The History of Life on Earth 18. Chapter 18 The History of Life on Earth Key Concepts 18.1 Events in Earths History Can Be Dated 18.2 Changes in Earths.

The History of Life on Earth

18

Chapter 18 The History of Life on Earth

Key Concepts• 18.1 Events in Earth’s History Can Be Dated

• 18.2 Changes in Earth’s Physical Environment Have Affected the Evolution of Life

• 18.3 Major Events in the Evolution of Life Can Be Read in the Fossil Record

Chapter 18 Opening Question

Can modern experiments test hypotheses about the evolutionary impact of ancient environmental changes?

Concept 18.1 Events in Earth’s History Can Be Dated

To understand long-term patterns of evolutionary change, we must think in time scales spanning many millions of years and consider conditions very different from today’s.

Earth is 4.6 billion years old.

Much of Earth’s history is recorded in rocks.


Ages of rocks relative to one another can be determined by stratigraphy:

In sedimentary rock layers (strata), the oldest layers are at the bottom, and successively higher strata are progressively younger.

Certain fossils are always found in younger rocks, others are found in older rocks.

Fossils in more recent strata are more similar to modern organisms.


Actual ages of rocks can be determined using radioisotopes—isotopes that decay in a predictable pattern.

Half-life is the time in which one half of the remaining radioisotope decays, changing into another element.

This is the basis for radiometric dating.

Figure 18.1 Radioactive Isotopes Allow Us to Date Ancient Rocks (Part 1)

Figure 18.1 Radioactive Isotopes Allow Us to Date Ancient Rocks (Part 2)


To date an event, the original concentration of the isotope must be known or estimated, and the half-life of the isotope must be known.

The amount of isotope remaining is indicative of how much time has passed since the event.


Sedimentary rocks can not be dated accurately; the materials that form the rocks existed for varying lengths of time before being transported and converted to rock.

Dating rocks older than 50,000 years requires estimating isotope concentrations in igneous rocks (formed when molten material cools).


Paleomagnetic dating relates ages of rocks to patterns in Earth’s magnetism.

Earth’s magnetic poles move and occasionally reverse.

Sedimentary and igneous rocks preserve a record of Earth’s magnetic field at the time they were formed.


Using the various dating methods and fossil stratigraphy, a geological time scale was developed.

The history of life is divided into eras and subdivided into periods.

Boundaries between the divisions are based on abrupt changes in fossil organisms.

Table 18.1 Earth’s Geological History (Part 1)

Table 18.1 Earth’s Geological History (Part 2)

Concept 18.2 Changes in Earth’s Physical EnvironmentHave Affected the Evolution of Life

Physical changes in the Earth and its atmosphere have influenced the evolution of life.

And life has also influenced Earth’s physical environment.


The continents have moved.

The idea that land masses have moved over time was first suggested by Alfred Wegener in 1912.

By the 1960s, evidence of plate tectonics (geophysics of the movement of land masses) convinced geologists that he was right.


Earth’s crust consists of solid plates about 40 km thick—the lithosphere.

The plates float on a fluid layer of liquid rock, or magma.

Heat from radioactive decay in Earth’s core causes the magma to circulate in convection currents. This exerts pressure on the plates and causes them to move.

Figure 18.2 Plate Tectonics and Continental Drift


Movement of the lithospheric plates is continental drift.

Position and size of the continents influences oceanic circulation patterns, global climates, and sea levels.

Dramatic physical changes resulted in mass extinctions, during which a large proportion of species living at the time disappeared.


Earth’s climate has changed through time; sometimes much warmer than today, sometimes much colder.

Drops in sea level were related to glaciation and often resulted in mass extinctions.

Earth’s cold periods were separated by long periods of milder climates.

The Quaternary period has been marked by a series of glacial advances interspersed with warmer interglacial intervals.

Figure 18.3 Sea Levels Have Changed Repeatedly


Some major climatic shifts are rapid (5,000 to 10,000 yrs), as a result of changes in Earth’s orbit around the sun.


Today’s rapid climate change is due to increasing CO2 concentrations, mostly from burning fossil fuels.

We are reversing the process of organic burial that occurred in the Carboniferous and Permian. It took millions of years for these deposits to accumulate.


The current rate of increase of atmospheric CO2 is unprecedented in Earth’s history.

If CO2 concentration doubles, average Earth temperature will increase, causing droughts, rising sea level, melting ice caps, and other major changes.


Large volcanic eruptions can have major impacts.

