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The History of Life on Earth

Early Earth showing volcanic activity and photosynthetic prokaryotes in dense mats

21 The History of Life on Earth In this chapter, we continue our discussion on how life changes over the span of 4 billion years, some major milestones, and of course, how scientists use a variety of tools available to confidently date events that happens hundreds of million years ago.

1 How Do Scientists Date Ancient Events? 2 How Have Earth’s Continents and Climates Changed over Time? 3 What Are the Major Events in Life’s History?

4 Why Do Evolutionary Rates Differ among Groups of Organisms?

Chapter Opener 2 Younger Rocks Lie on Top of Older Rocks

Fossils are preserved remains of ancient organisms, they tell us about body form or morphology, and where and how they lived. Earth’s history is recorded in rocks. Layers of rocks are called strata. Relative ages of rocks can be determined by looking at strata of undisturbed sedimentary rock. The oldest layers are at the bottom, youngest at the top. First observed in the 17th century by Nicolaus Steno.
Preserved bones Plants and reptile fossils Marine fossils

Primitive fish bones Shell fossils

Determining Earth’s Age

• Radioactivity provides a way to date rocks. • Radioactive isotopes decay in a predictable pattern over long periods of time. • The time it takes for half of a radioactive isotope to decay is that isotope’s half-life. Iodine 125 Iodine 131 Hydrogen 3 Carbon 14 57 days 8 days 12.3 years 5,730 years

• Each radioisotope has a characteristic half-life.

Potassium 40 1.3 bil years Uranium 238 4.5 bil years

Figure 21.1 Radioactive Isotopes Allow Us to Date Ancient Rocks

To use a radioisotope to date a past event, the concentration of the isotope at the time of that event must be known or estimated.


can be used to date fossils (and sedimentary rocks they were deposited in) less than 50,000 years old.

In the case of 14C, it is known that the ratio of 14C to 12C is relatively constant in the environment and living organisms. When an organism dies, 14C is no longer taken up by the cells, and the ratio of 14C to 12C decreases through time.

Carbon 14: C12 = 0.01 T1/2 for C14 is 5,730 years
Time T Alive 0.01 Environment 0.01

Time X Fossil 0.005 0.01

Time Y

Fossil 0.0025


Time Z

Fossil 0.00125


Determining Earth’s Age

• Sedimentary rocks are unreliable for dating (not stable). • To date sedimentary rocks, geologists look for lava flows between sedimentary layers. The lava can be dated by the decay of potassium40 to argon40. • When radioactive dating methods are not applicable, alternative approaches and observations are used, including paleomagnetism (based on movement of Earth’s magnetic field), continental drift, sea level changes, and molecular clocks.

Determining Earth’s Age

• Using information from these dating methods, geologists have divided Earth’s history into Eras and Periods. • Boundaries between the divisions are based on major differences in the fossil organisms contained in the layers. • The divisions (Eras) were established before the actual ages of the eras and periods were known. • In the Precambrian era, early life evolved.

Table 21.2 (Part 1)

Table 21.2 (Part 2)

21.2 How Have Earth’s Continents and Climates Changed over Time?

The idea that land masses have moved over time was first suggested by Alfred Wegener in 1912. By the 1960s, evidence of plate tectonics convinced geologists that he was right. Earth’s crust is divided into solid plates about 40 km thick—collectively, 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, setting up convection currents.

Figure 21.2 Plate Tectonics and Continental Drift

The movement of plates is called continental drift.
Where plates are pushed together, they move sideways past one another, or one is pushed underneath the other. Mountain ranges are pushed up, and deep rift valleys or trenches are formed. Where plates are pushed apart, ocean basins form.

The History of Continental Drift About 250 million years ago, all of Earth’s landmasses were locked together in a supercontinent named Pangaea.

About 180 million years ago, Pangaea began to split into northern (Laurasia) and southern (Gondwana) landmasses, which later separated into the modern continents.
India collided with Eurasia just 10 million years ago, forming the Himalayas, the tallest and youngest of Earth’s major mountain ranges. The continents continue to drift.

Figure 21.3 Sea Levels Have Changed Repeatedly

Position of the continents has changed dramatically over time. Influences of ocean circulation patterns, sea level, and global climate Mass extinctions of marine animals have occurred when sea level dropped, exposing the continental shelves.

21.2 How Have Earth’s Continents and Climates Changed over Time?
Earth’s atmosphere has also changed. Early atmosphere probably contained little or no free oxygen (O2). O2 began to increase when certain bacteria evolved the ability to use H2O as a source of H+ ions in photosynthesis. O2 was a waste product. Cyanobacteria formed rock-like structures called stromatolites which are abundant in the fossil record. Enough O2 was liberated to allow evolution of oxidation reactions to synthesize ATP.

