BIG IDEA I The process of evolution drives the diversity and unity of life.

1. BIG IDEA I The process of evolution drives the diversity and unity of life. Enduring Understanding 1.D The origin of living systems is explained by natural processes. Essential Knowledge 1.D.1 There are several hypotheses about the natural origin of life on Earth, each with supporting scientific evidence.

2. Essential Knowledge 1.D.1: There are several hypotheses about the natural origin of life on Earth, each with supporting scientific evidence. • Learning Objectives: • (1.27) The student is able to describe a scientific hypothesis about the origin of life of Earth. • (1.28) The student is able to evaluate scientific questions based on hypotheses about the origin of life on Earth. • (1.29) The student is able to describe the reasons for revisions of scientific hypotheses of the origin of life on Earth. • (1.30) The student is able to evaluate scientific hypotheses about the origin of life on Earth. • (1.31) The student is able to evaluate the accuracy and legitimacy of data to answer scientific questions about the origin of life on Earth.

3. Overview: Lost Worlds • Fossils in all parts of the world tell a similar, surprising story: past organisms were very different from those now alive. • The sweeping changes in life on Earth revealed by fossils illustrate macroevolution, the pattern of evolution over large time scales. • Specific examples of macroevolutionary change include the origin of key biochemical processes such as photosynthesis, the emergence of the first terrestrial vertebrates, and the long-term impact of a mass extinction on the diversity of life. • Taken together, such changes provide a grand view of the evolutionary history of life on Earth.

4. Early Earth Conditions • Conditions on early Earth made the origin of life possible. • The earliest evidence of life on Earth comes from fossils of microorganisms that are about 3.5 billion years old. • The current theory about how life arose indicates that chemical and physical processes on early Earth may have produced simple cells in a sequence of four main stages: • Primitive Earth provided inorganic precursors from which small organic molecules were abiotically synthesized due to the presence of available free energy and the absence of a significant quantity of oxygen. • These molecules served as monomers for the formation of more complex molecules, such as nucleic acids and nucleic nucleotides. • All these molecules were packaged into protobionts, membrane-containing droplets, whose internal chemistry differed from that of the external environment. • The joining of these monomers produced polymers with the ability to replicate, store and transfer information – which made inheritance possible.

5. Synthesis of Organic Compounds on Early Earth • There is scientific evidence that Earth and the other planets of the solar system formed about 4.6 billion years ago. • The first atmosphere was probably thick with water vapor; along with various compounds released by volcanic eruptions, including nitrogen and oxides, carbon dioxide, methane, ammonia, hydrogen, and hydrogen sulfide. • As Earth cooled, the water vapor condensed into the oceans, and much of the hydrogen quickly escaped into space. • In the 1920s, Russian and British chemists Oparin and Haldane hypothesized that Earth’s early atmosphere was a reducing (electron-adding) environment, in which organic compounds could have formed from simple molelcules. • They suggested that the early oceans were a solution of organic molecules, a “primitive soup” from which life arose.

6. Abiotic Synthesis of Macromolecules • The presence of small organic molecules, such as amino acids, is not sufficient for the emergence of life as we know it. • Every cell has an assortment of macromolecules – including enzymes and other proteins and nucleic acids that are essential for self-replication. • Experiments suggest that such molecules could have formed in early Earth. • Some models suggest that primitive life developed on biogenic surfaces, such as clay, that served as templates and catalysts for assembly of macromolecules. • By dripping solutions of amino acids into hot sand, clay, or rock, researchers have been able to produce amino acid polymers. The polymers formed spontaneously, without the help of enzymes or ribosomes. • It is possible that such polymers may have acted as weak catalysts for a variety of reactions on early Earth.

7. Protobionts • The necessary conditions for replication and metabolism early in life’s history may have been met by protobionts. • Protobionts are aggregates of abiotically produced molecules surrounded by a membrane or membrane-like structure. • Protobionts may exhibit some properties of life, such as simple reproduction and metabolism, as well as the maintenance of an internal chemical environment different from that of their surroundings. • Experiments demonstrate that protobionts could have formed spontaneously from abiotically produced organic compounds.

8. Protobionts

9. Self Replicating RNA and the Dawn of Natural Selection • According to the RNA World hypothesis, the first genetic material was most likely RNA, not DNA. • RNA molecules called ribozymeshave been found to catalyze many different reactions: • For example, ribozymes can make complementary copies of short stretches of their own sequence or other short pieces of RNA. • Early protobionts with self-replicating, catalytic RNA would have been more effective at using resources and would have increased in number through natural selection. • The early genetic material might have formed an “RNA world”.

