The Cambrian Explosion The Cambrian explosion refers to the sudden appearance of fossils resembling modern phyla in the Cambrian period (535 to 525 million years ago) The Cambrian explosion provides the first evidence of predator-prey interactions

Fig 25-UN6 Animals 1 4 Billions of years ago 2 3

500 Sponges Cnidarians Annelids Molluscs Chordates Arthropods Fig. 25-10 500 Sponges Cnidarians Annelids Molluscs Chordates Arthropods Echinoderms Brachiopods Early Paleozoic era (Cambrian period) Millions of years ago 542 Figure 25.10 Appearance of selected animal phyla Late Proterozoic eon

DNA analyses suggest that many animal phyla diverged before the Cambrian explosion, perhaps as early as 700 million to 1 billion years ago Fossils in China provide evidence of modern animal phyla tens of millions of years before the Cambrian explosion The Chinese fossils suggest that “the Cambrian explosion had a long fuse”

(a) Two-cell stage (b) Later stage 150 µm 200 µm Fig. 25-11 Figure 25.11 Proterozoic fossils that may be animal embryos (SEM) (a) Two-cell stage (b) Later stage 150 µm 200 µm

The Colonization of Land Fungi, plants, and animals began to colonize land about 500 million years ago Plants and fungi likely colonized land together by 420 million years ago Arthropods and tetrapods are the most widespread and diverse land animals Tetrapods evolved from lobe-finned fishes around 365 million years ago

Fig 25-UN7 Colonization of land 1 4 Billions of years ago 2 3

Video: Volcanic Eruption Concept 25.4: The rise and fall of dominant groups reflect continental drift, mass extinctions, and adaptive radiations The history of life on Earth has seen the rise and fall of many groups of organisms Video: Volcanic Eruption Video: Lava Flow

Continental Drift At three points in time, the land masses of Earth have formed a supercontinent: 1.1 billion, 600 million, and 250 million years ago Earth’s continents move slowly over the underlying hot mantle through the process of continental drift Oceanic and continental plates can collide, separate, or slide past each other Interactions between plates cause the formation of mountains and islands, and earthquakes

(a) Cutaway view of Earth (b) Major continental plates Fig. 25-12 North American Plate Eurasian Plate Crust Juan de Fuca Plate Caribbean Plate Philippine Plate Arabian Plate Indian Plate Mantle Cocos Plate Pacific Plate South American Plate Nazca Plate Outer core African Plate Australian Plate Inner core Figure 25.12 Earth and its continental plates Scotia Plate Antarctic Plate (a) Cutaway view of Earth (b) Major continental plates

(a) Cutaway view of Earth Fig. 25-12a Crust Mantle Outer core Figure 25.12 Earth and its continental plates Inner core (a) Cutaway view of Earth

(b) Major continental plates Fig. 25-12b North American Plate Eurasian Plate Juan de Fuca Plate Caribbean Plate Philippine Plate Arabian Plate Indian Plate Cocos Plate South American Plate Pacific Plate Nazca Plate African Plate Australian Plate Figure 25.12 Earth and its continental plates Scotia Plate Antarctic Plate (b) Major continental plates

Consequences of Continental Drift Formation of the supercontinent Pangaea about 250 million years ago had many effects A reduction in shallow water habitat A colder and drier climate inland Changes in climate as continents moved toward and away from the poles Changes in ocean circulation patterns leading to global cooling

Present Cenozoic 65.5 135 Millions of years ago Mesozoic 251 Paleozoic Fig. 25-13 Present Cenozoic North America Eurasia 65.5 Africa South America India Madagascar Australia Antarctica Laurasia 135 Mesozoic Millions of years ago Gondwana Figure 25.13 The history of continental drift during the Phanerozoic eon 251 Pangaea Paleozoic

Present Cenozoic Millions of years ago 65.5 North America Eurasia Fig. 25-13a Present Cenozoic North America Eurasia Millions of years ago 65.5 Africa Figure 25.13 The history of continental drift during the Phanerozoic eon India South America Madagascar Australia Antarctica

135 Mesozoic Millions of years ago 251 Paleozoic Laurasia Gondwana Fig. 25-13b Laurasia 135 Gondwana Mesozoic Millions of years ago Figure 25.13 The history of continental drift during the Phanerozoic eon 251 Pangaea Paleozoic

The break-up of Pangaea lead to allopatric speciation The current distribution of fossils reflects the movement of continental drift For example, the similarity of fossils in parts of South America and Africa is consistent with the idea that these continents were formerly attached

