Homework #5 Due Tomorrow, 6:00 pm

1. An image of Saturn's ice-spewing moon Enceladus taken from an altitude of 1,700 kilometers by the Cassini. Features the size of a bus are resolvable in this image of the active tiger stripe region of Enceladus. Although appearing dark in the contrast-enhanced image, the surface of Enceladus is covered with some of the brightest ice in the entire Solar System, reflecting about 99 percent of the light it receives.

2. Homework #5 Due Tomorrow, 6:00 pm Homework #6 will be posted by tomorrow morning. It will be due by 6:00 pm, Monday, Oct. 31 Exam #2 Wednesday, November 2

3. Understanding the history of life on Earth gives insight into likelihood and characteristics of life elsewhere

4. 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.

5. 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

6. 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

7. 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

8. 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!!!!

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

10. 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)

11. 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 • Single celled organisms

12. Photosynthesis • Important new metabolic process evolved gradually • Organisms living close to ocean surface probably developed means of absorbing sunlight (UV in particular) • Once absorbed, developed method of turning it into energy • Modern organisms of purple sulfur bacteria and green sulfur bacteria much like early photosynthetic microbes, use H2S instead of H2O for photosynthesis

13. Using water for photosynthesis developed later, perhaps 3.5 billion years ago carbon dioxide + water + light (energy) → glucose + oxygen • First appearing in cyanobacteria (blue-green algae) • O2 released into atmosphere • Changed the world!

14. The Rise of Oxygen in the atmosphere • O2 is highly reactive • All initial O2 would react with rock and minerals in water • O2 could not accumulate in atmosphere until surface rock was saturated • Rocks 2-3 bill. Yr old (banded iron formations), show atmosphere had <1% of current O2 To this day, the majority of oxygen produced over time is locked up in the ancient "banded rock" and "red bed" formations.

15. The Rise of Oxygen in the atmosphere • Clear evidence of O2 near current levels appears only 200 million yr ago • Find charcoal (fossil fuel) • Indicates enough O2 in atmosphere for fires to burn

16. The Oxygen Crisis • Rise of O2 would have created a crisis for life • O2 reacts with bonds of organic materials • Surviving species avoided effects of O2 because they lived or migrated to underground locations • Many anaerobic microbes found in such locales today

17. Early Eukaryotes • Fossil evidence dates to 2.1 bill. Yr ago • Dates to when O2 rising in atmosphere • DNA evidence suggests that prokaryotes and eukaryotes separated from common ancestor much earlier • O2 played a key role in eukaryote evolution • Cells can produce energy more efficiently using aerobic metabolism than anaerobic metabolism • Adaptations of aerobic organisms could develop adaptations that required more energy that would be available for anaerobic organisms

18. The Cambrian ExplosionBegan: ~ 545 million Yr ago Duration: ~ 40 million years • Explosion of life forms • Animal branch of the tree of life • Different classifications based on body plan • All known body plans made appearance in fossil record in a time span of 40 million years • <1% of Earth’s age • Animal diversity began 545 million Yr ago

19. Colonization of Land • Life flourished where liquid water existed • Life on land was more complicated • Had to develop means of collecting solar energy above ground and nutrients below • Life in shallow ponds or edges of lakes • Water evaporates • Natural selection favored that which could withstand periods of drought

20. DNA evidence suggests that plants evolved from an algae • It took only 75 million Yrs for animals to follow plants out of water

21. While life has expanded greatly in its diversity, this has not always been the case: mass extinctions

22. Mass Extinctions The Earth has experienced five major mass extinctions in the past half-billion years.

23. Mass Extinctions • Possible Causes • Impacts • Impact sites found for K-T boundary • Chicxulub Crater • Suspected for Permian extinction 245 mill yr ago

24. Mass Extinctions • Possible Causes • Impacts • Impact sites found for K-T boundary • Suspected for Permian extinction 245 mill yr ago • Active volcanism • Climate change • External influence for copying errors • Increase in solar particles or radiation hitting surface • Local supernova

