Lesson Overview 19.1 The Fossil Record

Fossils and Ancient Life What do fossils reveal about ancient life? From the fossil record, paleontologists learn about the structure of ancient organisms, their environment, and the ways in which they lived.

Fossils and Ancient Life Fossils are the most important source of information about extinct species, ones that have died out. Fossils vary enormously in size, type, and degree of preservation. They form only under certain conditions. For every organism preserved as a fossil, many died without leaving a trace, so the fossil record is not complete.

Types of Fossils Fossils can be as large and perfectly preserved as an entire animal, complete with skin, hair, scales, or feathers. They can also be as tiny as bacteria, developing embryos, or pollen grains. Many fossils are just fragments of an organism—teeth, pieces of a jawbone, or bits of leaf. Sometimes an organism leaves behind trace fossils—casts of footprints, burrows, tracks, or even droppings. Although most fossils are preserved in sedimentary rocks, some are preserved in other ways, like in amber.

Fossils in Sedimentary Rock Most fossils are preserved in sedimentary rock. Sedimentary rock usually forms when small particles of sand, silt, clay, or lime muds settle to the bottom of a body of water. As sediments build up, they bury dead organisms that have sunk to the bottom.

Fossils in Sedimentary Rock As layers of sediment continue to build up over time, the remains are buried deeper and deeper. Over many years, water pressure gradually compresses the lower layers and turns the sediments into rock.

Fossils in Sedimentary Rock The preserved remains may later be discovered and studied.

Fossils in Sedimentary Rock Usually, soft body structures decay quickly after death, so usually only hard parts like wood, shells, bones, or teeth remain. These hard structures can be preserved if they are saturated or replaced with mineral compounds.

Fossils in Sedimentary Rock Sometimes, however, organisms are buried so quickly that soft tissues are protected from aerobic decay. When this happens, fossils may preserve imprints of soft-bodied animals and structures like skin or feathers.

What Fossils Can Reveal The fossil record contains an enormous amount of information for paleontologists, researchers who study fossils to learn about ancient life. By comparing body structures in fossils to body structures in living organisms, researchers can infer evolutionary relationships and form hypotheses about how body structures and species have evolved. Bone structure and trace fossils, like footprints, indicate how animals moved.

What Fossils Can Reveal Fossilized plant leaves and pollen suggest whether the area was a swamp, a lake, a forest, or a desert. When different kinds of fossils are found together, researchers can sometimes reconstruct entire ancient ecosystems.

Dating Earth’s History How do we date events in Earth’s history? Relative dating allows paleontologists to determine whether a fossil is older or younger than other fossils. Radiometric dating uses the proportion of radioactive to nonreactive isotopes to calculate the age of a sample.

Relative Dating Lower layers of sedimentary rock, and fossils they contain, are generally older than upper layers. Relative dating places rock layers and their fossils into a temporal sequence.

Relative Dating To help establish the relative ages of rock layers and their fossils, scientists use index fossils. Index fossils are distinctive fossils used to establish and compare the relative ages of rock layers and the fossils they contain. If the same index fossil is found in two widely separated rock layers, the rock layers are probably similar in age.

Relative Dating A good index fossil species must be easily recognized and will occur in only a few rock layers (meaning the organism lived only for a short time). These layers, however, will be found in many places (meaning the organism was widely distributed). Trilobites, a large group of distinctive marine organisms, are often useful as index fossils.

Radiometric Dating Relative dating is important, but provides no information about a fossil’s absolute age in years. One way to date rocks and fossils is radiometric dating. Radiometric dating relies on radioactive isotopes, which decay, or break down, into nonradioactive isotopes at a steady rate. Radiometric dating compares the amount of radioactive to nonreactive isotopes in a sample to determine its age.

Radiometric Dating A half-life is the time required for half of the radioactive atoms in a sample to decay. After one half-life, half of the original radioactive atoms have decayed. After another half-life, another half of the remaining radioactive atoms will have decayed.

Radiometric Dating Different radioactive elements have different half-lives, so they decay at different rates. The half-life of potassium-40 is 1.26 billion years.

Radiometric Dating Carbon-14, which has a short half-life, can be used to directly date very young fossils. Elements with long half-lives can be used to indirectly date older fossils by dating nearby rock layers, or the rock layers in which they are found.