10. BIG IDEA I The process of evolution drives the diversity and unity of life. Enduring Understanding 1.D The origin of living systems is explained by natural processes. Essential Knowledge 1.D.2 Scientific evidence from many different disciplines supports models of the origin of life.

11. Essential Knowledge 1.D.2: Scientific evidence from many different disciplines supports models of the origin of life. • Learning Objectives: • (1.32) The student is able to justify the selection of geological, physical, and chemical data that reveal early Earth conditions.

12. Geologic evidence provides support for models of the origin of life on Earth. • The Earth formed approximately 4.6 billion years ago, and the environment was too hostile for life until 3.9 billion years ago. • The earliest fossil evidence for life dates to 3.5 billion years ago. • Taken together, this evidence provides a plausible range of dates when the origin of life could have occurred.

13. The Fossil Record • The fossil record is the sequence in which fossils appear in the layers of sedimentary rock that constitute Earth’s surface. • The fossil record reveals changes in the history of life on earth. Fossils can also document how new groups of organisms arose from previously existing ones. • Sedimentary rocks are deposited into layers called strata and are the richest source of fossils. • 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, and had parts capable of fossilizing.

14. How Rocks and Fossils Are Dated • Sedimentary strata reveal the relative ages of fossils: • In relative dating, the order of rock strata is used to determine the relative age of fossils. • The absolute ages of fossils can be determined by radiometric dating • Radiometiric dating uses the decay of radioactive isotopes to determine the age of the rocks or fossils. • It is based on the rate of decay, or half-life of the isotope (the time required for half the parent isotope to decay).

15. Key Events in Life’s History

16. Key Events in Life’s History • Key events in life’s history include the origins of single-celled and multicelled organisms. • The earliest living organisms were prokaryotes. • About 2.7 billion years ago, oxygen began to accumulate in Earth’s atmosphere as a result of photosynthesis. • Eukaryotes appeared about 2.1 billion years ago. • Multicellular eukaryotes evolved about 1.2 billion years ago. • The colonization of land occurred about 500 million years ago, when plants, fungi, and animals began to appear on Earth.

17. The First Single-Celled Organisms • The earliest evidence of life (3.5 billion years ago) comes from fossilized stromatolites. • These are layered rocks that form when certain prokaryotes bind thin films of sediment together. • Early prokaryotes were Earth’s sole inhabitants from about 3.5 to 2.1 billion years ago. • These prokaryotes transformed life on our planet.

18. Photosynthesis and the Oxygen Revolution • Most atmospheric oxygen gas is of biological origin, produced during the water-splitting steps of photosynthesis. • When oxygenic photosynthesis first evolved, the free O2 produced probably dissolved in the surrounding water until it reached a high enough concentration to react with dissolved iron. • This would have caused the iron to precipitate as iron oxide, which accumulated as sediments. Once all of the dissolved iron had precipitated, additional O2 dissolved in the water until the seas and lakes became saturated. • After this, O2 began to “gas out” of the water and enter the atmosphere.

19. Photosynthesis and the Oxygen Revolution

20. Effects of the Oxygen Revolution • The “oxygen revolution” had an enormous impact on life. • In certain chemical forms, oxygen attacks chemical bonds and can inhibit enzymes and damage cells. • As a result the rising concentrations of atmospheric O2 probably doomed many prokaryotic groups. • Some species survived in anaerobic habitats, where we find their descendants living today. • Among other survivors, diverse adaptations to the changing atmosphere evolved, including cellular respiration, which uses O2 in the process of harvesting the energy stored in organic molecules.

21. Endosymbiosis and the First Eukaryoteshttp://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter4/animation_-_endosymbiosis.html • The oldest fossils of eukaryotic cells date back 2.1 billion years. • The hypothesis of endosymbiosisproposes 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.

22. Endosymbiosis Theory

23. Evidence Supporting the Endosymbiotic Theory • The endosymbiotic hypothesis proposes that mitochondria and plastids (chloroplasts) were formerly small prokaryotes that began living within larger cells. Evidence for this hypothesis includes: • Both organelles have enzymes and transport systems homologous to those found in the plasma membranes of living prokaryotes. • Both replicate by a splitting process similar to prokaryotes. • Both contain a single, circular DNA molecule, not associated with histone proteins. • Both have their own ribosomes which translate their DNA into proteins.

24. 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. • Comparisons of DNA sequences date the common ancestor of multicellular eukaryotes to 1.5 billion years ago. • The oldest known fossils of multicellular eukaryotes are of small algae that lived about 1.2 billion years ago.

