21 The History of Life on Earth 21.1 How Do Scientists Date Ancient Events?21.2 How Have Earths Continents and Climates Changed over Time?21.3 What Are the Major Events in Lifes History?21.4 Why Do Evolutionary Rates Differ among Groups of Organisms?

21.1 How Do Scientists Date Ancient Events?Many evolutionary changes take place over long periods of time.To study long-term evolutionary change, we must think in time frames spanning millions of years; and imagine conditions very different from todays.

21.1 How Do Scientists Date Ancient Events?Fossils are preserved remains of ancient organisms, they tell us about body form or morphology, and where and how they lived.Earths history is recorded in rocks. Layers of rocks are called strata.

21.1 How Do Scientists Date Ancient Events?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.

Chapter Opener 2 Younger Rocks Lie on Top of Older Rocks

21.1 How Do Scientists Date Ancient Events?In the eighteenth century, geologists realized that fossils could also be used to age rocks.Certain fossils were always found in younger rocks, others were found in older rocks.Fossils in more recent strata were more similar to modern organisms.

21.1 How Do Scientists Date Ancient Events?Radioisotopes can be used to determine the actual age of rocks.Radioisotopes decay in a predictable pattern.Half-life is the time interval over which one half of the remaining radioisotope decays, changing into another element.

Figure 21.1 Radioactive Isotopes Allow Us to Date Ancient Rocks

Table 21.1Each radioisotope has a characteristic half-life.

21.1 How Do Scientists Date Ancient Events?To date an event, we must know (or be able to estimate) the concentration of the radioisotope at the start of the event.For 14C, production in the upper atmosphere is about equal to its natural decay.In an organism, the ratio of 14C to 12C stays constant during its lifetime.

21.1 How Do Scientists Date Ancient Events?When an organism dies, it is no longer incorporating 14C from the environment.The 14C that was present in the body decays with no replacement and the ratio of 14C to 12C decreases.This ratio can then be used to date fossils, up to about 50,000 years old.

21.1 How Do Scientists Date Ancient Events?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.But igneous rocks (e.g., lava or volcanic ash), that have intruded into layers of sedimentary rock can be dated.

21.1 How Do Scientists Date Ancient Events?Other radioisotopes are used to date older rocks.Decay of potassium-40 to argon-40 is used for the most ancient rocks.Radioisotope dating is combined with fossil analysis.

21.1 How Do Scientists Date Ancient Events?Other dating methods include paleomagnetic dating:Movement and reversals of Earths magnetic poles are recorded in igneous and sedimentary rocks at the time they were formed, by alignment of mineral grains and other characteristics.

21.1 How Do Scientists Date Ancient Events?The history of life is divided into geologic eras, which are subdivided into periods.Boundaries are based on changes in fossils.The eras were established before actual ages of rocks were known.

Table 21.2 (Part 1)

Table 21.2 (Part 2)

21.2 How Have Earths Continents and ClimatesChanged 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.

21.2 How Have Earths Continents and ClimatesChanged over Time?Earths crust is divided into solid plates about 40 km thickcollectively, the lithosphere.The plates float on a fluid layer of liquid rock or magma.Heat from radioactive decay in Earths core causes the magma to circulate, setting up convection currents.

21.2 How Have Earths Continents and ClimatesChanged over Time?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.

Figure 21.2 Plate Tectonics and Continental Drift

21.2 How Have Earths Continents and ClimatesChanged over Time?Position of the continents has changed dramatically over time.Influences of ocean circulation patterns, sea level, and global climateMass extinctions of marine animals have occurred when sea level dropped, exposing the continental shelves.

Figure 21.3 Sea Levels Have Changed Repeatedly

21.2 How Have Earths Continents and ClimatesChanged over Time?Earths 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.

21.2 How Have Earths Continents and ClimatesChanged over Time?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.

Figure 21.4 Stromatolites (Part 1)

Figure 21.4 Stromatolites (Part 2)

21.2 How Have Earths Continents and ClimatesChanged 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.

21.2 How Have Earths Continents and ClimatesChanged over Time?Some evolved the ability to metabolize the O2.Advantages: aerobic metabolism is faster and more energy is harvested.Aerobes replaced anaerobes in most environments.

21.2 How Have Earths Continents and ClimatesChanged over Time?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

21.2 How Have Earths Continents and ClimatesChanged over Time?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.

21.2 How Have Earths Continents and ClimatesChanged over Time?Earths climate has changed over time.Sometimes Earth was considerably hotter than today; sometimes colder, with extensive glaciation.