Radiometric Dating Carbon-14 is a radioactive form of carbon naturally found in the atmosphere. It is taken up by living organisms along with “regular” carbon, so it can be used to date material that was once alive, such as bones or wood. After an organism dies, carbon-14 in its body begins to decay to nitrogen-14, which escapes into the air. Researchers compare the amount of carbon-14 in a fossil to the amount of carbon-14 in the atmosphere, which is generally constant. This comparison reveals how long ago the organism lived. Carbon-14 has a half-life of only about 5730 years, so it’s only useful for dating fossils no older than about 60,000 years.

Radiometric Dating For fossils older than 60,00 years, researchers estimate the age of rock layers close to fossil-bearing layers and infer that the fossils are roughly same age as the dated rock layers. A number of elements with long half-lives are used for dating very old fossils, but the most common are potassium-40 (half-life: 1.26 billion years) and uranium-238 (half-life: 4.5 billion years).

Geologic Time Scale How was the geologic time scale established, and what are its major divisions? The geologic time scale is based on both relative and absolute dating. The major divisions of the geologic time scale are eons, eras, and periods.

Geologic Time Scale Geologists and paleontologists have built a time line of Earth’s history called the geologic time scale. The basic divisions of the geologic time scale are eons, eras, and periods.

Establishing the Time Scale By studying rock layers and index fossils, early paleontologists placed Earth’s rocks and fossils in order according to their relative age. They noticed major changes in the fossil record at boundaries between certain rock layers.

Establishing the Time Scale Geologists used these boundaries to determine where one division of geologic time ended and the next began. Years later, radiometric dating techniques were used to assign specific ages to the various rock layers.

Divisions of the Geologic Time Scale The time scale is based on events that did not follow a regular pattern. The Cambrian Period, for example, began 542 million years ago and continued until 488 million years ago, which makes it 54 million years long. The Cretaceous Period was 80 million years long.

Divisions of the Geologic Time Scale Geologists now recognize four eons of unequal length. The Hadean Eon, during which the first rocks formed, began about 4.6 billion years ago. The Archean Eon, when life first appeared, began about 4 billion years ago. The Proterozoic Eon began 2.5 billion years ago and lasted until 542 million years ago. The Phanerozoic Eon began at the end of the Proterozoic and continues to the present.

Divisions of the Geologic Time Scale Eons are divided into eras. The Phanerozoic Eon, for example, is divided into the Paleozoic, Mesozoic, and Cenozoic Eras. Eras are subdivided into periods, which range in length from nearly 100 millions of years to just under 2 million years. The Paleozoic Era, for example, is divided into six periods.

Naming the Divisions Geologists started to name divisions of the time scale before any rocks older than the Cambrian Period had been identified. For this reason, all of geologic time before the Cambrian is simply called Precambrian Time.

Naming the Divisions The Precambrian actually covers about 90 percent of Earth’s history. In this figure, the history of Earth is depicted as a 24-hour clock. Notice the relative length of Precambrian Time—almost 22 hours.

Life on a Changing Planet How have our planet’s environment and living things affected each other to shape the history of life on Earth? Building mountains, opening coastlines, changing climates, and geological forces have altered habitats of living organisms repeatedly throughout Earth’s history. In turn, the actions of living organisms over time have changed conditions in the land, water, and atmosphere of planet Earth.

Life on a Changing Planet Earth and its climate has been constantly changing, and organisms have evolved in ways that responded to those new conditions. The fossil record shows evolutionary histories for major groups of organisms as they have both responded to changes on Earth and how they have changed Earth.

Physical Forces Climate is one of the most important aspects of Earth’s physical environment. Earth’s climate has undergone dramatic changes over time. Many of these changes were triggered by fairly small shifts in global temperature. During the global “heat wave” of the Mesozoic Era, Earth’s average temperatures were only 6°C to 12°C higher than they were during the twentieth century. During the ice ages, world temperatures were only about 5°C cooler than they are now. These relatively small temperature shifts changed the shape of life on Earth.

Physical Forces Geological forces have transformed life on Earth, producing new mountain ranges and moving continents. Volcanic forces have altered landscapes and even formed entire islands. Local climates are shaped by the interaction of wind and ocean currents with geological features such as mountains and islands.

