Animal Evolution and Diversity

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Evolution Q: How have animals descended from earlier forms through the process of evolution? WHAT I KNOW WHAT I LEARNED

SAMPLE ANSWER: Invertebrates SAMPLE ANSWER: The first animals 26.1 How did evolved before vertebrates, began evolving before the invertebrates millions of years ago. Cambrian Explosion. Over a evolve? period of 10–15 million years, animals evolved complex body plans, appendages, skeletons, and other hard body parts. By the end of the Cambrian Period, all the basic body plans of modern animal phyla had been established.

SAMPLE ANSWER: Chordates SAMPLE ANSWER: Each time 26.2 How did evolved more recently than chordate ancestors evolved a chordates evolve? invertebrates. new adaptation in their body plan, a major adaptive radiation occurred. Today, there are six groups of modern chordates—one group of nonvertebrate chordates and five groups of vertebrate chordates (fishes, amphibians, reptiles, birds, and mammals).

SAMPLE ANSWER: Primates SAMPLE ANSWER: Humans and evolved later than other other primates evolved from 26.3 How did animals. There are many a common ancestor that lived primates evolve? different kinds of primates, more than 65 million years including monkeys, apes, and ago. Many species of the humans. genus Homo existed before our species, Homo sapiens, appeared. Our genus originated in Africa and spread to the rest of the world.

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26.1 Invertebrate Evolution and Diversity

Lesson Objectives Explain what fossil evidence indicates about the timing of the evolution of the first animals. Interpret the cladogram of invertebrates. Lesson Summary Origins of the Invertebrates It is not known when the first multicellular animals evolved from single-celled eukaryotes. ▶ Animals probably evolved from ancestors they shared with choanoflagellates. Fossil evidence indicates that animals began evolving long before the Cambrian Explosion, which occurred between 530 and 515 million years ago. ▶ Fossils from the Ediacara Hills of Australia date from roughly 565 to 544 million years ago. Their body plans are different from those of anything alive today. Some seem to be related to invertebrates such as jellyfishes and worms. ▶ Cambrian fossils (dating back about 542 million years ago) show how animals evolved complex body plans over a span of 10–15 million years. Many had body symmetry, a front and back end, specialized cells, and appendages, structures such as legs or antennae protruding from the body. Some had hard body parts that became fossilized. ▶ By the end of the Cambrian, the basic body plans of the modern phyla were established. ▶ Today, invertebrates are the most abundant animals on Earth.

Cladogram of Invertebrates The cladogram of invertebrates presents current hypotheses about evolutionary relationships among major groups of modern invertebrates. ▶ The major invertebrate phyla are the sponges, cnidarians, arthropods, nematodes (roundworms), flatworms, annelids, mollusks, and echinoderms. • Sponges have pores in their bodies. • Cnidarians are radially symmetrical animals with stinging tentacles. • Arthropods have segmented bodies, a hard external skeleton, jointed appendages, and cephalization. • Nematodes, or roundworms, are nonsegmented worms with pseudocoeloms. Their digestive tracts have two openings. • Platyhelminthes, or flatworms, are the simplest animals to have three germ layers, bilateral symmetry, and cephalization. • Annelids are worms with segmented bodies and a true coelom. ▶ Larvae are the immature stages of development in some animals, such as mollusks. Many mollusks have a free-swimming larval stage called a trochophore. The trochophore is also characteristic of many annelids, indicating that annelids and mollusks are closely related. • Mollusks are soft-bodied animals that usually have a shell. They also have a true coelom and complex organ systems. • Echinoderms have spiny skin and exhibit radial symmetry.

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Origins of the Invertebrates 1. How much time passed between the appearance of the first prokaryotic cells and the emergence of multicellular organisms? About three billion years passed 2. What are choanoflagellates? What is their significance in animal evolution? They are single-celled eukaryotes that sometimes grow in colonies. They may have evolved from ancestors they shared with animals. 3. How old is our oldest evidence of multicellular life? roughly 600 million years old

For Questions 4–9, write the letter of the correct answer on the line at the left. B 4. The first animals were tiny and soft-bodied, so A. no fossilized bodies exist. B. few fossilized bodies exist. C. fossilized bodies are plentiful. D. the only fossils that exist are “trace fossils.” B 5. Fossil evidence indicates that the first animals began evolving A. during the Cambrian Period. C. after the Cambrian Explosion. B. before the Cambrian Explosion. D. after the Cambrian Period. A 6. Why are the fossils of the Ediacara Hills of Australia important? A. Their body plans are different from those of anything alive today. B. Some had cells, tissues, and specialized organs. C. Some were differentiated into a front and back end. D. Some were autotrophic. C 7. Over a period of 10–15 million years in the Cambrian Period, animals evolved A. into eukaryotic, photosynthetic forms. B. the ability to survive on the bottom of shallow seas. C. complex body plans, including cells, tissues, and organs. D. into modern, vertebrate forms. B 8. Structures such as legs or antennae that protrude from the body are A. trace fossils. B. appendages. C. shells, skeletons, and other hard body parts. D. evidence of an extinct phylum. D 9. Which animals are the most abundant on Earth? A. arthropods C. sponges B. mollusks D. invertebrates

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Cladogram of Invertebrates 10. Write “yes” or “no” to indicate how certain features distinguish each phylum of multicellular invertebrates. The first row is completed as an example.

