Multicellular Organisms and the Cambrian Explosion For the first 3 billion years of life on Earth, all organisms were unicellular. Eubacteria gave rise to aerobic and photosynthetic lineages, while archaebacteria evolved into three main groups: methanogens, extreme halophiles,and extreme thermophiles.once eukaryotic organisms evolved complex structures and processes, including mitosis and sexual reproduction, they would have had the benefit of much more extensive genetic recombination than would have been possible among prokaryotic cells. Photosynthesis continued to increase the oxygen concentration in the atmosphere to the benefit of aerobic organisms. Multicellular organisms, including plants, fungi, and animals, are thought to have evolved less than 750 million years ago. The oldest fossils of multicellular animals date from about 640 million years ago. However, during a 40-million-year period beginning about 565 million years ago, a massive increase in animal diversity occurred, referred to as the Cambrian explosion. Fossil evidence dating from this period shows the appearance of early arthropods, such as trilobites, as well as echinoderms and molluscs; primitive chordates which were precursors to the vertebrates also appeared. Animals representing all present-day major phyla, as well as many that are now extinct, first appeared during this period, a time span that represents less than 1% of Earth s history. Figure 7 Creatures from Burgess Shale in Yoho National Park, B.C. methanogens species of archaebacteria that produce methane as a waste product extreme halophiles species of archaebacteria that live in such extremely saline environments as landlocked saline lakes extreme thermophiles species of archaebacteria that live in very hot aquatic environments, such as hot springs and hydrothermal vents on the ocean floor Cambrian explosion a period, beginning about 565 million years ago and lasting about 40 million years, during which the animal kingdom underwent rapid speciation and diversification; the origin of almost all major groups of animals can be traced to this period DID YOU KNOW? B.C. s Burgess Shales Fossil beds in the Burgess Shale in British Columbia likely formed through a series of mudslides, which buried living organisms in moving sediment. The resulting fossils of early Cambrian animals are superbly preserved (Figure 7). Many of the animals had unusual body forms not found in presentday animals. Discovered in 1909 by Charles D. Walcott of the Smithsonian Institute, it was immediately declared the most important geological find in the world. The Burgess Shale is now a World Heritage Site. Diversification and Mass Extinctions Figure 8 (page 596) provides a geological time scale and summarizes some of the most significant events in the evolutionary history of Earth since the Cambrian explosion. Geologists have established a geological time scale divided into five eras, each of which is further subdivided into periods and, in some cases, epochs. These time intervals are based on their distinctive fossil records, and dramatic changes in the fossil records mark the boundaries between these intervals. The eras of the Paleozoic (ancient life), Mesozoic (middle life), and Cenozoic (recent life) are remarkable for rapid diversification of life forms, as well as widespread extinctions. The Paleozoic era, for instance, begins with the Cambrian explosion and ends with the Permian extinction, believed to be the most massive extinction in Earth s history. The Evolutionary History of Life 595
Moving Continents Era Period Epoch Age millions of years ago (mya) 10 mya 65 mya Cenozoic Mesozoic Quaternary Tertiary Cretaceous Jurassic Recent Pleistocene Pliocene Miocene Oligocene Eocene Paleocene Late Early 0.01 1.8 5 24 37 58 65 100 144 240 mya Triassic Permian 208 245 286 Carboniferous 360 370 mya Paleozoic Devonian Silurian 408 438 Ordovician 505 Cambrian 420 mya 570 Proterozoic Oxygen(O ) abundant 2 2000 2500 540 mya Oldest fossils known 3500 Archean Oldest dated rocks 3800 Approximate origin of the earth 4600 Figure 8 Major events of life on Earth. Mass extinction events have occurred along with a general trend of increasing biological diversity. 596 Chapter 13