Physical Forces The theory of plate tectonics explains how solid continental “plates” move slowly above Earth’s molten core—a process called continental drift. Over the long term, continents have collided to form “supercontinents.” Later, these supercontinents have split apart and reformed.

Geological Cycles and Events Continental drift has affected the distribution of fossils and living organisms worldwide. As continents drifted apart, they carried organisms with them. For example, the continents of South America and Africa are now widely separated. But fossils of Mesosaurus, a semiaquatic reptile, have been found in both South America and Africa. The presence of these fossils on both continents, along with other evidence, indicates that South America and Africa were joined at one time.

Physical Forces Evidence indicates that over millions of years, giant asteroids have crashed into Earth. Many scientists agree that these kinds of collisions would toss up so much dust that it would blanket Earth, possibly blocking out enough sunlight to cause global cooling. This could have contributed to, or even caused, worldwide extinctions.

Biological Forces The activities of organisms have affected global environments. For example, Earth’s early oceans contained large amounts of soluble iron and little oxygen. During the Proterozoic Eon, however, photosynthetic organisms produced oxygen gas and also removed large amounts of carbon dioxide from the atmosphere. The removal of carbon dioxide reduced the greenhouse effect and cooled the globe. The iron content of the oceans fell as iron ions reacted with oxygen to form solid deposits. Organisms today shape the landscape by building soil from rock, and sand and cycle nutrients through the biosphere.

Speciation and Extinction What processes influence whether species and clades survive or become extinct? If the rate of speciation in a clade is equal to or greater than the rate of extinction, the clade will continue to exist. If the rate of extinction in a clade is greater than the rate of speciation, the clade will eventually become extinct.

Speciation and Extinction Grand transformations in anatomy, phylogeny, ecology, and behavior—which usually take place in clades larger than a single species—are known as macroevolutionary patterns. The ways new species emerge through speciation, and the ways species disappear through extinction, are both examples of macroevolutionary patterns. The emergence, growth, and extinction of larger clades, such as mammals or dinosaurs, are also macroevolutionary patterns.

Macroevolution and Cladistics Paleontologists study fossils to learn about patterns of macroevolution and the history of life. Fossils are classified using the same cladistic techniques, based on shared derived characters that are used to classify living species. In some cases, fossils are placed in clades that contain only extinct organisms. In other cases, fossils are classified into clades that include living organisms.

Adaptation and Extinction Throughout the history of life, organisms have faced changing environments. Some species can adapt to new conditions and thrive. Other species fail to adapt and become extinct. The rates at which species appear, adapt, and become extinct vary among clades and from one geologic time to another.

Adaptation and Extinction The emergence of new species with different characteristics can serve as the “raw material” for macroevolutionary change within a clade. If the “birth” rate of new species in a clade is equal to the “death” rate, or extinction, the clade will survive. If the “death” rate of the species exceeds the birth rate, the clade will die out. In some cases, the more varied the species in a particular clade are, the more likely the clade is to survive environmental change. The clade Reptilia is one example of a highly successful clade. It includes living organisms like crocodiles, but also dinosaurs and the surviving members of the dinosaur clade—birds.

Patterns of Extinction Species are always evolving and competing—and some species become extinct because of the slow but steady process of natural selection, referred to as background extinction. In contrast, a mass extinction affects many species over a relatively short period of time. In a mass extinction, entire ecosystems vanish and whole food webs collapse. Species become extinct because their environment breaks down and the ordinary process of natural selection can’t compensate quickly enough.

This graph shows how the rate of extinction has changed over time.

Patterns of Extinction Until recently, researchers looked for a single cause for each mass extinction. For example, geologic evidence shows that at the end of the Cretaceous Period, a huge asteroid crashed into Earth and caused global climate change. At about the same time, dinosaurs and many other species became extinct. It is reasonable to infer, then, that the asteroid played a significant role in this mass extinction. Many mass extinctions, however, were probably caused by several factors working in combination: volcanic eruptions, moving continents, and changing sea levels, for example.

Patterns of Extinction After a mass extinction, biodiversity is dramatically reduced. Extinction, however, offers new opportunities to survivors. As speciation and adaptation produce new species to fill empty niches, biodiversity recovers. This recovery takes a long time—typically between 5 and 10 million years. Some groups of organisms survive a mass extinction, while other groups do not.

