The Origin of Life

1. The Origin of Life Chapter 26 The History of Life on Earth

2. Spontaneous Generation • Concept stating that life generates from other things unlike itself • Ex: rotting meat gives rise to maggots and then to flies

3. Francesco Redi (1668) • 2 jars with rotting meat; 1 open to the air, the other covered with gauze

4. Lazzaro Spallanzani (1700’s) • 2 Flasks with gravy, both boiled. One sealed, the other open to the air

5. Louis Pasteur • Father of Microbiology and its effect on life • In 1862, he too used a broth and boiled the substance

7. So, if it has been shown that life must come from pre-existing life (biogenesis), then which came first, the chicken or the egg? Where/when/how did the first life appear on Earth? • One credible hypothesis is that chemical and physical processes in Earth’s primordial environment eventually produced simple cells

8. It’s important to understand that no one knows exactly how life arose on Earth • Just like any investigator (Ex: CSI), you must start with one piece of evidence and try to explain it • That means be able to replicate in the lab how that piece of evidence came to be • Then, when enough experimentally supported pieces of evidence have been gathered, an all-encompassing conclusion can be drawn (theory)

9. Under one hypothetical scenario, this occurred in four stages: (1) the abiotic synthesis of small organic molecules; (2) joining these small molecules into polymers: (3) origin of self-replicating molecules; (4) packaging of these molecules into “protobionts”

10. Abiotic synthesis of small O-molecules • Origin of the universe • Big Bang – lighter elements (mostly hydrogen) • Stars (fusion) – up to Carbon • Super Novae – heavier elements • Origin of Earth • Crust solidified • Volcanoes spewed inorganics, creating early atmosphere

11. Abiotic synthesis of small O-molecules • Earth’s first atmosphere most likely contained: CO, CO2, H2, and H20 mixed with some N2 and possibly other gases such as ammonia (NH3) and methane (CH4) • All inorganic molecules

12. Abiotic synthesis of small O-molecules • What is missing? • Little or no O2. Why not? • No photosynthetic organisms to produce O2 • O2 binds easily to other compounds – it doesn’t stay O2 for very long • Ex: CO2, H2O

13. Abiotic synthesis of small O-molecules • In the 1920’s, A.I. Oparin and J.B.S. Haldane independently proposed idea • Earth’s early atmosphere was much different that today; conditions could have been conducive to the formation of simple organic materials

14. Abiotic synthesis of small O-molecules • In 1953, American scientist Stanley Miller tested Oparin’s hypothesis by recreating Earth’s early environment with all of the inorganic molecules • He then exposed the environment to electric sparks (simulated lightning)

16. Abiotic synthesis of small O-molecules • In a few days, organic molecules started to form • Every run of the experiment provided amino acids, ATP, and Adenine

17. Abiotic synthesis of small O-molecules • Alternate sites proposed for the synthesis of organic molecules include submerged volcanoes and deep-sea vents where hot water and minerals gush directly into the deep, cool ocean

18. Abiotic synthesis of small O-molecules • Another possible source for organic monomers on Earth is from space, including via meteorites containing organic molecules that crashed to Earth • Panspermia

19. From monomers to polymers • With constant energy sources and enough time (millions/billions of years), the newly born Earth’s oceans would have been teeming with simple O-molecules • These monomers just needed a way to combine to become polymers

20. From monomers to polymers • Clay theory: clay acted as a template from which O-molecules replicated themselves • Dissolved O-molecules splash on hot sand, clay, or lava (or around deep sea vents) • Water evaporates, leaving the O-molecules behind • UV radiation and iron pyrite catalyze

21. From monomers to polymers

22. Self-replicating molecules • DNA, RNA, or Protein first? • Combination? • Many believe the first hereditary material was RNA, not DNA • RNA can also function as enzymes

23. Self-replicating molecules • Short RNA polymers can be synthesized abiotically in the lab • If these polymers are added to a solution of ribonucleotide monomers, sequences up to 10 bases long are copied • If zinc is added, the copied sequences may reach 40 nucleotides with less than 1% error

24. Self-replicating molecules • In the 1980’s Thomas Cech discovered RNA molecules are important catalysts in modern cells • RNA catalysts (ribozymes) remove introns from RNA • Ribozymes also help catalyze the synthesis of new RNA polymers • In the pre-biotic world, RNA molecules may have been fully capable of ribozyme-catalyzed replication

