Chapter 25: The History of Life on Earth

Question 1

In what type of rock are fossils most often found?

Answer 1

Sedimentary rocks are the richest source of fossils. As a result, the fossil record is based primarily on the sequence in which fossils have accumulated in sedimentary rock layers, called strata.

Question 2

What technique is most frequently used to date fossils?

Answer 2

One of the most common techniques is radiometric dating, which is based on the decay of radioactive isotopes. (A method for determining the absolute age of rocks and fossils, based on the half-life of radioactive isotopes).

Question 3

Both reptiles and mammals belong to a group of animals called ____. What three anatomical features are found in mammal fossils but are not found in other tetrapods?

Answer 3

Along with amphibians and reptiles, mammals belong to the group of animals called tetrapods (from the Greek tetra, four, and pod, foot), named for having four limbs. For example, the lower jaw is composed of one bone (the dentary) in mammals but several bones in other tetrapods. In addition, the lower and upper jaws in mammals hinge between a different set of bones than in other tetrapods. Mammals also have a unique set of three bones that transmit sound in the middle ear, the hammer, anvil, and stirrup, whereas other tetrapods have only one such bone, the stirrup (see Concept 34.6). Finally, the teeth of mammals are differentiated into incisors (for tearing), canines (for piercing), and the multi-pointed premolars and molars (for crushing and grinding). In contrast, the teeth of other tetrapods usually consist of a row of undifferentiated, single-pointed teeth.

Question 4

Study the Figure Exploring the Origin of Mammals. What evolutionary changes happen to the bones color-coded green and orange over time in the mammal lineage?

Orange: Articular, Green: Quadrate

Answer 4

Over the course of 120 million years, mammals originated gradually from a group of tetrapods called synapsids. Early synapsids (300 mya) had multiple bones in the lower jaw and single-pointed teeth. The jaw hinge was formed by the articular and quadrate bones. Later, a group of synapsids called therapsids appeared (280 mya). As in earlier synapsids, the jaw in cynodont (260 mya) had an articular-quadrate hinge. Later cynodont (220 mya) had teeth with complex cusp patters, and their lower and upper jaws hinged in two locations: they retained the original articular-quadrate hinge and formed a new, second hinge between the dentary and squamosal bones. In some very late (non-mammalian) cynodonts (195 mya) and early mammals, the original articular-quadrate hinge was lost, leaving the dentary-squamosal hinge as the only hinge between the lower and upper jaws, as in living mammals. The articular and quadrate bones migrated into the ear region, where they functioned in transmitting sound. In the mammal lineage, these two bones later evolved into the familiar hammer (malleus) and anvil (incus) bones of the ear.

Question 5

We are currently living in the Phanerozoic Eon. What are the three eras into which this eon is divided? Which one is the current era? During which of these eras were dinosaurs dominant?

Answer 5

The three eras into which the Phanerozoic Eon is divided include: Paleozoic Era, Mesozoic Era, and Cenozoic Era. We are currently living in the Cenozoic Era, which is itself broken down into three periods: the Paleogene Period, the Neogene Period, and the Quaternary Period. Dinosaurs were dominant during the Mesozoic Era, often called the “Age of the Dinosaurs.”

Question 6

Describe Earth’s first organisms and where they lived.

Answer 6

The Archaean Eon extends from 4 billion years ago to 2.5 billion years ago. During the Archaean, prokaryotes appeared around 3.5 billion years ago, and atmospheric oxygen developed around 2.7 billion years ago. Some prokaryotes bind thin films of sediment together, producing layered rocks called stromatolites. Earth’s first organisms were single-celled prokaryotes that lived in the ocean. The earliest direct evidence of these organisms, dating from 3.5 billion years ago, comes from fossilized stromatolites.

Question 7

Describe the “oxygen revolution,” its cause, and its effect on the atmosphere and on other organisms.