Rate of Evolution How fast does evolution take place? Evidence shows that evolution has often proceeded at different rates for different organisms at different times over the long history of life on Earth.

Gradualism Gradualism involves a slow, steady change in a particular line of descent. The fossil record shows that many organisms have indeed changed gradually over time.

Punctuated Equilibrium The pattern of slow, steady change does not always hold. Horseshoe crabs, for example, have changed little in structure from the time they first appeared in the fossil record. This species is said to be in a state of equilibrium, which means that the crab’s structure has not changed much over a very long stretch of time.

Punctuated Equilibrium Punctuated equilibrium is the term used to describe equilibrium that is interrupted by brief periods of more rapid change. The fossil record reveals periods of relatively rapid change in many groups of organisms. Some biologists suggest that most new species are produced during periods of rapid change.

Rapid Evolution After Equilibrium Rapid evolution may occur after a small population becomes isolated from the main population. This small population can evolve faster than the larger one because genetic changes spread more quickly among fewer individuals. Rapid evolution may also occur when a small group of organisms migrates to a new environment. That’s what happened with the Galápagos finches.

Rapid Evolution After Equilibrium Mass extinctions open many ecological niches, creating new opportunities for those organisms that survive. Groups of organisms that survive mass extinctions evolve rapidly in the several million years after the extinction.

Adaptive Radiation and Convergent Evolution What are two patterns of macroevolution? Two important patterns of macroevolution are adaptive radiation and convergent evolution.

Adaptive Radiation Studies often show that a single species or a small group of species has diversified over time into a clade containing many species. These species display variations on the group’s ancestral body plan. They often occupy different ecological niches. These differences are the product of adaptive radiation, an evolutionary process by which a single species or a small group of species evolves over a relatively short time into several different forms that live in different ways.

This diagram shows part of the adaptive radiation of mammals.

Adaptive Radiation An adaptive radiation may occur when species migrate to a new environment or when extinction clears an environment of a large number of inhabitants. A species may also evolve a new feature that enables it to take advantage of a previously unused environment.

Adaptive Radiations in the Fossil Record Dinosaurs flourished for about 150 million years during the Mesozoic Era. The fossil record documents that during this time, mammals diversified but remained small. After most dinosaurs became extinct, however, an adaptive radiation began and produced the great diversity of mammals of the Cenozoic Era.

Modern Adaptive Radiations Both Galápagos finches and Hawaiian honeycreepers evolved from a single bird species. Both finches and honeycreepers evolved different beaks and different behaviors that enable each of them to eat different kinds of food.

Convergent Evolution Sometimes groups of organisms evolve in different places or at different times, but in similar environments. These organisms start out with different structures, but they face similar selection pressures. In these situations, natural selection may mold different body structures in ways that perform similar functions. Because they perform similar functions, these body structures may look similar. Evolution produces similar structures and characteristics in distantly-related organisms through the process of convergent evolution. Convergent evolution has occurred often in both plants and animals.

Convergent Evolution For example, mammals that feed on ants and termites evolved five times in five different regions as shown in the figure below. They all developed the powerful front claws, long hairless snout, and tongue covered with sticky saliva that are necessary adaptations for hunting and eating insects.

Coevolution What evolutionary characteristics are typical of coevolving species? The relationship between two coevolving organisms often becomes so specific that neither organism can survive without the other. Thus, an evolutionary change in one organism is usually followed by a change in the other organism.

Coevolution Sometimes, the life histories of two or more species are so closely connected that they evolve together. The process by which two species evolve in response to changes in each other over time is called coevolution.

Flowers and Pollinators Coevolution of flowers and pollinators is common and can lead to unusual results. For example, Darwin discovered an orchid whose flowers had a 40-centimeter-long structure called a spur with a supply of nectar at the bottom. Darwin predicted that some pollinating insect must have some kind of feeding structure that would allow it to reach the nectar. Darwin never saw that insect. About 40 years later, researchers discovered a moth with a 40-centimeter-long feeding tube that matched Darwin’s prediction.

Plants and Herbivorous Insects Plants and herbivorous insects also demonstrate close coevolutionary relationships. Over time, many plants have evolved bad-tasting or poisonous compounds that discourage insects from eating them. Once plants began to produce poisons, natural selection on herbivorous insects favored any variants that could alter, inactivate, or eliminate those poisons. Milkweed plants, for example, produce toxic chemicals. But monarch caterpillars not only can tolerate this toxin, they also can store it in their body tissues to use as a defense against their predators.