21.2 How Have Earth’s Continents and Climates Changed over Time?

The evolution of life changed the physical nature of Earth. These changes in turn influenced the evolution of life. When O2 first appeared in the atmosphere it was poisonous to the anaerobic prokaryotes. Some evolved the ability to metabolize the O2. Advantages: aerobic metabolism is faster and more energy is harvested. Aerobes replaced anaerobes in most environments. Atmospheric O2 also made possible larger and more complex cells. About 1 billion years ago, eukaryote cells appeared.

Figure 21.5 Larger Cells Need More Oxygen

Figure 21.6 Hot/Humid and Cold/Dry Conditions Have Alternated over Earth’s History

Change in atmospheric O2 concentrations was unidirectional. Most physical conditions have oscillated over time in response to drifting continents, volcanic activity, and even extraterrestrial events such as meteorite impacts. Sometimes these events caused mass extinctions. Earth’s climate has changed over time. Sometimes Earth was considerably hotter than today; sometimes colder, with extensive glaciation.

21.2 How Have Earth’s Continents and Climates Changed over Time? For Earth to be cold and dry, atmospheric CO2 must have been much lower, but it is unclear what would cause low concentrations. Some climate changes have been very rapid. Extinctions caused by them appear to be ―instantaneous‖ in the fossil record. Today we are in a period of rapid climate change due to increasing CO2 concentrations, mostly from burning fossil fuels. Current CO2 concentration is greater than it has been for several thousand years. If CO2 concentration doubles, average Earth temperature will increase, causing droughts, sea increase, melting ice caps, and other major changes.
380 370 CO2 concentration (ppm) Temperature 0.60 0.45 350 0.30 1.05 0.90 Temperature variation (°C) 0.75


0.15 330 320 310 300 1960 1965 1970 1975 1980 1985 1990 Year 1995 2000 2005 CO2 0
–0.15 –0.30 –0.45

The Changing Face of Earth

Polar Bears Shored by Arctic Melting
Larry O'Hanlon, Discovery News

21.2 How Have Earth’s Continents and Climates Changed over Time?

Volcanic eruptions can trigger major climate change. When continents came together to form Pangaea in the Permian period, many volcanic eruptions reduced sunlight penetration and thus photosynthesis. Massive glaciation resulted. Collisions with large meteorites are probably the cause of several mass extinctions. Evidence of impacts include large craters and disfigured rocks; molecules with helium and argon isotope ratios characteristic of meteorites.

21.2 How Have Earth’s Continents and Climates Changed over Time?

A meteorite is thought to have caused or contributed to the mass extinction at the end of the Cretaceous period, 65 million years ago. First evidence was from a thin layer containing the element iridium. This element is very rare on Earth but abundant in some meteorites.

An Artist’s Conception of the Presumed Meteorite Impact of 65 Million Years Ago

A large crater has been located beneath the northern coast of the Yucatán Peninsula, Mexico. A massive plume of debris from the impact heated the atmosphere, ignited fires, and blocked the sunlight. Settling debris formed the iridium-rich layer.

Diversity of Life and Periods of Mass Extinction

21.3 What Are the Major Events in Life’s History?

Life first evolved about 3.8 billion years ago. Eukaryotic organisms had evolved by about 1.5 billion years ago. The number of individuals and species increased dramatically in the late Precambrian. The assemblage of all kinds of organisms alive at one time (or in one place) is called the biota. All the plants are the flora and all the animals are the fauna. Although about 300,000 species of fossils have been described, they are only a tiny fraction of species that have existed on Earth. Only a tiny fraction of organisms become fossils, and only a fraction of those are studied by paleontologists.

21.3 What Are the Major Events in Life’s History?

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. A large number of fossil species are marine organisms that had hard shells or skeletons that resist decomposition. Insects and spiders are also well represented in the fossil record.

21.3 What Are the Major Events in Life’s History?

The Precambrian Era
For most of this era, life was microscopic, prokaryote cells living in oceans. Eukaryotes evolved about 2/3 through the Precambrian. By the late Precambrian, soft-bodied multicellular animals had evolved.

21.3 What Are the Major Events in Life’s History?

Paleozoic Era
Cambrian Period Beginning of the Paleozoic Era O2 concentration was approaching modern levels. Continents formed large land masses, the largest called Gondwana. Rapid diversification of life took place—called the Cambrian explosion. Most of the major groups of animals living today appeared in the Cambrian.

21.3 What Are the Major Events in Life’s History?

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. Three groups of phytoplankton became ecologically important: dinoflagellates, coccolithophores, and diatoms. New seed plants replaced the trees of the Permian forests. Earth’s biota became increasingly provincialized—distinct biota’s evolved on each continent.

21.3 What Are the Major Events in Life’s History?

The Cenozoic Era
Characterized by an extensive radiation of mammals.

Flowering plants came to dominate forests except in cool regions.
Mutations in one group of plants allowed them to form symbiotic associations with N-fixing bacteria. This dramatically increased N available for terrestrial plants.

Figure 21.19 Evolutionary Faunas

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