Figure 21.6 Hot/Humid and Cold/Dry Conditions Have Alternated over Earths History

21.2 How Have Earths Continents and ClimatesChanged 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.

21.2 How Have Earths Continents and ClimatesChanged over Time?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.

21.2 How Have Earths Continents and ClimatesChanged 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.

21.2 How Have Earths Continents and ClimatesChanged over Time?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 Earths Continents and ClimatesChanged 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.

Figure 21.7 Evidence of a Meteorite Impact

21.2 How Have Earths Continents and ClimatesChanged over Time?A large crater has been located beneath the northern coast of the Yucatn 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.

An Artists Conception of the Presumed Meteorite Impact of 65 Million Years Ago

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Photo 21.1 Lava dome on Krakatoa Island, Indonesia, has formed since the eruption in 1883.

Photo 21.2 A forest is becoming re-established on Krakatoa Island.

Photo 21.3 Barringer Crater, a meteor impact crater east of Flagstaff, AZ.

Photo 21.4 Fossil sea urchin from Cretaceous vs. test of recent sea urchin.

Photo 21.5 Fly in Baltic amber from the Eocene (50 million years old).

Photo 21.6 50-million-year-old flower preserved in Baltic amber.

Photo 21.7 Precambrian fossil invertebrate (Dickinsonia sp.), from Ediacara, So. Australia.

Photo 21.8 Another Precambrian invertebrate (Mawsonites spriggi) from Ediacara.

Photo 21.9 This Cambrian fossil (Opabinia regalis) has no known relationship to current animals.

Photo 21.10 Hyolithes carinata from Cambrian Burgess Shale may be an early mollusk.

Photo 21.11 Fossil arthropod (Waptia fieldensis), from the Burgess Shale, British Columbia.

Photo 21.12 Fossil trilobite (Olenellus vermontanus), from early Cambrian.

Photo 21.13 Fossil crinoid (Locrinus subcrassus), an echinoderm from the Ordovician.

Photo 21.14 Trilobite fossils (Hometelus bromidensis), from mid-Ordovician.

Photo 21.15 Fossil trilobites (Dalamites limulurus), from the Silurian period.

Photo 21.16 Fossil cystoid echinoderm (Caryocrinus ornatus), a crinoid relative; Silurian period.

Photo 21.17 Fossil of a seed fern frond (Alethopteris serlii); late Carboniferous.

Photo 21.18 Fossil plant (Cladophlebis australis); middle Jurassic.

Photo 21.19 Fossil skeleton of Champosaur natatos, fish-eating marine vertebrate; late Cretaceous.

Photo 21.20 Fossil fish (Knightia sp.), from early Eocene.

Photo 21.21. Cast of fossil bird (Primobucco mcgrewi), from early Eocene.

Photo 21.22 Fossil wood of an elm (Ulmus sp.), an angiosperm from the Miocene.

Photo 21.23 Grand Canyon, from Precambrian rocks at river level to highest Mesozoic rocks.

Photo 21.24 Argon laser; isotopic ratios of argon gas are used to calculate the rock's age.

Photo 21.25 Portion of a 3.5-billion-year-old stromatolite from the Warrawoona Group, Australia.

Photo 21.26 Laminae of stromatolites, 2 billion years old; Gunflint Formation, Lake Superior. LM.

Photo 21.27 Microfossils of coccoid cells, 750 million years old; Tindir Group, Yukon. LM.

Photo 21.28 Recent stromatolites off the Bahamas; columns about 1 m tall.

21.3 What Are the Major Events in Lifes 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.

21.3 What Are the Major Events in Lifes History?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.

21.3 What Are the Major Events in Lifes History?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 Lifes 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.

21.3 What Are the Major Events in Lifes History?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.

Figure 21.8 Insect Fossils

21.3 What Are the Major Events in Lifes History?The Precambrian EraFor 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.

Figure 21.9 Ediacaran Animals

21.3 What Are the Major Events in Lifes History?Cambrian PeriodBeginning of the Paleozoic EraO2 concentration was approaching modern levels.Continents formed large land masses, the largest called Gondwana.

Figure 21.10 Cambrian Continents and Fauna (Part 1)

Figure 21.10 Cambrian Continents and Fauna (Part 2)

21.3 What Are the Major Events in Lifes History?Rapid diversification of life took placecalled the Cambrian explosion.Most of the major groups of animals living today appeared in the Cambrian.Three different Cambrian fossil beds have preserved the soft parts of many animalsthe Burgess Shale, Sirius Passet, and Chengjiang site.