The Mysteries of Life’s Origins What do scientists hypothesize about early Earth and the origin of life? Earth’s early atmosphere contained little or no oxygen. It was principally composed of carbon dioxide, water vapor, and nitrogen, with lesser amounts of carbon monoxide, hydrogen sulfide, and hydrogen cyanide. Miller and Urey’s experiment suggested how mixtures of the organic compounds necessary for life could have arisen from simpler compounds on a primitive Earth.

The Mysteries of Life’s Origins “The RNA world” hypothesis proposes that RNA existed by itself before DNA. From this simple RNA-based system, several steps could have led to DNA-directed protein synthesis.

The Mysteries of Life’s Origins Geological and astronomical evidence suggests that Earth formed as pieces of cosmic debris collided with one another. While the planet was young, it was struck by one or more huge objects, and the entire globe melted. For millions of years, violent volcanic activity shook Earth’s crust. Comets and asteroids bombarded its surface. About 4.2 billion years ago, Earth cooled enough to allow solid rocks to form and water to condense and fall as rain. Earth’s surface became stable enough for permanent oceans to form.

The Mysteries of Life’s Origins This infant planet was very different from Earth today. Earth’s early atmosphere contained little or no oxygen. It was principally composed of carbon dioxide, water vapor, and nitrogen, with lesser amounts of carbon monoxide, hydrogen sulfide, and hydrogen cyanide. Because of the gases in the atmosphere, the sky was probably pinkish-orange. Because they contained lots of dissolved iron, the oceans were probably brown.

The First Organic Molecules In 1953, chemists Stanley Miller and Harold Urey tried recreating conditions on early Earth to see if organic molecules could be assembled under these conditions. They filled a sterile flask with water, to simulate the oceans, and boiled it.

The First Organic Molecules To the water vapor, they added methane, ammonia, and hydrogen, to simulate what they thought had been the composition of Earth’s early atmosphere. They passed the gases through electrodes, to simulate lightning.

The First Organic Molecules Next, they passed the gases through a condensation chamber, where cold water cooled them, causing drops to form. The liquid continued to circulate through the experimental apparatus for a week. After a week, they had produced 21 amino acids—building blocks of proteins.

The First Organic Molecules Miller and Urey’s experiment suggested how mixtures of the organic compounds necessary for life could have arisen from simpler compounds on a primitive Earth. We now know that Miller and Urey’s ideas on the composition of the early atmosphere were incorrect. But new experiments based on current ideas of the early atmosphere have produced similar results.

Formation of Microspheres Geological evidence suggests that during the Archean Eon, 200 to 300 million years after Earth cooled enough to carry liquid water, cells similar to bacteria were common. How did these cells originate? Large organic molecules form tiny bubbles called proteinoid microspheres under certain conditions. Microspheres are not cells, but they have some characteristics of living systems. Like cells, microspheres have selectively permeable membranes through which water molecules can pass. Microspheres also have a simple means of storing and releasing energy. Several hypotheses suggest that structures similar to proteinoid microspheres acquired the characteristics of living cells as early as 3.8 billion years ago.

Evolution of RNA and DNA Cells are controlled by information stored in DNA, which is transcribed into RNA and then translated into proteins. The “RNA world” hypothesis about the origin of life suggests that RNA evolved before DNA. From this simple RNA-based system, several steps could have led to DNA-directed protein synthesis.

Evolution of RNA and DNA A number of experiments that simulated conditions on early Earth suggest that small sequences of RNA could have formed from simpler molecules. Under the right conditions, some RNA sequences help DNA replicate. Other RNA sequences process messenger RNA after transcription. Still other RNA sequences catalyze chemical reactions. Some RNA molecules even grow and replicate on their own.

Evolution of RNA and DNA One hypothesis about the origin of life suggests that RNA evolved before DNA.

Production of Free Oxygen Microscopic fossils, or microfossils, of prokaryotes that resemble bacteria have been found in Archean rocks more than 3.5 billion years old. Those first life forms evolved in the absence of oxygen because at that time, Earth’s atmosphere contained very little of that highly reactive gas.