Range of Global Diversity (Marine and Terrestrial) Mass Extinction 65 mya extinction of large reptiles mammal radiation begins angiosperm plants dominate 65 mya 208 mya 245 mya 135 180 mya birds appear reptiles rule land, air, and sea mammals appear angiosperm plants appear 180 225 mya cycad-like and conifer trees dominate mammal-like reptiles appear early dinosaurs appear 225 280 mya reptiles radiate coniferous trees radiate and modernize 360 mya 280 345 mya reptiles appear amphibians and insects radiate coniferous trees appear 438 mya 345 395 mya amphibians appear trees and forests appear insects appear first bony fish appear land plants radiate 395 435 mya land plants appear arthropods invade land jawed fish appear armoured fish dominate 435 500 mya vertebrates appear armored jawless fish appear shell-bearing marine invertebrates dominate 500 570 mya shell-bearing animals appear marine invertebrates radiate Note also the dramatic changes that have occurred in the arrangements of the Earth s land surface. The Evolutionary History of Life 597
Figure 9 A computer-generated image of the 200-km-wide Chicxulub Crater located in the ocean floor at the edge of the Yucatan peninsula. It is thought to be the impact site of an asteroid that caused a mass extinction 65 million years ago, ending the Cretaceous period and the domination of the dinosaurs. Fossil evidence of diversification of marine invertebrates early in the Paleozoic era is very extensive. The first vertebrates are thought to have evolved later, followed by bony fish and amphibians. By the mid-paleozoic era, plants had invaded land surfaces and the first reptiles and insects had evolved. Around 245 million years ago, a series of cataclysmic events eradicated more than 90% of known marine species, as indicated by their disappearance from the fossil record after this period. Although uncertainty remains about causes of this Permian extinction, many scientists suspect that tectonic movements were a primary cause. The formation of the supercontinent Pangea, which occurred during the Permian period, would have produced major changes in terrestrial and coastal environments as well as in global climate (Figure 8, page 596).Ongoing research by Kunio Kaiho of Tohoku University, Japan, and his colleagues has uncovered evidence in southern China of a 60-km-wide asteroid that may have collided with Earth in this period. These researchers believe that the impact may have vaporized enough sulfur to consume a third of the atmospheric oxygen and generate enough acid rain to make the ocean surface water as acidic as lemon juice. If such a catastrophic impact did occur, it would have been the primary cause of the biggest extinction event in history. Despite the harsh conditions responsible for mass extinctions, life on earth continued. The Mesozoic era is well known for dinosaurs, a diverse group of often very large animals that dominated earth from about the mid Triassic to the late Cretaceous period. Oceans were home to many bony fish, hard-shelled molluscs, and crabs. On land, at first dominated by gymnosperms, early mammals evolved alongside dinosaurs and insects. Placental mammals, birds, and flowering plants also evolved within the Mesozoic era. After this time, the remaining dinosaurs and many other species suddenly disappear from the fossil record. Considerable evidence supports the hypothesis that an asteroid impact caused this best-known mass extinction. The Chicxulub Crater, almost 10 km deep and 200 km in diameter at the edge of the Yucatan peninsula, is thought to be the impact zone for such an asteroid (Figure 9). Some theorize that the asteroid would have been moving at about 160 000 km/h and would have blasted 200 000 km 3 of vaporized debris and dust into Earth s atmosphere. The debris and energy released in the resulting fireball equivalent to 100 million nuclear bombs would have killed most of the plants and animals in the continental Americas within minutes. Tidal waves 120 m high would have devastated coastlines around the world and atmospheric debris would have blocked out much of the sunlight for months. Among the strong evidence for the impact hypothesis is the presence of unusually high concentrations of iridium in sedimentary rock dated at 65 million years old, the boundary between the Mesozoic and Cenozoic eras. Rock samples from 95 locations worldwide show these same elevated levels. Iridium, a rare metal in the Earth s crust, is abundant in meteorite samples. These findings suggest that a large asteroid may have been the source of a great quantity of iridium-bearing dust, deposited on a global scale. Although the mass extinctions that ended the Permian and the Mesozoic eras are dramatic in scope, it is important to keep in mind that most species extinctions result from ongoing evolutionary forces of competition and environmental change. Amazingly, even the five major mass extinction events since the Cambrian explosion account for about only 4% of all extinctions that took place during this time. Scientists have also noted that periods of widespread extinction are followed by periods of very rapid diversification. In the present Cenozoic era, life forms have attained the greatest diversity in Earth s history. Flowering plants have out-competed gymnosperms in many habitats and now number more than 250 000 species. Millions of species of insects now dominate the animal kingdom. Are natural extinctions as much a part of evolution as diversification? It is probable that, had the dinosaurs not become extinct, the ancestors of humans may not have met with later successes which means that humans might have never existed. 598 Chapter 13