21.3 What Are the Major Events in Lifes History?Ordovician PeriodA great radiation of marine organisms occurred, especially among the brachiopods and mollusks.At the end of the period, massive glaciers formed over Gondwana, sea levels were lowered, and a mass extinction occurred.

21.3 What Are the Major Events in Lifes History?Silurian PeriodMarine life rebounded from the late Ordovician extinction.The first vascular plants appeared in the late Silurian, as well as some terrestrial arthropodsscorpions and millipedes.

Figure 21.11 Cooksonia, the Earliest Known Vascular Plant

21.3 What Are the Major Events in Lifes History?Devonian PeriodThe northern landmass (Laurasia) and southern landmass (Gondwana) moved towards each other.There were evolutionary radiations of corals and squid-like cephalopods.Jawed fishes replaced jawless forms.

Figure 21.12 Devonian Continents and Marine Communities (Part 1)

Figure 21.12 Devonian Continents and Marine Communities (Part 2)

21.3 What Are the Major Events in Lifes History?Club mosses, horsetails, and tree ferns became common in terrestrial habitats.Their roots accelerated weathering of rocks and soil formation.Ancestors of gymnosperms appeared.First known fossils of centipedes, spiders, mites, and insects.Fish-like amphibians began to occupy land.

21.3 What Are the Major Events in Lifes History?An extinction at the end of the Devonian resulted in loss of 75 percent of marine animals.Two meteorite impacts may have contributed to this extinction. The craters are in Nevada and western Australia.

21.3 What Are the Major Events in Lifes History?Carboniferous PeriodLarge glaciers formed over high latitudes but great swamp forests of horsetails and tree ferns grew on the tropical continents.These swamp plants became fossilized as coal.

21.3 What Are the Major Events in Lifes History?Diversity of terrestrial animals increased. Snails, centipedes, scorpions, and insects were abundant.Insects developed wings. Flight gave them access to tall plants.Amphibians became larger; their lineage split from the amniotesvertebrates with well-protected eggs.In the oceans, crinoids reached their greatest diversity.

Figure 21.13 Evidence of Insect Diversification

Figure 21.14 A Carboniferous Crinoid Meadow

21.3 What Are the Major Events in Lifes History?Permian PeriodContinents came together to form the supercontinent Pangaea.Reptiles outnumbered amphibians at the end of that period.Ray-finned fishes diversified.

Figure 21.15 Pangaea Formed in the Permian Period

21.3 What Are the Major Events in Lifes History?Near the end of the Permian, massive volcanic eruptions poured lava over large areas of Earth.Volcanic ash blocked sunlight and caused climate cooling, resulting in the largest glaciers in Earths history.

21.3 What Are the Major Events in Lifes History?O2 concentrations dropped to about 12 percentmost animals would have been unable to survive at elevations above 500 m.A combination of factors resulted in the greatest mass extinction in Earths history.

21.3 What Are the Major Events in Lifes 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.

21.3 What Are the Major Events in Lifes History?New seed plants replaced the trees of the Permian forests.Earths biota became increasingly provincializeddistinct biotas evolved on each continent.

21.3 What Are the Major Events in Lifes History?Triassic PeriodPangaea began to break apart.On land, conifers and pteridosperms became dominant.A great radiation of reptiles began, which gave rise to crocodilians, dinosaurs, and birds.A mass extinction at the end may have been caused by a meteorite impact in present-day Quebec.

21.3 What Are the Major Events in Lifes History?Jurassic PeriodLand once again in two continents, Laurasia and Gondwana.Ray-finned fishes began a great radiation.First salamanders, lizards, and flying reptiles (pterosaurs).

21.3 What Are the Major Events in Lifes History?Dinosaur lineages evolved into predators on two legs, and large herbivores on four legs.Several groups of mammals appeared.Flowering plants appeared.

Figure 21.16 Jurassic Parkland

21.3 What Are the Major Events in Lifes History?Cretaceous PeriodA continuous sea encircled the tropics. Earth was warm and humid.Dinosaurs continued to diversify.Flowering plants began the radiation that led to their current dominance.

Figure 21.17 Positions of the Continents during the Cretaceous Period

Figure 21.18 Flowering Plants of the Cretaceous

21.3 What Are the Major Events in Lifes History?By the end of the period, many mammal groups had evolved.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 went extinct, perhaps because of lack of food plants.

21.3 What Are the Major Events in Lifes History?The Cenozoic EraCharacterized 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.

Table 21.3

21.3 What Are the Major Events in Lifes History?Tertiary PeriodClimate was hot and humid at the beginning, but became cooler and drier about half way through.Many flowering plants evolved herbaceous forms. Grasslands spread.Snakes, lizards, birds, and mammals underwent extensive radiations.