Mass Extinctions The fossil record shows that most species that have ever lived are now extinct At times, the rate of extinction has increased dramatically and caused a mass extinction

The “Big Five” Mass Extinction Events In each of the five mass extinction events, more than 50% of Earth’s species became extinct

(families per million years): Fig. 25-14 20 800 700 15 600 500 Number of families: (families per million years): Total extinction rate 10 400 300 5 200 100 Figure 25.14 Mass extinction and the diversity of life Era Period Paleozoic Mesozoic Cenozoic E O S D C P Tr J C P N 542 488 444 416 359 299 251 200 145 65.5 Time (millions of years ago)

The Permian extinction defines the boundary between the Paleozoic and Mesozoic eras This mass extinction occurred in less than 5 million years and caused the extinction of about 96% of marine animal species This event might have been caused by volcanism, which lead to global warming, and a decrease in oceanic oxygen

The Cretaceous mass extinction 65 The Cretaceous mass extinction 65.5 million years ago separates the Mesozoic from the Cenozoic Organisms that went extinct include about half of all marine species and many terrestrial plants and animals, including most dinosaurs

NORTH AMERICA Chicxulub crater Yucatán Peninsula Fig. 25-15 Figure 25.15 Trauma for Earth and its Cretaceous life For the Discovery Video Mass Extinctions, go to Animation and Video Files.

The presence of iridium in sedimentary rocks suggests a meteorite impact about 65 million years ago The Chicxulub crater off the coast of Mexico is evidence of a meteorite that dates to the same time

Is a Sixth Mass Extinction Under Way? Scientists estimate that the current rate of extinction is 100 to 1,000 times the typical background rate Data suggest that a sixth human-caused mass extinction is likely to occur unless dramatic action is taken

Consequences of Mass Extinctions Mass extinction can alter ecological communities and the niches available to organisms It can take from 5 to 100 million years for diversity to recover following a mass extinction Mass extinction can pave the way for adaptive radiations

(percentage of marine genera) Fig. 25-16 50 40 30 (percentage of marine genera) Predator genera 20 10 Paleozoic Era Period Mesozoic Cenozoic Figure 25.16 Mass extinctions and ecology E O S D C P Tr J C P N 542 488 444 416 359 299 251 200 145 65.5 Time (millions of years ago)

Adaptive Radiations Adaptive radiation is the evolution of diversely adapted species from a common ancestor upon introduction to new environmental opportunities

Worldwide Adaptive Radiations Mammals underwent an adaptive radiation after the extinction of terrestrial dinosaurs The disappearance of dinosaurs (except birds) allowed for the expansion of mammals in diversity and size Other notable radiations include photosynthetic prokaryotes, large predators in the Cambrian, land plants, insects, and tetrapods

Ancestral mammal Monotremes (5 species) ANCESTRAL CYNODONT Marsupials Fig. 25-17 Ancestral mammal Monotremes (5 species) ANCESTRAL CYNODONT Marsupials (324 species) Eutherians (placental mammals; 5,010 species) Figure 25.17 Adaptive radiation of mammals 250 200 150 100 50 Millions of years ago

Regional Adaptive Radiations Adaptive radiations can occur when organisms colonize new environments with little competition The Hawaiian Islands are one of the world’s great showcases of adaptive radiation

Close North American relative, the tarweed Carlquistia muirii Fig. 25-18 Close North American relative, the tarweed Carlquistia muirii KAUAI 5.1 million years MOLOKAI 1.3 million years Dubautia laxa MAUI OAHU 3.7 million years Argyroxiphium sandwicense LANAI HAWAII 0.4 million years Figure 25.18 Adaptive radiation on the Hawaiian Islands Dubautia waialealae Dubautia scabra Dubautia linearis

1.3 KAUAI MOLOKAI million 5.1 years million MAUI years OAHU 3.7 Fig. 25-18a 1.3 million years KAUAI 5.1 million years MOLOKAI MAUI OAHU 3.7 million years LANAI Figure 25.18 Adaptive radiation on the Hawaiian Islands HAWAII 0.4 million years

Close North American relative, the tarweed Carlquistia muirii Fig. 25-18b Figure 25.18 Adaptive radiation on the Hawaiian Islands Close North American relative, the tarweed Carlquistia muirii

Dubautia waialealae Fig. 25-18c Figure 25.18 Adaptive radiation on the Hawaiian Islands Dubautia waialealae

Fig. 25-18d Figure 25.18 Adaptive radiation on the Hawaiian Islands Dubautia laxa

Fig. 25-18e Figure 25.18 Adaptive radiation on the Hawaiian Islands Dubautia scabra