25. The Rise and Fall of Dominant Groups • Anaerobic prokaryotes originated, flourished, and then declined as the oxygen content of the atmosphere rose. • Billions of years later, the first tetrapods emerged from the sea, giving rise to amphibians that went on to dominate life on land for 100 millions years – until other tetrapods (dinosaurs and later, mammals) replaced them as the dominant terrestrial vertebrates. • These and other major changes in life on Earth have been influenced by large-scale processes such as continental drift, mass extinctions, and adaptive radiations.

26. Continental Drift • Continental drift is the movement of Earth’s continents on great plates that float on the hot, underlying mantle. • Plate movements rearrange geography slowly, but their cumulative effects are dramatic. In addition to reshaping physical features of our planet, continental drift has a major impact on life on Earth. • 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.

27. The “Big Five” Mass Extinction Events

28. Consequences of Mass Extinctions

29. Adaptive Radiations • Adaptive radiations are periods of evolutionary change in which groups of organisms form many new species whose adaptations allow them to fill different ecological niches. • Large-scale adaptive radiations occurred after each of the big five mass extinctions, when survivors became adapted to the many vacant ecological niches. • Fossil evidence indicates that 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.

30. Major Changes in Body Form • The fossil record tells us what the great changes in the history of life have been and when they occurred. • Our understanding of continental drift, mass extinction, and adaptive radiation provides a picture of how those changes came about. • We now must seek to understand the intrinsic biological mechanisms that underlie changes seen in the fossil record. • For this, we focus on genetic mechanisms of change, paying particular attention to genes that influence development.

31. Evolutionary Novelty • Evolutionary novelty can arise when structures that originally played one role gradually acquire a different one. • Structures that evolve in one context but become co-opted for another function are referred to as exaptations. • For example, it is possible that feathers of modern birds were co-opted for flight after functioning in some other capacity, such as thermoregulation.

32. “Evo-Devo” • “Evo-devo” is a field of study in which evolutionary biology and developmental biology converge. • This field is illuminating how slight genetic divergences can be magnified into major morphological differences between species.

33. Heterochrony • Heterochrony is an evolutionary change in the rate or timing of developmental events. • Change relative rates of growth even slightly can change the adult form of an organisms substantially, thus contributing to the potential for evolutionary change.

34. Homeotic Genes • Homeotic genes are master regulatory genes that determine the location and organization of body parts. • Hox genes are one class of homeotic genes. • Changes in Hox genes and in the genes that regulate them can have a profound effect on morphology, thus contributing to the potential for evolutionary change.

35. Fig. 25-21 Hypothetical vertebrate ancestor (invertebrate) with a single Hox cluster First Hox duplication Duplication of the single Hox complex occurs and provides genetic material associated with origin of first vertebrate. Dulpicate set of genes takes on new roles – such as development of backbone. Hypothetical early vertebrates (jawless) with two Hox clusters Second Hox duplication Second duplication of Hox complex may have allowed the development of even greater structural complexity – such as jaws and limbs. Vertebrates (with jaws) with four Hox clusters

36. Fig. 25-22 Hox gene 6 Hox gene 7 Hox gene 8 Ubx About 400 mya Artemia Drosophila

37. 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: • Most novel biological structures evolve in many stages from previously existing structures. • Natural selection can only improve a structure in the context of its current utility.

38. Evidence for the Origin of Life Hypotheses • Chemical experiments have shown that it is possible to form complex organic molecules from inorganic molecules in the absence of life. • In the 1920s, Russian and British chemists Oparin and Haldane hypothesized that Earth’s early atmosphere was a reducing (electron-adding) environment, in which organic compounds could have formed from simple molecules. • They suggested that the early oceans were a solution of organic molecules, a “primitive soup” from which life arose.

39. The Miller-Urey experimenthttp://bcs.whfreeman.com/thelifewire/content/chp03/0301s.swf

40. Sidney Fox and Proteinoids • In the 1960s, Sidney Fox synthesized organic polymers such as polypeptides by dripping dilute solutions of organic monomers over hot sand, clay, or rock. • This method mimics the condensation of the Miller-Urey model, but with the idea that rain falling from the early atmosphere or waves washing onto hot substrate would be favorable to the formation of polypeptides and other organic polymers. • Once these polymers have formed, they can form aggregates, which spontaneously form into proteinoids (protobiont structures similar to living organisms).

41. Evidence for the Origin of Life Hypotheses • Molecular and genetic evidence from extant and extinct organisms indicates that all organisms on Earth share a common ancestral origin of life. • Scientific evidence includes molecular building blocks that are common to all life forms (carbohydrates, proteins, lipids, amino acids). • Scientific evidence includes a common genetic code (DNA and RNA).