Chapter 22

1. Chapter 22 The Origin and History of Life Early Evolution

2. The universe began with the Big Bang about 13.7 bya (billion years ago) Our solar system began about 4.6 bya The Earth is 4.55 billion years old By 4 bya the Earth had cooled enough for outer layers to solidify and oceans to form By 4-3.5 bya life emerged bya

3. Proposed Stages of Life’s Origins Polymers became enclosed in membranes  protobionts Nucleotides polymerized to form DNA and RNA, amino acids polymerized to form proteins Nucleotides and amino acids were produced prior to the existence of cells Polymers enclosed in membranes acquired cellular properties

4. Stage 1: Origin of organic molecules Conditions on primitive Earth may have been more conducive to spontaneous formation of organic molecules Prebiotic or abiotic synthesis Little free oxygen gas Dilute solution called “prebiotic soup” Several hypotheses on where and how organic molecules originated

5. Miller-Urey experiment Electrical discharge Electrodes Reducing atmosphere thought to be like early earth Gases H2O To vacuum H2 Cold water CH4 NH3 Condenser Precipitating droplets Products included amino acids, purines, pyrimidines Trap Boiling water Sample containing organic molecules such as amino acids

6. Reducing atmosphere hypothesis Based on geological data Atmosphere rich in water vapor, H2, CH4, NH3 (and little if any O2) Miller and Urey Chamber simulates atmosphere and bolts of lightning Formed precursors, amino acids, sugars and nitrogenous bases First attempt to apply scientific experiments to understand origin of life Since 1950s, ideas about early Earth atmosphere have changed, may have been a neutral environment Still similar results for abiotic synthesis

7. Extraterrestrial hypothesis Meteorites brought organic carbon to Earth Including amino acids and nucleic acid bases Opponents argue that most of this would be destroyed in the intense heating and collision • Deep-sea vent hypothesis • Biologically important molecules may have been formed in the temperature gradient between extremely hot vent water and cold ocean water • Supported by experiments • Complex biological communities found here that derive energy from chemicals in the vent (not the sun)

8. Polymerization Monomers are joined to form long chains. Sugars form carbohydrates, amino acids form proteins, and nucleotides form nucleic acids. Reaction: Condensation or Dehydration Synthesis

9. Stage 2: Organic polymers Experimentally, prebiotic synthesis of polymers is usually not possible in aqueous solutions Hydrolysis competes with polymerization Experiments have shown formation of nucleic acid polymers and polypeptides (proteins) on clay surfaces

10. Stage 3: Formation of boundaries • Protobionts are cell-like collections of polymers • 4 characteristics • Boundary separated external environment from internal contents • Polymers inside the protobiont contained information • Polymers inside the protobiont had enzymatic function • Protobionts capable of self-replication

11. Types of Protobionts 57 µm 200 nm 11 • Coacervates • Droplets that form spontaneously from the association of charged polymers • Enzymes trapped inside can perform primitive metabolic functions • Liposomes • Vesicles surrounded by a lipid layer • Clay can catalyze formation of liposomes that grow and divide • Can enclose RNA

12. Stage 4: RNA world Most scientists favor RNA as the first macromolecule of protobionts Important RNA functions Ability to store information Capacity for self-replication Enzymatic function – ribozymes DNA and proteins do not have all 3 functions

13. Information storage DNA would have relieved RNA of informational role and allowed RNA to do other functions DNA is less likely to suffer mutations Metabolism and other cellular functions Proteins have a greater catalytic potential and efficiency Proteins can fulfill other functions such as transport and stabilizing cell structure Advantages of DNA/RNA/protein world

14. How did metabolism develop? • Reactions occurred by chance in protobionts • Useful reactions could be retained by selection for successful protobionts • Metabolic pathways evolved backward • Use of ATP and breakdown of glucose by glycolysis represent early pathways now shared by all living organisms

15. From Protobionts to Living Cells • In contrast to protobionts, cells contain • specific and reproducible reaction sequences to maintain metabolism • specific macromolecules to maintain cell structure • ability to control internal processes • ability to reproduce • The transition from protobionts to living cells has not been demonstrated in the laboratory

16. MYA Eons History of life on Earth 0 Quaternary 1.8 Tertiary Cretaceous 144 Jurassic Triassic Phanerozoic 248 Permian Carboniferous 354 Devonian • Geological time scale • From 4.55 bya to present • 4 eons • Hadean, Archaean, Proterozoic, Phanerozoic • 1st three are called Precambrian • Each eon is further divided into eras Silurian 443 Ordovician Cambrian 543 Late 900 Middle 1,600 Proterozoic Early PRECAMBRIAN 2,500 Late 3,000 Middle Archaean 3,400 Early 3,800 Hadean 4,550

17. Earth’s History Condensed Into a One Year Timeline Earth’s scientists estimate the age of the earth to be ~4.6 billion years old. To make this time span a bit more tangible, I'll be marking events in earth’s history on a one year timeline using these scales: One month represents ~375 million yearsOne day represents ~12.3 million years Assuming that the earth formed on January 1st….

