Homework #5 Due Tuesday, October 25, 6:00 pm

1. Homework #5 Due Tuesday, October 25, 6:00 pm Exam #2 date Wednesday, November 2 Exam #3 tentative date Wednesday, November 30

2. Extremophiles • Life that exists under “extreme” conditions, conditions that until recently were thought to be inhospitable to life.

3. Extremophiles • Volcanic vents: • Water temperature reaches 400°C (750°F), possible because of the large pressure • Black smokers: mixed with volcanic chemicals

4. Extremophiles • Antarctic dry valleys: • Microbes in small pockets of water in rocks

5. Extremophiles • Lithophiles(rock lovers): • Several kilometers below the surface • Chemical energy from rocks • Carbon from CO2 filtering down

6. Extremophiles • Endospores(e.g., anthrax) • Can lay dormant for long periods • Can survive lack of water, extreme heat and cold, and poisons • Some can survive in vacuum

7. Implications for Extraterrestrial Life • Sufficient atmospheric oxygen for eukarya has existed for only ~10% of the Earth’s lifetime – complex life not possible before this time • What is the probability that eukarya-like organisms would develop? • We are more likely to find extremophiles elsewhere • Extremophiles may be the norm, not the exception

8. The Origin and Evolution of Life on Earth When Did Life Begin?

9. Stromatolites • Living: colonies of bacteria living in outer layer of sedimentary rocks • 3.5 Byr old rocks: almost identical layered structure • Inconclusive evidence: sedimentation layering may mimic stromatolites • Fossil evidence • 3.5 Byr old Australian rock shows “cells” • Could this form naturally from minerals? • Younger sites: at least two more (3.2-3.5 byr old) • Older sites: sedimentary rock too altered to be useful

10. 13C/12C ratio • Normal abundance ratio 1/89 • Living tissue and fossils show less 13C • Some rocks older than 3.85 byr show the low 13C abundance • Sterilization • Last sterilization: 3.9-4.2 byr ago (Late Heavy Bombardment)

11. The evidence indicates life formed quickly after the Earth became habitable. Within a few 100 million years, Perhaps as short as 100 million years

12. The Origin and Evolution of Life on Earth How Did Life Begin?

13. How did life get here? ? Life No Life few hundred million years

14. Simplest organisms today and those dated 3.5 billion years ago are remarkable advanced • What are the natural chemical processes that could have led to life? • Assumptions • Life began under chemical conditions of early Earth • Life did not migrate to Earth

15. Certain chemical processes are energetically favored given specific circumstances, i.e., in the presence of specific chemical elements, energy sources & liquid(s) • Can the presence of specific elements and energy inevitably lead to the formation of life? • We know it can lead to the building blocks

16. Simulated conditions thought to exist on the early Earth • Tested for the occurrence of chemical evolution • Considered to be the classic experiment on the origin of life

17. After one week, Miller and Urey observed that 10–15% of the carbon was now in the form of organic compounds. • Recent re-analysis of Miller's archived solutions found 22 amino acids

18. Miller-Urey Experiments • Different mixes of gases used for atmosphere • Different energy sources, like UV (sunlight) • Results: With the exception of when oxygen was present, ALL VARIATIONS PRODUCE AMINO ACIDS AND COMPLEX ORGANIC MOLECULES • Proper mix produces less than original experiment • THERE MUST be additional sources of organic material – the atmosphere is not sufficient!

19. The Origin and Evolution of Life on Earth Where Did Life Begin?

20. Sources of Organic Molecules • Chemical reactions in atmosphere • Lab experiments show this is probably insufficient • Organic material brought by impacts • Chemical analysis of comets and carbonaceous chondrites show that they have organic molecules • Chemical reactions near deep-sea vents • Heat from undersea volcanoes and vents can fuel chemical reactions between water and minerals

21. Early Organic Chemistry • No atmospheric oxygen • Helps: Oxygen destroys many organic compounds • Atmosphere is reducing, not oxidizing • Miller-Urey experiments • Can form amino-acid soup from methane (CH4) and ammonia (NH3) with electric energy (mimicking lightning) • Current thinking: early atmosphere was dominated by CO2 • Low yield • Shallow ponds close to surface -> UV sterilization • Alternative sources of amino acids • Extraterrestrial: amino acids are abundant in meteorites • Deep sea vents: abundant chemical energy & protected from UV