EXPLORE an issue Take a Stand: The Human Meteorite? Evaluate Many people are concerned about species now at risk for extinction. Since the 17th century, scientists have documented the extinction of more than 1000 plants and animals. The current list of endangered species worldwide is greater than 25 000. The dodo, great auk, passenger pigeon, Stellar s sea cow, and Banff long-nose dace are examples of species that have become extinct in recent time as a result of human activity. The causes of such human-driven extinctions are numerous, including habitat destruction, introduction of exotic species, overhunting, and commercial harvesting. Statement Because extinction is a natural process of evolution, and because the extinction of one species can benefit others, people should not be concerned about the loss of species even as a result of human activity. In your group, define the issue within the scope of humancaused extinctions. Decision-Making Skills Define the Issue Analyze the Issue Research Defend the Position Identify Alternatives Evaluate Research the issue, searching for information in print and electronic resources. GO www.science.nelson.com You might consider how the current pace of human-caused extinction compares with extinction rates in nature. Find out what tropical and conservation biologists, such as E.O. Wilson, think about this issue. What impact might the preservation of the genomes of endangered species have? What are possible effects of extinctions on ecosystems and on human health and welfare? What species are likely to benefit? How long might it take new species to fill the ecological gap left by species that become extinct? Write a position paper outlining your stand and be prepared to present your ideas to the class. The Rate of Evolution Biologists are keenly interested in the pace at which evolution may be occurring. Until recently, most supported the idea that changes to species were slow and steadily paced over time. The theory of gradualism contends that when new species first evolve, they appear very similar to the originator species and only gradually become more distinctive, as natural selection and genetic drift act independently on both species. One would expect to find, according to this theory, as a result of slow incremental changes, numerous fossil species representing transitional forms (also called intermediate forms). Many very distinct species, however, seem to appear suddenly in the fossil record with little evidence of gradual transitions from one species to another. Their sudden appearance is often followed by little change over very long periods of time. The most accepted explanation for these deviations from a gradualism model was that the fossil record is incomplete, and intermediate forms may not have been preserved. Niles Eldredge of the American Museum of Natural History and Stephen Jay Gould of Harvard University rejected this explanation and, in 1972, proposed an alternative theory called the theory of punctuated equilibrium.it consists ofthree main assertions: Species evolve very rapidly in evolutionary time. Speciation usually occurs in small isolated populations and thus intermediate fossils are very rare. After the initial burst of evolution, species do not change significantly over long periods of time. These contrasting theories about the rate of evolution are represented in Figure 10 (page 600). To some extent, the differences between them are a matter of perspective. To many population biologists, the word rapid in relation to species evolution suggests changes that can be measured in a few generations or, perhaps, decades. To paleontologists, rapid might represent the appearance of a new species in the fossil record within a thousand generations or 100 000 years. In fact, both theories are needed to understand the fossil record while remaining compatible with many other forms of evidence. Consider, for instance, how both theories apply to the evolution of species before and after a major extinction event. theory of gradualism a theory that attributes large evolutionary changes in species to the accumulation of many small and ongoing changes and processes transitional forms a fossil or species intermediate in form between two other species in a direct line of descent theory of punctuated equilibrium a theory that attributes large evolutionary changes to relatively rapid spurts of change followed by long periods of little or no change The Evolutionary History of Life 599