Production of Free Oxygen During the early Proterozoic Eon, photosynthetic bacteria became common. By 2.2 billion years ago, these organisms were producing oxygen.

Production of Free Oxygen At first, the oxygen combined with iron in the oceans, producing iron oxide, or rust. Iron oxide, which is not soluble in water, sank to the ocean floor and formed great bands of iron that are the source of most iron ore mined today. Without iron, the oceans changed color from brown to blue-green.

Production of Free Oxygen Next, oxygen gas began to accumulate in the atmosphere. The ozone layer began to form, and the skies turned their present shade of blue. Over several hundred million years, oxygen concentrations rose until they reached today’s levels

Production of Free Oxygen Many scientists think that Earth’s early atmosphere may have been similar to the gases released by a volcano today. The graphs show the composition of the atmosphere today and the composition of gases released by a volcano.

Production of Free Oxygen To the first cells, which evolved in the absence of oxygen, this reactive oxygen gas was a deadly poison that drove this type of early life to extinction. Some organisms, however, evolved new metabolic pathways that used oxygen for respiration and also evolved ways to protect themselves from oxygen’s powerful reactive abilities.

Origin of Eukaryotic Cells What theory explains the origin of eukaryotic cells? The endosymbiotic theory proposes that a symbiotic relationship evolved over time, between primitive eukaryotic cells and the prokaryotic cells within them.

Origin of Eukaryotic Cells One of the most important events in the history of life was the evolution of eukaryotic cells from prokaryotic cells. Eukaryotic cells have nuclei, but prokaryotic cells do not. Eukaryotic cells also have complex organelles. Virtually all eukaryotes have mitochondria, and both plants and algae also have chloroplasts.

Endosymbiotic Theory It is believed that about 2 billion years ago, some ancient prokaryotes began evolving internal cell membranes. These prokaryotes were the ancestors of eukaryotic organisms. According to endosymbiotic theory, prokaryotic cells entered those ancestral eukaryotes. The small prokaryotes began living inside the larger cells.

Endosymbiotic Theory Over time a symbiotic relationship evolved between primitive eukaryotic cells and prokaryotic cells in them.

Endosymbiotic Theory The endosymbiotic theory was proposed more than a century ago. At that time, microscopists saw that the membranes of mitochondria and chloroplasts resembled the cell membranes of free-living prokaryotes. This observation led to two related hypotheses.

Endosymbiotic Theory One hypothesis proposes that mitochondria evolved from endosymbiotic prokaryotes that were able to use oxygen to generate energy-rich ATP molecules. Without this ability to metabolize oxygen, cells would have been killed by the free oxygen in the atmosphere.

Endosymbiotic Theory Another hypothesis proposes that chloroplasts evolved from endosymbiotic prokaryotes that had the ability to photosynthesize. Over time, these photosynthetic prokaryotes evolved within eukaryotic cells into the chloroplasts of plants and algae.

Modern Evidence During the 1960s, Lynn Margulis of Boston University noted that mitochondria and chloroplasts contain DNA similar to bacterial DNA. She also noted that mitochondria and chloroplasts have ribosomes whose size and structure closely resemble those of bacteria. In addition, she found that mitochondria and chloroplasts, like bacteria, reproduce by binary fission when cells containing them divide by mitosis. These similarities provide strong evidence of a common ancestry between free-living bacteria and the organelles of living eukaryotic cells.

Sexual Reproduction and Multicellularity What is the evolutionary significance of sexual reproduction? The development of sexual reproduction sped up evolutionary change because sexual reproduction increases genetic variation.

Significance of Sexual Reproduction When prokaryotes reproduce asexually, they duplicate their genetic material and pass it on to daughter cells. This process is efficient, but it yields daughter cells whose genomes duplicate their parent’s genome. Genetic variation is basically restricted to mutations in DNA.

Significance of Sexual Reproduction When eukaryotes reproduce sexually, offspring receive genetic material from two parents. Meiosis and fertilization shuffle and reshuffle genes, generating lots of genetic diversity. The offspring of sexually reproducing organisms are never identical to either their parents or their siblings (except for identical twins). Genetic variation increases the likelihood of a population’s adapting to new or changing environmental conditions.

Multicellularity Multicellular organisms evolved a few hundred million years after the evolution of sexual reproduction. Early multicellular organisms likely underwent a series of adaptive radiations, resulting in great diversity.