The document discusses the history of life on Earth from its origins through the development of early organisms and eventual emergence of humans. It describes how early Earth's atmosphere likely allowed for the formation of organic molecules through chemical reactions, and may have given rise to self-replicating molecules like RNA. The evolution of photosynthesis introduced oxygen to the atmosphere, and energy harvesting aerobic cells became dominant. Eukaryotic cells later emerged through endosymbiotic relationships between cells and engulfed prokaryotes like mitochondria and chloroplasts. Multicellular life then diversified over millennia as plants and animals evolved on land and in the seas.

1. History of Life on Earth

2. Spontaneous Generation Spontaneous generation is the proposal that living organisms can arise from nonliving matter Medieval beliefs Microbes were thought to arise from broth Maggots were thought to arise from meat Mice were thought to arise from mixtures of sweaty shirts and wheat

3. Spontaneous Generation Refuted The maggots-from-meat idea was disproved by Francesco Redi in 1668 He kept flies away from uncontaminated meat The broth-to-microorganism idea was disproved by Louis Pasteur and John Tyndall in the mid-1800s

4. Spontaneous Generation Refuted Broth in flask is boiled to kill preexisting microogranisms As broth cools, condensing water collects, sealing the mouth of the flask If neck is later broken off, outside air can carry microorganisms into broth

5. Spontaneous Generation Refuted Did spontaneous generation occur on early Earth? Pasteur did not prove that spontaneous generation never happened He only showed that it does not happen under present-day conditions in an oxygen-rich atmosphere

6. The First Living Things Alexander Oparin and John Haldane (1920s and 1930s) Noted that an oxygen-rich atmosphere would not have permitted the spontaneous formation of complex organic molecules Speculated that the atmosphere of early Earth contained little oxygen Proposed that prebiotic chemical evolution gave rise to life

7. The First Living Things Oparin and Haldane envisioned that prebiotic chemical evolution occurred in four stages Prebiotic synthesis and accumulation of small organic molecules Small organic molecules combined to form larger molecules Origin of self-replicating molecules Packaging of molecules within some kind of enclosing membrane

8. Organic Molecules Stanley Miller and Harold Urey (1953) Noted that the atmosphere of early Earth probably contained methane, ammonia, hydrogen, and water vapor, but no oxygen

9. Organic Molecules Miller and Urey (1953) Simulated early Earth’s atmosphere by mixing the above gases in a flask and adding an electrical discharge to simulate lightning Simple organic molecules appeared after a few days

10. The Experiment of Miller & Urey Electric spark simulates lightning storm Organic molecules appear after only a few days Condenser Cool water flow Electric spark chamber CH 4 NH 3 H 2 Boiling chamber Gases of primeval atmosphere Purified water H 2 O H 2 O

11. Organic Molecules Similar experiments by Miller and others have produced amino acids, short proteins, nucleotides, and ATP Exact composition of “atmosphere” was unimportant Must contain carbon, hydrogen, and nitrogen, and exclude oxygen Type of energy source was unimportant Electrical discharge, UV light, and heat were equally effective

12. Organic Molecules Accumulate The lack of both life and oxygen gas on early Earth allowed large quantities of organic molecules to accumulate in areas protected from UV radiation (beneath rock ledges, in oceans) UV radiation bombarded early Earth’s surface because there was no ozone to block it UV radiation can break apart organic molecules Accumulated simple organic molecules combined to form complex organic molecules

13. RNA May have been the first self-reproducing molecule Thomas Cech and Sidney Altman (1980s) discovered an RNA molecule ( ribozyme ) that could catalyze a chemical reaction, a role that was thought to be performed only by protein enzymes

14. RNA Since Cech and Altman’s initial discovery dozens of naturally-occurring ribozymes have been found that catalyze reactions including Cutting other RNA molecules Splicing together different RNA fragments Attaching amino acids to growing proteins

