Chapter 25

1. Chapter 25 The History of Life on Earth

2. Overview: Lost Worlds Past organisms were very different from those now alive • The fossil record shows macroevolutionary changes over large time scales including • The emergence of terrestrial vertebrates • The origin of photosynthesis • Long-term impacts of mass extinctions

3. Key Concepts • Chemical and physical conditions on the early earth differed from those here today. • Random chemical reactions built up supplies of organic chemicals which assembled into large aggregates. • These aggregates eventually came to have the organization, function, and reproduction characteristic of life.

4. Concept 25.1: Conditions on early Earth made the origin of life possible • Chemical and physical processes on early Earth may have produced very simple cells through a sequence of stages: 1. Abiotic synthesis of small organic molecules 2. Joining of these small molecules into macromolecules 3. Packaging of molecules into “protobionts” 4. Origin of self-replicating molecules

5. Conditions for the Origin of Life and the Synthesis of Organic Compounds on Early Earth • The right combination of chemicals. Earth’s early atmosphere likely contained water vapor and chemicals released by volcanic eruptions (nitrogen, nitrogen oxides, carbon dioxide, methane, ammonia, hydrogen, hydrogen sulfide) • An energy source to promote reactions. Possibly electrical storms. • Absence of oxygen because it is a strong oxidizing agent. • Eons of time. Earth formed about 4.6billion years ago, along with the rest of the solar system.

6. Stanley Miller and Harold Urey conducted lab experiments that showed that the abiotic synthesis of organic molecules in a reducing atmosphere is possible • However, the evidence is not yet convincing that the early atmosphere was in fact reducing • Instead of forming in the atmosphere, the first organic compounds may have been synthesized near submerged volcanoes and deep-sea vents • Amino acids have also been found in meteorites

7. Fig. 25-2 Hydrothermal vent Releases hydrogen sulfide and iron sulfide, which react and produce pyrite and hydrogen gas. Prokaryotes that live near the vent use the hydrogen as an energy source.

8. Abiotic Synthesis of Monomers to Polymers • Small organic molecules polymerize when they are concentrated on hot sand, clay, or rock • Replication and metabolism are key properties of life • Protobionts are aggregates of abiotically produced molecules surrounded by a membrane or membrane-like structure • Protobionts exhibit simple reproduction and metabolism and maintain an internal chemical environment • Experiments demonstrate that protobionts could have formed spontaneously from abiotically produced organic compounds

9. Fig. 25-3a 20 µm For example, small membrane-bounded droplets called liposomes can form when lipids or other organic molecules are added to water (a) Simple reproduction by liposomes

10. Fig. 25-3b Glucose-phosphate If enzymes are added to the solution from which the droplets self-assemble, some liposomes can carry out simple metabolic reactions and export the products. Glucose-phosphate Phosphatase Starch Amylase Phosphate Maltose Maltose (b) Simple metabolism

11. Self-Replicating RNA and the Dawn of Natural Selection • The first genetic material was probably RNA, not DNA • RNA carries genetic information but can act as a catalyst. • RNA polymers can form without enzymes • RNA strands complimentary to existing RNA polymers form readily. • RNA can catalyze RNA splicing and the joining of short RNA strands to longer RNA strands • Early protobionts with self-replicating, catalytic RNA would have been more effective at using resources and would have increased in number through natural selection

12. RNA Replaced by DNA • DNA is more stable and easier to copy faithfully. • The proteins of metabolism and protein synthesis, and the nucleic acids carrying the codes for these proteins, must have evolved together, in increasingly precise association.

13. Beginnings of Metabolism • Aggregates with the most stable combinations of chemicals and greatest efficiency at competing for scarce resources were chemically selected. Process similar to natural selection but in non living systems. • Aggregates which could convert an abundant material to a scarce one had a selective advantage • Light energy, H+ pumps housed in membranes, and possibly ATP, which was readily available at first, provided energy. • Anaerobic pathways like glycolysis are older than aerobic respiratory ones.