25. Primate Evolution • Monkeys, apes, lemurs and humans have common ancestor that lived in trees • Tree life • Limber arms for swinging between branches • Eyes in front of head for depth perception • Offspring would be born more helpless than other animals

26. Emergence of Humans • Did NOT evolve from gorillas or monkeys • Share a common ancestor that lived just a few million years ago • 98% of human genome is identical to genome of the chimpanzee • 2% difference in genome separates the success of humans verses chimps • Also indicates evolution of intelligence is complex

27. Emergence of Humans

28. Emergence of Humans • After hominids diverged from chimps and gorillas, evolution has followed a complex path • Numerous hominids species existed, some during the same time period • All humans are the same species • First skull fossils that are identical to modern human skulls dates to 100,000 yr old • Our ancestors shared the Earth with Neanderthals • Went extinct 35,000 years ago

29. Emergence of Humans

30. Cultural and Technological Evolution • Have not undergone biological evolution in 40,000 years • Mutation rates are slow • Dramatic cultural changes • Transmission of knowledge between generations • Spoken to written word, thousands of years • agriculture • Technological evolution • Result of coupling between science and technology • About 100 years between industrial revolution to landing on the Moon and generating weapons of mass destruction

31. Life in the Solar System?

32. Essentials for life • Chemicals / Nutrients • Energy • Liquid solvent

33. Chemical components of cells: • Oxygen • Carbon • Nitrogen • Hydrogen These elements make up 96% of the mass of living organisms on Earth.

34. Where can we expect to find the necessary ingredients/chemicals for life? • These elements are abundant in the Universe, except in older star systems • As long as condensation and accretion occur during a star system’s formation, we can expect these elements to be present • Organic molecules (building blocks for amino acids) have been found in asteroids and comets

35. What are the energy requirements for life? • Sunlight(photosynthesis) • Consuming organic molecules (ex. eating plants) • Chemical reactions with inorganic compounds ofiron, sulfur, or hydrogen

36. Light is most intense near the sun (or another star) Light intensity falls off with distance from the Sun/star, going as 1/(distance)2 Inefficient far from a star Sunlight?

37. Chemical Energy requires a means for chemicals to mix & interact Atmospheres and/or bodies of liquids can provide this means Where can we expect to find energy for life?

38. What to use as a solvent?

39. Liquids exist only in specific temperature and pressure ranges. On the surface of a planet or moon, temperature alone is insufficient for the presence of liquids.

40. On Earth, all life requires water: • Water dissolves organic molecules so they can be used for chemical reactions in cells • Water transports chemicals into and out of cells • Water is directly involved in many metabolic reactions within cells

41. On Earth, the only other common liquid is molten rock (too hot)!

42. Alternative Liquids for Life(under 1 atmosphere pressure) Water remains liquid over broader and higher range of temperatures The higher temperature range for liquid water allows faster rates of chemical reactions

43. ethane/methane lakes on Titan • On colder worlds (e.g., Titan), other liquids may be more common, such as • Ammonia NH3 • Methane CH4 • Ethane C2H6 Chemical reactions are highly temperature sensitive, so on colder worlds with other liquids, reaction rates may be too slow for life.

44. Advantages of water: • Water remains liquid over the widest range of temperatures • Water is liquid at a higher temperature than the others. High temp makes chemical reactions easier. • Water is less dense when it freezes (ice floats), allowing life to survive in the water under the ice

45. Ice insulates the water underneath it, making it less likely to totally freeze, promoting greater climate stability • Water is POLAR. Many substances can dissolve in water, but many do not. Cell membranes do not dissolve in water. Earth-like cells can only survive in water.

46. Water is the most likely candidate liquid as a solvent in life. • Searching for liquid water may be the best approach to searching for life. • We do not know if other liquids can support life.