Argyroxiphium sandwicense Fig. 25-18f Figure 25.18 Adaptive radiation on the Hawaiian Islands Argyroxiphium sandwicense

Dubautia linearis Fig. 25-18g Figure 25.18 Adaptive radiation on the Hawaiian Islands Dubautia linearis

Concept 25.5: Major changes in body form can result from changes in the sequences and regulation of developmental genes Studying genetic mechanisms of change can provide insight into large-scale evolutionary change

Evolutionary Effects of Development Genes Genes that program development control the rate, timing, and spatial pattern of changes in an organism’s form as it develops into an adult

Changes in Rate and Timing Heterochrony is an evolutionary change in the rate or timing of developmental events It can have a significant impact on body shape The contrasting shapes of human and chimpanzee skulls are the result of small changes in relative growth rates Animation: Allometric Growth

Figure 25.19 Relative growth rates of body parts Newborn 2 5 15 Adult Age (years) (a) Differential growth rates in a human Figure 25.19 Relative growth rates of body parts Chimpanzee fetus Chimpanzee adult Human fetus Human adult (b) Comparison of chimpanzee and human skull growth

(a) Differential growth rates in a human Fig. 25-19a Figure 25.19 Relative growth rates of body parts Newborn 2 5 15 Adult Age (years) (a) Differential growth rates in a human

(b) Comparison of chimpanzee and human skull growth Fig. 25-19b Chimpanzee fetus Chimpanzee adult Figure 25.19 Relative growth rates of body parts Human fetus Human adult (b) Comparison of chimpanzee and human skull growth

Heterochrony can alter the timing of reproductive development relative to the development of nonreproductive organs In paedomorphosis, the rate of reproductive development accelerates compared with somatic development The sexually mature species may retain body features that were juvenile structures in an ancestral species

Fig. 25-20 Gills Figure 25.20 Paedomorphosis

Changes in Spatial Pattern Substantial evolutionary change can also result from alterations in genes that control the placement and organization of body parts Homeotic genes determine such basic features as where wings and legs will develop on a bird or how a flower’s parts are arranged

Hox genes are a class of homeotic genes that provide positional information during development If Hox genes are expressed in the wrong location, body parts can be produced in the wrong location For example, in crustaceans, a swimming appendage can be produced instead of a feeding appendage

Evolution of vertebrates from invertebrate animals was associated with alterations in Hox genes Two duplications of Hox genes have occurred in the vertebrate lineage These duplications may have been important in the evolution of new vertebrate characteristics

Hypothetical vertebrate ancestor (invertebrate) Fig. 25-21 Hypothetical vertebrate ancestor (invertebrate) with a single Hox cluster First Hox duplication Hypothetical early vertebrates (jawless) with two Hox clusters Second Hox duplication Figure 25.21 Hox mutations and the origin of vertebrates Vertebrates (with jaws) with four Hox clusters

The Evolution of Development The tremendous increase in diversity during the Cambrian explosion is a puzzle Developmental genes may play an especially important role Changes in developmental genes can result in new morphological forms

Changes in Genes New morphological forms likely come from gene duplication events that produce new developmental genes A possible mechanism for the evolution of six-legged insects from a many-legged crustacean ancestor has been demonstrated in lab experiments Specific changes in the Ubx gene have been identified that can “turn off” leg development

Hox gene 6 Hox gene 7 Hox gene 8 Ubx About 400 mya Drosophila Artemia Fig. 25-22 Hox gene 6 Hox gene 7 Hox gene 8 Ubx About 400 mya Figure 25.22 Origin of the insect body plan Drosophila Artemia

Changes in Gene Regulation Changes in the form of organisms may be caused more often by changes in the regulation of developmental genes instead of changes in their sequence For example three-spine sticklebacks in lakes have fewer spines than their marine relatives The gene sequence remains the same, but the regulation of gene expression is different in the two groups of fish

Differences in the coding sequence of the Pitx1 gene? Result: No Fig. 25-23 RESULTS Test of Hypothesis A: Differences in the coding sequence of the Pitx1 gene? Result: No The 283 amino acids of the Pitx1 protein are identical. Test of Hypothesis B: Differences in the regulation of expression of Pitx1 ? Pitx1 is expressed in the ventral spine and mouth regions of developing marine sticklebacks but only in the mouth region of developing lake stickbacks. Result: Yes Marine stickleback embryo Lake stickleback embryo Figure 25.23 What causes the loss of spines in lake stickleback fish? Close-up of mouth Close-up of ventral surface