25. Self-replicating molecules • Because RNA is only single stranded, its conformation can be quite different than DNA, based upon the nucleotide sequence • Varying conformations of RNA strands allows natural selection to favor some strands and “weed out” others • Occasional copying errors lead to mutations – the source of variation

26. Self-replicating molecules • RNA-directed protein synthesis may have begun as weak binding of specific amino acids to bases along RNA molecules, which functioned as simple templates holding a few amino acids together long enough for them to be linked • This is one function of rRNA today in ribosomes • If RNA synthesized a short polypeptide that behaved as an enzyme helping RNA replication, then early chemical dynamics would include molecular cooperation as well as competition

27. Self-replicating molecules • Eventually, an RNA template would have helped synthesize a single strand of DNA, which would have quickly made its complementary strand • DNA is a more stable molecule • If it was synthesized based upon an RNA code, it could still produce RNA replicas

28. Road to “Protobionts” • Protobionts are groups of abiotically produced molecules • Maintain separate internal environment • “Reproduce” • May contain required materials for some chemical rxns • i.e., they exhibit some attributes of living things

29. Road to “Protobionts” • Amphipathic lipids to form bilayers, which can wrap to form spheres • Can grow or shrink due to osmosis when placed in different salt concentrations • Can store E as a membrane potential • Can “eat” (engulf) smaller spheres

30. Road to “Protobionts” • Membranes separate internal from external environments • Provides stability and compartmentalization • If one metabolic process generates E, a membrane can keep the E for itself (nat. sel.) • Protobiont can evolve as a unit

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

33. Diversity of Life • Simple cells, with genetic info, that could replicate now found on Earth • Mutations driving force behind nat. sel.

34. Diversity of Life • Geology dictated what life could evolve • Pangea allowed mixing of gene pools • Breakup of Pangea isolated populations • Life dictated what life could evolve • Lack of O2 drove anaerobic resp. • Photosynthesizers put O2 in air, driving evolution of aerobic resp.

35. Origins of Organelles • Because of the environment, heterotrophic life could have lived off this organic mix for some time • If one organism engulfed another to eat it, but the prey turned out to benefit the predator, a mutualism could be formed (Endosymbiont Theory) • Heterotrophic eukaryotes could have formed

36. Aerobic bacterium Anaerobic, predatory prokaryotic cell engulfs an aerobic bacterium Descendents of engulfed bacterium evolve into mitochondria

37. Origins of Organelles • But Natural Selection would have favored organisms that could harness an outside E source to survive • At some point, an ancient form of photosynthesis evolved • The first autotrophs were very successful and spread throughout the environment

38. Photosynthetic bacterium Mitochondria-containing cell engulfs photosynthetic bacteria Descendents of photosynthetic bacteria evolve into chloroplasts

39. Paramecium sp. Chlorella sp,a green algae

40. Origins of Diversity • Earth formed ~4.5 Bya • Earth’s crust didn’t form until ~4Bya • Oldest fossils found formed ~3.5Bya • So, life had to have originated sometime between ~4-3.5Bya • Crust, cooler temps, liquid water • The life resembled bacteria

41. Origins of Diversity • Prokaryotes dominated from ~3.5-2Bya • Stromatolites are sources of prokaryotic fossils • Cyanobacteria that lived in huge floating mats • They’d deposit CaCO3, which left layered effect • Probably responsible for Earth’s O2 atmosphere

42. Origins of Diversity • Most of the O2 liberated from H2O probably reacted with Fe to form iron oxide • Seen in many “rusted” banded patterns • ~2.7Bya, enough O2 was being formed to change the atmospheric compositions

43. Origins of Diversity • O2 oxidizes so much that most of the existing prokaryotic life died off • Others evolved mechanisms to utilize O2 • First eukaryotic cells formed ~2.7-2.1Bya – right about the time O2 was becoming dominant • This “coincidence” could help explain how aerobic respiration evolved (environmental factors putting pressures on the organisms)

44. Origins of Diversity • Multicellular organisms appear ~1.5-1.2Bya • Most cnidarians and poriferans were present in late Precambrian • The “Cambrian Explosion” is where the real animal diversity that we see today came from • ~550-510Mya • Could be due to a global “thawing” period

45. Origins of Diversity • Land invasion took place ~500Mya • Organisms had to evolve ways to prevent water loss • Plants helped “bring” animals to land by providing food sources • Herbivores “brought” their predators to land • Terrestrial vertebrates, tetrapods, evolved from fishes • Most modern mammals appeared ~60-50Mya • Hominids diverged only ~5Mya

46. Write one paragraph explaining the significance of this cartoon