15. RNA Since Cech and Altman’s initial discovery researchers have synthesized ribozymes that catalyze the replication of small RNA molecules Discovery of ribozymes led to hypothesis that RNA preceded the origin of DNA RNA served as The information-carrying genetic molecule The enzyme catalyst for its own replication

16. RNA Over time, DNA replaced RNA as the information-carrying genetic molecule and RNA took on its present role as an intermediary between DNA and protein

17. Membrane-Like Vesicles Vesicles are small, hollow spheres formed from proteins or proteins complexed with other compounds Have been formed artificially by agitating water-containing proteins and lipids

18. Membrane-Like Vesicles Vesicles resemble living cells Have a well-defined outer boundary that separates internal and external environments Depending on composition, membrane may be remarkably similar to that of a real cell Under certain conditions, may absorb material from the external solution, grow, and divide

19. Membrane-Like Vesicles Certain vesicles ( protocells ) may have been the precursors of living cells

20. Microspheres as Proto-Cells

21. When Did Life Arise on Earth? Earth formed about 4.5 billion years ago Life arose 3.9 to 3.5 billion years ago during the Precambrian era Oldest fossil organisms found to date are estimated to be about 3.5 billion years old

22. Earth's History Projected on a 24-hour Day Formation of Earth First Earth rocks 12 1 2 3 4 5 8 9 10 11 12 a.m. 6 7 1 2 3 4 5 7 8 9 10 11 MIDNIGHT NOON 6 p.m. First prokaryotes First atmospheric oxygen First eukaryotes First multicellular organisms First flowers First humans (11:59:40) First humans (11:59:40) Billions of years ago 4 3 2 1

23. Capturing the Sun’s Energy The first photosynthesizing organisms (ancestors of cyanobacteria) appeared about 3.5 billion years ago Photosynthesis requires sunlight, CO 2 , and hydrogen Earliest source of hydrogen believed to be hydrogen sulfide Eventually, water replaced hydrogen sulfide as the source of hydrogen and photosynthesis became water-based

24. Increased Oxygen in Atmosphere Water-based photosynthesis resulted in the release of oxygen gas as a by-product Initially, oxygen combined with iron in the Earth’s crust to form iron oxide Subsequently, oxygen began accumulating in the atmosphere Chemical analysis of rocks suggests that significant levels of atmospheric oxygen first appeared about 2.2 billion years ago

25. Aerobic Metabolism The accumulation of oxygen in Earth’s atmosphere probably Exterminated many anaerobic organisms Provided the environmental pressure for the evolution of aerobic metabolism The evolution of aerobic metabolism was significant because aerobic organisms can harvest more energy per food molecule than anaerobic organisms

26. Membrane-Enclosed Organelles The first eukaryotes (cells that possess membrane-bound organelles) appeared about 1.7 billion years ago Several organelles (mitochondria, chloroplasts, centrioles) may have arisen when primitive cells engulfed certain types of bacteria (the endosymbiont hypothesis )

27. Probable Origin of Mitochondria & Chloroplasts Anaerobic, predatory prokaryotic cell engulfs an aerobic bacterium Aerobic bacterium Descendents of engulfed bacterium evolve into mitochondria Photosynthetic bacterium Mitochondria-containing cell engulfs photosynthetic bacteria Descendents of photosynthetic bacteria evolve into chloroplasts

28. Evolution of Mitochondria Anaerobic, predatory prokaryotic cell engulfs an aerobic bacterium that it failed to digest Predatory cell and bacterium gradually enter into a symbiotic relationship Descendants of engulfed bacterium evolve into mitochondria

29. Evolution of Chloroplasts Mitochondria-containing predatory prokaryotic cell engulf a photosynthetic bacterium Predatory cell and bacterium gradually enter into a symbiotic relationship Descendants of engulfed bacterium evolve into chloroplasts

30. Evidence for Endosymbionts Many biochemical features are shared by eukaryotic organelles and living bacteria Mitochondria, chloroplasts, and centrioles contain their own supply of DNA Living intermediates (modern cells that host bacterial endosymbionts) Pelomyxa palustris harbors aerobic bacteria Paramecium harbors photosynthetic bacteria

