first genome

protocells in an RNA world

…are subject to selection that favors a DNA genome

DNA-based cells

Figure 19-6 p312

Origin of Metabolism

• Before cells, nonbiological process that concentrate organic subunits might increase the chance of polymer formation

• Concentration of molecules on clay particles in tidal flats may have caused organic subunits to bond as polymers

• The iron–sulfur world hypothesis holds that the first metabolic reactions began on the surface of rocks around hydrothermal vents

Origin of the Genome

• An RNA-based system of inheritance may have preceded DNA-based systems

• RNA world hypothesis• RNA may have stored genetic information and functioned

like an enzyme in protein synthesis

• RNAs that function as enzymes (ribozymes) are common in living cells today

Origin of the Plasma Membrane

• The cell’s plasma membrane allows organic molecules to concentrate and undergo reactions

• Membranous sacs (protocells) containing interacting organic molecules may have formed prior to the earliest life forms

• In experiments, small organic molecules can react with minerals and seawater to form vesicles with a bilayer membrane

Laboratory-Produced Protocells

Testing a Hypothesis

Take-Home Message: What do experiments reveal about steps that led to the first cells?

• All living cells carry out metabolic reactions, are enclosed within a plasma membrane, and can replicate themselves

• Metabolic reactions may have begun when molecules became concentrated on clay particles or in tiny rock chambers near hydrothermal vents

• RNA can serve as an enzyme, as well as a genome. An RNA world may have preceded evolution of DNA-based genomes

• Vesicle-like structures with outer membranes form spontaneously when certain organic molecules are mixed with water

19.5 Life’s Early Evolution

• Fossils and molecular comparisons among modern organisms inform us about the early history of life

Origin of Bacteria and Archaea

• Life that arose 3-4 billion years ago was probably anaerobic and used dissolved carbon dioxide as a carbon source

• Early fossil cells are similar in size and structure to modern archaea and bacteria

• The first photosynthetic cells were bacteria that used the cyclic pathway (does not produce O2)

Fossil Prokaryotic Cells

The Proterozoic Era

• The oxygen-producing, non-cyclic pathway of photosynthesis first evolved in cyanobacteria

• In the Proterozoic era Layers of photosynthetic bacteria formed large dome-shaped, layered called stromatolites

• Oxygen accumulation in air and seas halted spontaneous formation of molecules of life, formed a protective ozone layer, and spurred evolution of organisms using aerobic respiration

Stromatolites

The Rise of Eukaryotes

• The earliest evidence of eukaryotes is lipids in 2.7-billion-year-old rocks – the lipids are biomarkers for eukaryotes

• A red alga that lived 1.2 billion years ago is the oldest species known to reproduce sexually, a trait unique to eukaryotes

• Multicellularity and cellular differentiation allowed evolution of larger bodies with specialized parts

• Spongelike animals evolved about 870 million year ago; animals with more complex bodies existed about 570 mya

Fossils of Some Early Eukaryotes

Take-Home Message: What was early life like and how did it change Earth?

• Life arose by 3–4 billion years ago; it was probably anaerobic and did not have a nucleus

• An early divergence separated ancestors of modern bacteria from the lineage that lead to archaea and eukaryotic cells

• The first photosynthetic cells were bacteria that used the cyclic pathway; later, the oxygen-producing, noncyclic pathway evolved in cyanobacteria

• Oxygen accumulation in air and seas halted spontaneous formation of the molecules of life, formed a protective ozone layer, and favored organisms that carried out the highly efficient pathway of aerobic respiration

19.6 How Did Eukaryotic Traits Evolve?

• Eukaryotic cells have a composite ancestry, with different components derived from different lineages

• Archaea-like nuclear genes govern genetic processes (DNA replication, transcription, translation)

• Bacteria-like nuclear genes govern metabolism and membrane formation

Origins of Internal Membranes

• In eukaryotes, DNA resides in a nucleus bordered by a nuclear envelope – a double membrane with pores that control the flow of material into and out of the nucleus

• A few modern bacteria also have internal membrane-enclosed compartments

• The nucleus and endomembrane system probably evolved from infoldings of plasma membrane

19-10a p1910

nuclear envelope

infolding of plasma membrane

ER

Bacteria with Internal Membranes

Evolution of Mitochondria and Chloroplasts

• The endosymbiont hypothesis holds that mitochondria and chloroplasts descended from bacteria that were prey or parasites of early eukaryotic cells

• Mitochondria are genetically similar to aerobic bacteria called rickettsias; chloroplasts are similar to photosynthetic bacteria called cyanobacteria

Rickettsia prowazekii

Evidence of Endosymbiosis

• Endosymbiosis can occur when a bacterium infects a eukaryotic cell

• Eventually, host and symbiont become incapable of living independently

• Example: The photosynthetic organelles of glaucophytes are dependent on their host – they can’t survive on their own

Figure 19-12a p317

photosynthetic organelle thatresembles a cyanobacterium

Figure 19-12b p317

photosynthetic organelle thatresembles a cyanobacterium

Take-Home Message: How might eukaryotic organelles have evolved?

• A nucleus and other organelles are defining features of eukaryotic cells

• The nucleus and ER may have arisen through modification of infoldings of the plasma membrane

• Mitochondria and chloroplasts most likely descended from bacteria

19.7 Time Line for Life’s Origin and Evolution

Table 19-1 p320

ANIMATION: Evolutionary tree of life

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ANIMATED FIGURE: Milestones in the history of life

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