14. Autotrophs and Heterotrophs • The earliest prokaryotes, which were Earth’s sole inhabitants from 3.5 billion years ago to about 2.1 billion years ago, were chemoautotrophs that obtained energy from inorganic chemicals and made their own energy currency molecules instead of absorbing ATP. • Autotroph numbers increased and in so doing increased the amount of life the earth could support. • As the amount of organic molecules increased heterotrophs evolved.

15. Table 27-1

16. Photosynthesis and the Oxygen Revolution • Early photosynthetic organisms used light energy to create a H+ membrane potential which provided energy to make ATP and form membrane transport. • Later organisms used light energy to get H atoms to reduce CO2. Organisms using water as a hydrogen donor evolved much later. • The photosynthetic organisms (probably similar to cyanobacteria) using water as the H donor released small amounts of O2. *****Changed the World*****

17. By about 2.7 billion years ago, O2 began accumulating in the atmosphere and rusting iron-rich terrestrial rocks • Anaerobic respiration evolved before aerobic respiration. Aerobic respiration probably began with cells capable of combining their wastes with O2 • This “oxygen revolution” from 2.7 to 2.2 billion years ago • Posed a challenge for life • Provided opportunity to gain energy from light • Allowed organisms to exploit new ecosystems

18. Fig. 25-8 Banded iron formations: evidence of oxygenic photosynthesis

19. CONCEPT SUMMARY • Autotrophs removed CO2 from the atmosphere and produced O2. • Because of increased O2 levels there was the evolution of aerobic respiration from anaerobic respiration. • When UV light reacts with O2 the product is O3, and the formation of the ozone layer.

20. Fig. 25-7 Some key events in earth’s history Ceno- zoic Meso- zoic Humans Paleozoic Colonization of land Animals Origin of solar system and Earth – 4.6 billion years 4 1 Proterozoic Archaean Prokaryotes years ago Billions of 3 2 Multicellular eukaryotes Single-celled eukaryotes Atmospheric oxygen

21. Prokaryotes evolved before Eukaryotes • The oldest fossils of eukaryotic cells date back 2.1 billion years • The hypothesis of endosymbiosis proposes that mitochondria and plastids (chloroplasts and related organelles) were formerly small prokaryotes living within larger host cells • An endosymbiont is a cell that lives within a host cell • Thus, animals, plants, fungi, and protists probably evolved from symbiotic associations of ancestral cells.

22. The prokaryotic ancestors of mitochondria and plastids probably gained entry to the host cell as undigested prey or internal parasites • In the process of becoming more interdependent, the host and endosymbionts would have become a single organism • Serial endosymbiosis supposes that mitochondria evolved before plastids through a sequence of endosymbiotic events

23. Fig. 25-9-4 Cytoplasm Plasma membrane DNA Ancestral prokaryote Infolding of plasma membrane Endoplasmic reticulum Nucleus Cell with nucleus and endomembrane system Nuclear envelope Aerobic heterotrophic prokaryote Photosynthetic prokaryote Mitochondrion Mitochondrion Ancestral heterotrophic eukaryote Plastid Ancestral photosynthetic eukaryote

24. Key evidence supporting an endosymbiotic origin of mitochondria and plastids: • The inner membranes of both organelles have enzymes and transport systems that are homologous to those found in the plasma membranes of living prokaryotes. • Cellular division is similar in these organelles and some prokaryotes • Each organelle contains a single, circular DNA molecule that, like the chromosomes of bacteria, is not associated with histones. • These organelles transcribe and translate their own DNA • Their ribosomes are more similar to prokaryotic than eukaryotic ribosomes

25. The Origin of Multicellularity • The evolution of eukaryotic cells allowed for a greater range of unicellular forms • A second wave of diversification occurred when multicellularity evolved and gave rise to algae, plants, fungi, and animals

26. Concept 25.2: The fossil record documents the history of life • The fossil record reveals changes in the history of life on earth • Sedimentary rocks are deposited into layers called strata and are the richest source of fossils • Fossils illustrate representative organisms from different points in time • Few individuals have fossilized, and even fewer have been discovered • The fossil record is biased in favor of species that • Existed for a long time • Were abundant and widespread • Had hard parts