Ash and SO2 are injected into the atmosphere, which blocks sunlight and results in cooling.

Collision of continents during the Permian formed a single land mass and caused massive volcanic eruptions, leading to the greatest mass extinction in Earth’s history.

Figure 18.4 Volcanic Eruptions Can Cool Global Temperatures


Collision of meteorites and comets may have also caused mass extinctions.

Evidence includes impact craters and disfigured rocks with isotope ratios characteristic of meteorites.


A meteorite probably caused the mass extinction at the end of the Cretaceous (about 65 mya).

Evidence: A thin rock layer with high iridium content (common in meteorites); a huge impact crater beneath the northern coast of the Yucatán Peninsula.

The resulting tsunamis, debris plumes that blocked the sun, and massive fires had a devastating effect on biodiversity.

Figure 18.5 Evidence of a Meteorite Impact


Atmospheric oxygen concentration has also changed over time.

The early atmosphere probably had little or no O2.

Primitive Earth provided inorganic precursors from which organic molecules derived due to the presence of free energy and the absence of a significant amount of O2

• Miller and Urey experiment


These organic molecules served as monomers for the formation of amino acids and nucleotides.

The joining of these monomers produced polymers with the ability to replicate, store, and transfer information.

These complex reactions could have occurred in or out of solution.

The earliest genetic material could have been either DNA or RNA.


O2 first increased when certain bacteria evolved photosynthesis (about 2.4 bya).

Cyanobacteria formed stromatolites which are abundant in the fossil record.

Figure 18.6 Stromatolites (Part 1)

Figure 18.6 Stromatolites (Part 2)


O2 released by Cyanobacteria allowed evolution of oxidation reactions as the energy source for ATP synthesis.

Later, eukaryote cells with chloroplasts evolved, and O2 increased again.

Organisms with aerobic metabolism replaced anaerobes in most of Earth’s environments.


O2 also allowed larger and more complex organisms to evolve.

Larger cells have lower surface area-to-volume ratios and require higher O2 concentrations.

Further increases in O2 in the late Precambrian enabled evolution of multicellular organisms.

Figure 18.7 Larger Cells and Organisms Need More Oxygen


O2 concentrations increased again during the Carboniferous and Permian as large vascular plants evolved.

Plant debris was not oxidized but buried in swamps (forming coal deposits). Living plants were producing large quantities of O2.

High concentrations of O2 allowed evolution of giant flying insects and amphibians.


In experiments with hyperoxic conditions (high O2), Drosophila evolve larger body sizes over just a few generations.

The stabilizing selection on body size at present O2 concentrations can quickly switch to directional selection for increased body size in response to higher O2.

Concept 18.3 Major Events in the Evolution of LifeCan Be Read in the Fossil Record

The fossil record is used to reconstruct life’s history.

A biota—all organisms of all kinds living at a particular time or place.

All plants living at a particular time or place are its flora; all animals are its fauna.


The number of fossil species that have been found are only a tiny fraction of the species that have ever lived.

Only a tiny fraction of organisms become fossils, and only a fraction of those are found by paleontologists.


Most organisms are decomposed quickly after death.

If they are transported to sites with no oxygen, where decomposition is very slow, fossilization could occur.

Many geologic processes transform rocks and destroy the fossils they contain or bury them too deeply to be accessible.


The fossil record is most complete for marine animals with hard skeletons or shells.

Insects and spiders are also well represented.

Although the fossil record is incomplete, it is enough to document the history of the evolution of life.

Figure 18.9 Insect Fossils


Precambrian era

For most of this era, life consisted of microscopic prokaryotes living in oceans.

Life first appeared about 3.8 bya. Eukaryotes evolved about 1.5 bya.

Figure 18.10 A Sense of Life’s Time


By the late Precambrian, many kinds of multicellular soft-bodied animals had evolved.

Some were very different from any animals living today and may have no living descendants.

Figure 18.11 Precambrian Life


Cambrian period: Beginning of the Paleozoic era.

O2 concentration was near modern levels.

The Cambrian explosion was a rapid diversification of life.

Such periods of rapid diversification are known as evolutionary radiations.

Figure 18.12 A Brief History of Multicellular Life on Earth (Part 2)


Most of the major groups of animals living today appeared in the Cambrian.

Most life was aquatic; some fossil beds have preserved the soft parts of many organisms.


Ordovician period

Radiation of marine organisms, especially brachiopods and mollusks.