31. Modern Intracellular Symbiosis Paramecium sp. Chlorella sp, a green alga

32. Cell Size Once predation evolved, increased cell size became an advantage Larger cells could more easily engulf smaller cells and they could move faster However, organisms larger than a millimeter in diameter can survive only in one of two ways Have a low metabolic rate Be multicellular

33. Some Algae Become Multicellular The first multicellular organisms appeared in the seas about 1 billion years ago For plants, multicellularity allowed: Some protection from predation Specialization of cells (plants were able to anchor themselves in the brightly lit waters of the shoreline)

34. Some Algae Become Multicellular For animals, multicellularity allowed More efficient predation More effective escape from predators

35. Animal Diversity Fossil traces of animal tracks and burrows have been found in 1 billion-year-old rocks Fossils of invertebrate animals (animals lacking backbones) have been collected from rocks 610 million to 544 million years old The oldest rock layers included fossils of ancestral sponges and jellyfish Subsequent rock layers revealed fossils of ancestral worms, mollusks, and arthropods

36. The Cambrian Explosion Most of the major phyla of animals had made their appearance by the Cambrian period of the Paleozoic era (544 million years ago) The Cambrian period was marked by an “explosion” in animal diversity (may have resulted from coevolution of predator and prey) Great diversity of ocean life arose during the Silurian period…\

38. The Appearance of Fishes Fishes appeared in the fossil record about 530 million years ago They were the first vertebrates (animals with backbones) Over time, fish became the dominant predators in the oceans Faster than invertebrates Possessed more acute senses and larger brains than invertebrates

39. The Transition to Land The evolution of land plants The first land plants Mosses and ferns Continued water dependency Conifers - the invasion of dry habitats Flowering plants The dominant plant form today Pollination by insects

40. Evolution of Terrestrial Animals Arthropods Lobefin fish to amphibians Amphibians to reptiles The age of the dinosaurs Reptiles and maintenance of body temperature Birds Insulating feathers retain body heat Evolution of feathers for flight Mammals Insulating hair retains body heat Live births and mammary glands

41. Multicellular Organisms Advantages of multicellularity Challenges of multicellularity The first multicellular organisms Plants - primitive marine algae Animals - marine invertebrates The transition to land

42. Diversity over Time 200 0 400 600 800 Millions of Years Ago Cambrian Ordovician Silurian Devonian Carboniferous Permian Triassic Jurassic Cretaceous Tertiary Number of Families Mass Extinctions 500 400 300 200 100 0 600

43. Plate Tectonics & Climate Change

44. Human Evolution Primate evolution Grasping hands - precision grip and power grip Binocular and color vision with overlapping fields of view Large brain - allows fairly complex social systems

45. Hominid Evolution I The evolution of Dryopithecines - between 20 and 30 million years ago Australopithecines - the first true hominids Appeared 4 million years ago (fossils) Walked upright Large brains Homo habilis - 2 million years ago Larger body and brain Ability to make crude stone and bone tools

46. Hominid Evolution II Homo erectus - 1.8 million years ago Face of modern human More socially advanced Used fire & sophisticated stone tools Homo sapiens - 200,000 years ago Neanderthals evolved 100,000 years ago Similar to humans - muscular, fully erect, dexterous, large brains Developed ritualistic burial ceremonies Cro-Magnons evolved 90,000 years ago Direct descendants of modern humans Were artistic and made precision tools

47. Possible Human Line of Descent Millions of Years Ago Ardipithecus ramidus A. boisei A. africanus Australopithecus afarensis A. robustus Homo habilis H. erectus H. heidel- bergensis H. neander- thalensis Homo ergaster H. sapiens 5 4 3 2 1 0

48. The “Out of Africa” Theory H. erectus spread began ~1.8 mya H. sapiens spread began ~100 kya

49. The “Multiregional” Hypothesis Regional pops of H. erectus may have evolved into H. sapiens while intermingling.

50. The End