Answer 7

Most atmospheric oxygen gas is of biological origin, produced during the water-splitting step of photosynthesis. When oxygenic photosynthesis first evolved—in photosynthetic prokaryotes similar to today’s cyanobacteria—the free oxygen gas it produced probably dissolved in the surrounding water until it reached a high enough concentration to react with elements dissolved in water, including iron. This would have caused the iron to precipitate as iron oxide, which accumulated as sediments. These sediments were compressed into banded iron formations, red layers of rock containing iron oxide that are a source of iron ore today. Once all of the dissolved iron had precipitated, additional oxygen gas dissolved in the water until the seas and lakes became saturated with oxygen gas. After this occurred, the oxygen gas finally began to “gas out” of the water and enter the atmosphere. As shown in Figure 25.9, the amount of atmospheric oxygen gas increased gradually from about 2.7 to 2.4 billion years ago, but then shot up relatively rapidly to between 1% and 10% of its present level. This “oxygen revolution” had an enormous impact on life. As a result, the rising concentration of atmospheric oxygen gas probably doomed many prokaryotic groups. Some species survived in habitats that remained anaerobic, where we find their descendants living today (see Concept 27.4). Among other survivors, diverse adaptations to the changing atmosphere evolved, including cellular respiration, which uses oxygen gas in the process of harvesting the energy stored in organic molecules.

Question 8

The first dramatic rise in free oxygen in the atmosphere probably caused many prokaryotic species to go extinct. Why?

Answer 8

In some of its chemical forms, oxygen attacks chemical bonds and can inhibit enzymes and damage cells. As a result, the rising concentration of atmospheric oxygen gas probably doomed many prokaryotic groups.

Question 9

Explain the process of endosymbiosis that explains the origin of mitochondria and chloroplasts.

Answer 9

Current evidence indicates that the eukaryotes originated by endosymbiosis when a prokaryotic cell engulfed a small cell that would evolve into an organelle found in all eukaryotes, the mitochondrion. Mitochondria and chloroplasts display similarities with bacteria that led to the endosymbiont theory, illustrated in Figure 6.16. This theory states that an early ancestor of eukaryotic cells (a host cell) engulfed an oxygen using non-photosynthetic prokaryotic cell. Eventually, the engulfed cell formed a relationship with the host cell in which it was enclosed, becoming an endosymbiont (a cell living within another cell). Indeed, over the course of evolution, the host cell and its endosymbiont merged into a single organism, a eukaryotic cell with the endosymbiont having become a mitochondrion. At least one of these cells may have then taken up a photosynthetic prokaryote, becoming the ancestor of eukaryotic cells that contain chloroplasts. Fig. 6. 16. Ancestor of eukaryotic cells (host cell) drawn containing the nucleus with the nuclear envelope continuous with the Endoplasmic reticulum. Large arrow points to the host cell, which contains the nucleus and E R. Small arrows within the cell indicate the process of engulfing a formerly free-living cell oxygen-using prokaryote which, over many generations of cells, becomes a mitochondrion. This cell is now labeled a non-photosynthetic eukaryote. Large arrow points to a cell (at least one) labeled a photosynthetic eukaryote, which contains the nucleus, E R, and mitochondrion. Small arrows within the cell indicate the process of the host cell engulfing a formerly free-living photosynthetic prokaryote, which over time becomes a chloroplast. This cell is now labeled a photosynthetic eukaryote.

Question 10

What evidence suggests that mitochondria evolved before plastids?

Answer 10

Although all eukaryotes have mitochondria or remnants of these organelles, they do not all have plastids (a general term for chloroplasts and related organelles). Thus, the serial endosymbiosis hypothesis supposes that mitochondria evolved before plastids through a sequence of endosymbiotic events.

Question 11

How do pre-Cambrian animal fossils compare to those after the Cambrian explosion? What new features evolve in animal anatomy and their lifestyles?

Answer 11

Fossil evidence and DNA sequence data suggest that multicellular eukaryotes emerged about 1.3 billion years ago. The oldest known fossils of multicellular eukaryotes that can be resolved taxonomically are of relatively small red algae that lived 1.2 billion years ago. These fossils, referred to as the Ediacaran biota, were of mostly soft-bodied organisms—some over 1 m long—that lived from 635 to 541 million years ago (mya). The Ediacaran biota included both algae and animals, along with various organisms of unknown taxonomic affinity. Many present-day animal phyla appear suddenly in fossils formed 535–525 million years ago, early in the Cambrian period. This phenomenon is referred to as the Cambrian explosion. Prior to the Cambrian explosion, all large animals were soft-bodied. The fossils of large pre-Cambrian animals reveal little evidence of predation. Instead, these animals appear to have been grazers (feeding on algae), filter feeders, or scavengers, not hunters. The Cambrian explosion changed all of that. In a relatively short period of time (10 million years), predators over 1 m in length emerged that had claws and other features for capturing prey. Simultaneously, new defensive adaptations, such as sharp spines and heavy body armor, appeared in their prey.