21.3 What Are the Major Events in Lifes History?Three waves of mammals dispersed from Asia to North America across the Bering land bridge.Rodents, marsupials, primates, and hoofed mammals appeared in North America for the first time.

21.3 What Are the Major Events in Lifes History?Quaternary PeriodDivided into Pleistocene and Holocene epochs.Pleistocene was a time of drastic cooling and climate fluctuation.During four major and 20 minor ice ages, continental glaciers spread, shifting the ranges of plants and animals towards the equator.

21.3 What Are the Major Events in Lifes History?The last glaciers retreated from temperate latitudes about 15,000 years ago.The Pleistocene was also the time of hominid evolution and radiation.Many large mammal species became extinct in Australia and the Americas when Homo sapiens arrivedpossibly due to hunting pressure.

21.3 What Are the Major Events in Lifes History?Three great evolutionary radiations occurred that resulted in major new faunas.The Cambrian explosion60 million years later, the radiation that resulted in the Paleozoic faunaAfter the Permian extinction, in the Triassic

Figure 21.19 Evolutionary Faunas

21.4 Why Do Evolutionary Rates Differ among Groups of Organisms?The rate of evolutionary change has varied greatly at different times and in different lineages.Changes in the physical and biological environment are likely to stimulate evolutionary change.Climate change can shift ranges of organisms, bringing them into contact with previously unknown competitors or predators.

21.4 Why Do Evolutionary Rates Differ among Groups of Organisms?Species whose morphology has changed little over millions of years are called living fossils.Examples: horseshoe crabs, chambered nautilus, Gingko trees

Figure 21.20 Living Fossils

21.4 Why Do Evolutionary Rates Differ among Groups of Organisms?On average, rates of evolutionary change are very slow.There are many series of fossils that show gradual change.Example: Eight lineages of trilobites show gradual change in the number of rear dorsal ribs on the exoskeleton.

Figure 21.21 Rib Number Evolved Gradually in Trilobites (Part 1)

Figure 21.21 Rib Number Evolved Gradually in Trilobites (Part 2)

21.4 Why Do Evolutionary Rates Differ among Groups of Organisms?Gradual change appears to dominate the fossil record.One explanation is that climate change is usually slow.Ranges of organisms shift accordingly, so the environment in which an individual lived actually changed very little.As the climate warmed after the last glaciers, plants and animals shifted their ranges northward.

Figure 21.22 Some Species Expanded Their Ranges as Continental Glaciers Retreated (Part 1)

21.4 Why Do Evolutionary Rates Differ among Groups of Organisms?If the environment changes rapidly, some lineages may change rapidly.Example: The house finch lived in semiarid regions of western North America.It was released in New York City in 1939 and formed a small population there.Now birds in the eastern populations have already evolved distinct differences.

Figure 21.23 House Finches Changed Rapidly as Their Range Expanded (Part 1)

Figure 21.23 House Finches Changed Rapidly as Their Range Expanded (Part 2)

21.4 Why Do Evolutionary Rates Differamong Groups of Organisms?Rates of extinction have also varied.The five major extinction events reduced the biota, and were followed by high rates of evolution.But some groups have had high rates of extinction while others are proliferating.

21.4 Why Do Evolutionary Rates Differamong Groups of Organisms?At the end of the Cretaceous, groups of related mollusk species with large geographic ranges survived better than groups with small ranges, even if individual species in the group had small ranges.

21.4 Why Do Evolutionary Rates Differamong Groups of Organisms?An organisms diet can also affect extinction rates.Animals with specialized diets are more vulnerable to loss of their food supply.Large, specialized carnivores may be more likely to go extinct than small carnivores with generalized diets.This hypothesis has been tested using canid fossils.

Figure 21.24 Large, Specialized Canids Survived Shorter Times (Part 1)

Figure 21.24 Large, Specialized Canids Survived Shorter Times (Part 2)

21.4 Why Do Evolutionary Rates Differamong Groups of Organisms?Although agents of evolutionary change are operating today as they have in the past, the dramatic increase in human population is driving major changes.Hunting has caused extinction of many species.Humans drastically alter the vegetation of Earthconverting forests and grasslands to agricultural land.

21.4 Why Do Evolutionary Rates Differamong Groups of Organisms?Humans move thousands of species around the globe, deliberately and accidentally changing the ranges of species.Humans practice artificial selection and biotechnology that influences the evolution of some species.Humans have become a dominant agent of evolutionary change.