Before the event, an environment might be host to many well-adapted species that have evolved to occupy specific ecological niches. They are largely exposed to the pressures of stabilizing selection and evolutionary change would be very slow. An environmental crisis results in the extinction of most species, leaving many niches empty. Surviving species have many new opportunities and experience strong disruptive selection. These survivors can evolve rapidly into many new species, filling these empty niches. Once the new species become well adapted to their new niches in a relatively stable environment, they again experience stabilizing selection pressures. Thereafter, they show little, or more gradual, change until another crisis opens opportunities for diversification. It is now widely accepted that both gradual and rapid evolutionary processes are at work. Although many species have evolved rapidly at times, the fossil record of some organisms show very gradual change over extended periods of time. Time Figure 10 Two theories of the rate of evolution (a) punctuated equilibrium (b) gradualism TRY THIS activity Gradual? Or Rapid and Punctuated? Analyze and outline the evidence in sections 13.1 and 13.2 that supports and refutes the roles that both gradualism and punctuated equilibrium may have played in each of these instances: (i) the earliest evolution of single-celled organisms (from 3.5 to 1.0 billion years ago) (ii) the Cambrian explosion (iii) the evolution of photosynthetic eukaryotic cells (iv) the general evolutionary pattern from 350 to 250 million years ago (Figure 8, page 596) (v) the general evolutionary pattern from 55 to 38 million years ago (Figure 8) (vi) the early evolution of terrestrial plants and animals (vii) changes in species diversity before, during, and after a period of glaciation Be prepared to defend in a class discussion your conclusions about the roles gradualism and punctuated equilibrium have played in the evolution of life on earth. 600 Chapter 13
SUMMARY Earliest Evolutionary Patterns of Life Evidence suggests that, under the conditions present on Earth more than 4 billion years ago, organic molecules formed spontaneously, some able to form celllike structures, while others had enzymatic properties. Endosymbiosis is likely responsible for the evolution of both aerobic and photosynthetic eukaryotic cells. All life was unicellular for at least 2 billion years until about 600 million years ago, when life diversified dramatically. The history of multicellular life is a record of the ongoing evolution of millions of new species and the extinction of many others. Both gradualism and punctuated equilibrium account for the patterns seen in the evolution of life on Earth. Section 13.2 Questions Understanding Concepts 1. Draw to scale a time line to show key evolutionary processes and events as presented in this chapter, beginning with biochemical reactions 4 billion years ago and ending with the events of 65 million years ago. 2. Below are listed two time periods during which significant evolutionary events occurred. Describe at least two different kinds of evidence scientists used to develop hypotheses about these evolutionary events: (a) between 3 and 1 billion years ago (b) between 650 and 50 million years ago Applying Inquiry Skills 3. The Barringer Meteorite Crater, also called the Meteor Crater, in Arizona (Figure 11) is thought to have been created about 25 000 years ago by an asteroid about 45 m in diameter. Brainstorm some local and global evolutionary impacts that might have resulted from this asteroid s collision with Earth. Note scientific evidence that might support your reasoning. 4. The ability of ribozymes to recognize and cut specific RNA molecules makes them exciting candidates for human therapy. For example, one target for ribozymes might be the mrna that encodes vascular endothelial growth factor (VEGF). VEGF stimulates the production of blood vessels necessary for the rapid growth of cancer tumours. A ribozyme that destroys this mrna might prove valuable in the treatment of many cancers. Find out what other exciting research is underway regarding potential applications of ribozymes. Report your findings to the class. GO 5. Draw a fully labelled set of diagrams to illustrate and describe the evolution of an aerobic eukaryote by the process of endosymbiosis. Draw and clearly label all membrane and chromosomes. Figure 11 www.science.nelson.com The Evolutionary History of Life 601