Question 12

Describe the colonization of land by plants. What adaptations evolved that made the transition possible? How were fungi involved in enabling plant life on land?

Answer 12

However, larger forms of life, such as fungi, plants, and animals, did not begin to colonize land until about 500 million years ago. This gradual evolutionary venture out of aquatic environments was associated with adaptations that made it possible to reproduce on land and that helped prevent dehydration. For example, many plants today have a vascular system for transporting materials internally and a waterproof coating of wax on their leaves that slows the loss of water to the air. Early signs of these adaptations were present 420million years ago, at which time small plants (about 10 cm high) existed that had a vascular system but lacked true roots or leaves. Plants appear to have colonized land in the company of fungi. Even today, the roots of most plants are associated with fungi that aid in the absorption of water and minerals from the soil (see Concept 31.1). These root fungi (or mycorrhizae), in turn, obtain their organic nutrients from the plants.

Question 13

Approximately how long were arthropods common on land before the first tetrapods moved to land?

Answer 13

Although many animal groups are now represented in terrestrial environments, the most widespread and diverse land animals are arthropods (particularly insects and spiders) and tetrapods. Arthropods were among the first animals to colonize land, roughly 450 million years ago. The earliest tetrapods found in the fossil record lived about 365 million years ago and appear to have evolved from a group of lobe-finned fishes (see Concept 34.3).

Question 14

List the events in order, from earliest to most recent.

Question 15

List the events in order, from earliest to most recent.

Question 16

In a couple of sentences, describe what plate tectonics is, how it has affected climate, and how that can affect patterns of evolution.

Answer 16

The theory that the continents are part of great plates of Earth’s crust that float on the hot, underlying portion of the mantle. Movements in the mantle cause the continents to move slowly over time. According to the theory of plate tectonics, the continents are part of great plates of Earth’s crust that essentially float on the hot, underlying portion of the mantle (Figure 25.15). Movements in the mantle cause the plates to move over time in a process called continental drift. About 250 million years ago, plate movements brought previously separated landmasses together into a supercontinent named Pangaea. Overall, the formation of Pangaea greatly altered the physical environment and climate, which drove some species to extinction and provided new opportunities for groups of organisms that survived the crisis.

Question 17

What is mass extinction? How many are generally identified by paleontologists? What are some causes?

Answer 17

Mass extinction is the elimination of a large number of species throughout Earth, the result of global environmental changes. Five mass extinctions are documented in the fossil record over the past 500 million years (Figure 25.18). In each mass extinction, 50% or more of marine species became extinct. The five generally recognized mass extinction events, represent peaks in the extinction rate of marine animal families. These mass extinctions interrupted the overall increase, over time, in the number of extant families of marine animals. The Permian mass extinction, which defines the boundary between the Paleozoic and Mesozoic eras (252 million years ago), claimed about 96% ofmarine animal species and drastically altered life in the ocean. Terrestrial lifewasalso affected. The Permian mass extinction occurred during the most extreme episode of volcanism in the past 500 million years. Geologic data indicate that 1.6 millionsquare kilometers (roughly half the size of Western Europe) in Siberia was covered with lava hundreds of meters thick. The eruptions are thought to have produced enough carbon dioxide to warm the global climate by an estimated 6 °C, harming many temperature-sensitive species. The rise in atmospheric carbon dioxide levels would also have led to ocean acidification, thereby reducing the availability of calcium carbonate, which is required by reef-building corals and many shell-building species (see Figure 3.12). The eruptions would also have added nutrients such as phosphorus to marine ecosystems, stimulating the growth of microorganisms. Upon their deaths, these microorganisms would have provided food for bacterial decomposers. Bacteria use oxygen as they decompose the bodies of dead organisms, thus causing oxygen concentrations to drop. This would have harmed oxygen-breathers and promoted the growth of anaerobic bacteria that emit a poisonous metabolic by-product, hydrogen sulfide gas. Mass extinctions have significant and long-term effects. By eliminating large numbers of species, a mass extinction can reduce a thriving and complex ecological community to a pale shadow of its former self. Mass extinctions can also alter ecological communities by changing the types of organisms residing there.

