CHAPTER 2 Physiological Evolution of Animals Learning Objectives After reading this chapter, you should be able to: 1 Explain the evolutionary relationship FIGURE 2.1 Yeti Crab (Kiwa hirsuta) between protozoans and metazoans. Photo source: Ifremer, A. Fifis/AP Images. 2 Demonstrate familiarity with the evolutionary relationships among animals. 3 Identify the major events in animal evolution. 4 Discuss the evolutionary origins of specific he diversity of life on Earth inspires many to learn more physiological abilities. about biology. Whether your awareness of animals comes from your own experience or watching nature shows on television, you have some appreciation for the breadth of T animal diversity. Remarkably, there are a great many living animals yet to be discovered, and paleontologists regularly uncover new types of animals in the fossil record. One challenge for scientists is to figure out how all of these animals are related, and thus what they tell us about the evolutionary origins of animals. More than 10 years ago the Census of Marine Life began a project of exploring the world to catalog the many animals living in oceans, as well as to find new species. The strange animals the Census found, such as the “fur”- covered Yeti crab (Figure 2.1), spark the imagination about the as-yet undis- covered life forms that likely exist on the planet. Though living animals are indeed remarkably diverse, those alive today represent only a snapshot in time. 20 DESIGN SERVICES OF # 153477 Cust: Pearson Au: Moyes Pg. No. 20 C/M/Y/K S4CARLISLE M02_MOYE8179_03_SE_CH02.inddTitle: Principles of Animal20 Physiology Short / Normal / Long Publishing Services 12/3/14 8:18 PM A far richer world is evident from explorations of the his 1990 book, Wonderful Life: The Burgess Shale and the fossil record. In the early 1900s, a paleontologist named Nature of History, in which he proposed that many of the Charles Walcott uncovered an extraordinary fossil bed in the Burgess Shale fossils were members of phyla that are now Canadian Rockies. The deposit was remarkable because extinct. A more conservative interpretation is that these fos- of the richness of the collection of soft-bodied animals, of sils all belong to extant phyla. Though many lineages within types that had largely escaped preservation in other fossil the phyla may have disappeared entirely, this approach as- beds. Analysis of this Burgess Shale collection reveals a sumes that the number of phyla has not changed markedly. rich diversity in animals present in the area around 500 mya An awareness of the evolutionary origins and phyloge- (million years ago). When the fossils were discovered, netic diversity of animals is essential for an understanding of Walcott assigned each to the most similar groups of extant the conservation and divergence in animal traits, including animals. As it turned out, many of the original taxonomic physiological traits. In this chapter, we provide a survey of assignments are now thought to be wrong, and the confusion animal diversity, albeit a general one, focusing on the origins about evolutionary affinities led Stephen J. Gould to prepare of physiological traits. ■ INTRODUCTION Within the next billion years, the progenote’s descen- dants diverged to form three distinct groups of organisms: About 4.5 bya (billion years ago) the planet Earth coalesced Eubacteria, Archaea, and Eukaryota. Each lineage diversi- from clumps of debris floating through space after the Big fied independently over the next 3 billion years. The two Bang that occurred about 14 bya. For another billion years, prokaryote lineages, Eubacteria and Archaea, remained Earth’s surface was a harsh place: Asteroid bombardment and single-celled organisms with little intracellular organization. volcanic eruptions were constantly remodeling the surface of In contrast, the ancestral eukaryotes experienced evolution- the planet. Yet it was during this tumultuous period that life on ary changes that resulted in the production of membranous, Earth began. Some researchers believe that organic molecules subcellular compartments, thereby increasing intracellular arose from a “primordial soup” of methane, ammonia, and organization. This is thought to have begun when the ear- water, energized by atmospheric electrical discharges. Others liest eukaryotes found a way to package their DNA into a believe that the first organic molecules arose from chemical membrane-bound compartment: the nucleus. Later, around reactions of products of deep-sea volcanoes. Regardless of 3 bya, a eukaryote engulfed a bacterium that likely re- the origins of the first small organic molecules, the pathway sembled a modern purple bacterium. Although the purple to living organisms required the formation of larger