At the end of the period, massive glaciers formed, sea levels were lowered, and a mass extinction occurred.


Silurian period

Marine life rebounded from the late Ordovician extinction.

Vascular plants appeared, as well as some terrestrial arthropods—scorpions and millipedes.


Devonian period

Rates of evolutionary change accelerated in many groups.

Club mosses, horsetails, and tree ferns became common; forest soils developed. First seed plants.

Earliest insect, spider, and amphibian fossils.

Evolutionary radiations of corals and cephalopods. Jawed fishes replaced jawless forms.



An extinction at the end of the Devonian resulted in loss of 75% of marine species.

Two meteorite impacts may have contributed to this extinction. The craters are in Nevada and Western Australia.


Carboniferous period

Large glaciers at high latitudes but great swamp forests on tropical continents, which became fossilized as coal.

Terrestrial diversity increased; insects evolved wings; plant fossils show evidence of chewing by insect herbivores.

Amphibians became more terrestrial; a sister lineage led to the amniotes—vertebrates with well-protected eggs.

Figure 18.13 Evidence of Insect Diversification


Permian period

Continents came together to form the super-continent Pangaea.

Reptiles split from a second amniote lineage (which would lead to the mammals).

Ray-finned fishes became common in fresh waters.



End of the Permian: massive volcanic eruptions blocked sunlight and caused climate cooling, resulting in the largest glaciers in Earth’s history.

Atmospheric O2 concentrations dropped, making land above 500 m elevation uninhabitable.

These changes resulted in the greatest mass extinction in Earth’s history. About 96% of all multicellular species became extinct.


At the start of the Mesozoic era, the surviving organisms inhabited a relatively empty world.

The continents began to drift apart; sea levels rose and flooded the continents, forming large shallow seas.

Phytoplankton groups arose that dominate today’s oceans: dinoflagellates, coccolithophores, and diatoms. Their remains form oil deposits.


Triassic periodPangaea began to break apart. Conifers and

seed plants became dominant on land.Radiation of reptiles began, which gave rise to

crocodilians, dinosaurs, and birds.

A mass extinction at the end eliminated about 65% of species.



Jurassic period

Pangaea divided into Laurasia, which drifted northward, and Gondwana, which drifted southward.

First lizards and flying reptiles (pterosaurs) appeared; most large terrestrial animals were dinosaurs.

Flowering plants and several mammal groups appeared.


Cretaceous period

A continuous sea encircled the tropics. Earth was warm and humid.

Dinosaurs continued to diversify; snakes appeared.

Flowering plants began the radiation that led to their current dominance.



Another mass extinction at the end of the Cretaceous was caused by a meteorite.

On land, all animals larger than about 25 kg became extinct.

Many insects became extinct, perhaps because of lack of food plants.


Cenozoic era

The positions of the continents resembled those of today.

Extensive radiation of mammals.

Flowering plants dominated forests except in cool regions.

Evolution of symbiosis between legume plants and nitrogen-fixing bacteria.

Table 18.2 Subdivisions of the Cenozoic Era


Tertiary period

Climate was hot and humid at the beginning but became cooler and drier.

Many flowering plants evolved herbaceous forms. Grasslands spread.

Snakes, lizards, birds, and mammals underwent extensive radiations.

Mammals dispersed from Asia to North America across the Bering land bridge.



Quaternary period: Pleistocene and Holocene epochs.

During Pleistocene “ice ages,” continental glaciers spread, shifting the ranges of plants and animals.

Hominid evolution and radiation occurred.

Many large mammal species became extinct in Australia and the Americas when Homo sapiens arrived—possibly due to hunting pressure.


The fossil record reveals broad patterns in life’s evolution.

We can use phylogeny and the fossil record to estimate timing of major events in evolution.

The changing physical environment on Earth has clearly influenced the great diversity of life we see today.

Scientific evidence supports the idea that evolution continues to occur.

• Chemical resistance

• Emergent diseases

• Darwin’s finches observed phenotypic changes in a populations

Answer to Opening Question

Experiments have been conducted to test the hypothesis that atmospheric O2 concentrations affected evolution of body size.

The hypothesis has been supported by both hyperoxic and hypoxic experimental conditions.

Experiments with hypoxic conditions explain the extinction of many large flying insects at the end of the Permian, a result of rapidly decreasing O2 concentrations.

The History of Life on Earth 18. Chapter 18 The History of Life on Earth Key Concepts 18.1 Events in Earths History Can Be Dated 18.2 Changes in Earths.

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