Question 18

What caused the Cretaceous mass extinction?

Answer 18

The Cretaceous mass extinction occurred 66 million years ago. This event extinguished more than half of all marine species and eliminated many families of terrestrial plants and animals, including all dinosaurs (except birds, which are members of the same group). One clue to a possible cause of the Cretaceous mass extinction is a thin layer of clay enriched in iridium that dates to the time of the mass extinction. Iridium is an element that is very rare on Earth but common in many of the meteorites and other extraterrestrial objects that occasionally fall to Earth. As a result, researchers proposed that this clay is fallout from a huge cloud of debris that billowed into the atmosphere when an asteroid collided with Earth. This cloud would have blocked sunlight and caused a sudden drop in global temperatures lasting for several months to years.

Question 19

How are mass extinctions related to adaptive radiations?

Answer 19

The fossil record shows that the diversity of life has increased over the past 250 million years (see blue line in Figure 25.18). This increase has been fueled by adaptive radiations, periods of evolutionary change in which groups of organisms form many new species whose adaptations allow them to fill different ecological roles, or niches, in their communities. Large-scale adaptive radiations occurred after each of the big five mass extinctions, when survivors became adapted to the many vacant ecological niches. Adaptive radiations have also occurred in groups of organisms that possessed major evolutionary innovations, such as seeds or armored body coverings, or that colonized regions in which they faced little competition from other species.

Question 20

Why do many scientists consider us to be in the middle of a mass extinction now? If we are, what is causing it?

Answer 20

Monitoring programs show that many species are declining at an alarming rate due to habitat loss, introduced species, overharvesting, and other factors. Recent studies on a variety of organisms, including lizards, pine trees, and polar bears, suggest that climate change may hasten some of these declines. The fossil record also highlights the potential importance of climate change: Over the last 500 million years, extinction rates have tended to increase when global temperatures were high (Figure25.20). Overall,the evidence suggests that unless dramatic actions are taken, a sixth, human-caused mass extinction is likely to occur within the next few centuries.

Question 21

What conditions lead to regional adaptive radiations?

Answer 21

Striking adaptive radiations have also occurred over more limited geographic areas. Such radiations can be initiated when a few organisms make their way to a new, often distant location in which they face relatively little competition from other organisms. TheHawaiian archipelago is one of the world’s great showcases of this type of adaptive radiation (Figure 25.23).

Question 22

What are developmental genes? What role do they play in embryonic development?

Answer 22

Major changes in body form can result from changes in the sequences and regulation of developmental genes. Developmental genes affect morphological differences between species by influencing the rate, timing, and spatial patterns of change in an organism’s form as it develops into an adult. The evolution of new forms can be caused by changes in the nucleotide sequences or regulation of developmental genes. In particular, large morphological differences can result from genes that alter the rate, timing, and spatial pattern of change in an organism’s form as it develops from a zygote into an adult.

Question 23

Some people argue that evolution could never produce a structure as complex as a human eye. Briefly explain how Darwin would argue against that position.

Answer 23

François Jacob’s view of evolution harkens back to Darwin’s concept of descent with modification. As new species form, novel and complex structures can arise as gradual modifications of ancestral structures. In many cases, complex structures have evolved in increments from simpler versions that performed the same basic function. For example, consider the human eye, an intricate organ constructed from numerous parts that work together in forming an image and transmitting it to the brain. How could the human eye have evolved in gradual increments? Some argue that if the eye needs all of its components to function, a partial eye could not have been of use to our ancestors. The flaw in this argument, as Darwin himself noted, lies in the assumption that only complicated eyes are useful. In fact, many animals depend on eyes that are far less complex than our own.