macro- bacterium was probably ingested as food, it developed a molecules with the capacity for catalysis and self-replication. symbiotic relationship with its host, replicating within the At some point around 4 bya, these purely chemical processes host cell. Over time, the bacterial endosymbiont lost its produced the earliest life form, the progenote. The proge- capacity to exist outside the cell, and the host cell became note was likely a chemoautolithotroph, capable of surviving reliant on the metabolic contributions of the endosymbiont, without oxygen and living on inorganic sources of energy and the ancestor of mitochondria. By 2 bya, all of the diverse carbon. The closest living relatives to the progenote are likely groups of unicellular organisms were established, including the Archaea. The modern Archaea are extremophiles, able to the many lineages of single-celled eukaryotes, collectively survive in the harshest environments that exist on Earth, such known as protists. as sulfuric hot springs and deep-sea vents. The origins of animals can be traced back about 600 mya, The progenote was the ancestor to all organisms on the with the appearance of sponges. In the time since, animal planet and, as a result, it is likely that many of the biologi- evolution occurred in concert with changing environmental cal features that are shared by all currently living organisms conditions (Figure 2.2). We cannot understand the basis of arose in the progenote. These shared features include de- animal diversity without an awareness of the evolutionary pendence on water, the role of nucleic acids, the use of only origins of animals in a changing environment. On the one 20 amino acids in proteins, and the basic pathways of inter- hand, many cellular processes are similar across major taxa, mediary metabolism. so what we learn from studies on model species of fungi and 21 DESIGN SERVICES OF # 153477 Cust: Pearson Au: Moyes Pg. No. 21 C/M/Y/K S4CARLISLE M02_MOYE8179_03_SE_CH02.inddTitle: Principles 21 of Animal Physiology Short / Normal / Long Publishing Services 15/12/14 5:19 PM 22 PART ONE Introduction to Physiology FIGURE 2.2 Biotic and abiotic events over geologic time membrane-bound nucleus and organelles. Protists are a Many evolutionary events coincide with periods of environmental collection of only distantly related organisms containing change over the geological record. The colors reflect periods more than 50 different phyla. The most familiar protists of global warmth (red) and cold (blue). are Euglena (with features of both animals and plants), Plasmodium (the single-celled flagellate parasites of blood Oxygen that cause malaria), Paramecium (ciliated hunters), and 40% 30% 20% amoebas (cells that are the namesake of amoeboid move- 0 mya Mammalian ment). Early researchers recognized that some protists Tertiary radiation were able to move from place to place, and because loco- motion was deemed to be a unique trait of animals, the Dinosaur 100 Cretaceous mobile protists were at one point considered to be the extinctions ancestors of animals, giving rise to the term protozoan. The term continues to be used in some contexts, but it Jurassic Dinosaur radiation; Appearance of birds has no meaningful evolutionary basis. The protist phyla 200 Reptile radiation; emerged prior to the origins of the three main eukary- Triassic First mammals ote kingdoms: plants, fungi, and animals. The term Permean Mass extinction; metazoan, which arose originally to distinguish multicel- Synapsids common lular animals from the single-celled protozoans, is now 300 First amniotes (reptiles); Carboniferous Amphibian radiation; used synonymously with “animal.” Flying insects The earliest steps in animal evolution involved the for- Amphibian appearance; mation of a multicellular entity, though the mechanisms Devonian Fish radiation by which this occurred remain uncertain. The colonial 400 First terrestrial invertebrates; Silurian Radiation of corals hypothesis suggests that genetically identical individual cells Ordovician Fish appearance; remained associated as colonies, a phenomenon that is com- Radiation of trilobites mon in flagellated protists. Amongst the protists, genetic 500 choanoflagellates Earliest vertebrates; studies show that the are the protists most Cambrian Appearance of trilobites closely related to metazoans. They are single-celled organ- Appearance of protists, isms that possess a flagellum emerging from a cup-shaped Precambrian sponges, and cnidarians collar extending from a more spherical cell body. Remark- 600 ably, they are very similar in appearance to choanocytes, a Figure source: Oxygen patterns are based on Berner, R. A. (1999). Atmospheric flagellated cell in sponges (Figure 2.3). oxygen over Phanerozoic time. Proceedings of