27. Fig. 25-4 Rhomaleosaurus victor, a plesiosaur Present Dimetrodon 100 million years ago Casts of ammonites 175 200 270 300 Hallucigenia 4.5 cm 375 Coccosteus cuspidatus 400 1 cm Dickinsonia costata 500 525 2.5 cm 565 Stromatolites Tappania, a unicellular eukaryote 600 3,500 1,500 Fossilized stromatolite

28. How Rocks and Fossils Are Dated • Sedimentary strata reveal the relative ages of fossils • The absolute ages of fossils can be determined by radiometric dating • A “parent” isotope decays to a “daughter” isotope at a constant rate • Each isotope has a known half-life, the time required for half the parent isotope to decay • Radiocarbon dating can be used to date fossils up to 75,000 years old • For older fossils, some isotopes can be used to date sedimentary rock layers above and below the fossil • The magnetism of rocks can provide dating information • Reversals of the magnetic poles leave their record on rocks throughout the world

29. Fig. 25-5 Accumulating “daughter” isotope Fraction of parent isotope remaining 1/2 Remaining “parent” isotope 1/4 1/8 1/16 4 3 2 1 Time (half-lives)

30. 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 • 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”

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

32. Fig. 25-11 Fossils that may be animal embryos (a) Two-cell stage (b) Later stage 200 µm 150 µm

33. 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

34. 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 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

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

36. 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 • 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

37. Fig. 25-13 Present Cenozoic Eurasia North America Africa 65.5 India South America Madagascar Australia Antarctica Laurasia 135 Mesozoic Gondwana Millions of years ago Pangaea 251 Paleozoic

38. 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

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

40. Fig. 25-14 800 20 700 600 15 500 Number of families: 400 Total extinction rate (families per million years): 10 300 200 5 100 0 0 Mesozoic Paleozoic Cenozoic Era Period E C Tr C O S D P J P N 200 145 65.5 0 542 488 444 416 359 299 251 Time (millions of years ago)

41. Fig. 25-15 NORTH AMERICA Chicxulub crater Yucatán Peninsula

42. 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

43. Fig. 25-16 50 40 30 Predator genera (percentage of marine genera) 20 10 0 Cenozoic Paleozoic Mesozoic Era Period P D C P C N E O J S Tr 200 145 359 65.5 0 488 444 542 416 299 251 Time (millions of years ago)

44. Adaptive Radiations • Adaptive radiation is the evolution of diversely adapted species from a common ancestor upon introduction to new environmental opportunities. This can occur on a worldwide scale or a regional scale • Adaptive radiations can occur when organisms colonize new environments with little competition • Mammals underwent an adaptive radiation after the extinction of terrestrial dinosaurs (worldwide) • The Hawaiian Islands are one of the world’s great showcases of adaptive radiation (regional)

45. Fig. 25-18 Close North American relative, the tarweed Carlquistia muirii 1.3 million years MOLOKAI KAUAI 5.1 million years Dubautia laxa MAUI OAHU 3.7 million years Argyroxiphiumsandwicense LANAI HAWAII 0.4 million years Dubautia waialealae Dubautia scabra Dubautia linearis

46. 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

47. Concept 25.5: Major changes in body form can result from changes in the sequences and regulation of developmental genes Changes in Rate and Timing • Heterochronyis 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 • 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

48. Fig. 25-19a Newborn 2 15 Adult 5 Age (years) Differential growth rates in a human – The arms and legs lengthen more during growth than the head and trunk.

49. Fig. 25-19b Chimpanzee fetus Chimpanzee adult Human adult Human fetus Comparison of chimpanzee and human skull growth – The fetal skulls are similar in shape, however growth in the jaw is accelerated in chimps relative to humans

50. Fig. 25-20 Gills This salamander, an aquatic species that grows to full size, becomes sexually mature, and reproducers while retaining certain larval (tadpole) characteristics, including gills.