Marine stickleback embryo Lake stickleback embryo Fig. 25-23a Marine stickleback embryo Lake stickleback embryo Close-up of mouth Figure 25.23 What causes the loss of spines in lake stickleback fish? Close-up of ventral surface

Concept 25.6: Evolution is not goal oriented Evolution is like tinkering—it is a process in which new forms arise by the slight modification of existing forms

Evolutionary Novelties Most novel biological structures evolve in many stages from previously existing structures Complex eyes have evolved from simple photosensitive cells independently many times Exaptations are structures that evolve in one context but become co-opted for a different function Natural selection can only improve a structure in the context of its current utility

(a) Patch of pigmented cells (b) Eyecup Fig. 25-24 Pigmented cells (photoreceptors) Pigmented cells Epithelium Nerve fibers Nerve fibers (a) Patch of pigmented cells (b) Eyecup Fluid-filled cavity Cellular mass (lens) Cornea Epithelium Optic nerve Pigmented layer (retina) Optic nerve (c) Pinhole camera-type eye (d) Eye with primitive lens Figure 25.24 A range of eye complexity among molluscs Cornea Lens Retina Optic nerve (e) Complex camera-type eye

Evolutionary Trends Extracting a single evolutionary progression from the fossil record can be misleading Apparent trends should be examined in a broader context

Figure 25.25 The branched evolution of horses Recent (11,500 ya) Equus Hippidion and other genera Pleistocene (1.8 mya) Nannippus Pliohippus Pliocene (5.3 mya) Hipparion Neohipparion Sinohippus Megahippus Callippus Archaeohippus Miocene (23 mya) Merychippus Anchitherium Hypohippus Parahippus Miohippus Oligocene (33.9 mya) Figure 25.25 The branched evolution of horses Mesohippus Paleotherium Epihippus Propalaeotherium Eocene (55.8 mya) Pachynolophus Orohippus Key Grazers Hyracotherium Browsers

Oligocene (33.9 mya) Eocene (55.8 mya) Fig. 25-25a Miohippus Oligocene (33.9 mya) Mesohippus Paleotherium Epihippus Propalaeotherium Eocene (55.8 mya) Pachynolophus Orohippus Key Figure 25.25 The branched evolution of horses Grazers Hyracotherium Browsers

Figure 25.25 The branched evolution of horses Fig. 25-25b Recent (11,500 ya) Equus Hippidion and other genera Pleistocene (1.8 mya) Nannippus Pliohippus Pliocene (5.3 mya) Hipparion Neohipparion Sinohippus Megahippus Callippus Archaeohippus Miocene (23 mya) Figure 25.25 The branched evolution of horses Merychippus Anchitherium Hypohippus Parahippus

According to the species selection model, trends may result when species with certain characteristics endure longer and speciate more often than those with other characteristics The appearance of an evolutionary trend does not imply that there is some intrinsic drive toward a particular phenotype

Millions of years ago (mya) Fig 25-UN8 1.2 bya: First multicellular eukaryotes 535–525 mya: Cambrian explosion (great increase in diversity of animal forms) 500 mya: Colonization of land by fungi, plants and animals 2.1 bya: First eukaryotes (single-celled) 3.5 billion years ago (bya): First prokaryotes (single-celled) 500 4,000 3,500 3,000 2,500 2,000 1,500 1,000 Present Millions of years ago (mya)

Ceno- Meso- zoic zoic Paleozoic Origin of solar system and Earth 1 4 2 Fig 25-UN9 Ceno- zoic Meso- zoic Paleozoic Origin of solar system and Earth 1 4 Proterozoic Archaean Billions of years ago 2 3

Flies and fleas Caddisflies Moths and butterflies Herbivory Fig 25-UN10 Flies and fleas Caddisflies Moths and butterflies Herbivory

Ceno- zoic Meso- zoic Paleozoic Origin of solar system and Earth 1 4 Fig 25-UN11 Ceno- zoic Meso- zoic Paleozoic Origin of solar system and Earth 1 4 Proterozoic Archaean Billions of years ago 2 3

You should now be able to: Define radiometric dating, serial endosymbiosis, Pangaea, snowball Earth, exaptation, heterochrony, and paedomorphosis Describe the contributions made by Oparin, Haldane, Miller, and Urey toward understanding the origin of organic molecules Explain why RNA, not DNA, was likely the first genetic material

Describe and suggest evidence for the major events in the history of life on Earth from Earth’s origin to 2 billion years ago Briefly describe the Cambrian explosion Explain how continental drift led to Australia’s unique flora and fauna Describe the mass extinctions that ended the Permian and Cretaceous periods Explain the function of Hox genes