Tissues Radial Bilateral symmetry Protostome Deuterostome development development

Sponges no no no no no

Cnidarians yes yes no no no

Arthropods yes no yes yes no Nematodes (Roundworms) yes no yes yes no

Flatworms yes no yes yes no

Annelids yes no yes yes no

Mollusks yes no yes yes no

Echinoderms yes yes yes no yes

11. Describe three evolutionary trends you see in invertebrates. Invertebrates developed complex body plans, including specialized tissues; they also developed body symmetry such as bilateral or radial symmetry; arthropods, annelids, and mollusks developed a protostome, while echinoderms developed a deuterostome.

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26.2 Chordate Evolution and Diversity

Describe the most ancient chordates. Interpret the cladogram of chordates. Lesson Summary Origins of the Chordates Embryological studies suggest that the most ancient chordates were related to the ancestors of echinoderms. ▶ Fossils of the earliest chordates (Cambrian Period) show muscles arranged in a series, traces of fins, sets of feathery gills, a head with paired sense organs, and a skull and skeletal structures likely made of cartilage, a strong connective tissue that is softer and more flexible than bone. Cartilage supports all or part of a vertebrate’s body. ▶ Modern chordates are very diverse, consisting of six groups: the nonvertebrate chordates and the five groups of vertebrates—fishes, amphibians, reptiles, birds, and mammals.

Cladogram of Chordates The cladogram of chordates presents current hypotheses about relationships among chordate groups. Major groups are: ▶ Nonvertebrate chordates: The tunicates and the lancelets lack backbones. ▶ Jawless fishes: Lampreys and hagfishes lack vertebrae and have notochords as adults. ▶ Sharks and their relatives: They have jaws and skeletons made of cartilage. ▶ Bony fishes: These animals have skeletons made of true bone. Most modern bony fishes are ray-finned fishes. One group of ancient lobe-finned fishes evolved into the ancestors of tetrapods, which are four-limbed vertebrates. ▶ Amphibians: Amphibians live in water as larvae but on land as adults. They breathe with lungs as adults, but most require water for reproduction. ▶ Reptiles: Reptiles have dry, scaly skin, well-developed lungs, strong limbs, and shelled eggs that do not develop in water. ▶ Birds: Birds can regulate their internal body temperature. They have an outer covering of feathers, strong yet lightweight bones, two legs covered with scales that are used for walking or perching, and front limbs modified into wings. ▶ Dinosaurs and birds are now considered to be in one clade, which is part of the larger reptiles clade. Modern birds are, therefore, reptiles. The traditional class Reptilia, which is not a clade, includes living reptiles and dinosaurs but not birds. ▶ Mammals: Mammals produce milk from mammary glands, have hair, breathe air, have four-chambered hearts, and regulate their internal body temperature. Origins of the Chordates For Questions 1–8, write True if the statement is true. If the statement is false, change the underlined word to make the statement true. True 1. Embryological evidence suggests that the most ancient chordates were related to the ancestors of echinoderms. chordate 2. Pikaia was an early vertebrate fossil.

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vertebrate 3. The earliest known chordate fossil was Myllokunmingia, which had muscles arranged in a series, traces of fins, sets of feathery gills, a head with paired sense organs, and a skull and skeletal structures. cartilage 4. The earliest vertebrate fossils had skeletons made of bone. True 5. Cartilage is a strong connective tissue that is more flexible than bone. True 6. Most modern chordates are vertebrates. True 7. Modern chordates include five groups of vertebrates. fishes 8. The most numerous group of vertebrates today is the mammals. Cladogram of Chordates 9. Write “yes” or “no” to indicate how certain features distinguish each subphylum of chordates. The first row is completed as an example.

Vertebrae Jaws and True Lungs Four Amniotic Endothermy Paired Bone Limbs Egg Appendages

Nonvertebrate no no no no no no no chordates

Jawless fishes yes no no no no no no

Sharks and their relatives yes yes no no no no no Bony fishes yes yes yes no no no no

Amphibians yes yes yes yes yes no no

Reptiles yes yes yes yes yes yes no

Birds yes yes yes yes yes yes yes

Mammals yes yes yes yes yes yes yes

10. Sharks and their relatives are the first group of animals with jaws. Why are jaws a significant evolutionary development? Jaws enabled the sharks and their relatives to more easily feed and defend themselves.

11. What three adaptations were needed for chordates to move from living in water to living on land? Animals on land needed legs to crawl around on, ways to breathe air, and ways to protect themselves from drying out.

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12. One group of feathered dinosaurs led to modern birds. What advantage might feathers have given these dinosaurs? Feathers may have evolved as a means of regulating body temperature. 13. How do mammals differ from all other chordates on the cladogram? Mammals feed their young with milk. 14. Which chordate groups can regulate their body temperatures? Birds and mammals

15. Much evidence supports the hypothesis that modern birds share a common ancestor with dinosaurs. Make a sketch to show the probable evolutionary relationships among modern birds, modern reptiles, and extinct dinosaurs. Circle the clades shown in your diagram.

Students’ sketches should resemble the bird-evolution cladogramcladogram in the textbook. They should circle the dinosaurs and birds as one clade, and the rereptilesptiles as a larlargerger cclade,lade, which includes the dinosaurs and birds as a subsesubset.t.

16. The order in which the major groups of chordates evolved makes sense. For example, a bony skeleton had to evolve before a vertebral column. A vertebral column had to develop before four limbs. Fish had to evolve before birds. Explain why certain traits had to evolve before the traits now seen in birds. SAMPLE ANSWER: Certain aspects of the bird body plan are derived from older forms. For example, one group of ancient fishes evolved skeletons of true bone. Another group of fishes evolved into the ancestors of four-limbed vertebrates. Today’s birds have bony skeletons and four limbs. Birds could not have evolved before these features became parts of the chordate body plan, as seen in fishes.

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