18. Prokaryotic cells arose during Archaeon Eon Archaeon Eon- when diverse microbial life flourished in primordial oceans First cells were prokaryotic Includes Bacteria and Archaea Organisms were anaerobic due to scarcity of free oxygen First cells were heterotrophs Prokaryotic autotrophs evolved as supply of organic molecules dwindled

19. Stromatolites First known fossils from 3.5 bya Autotrophic cyanobacteria were preserved while heterotrophic ancestors were not Stromatolite formation involves layers of calcium carbonate Cyanobacteria produce O2 as a by-product of photosynthesis Release of O2 spelled doom for many prokaryotic groups that were anaerobic Allowed the evolution of aerobic species March 18th in the evolutionary year

20. Photosynthesis and the Oxygen Revolution • Oxygen began accumulating in the atmosphere about 2.7 billion years ago. • Banded iron formations are evidence of the age of oxygenic photosynthesis – approximately 2 bya in photo

21. July 10th in the evolutionary year The First Eukaryotes • During the Proterozoic Eon evidence of fossils of eukaryotic cells appears ~2.1 bya • Note the presence of a nucleus • Both bacteria and archaea contributed substantially to nuclear genome • Endosymbiosis hypothesis • mitochondria and plastids (chloroplast precursors) were formerly small prokaryotes living within larger host cells • An endosymbiont is a cell that lives within a host cell

22. Endosymbiosis Hypothesis 2.5 BYA 3.5 BYA 1.0 BYA 3 BYA 2 BYA 1.5 BYA Origin of Eukaryotes Infolding of plasma membrane Endosymbiotic origin of mitochondria Aerobic prokaryote Endosymbiotic origin of chloroplasts Origin of Prokaryotes Photosynthetic prokaryote "Serial endosymbiosis"

23. Key evidence supporting an endosymbiotic origin of mitochondria and plastids: • Similarities in inner membrane structures and functions • Division is similar in these organelles and some prokaryotes • These organelles transcribe their own DNA into RNA and produce proteins from this RNA • Their ribosomes are more similar to prokaryotic ribosomes than to eukaryotic ribosomes

24. The Origin of Multicellularity • Also during the Proterzoic Eon a wave of diversification occurred when multicellularity evolved at ~1.5 bya • Comparisons of DNA sequences give evidence that multicellular ancestors gave rise to algae, plants, fungi, and animals August 30th in the evolutionary year unicellular alga 64+ cells, two types 1,000+ cells, 2 types 8 identical cells

25. Phanerzoic Eon Proliferation of multicellular eukaryotic life was extensive (from 543 mya to today) Includes the Cambrian explosion The Cambrian explosion refers to the sudden appearance of fossils resembling modern phyla in the Cambrian period(533 to 525 mya) The Cambrian explosion provides the first evidence of predator-prey interactions November 17th in the evolutionary year

26. November 20th in the evolutionary year The Colonization of Land ~500 mya • Adaptations developed for organisms to live on land • Plants produced waterproof coating and a vascular system for internal transport • Fungi followed plants • Arthropods are the most abundant land animals • Tetrapods arrived ~365 mya • Our species arrived ~170,000 years ago December 1st in the evolutionary year Last 23 minutes of December 31st!

27. Relative Dating of rock layers and fossils/environments: Which rocks and fossils are the oldest? Why? A B C D E F

28. Distribution of Fossils • The lowest stratum, or layer, in a cross section of Earth is oldest, while the top stratum is the most recent. • Fossils found within a single stratum are of the same approximate age. • Relative age of a fossil says that a given fossil is younger or older than another based on what stratum it is found • Absolute age could be estimated from radioisotope dating

29. Radioisotope dating Fossils can be dated using elemental isotopes in accompanying rock Half-life – length of time required for exactly one-half of original isotope to decay Measure amount of a given isotope as well as the amount of the decay product As paleontologists are unlikely to find the first member of a species, expect fossil record to underestimate actual date species came into existence

31. How Rocks and Fossils Are Dated • After every half-life, the amount of parent material decreases by one-half. • C-14 has a ½ life of ~5,730 years

32. Major environmental changes Climate/temperature Atmosphere Land masses Continental drift Flood Glaciation Volcanic eruptions Meteoric impacts These environmental changes Can allow for new types of organisms Responsible for many extinctions

33. Five Mass Extinctions so far

34. 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

35. 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

36. Adaptive radiation • An organism’s movement into a variety of different environments or exploitation of a variety of different food sources leads to adaptive radiation. • Adaptive radiation produces a wide array of descendant species from one type of ancestor. • The mass extinction of dinosaurs gave way to adaptive radiation of mammals 65 million years ago. • Hawaii is an excellent laboratory to study adaptive radiation.

37. Descendants of ancestral tarweed that arrived 5 mya from North America N Dubautia laxa 1.3 million years KAUAI 5.1 million years MOLOKAI MAUI OAHU 3.7 million years Argyroxiphium sandwicense LANAI HAWAII 0.4 million years Dubautia waialealae Dubautia scabra Dubautia linearis

38. Mammalian adaptive radiation

39. Evolution is not goal oriented

40. 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 • Leads to species that are adapted to a specific environment