22. Where Did Life Begin? • Unlikely on land • Solar UV radiation: protection today by ozone (O3) • But no atmospheric oxygen in the early Earth • In water: no problem, UV absorbed effectively • Shallow ponds • First evidence from Miller-Urey experiment • Recent evidence: incorrect atmospheric content • Thermophiles • DNA evidence shows early thermophiles • Have advantage of more chemical energy • Deeper sea vents better protected against bombardment

23. Chemical reactions near deep-sea vents: Considered the most likely sites where life on Earth originated • “Black smokers”

24. The Origin and Evolution of Life on Earth Chemistry to life

25. Search for Self-Replicating Molecule • Work backward from organisms that live today • DNA is double-stranded = complicated • RNA obvious candidate, simpler than DNA • Hereditary information • Can serve as template for replication • Fewer steps to produce backbone structure

26. Search for Self-Replicating Molecule • Problem: RNA and DNA require enzymes to replicate (chicken and egg problem?) • In 1980’s determined that RNA might catalyze their own replication instead of other enzymes • Conclusion: Early Earth-life probably used RNA to encode its structure

27. Early Earth: short strands of RNA-like molecules produced spontaneously partially or completely • RNA-like molecules that could replicate faster with less errors soon dominated population • Copying errors introduced mutations, ensuring the production of many variations of successful molecules • Allowed molecular evolution to continue • RNA-world gave way to DNA-world • DNA less prone to copying errors • DNA more flexible hereditary material • RNA kept some of its original functions

28. Assembling Complex Organic Molecules • Organic soup was too dilute to favor the creation of complex organic molecules • Lab experiment with possible solution: When hot sand, clay or rock is placed in dilute organic solution, complex molecules self-assemble • Organic molecules stick to surface of clay • Increases density and likelihood of reactions • Strands of RNA up to 100 bases have been spontaneously produced this way

29. Other inorganic minerals may have also had a similar role • Iron pyrite (fool’s gold) • Positive charges on surface which allows organic molecules to adhere • Formation of pyrite releases energy which could be used as fuel for chemical reactions

30. Early Cell-like Structures • There are advantages to enclosing enzymes with RNA molecules • Their close proximity increases the rate of reactions between them • Also, isolates contents from outside world

31. Lab experiments suggest that membrane structures existed on early Earth • These form spontaneously when… • Cool down warm-water solution of amino acids • Mix lipids (fats) with water

32. Nonliving Pre-Cells have Lifelike Behavior • Grow in size until unstable then split to form a ‘daughter’ cell • Selectively allow other types of molecules to pass in/out of membrane • Store energy in the form of electric voltage

33. Brief Summary • Synthesis of organic precursor molecules • Origin of self-replicating RNA • Origin of enclosed pre-cells • Origins of true cells with RNA genome • Evolution of modern cells with DNA genome

34. Migration of Life to Earth?“Panspermia” • Proposal: “Seeds of life” exist everywhere around the universe • Life on earth started when these ‘seeds’ came here, probably by a meteor. • It also suggests that these seeds are taken to other habitable places in the universe.

35. Panspermia argument: • Against: • No atmosphere or water in space • Solar and stellar radiation hazards in space • For: • organic material is everywhere, and some bacteria can withstand large amounts of radiation and go dormant under low atmospheric conditions • Fact: amino acids are found in some meteorites • Question is not “could” but “did” life originate elsewhere

36. Panspermia: 2 schools of thought • School 1: life did not evolve as easily as imagined on early Earth in timescales we’ve determined • School 2: life evolved easily and was everywhere with suitable conditions

37. School 1: life did not evolve as easily as imagined on early Earth in timescales we’ve determined • Problem: entire solar system was under heavy bombardment at the same time - hard to form life quickly in another location in Solar System • Other possibility: interstellar migration • Problem: rock to be ejected out of its own system, then fall into ours and hit the tiny planet of Earth - very dificult

38. School 2: life evolved easily and was everywhere with suitable conditions • Earth was not first planet with suitable conditions • Migration of life from another planet (say Mars) dominated before early life on Earth could • We’re Martians!!!!

39. Martian meteorites • Fossil evidence of life on Mars or geochemical structure?

40. Early Evolution and Rise of O2 • First organisms had simple metabolism • Atmosphere was O2 free, life must have been anaerobic • Probably chemoheterotrophs • Obtained nutrients from organic material • Obtained energy from inorganic material • Modern archaea appear to be close to the root of the tree of life • Obtaining energy from chemical reactions involving hydrogen, sulfur and iron compounds (all abundant on early Earth)

41. Early Evolution • Natural selection probably resulted in rapid diversification • Modern DNA has enzymes that reduce the rate of mutations • RNA is not so lucky, more likely to have copying errors • Higher mutation rate in early evolution than now