Chapter 16

1. Chapter 16 Origins of Life and Precambrian Evolution

2. What are the two main categories of reproductive barriers, also give an example of each? • What is a secondary hybrid zone? • Name the 4 species concepts discussed in class. • What is a ring species? • What is senescence?

3. Origins of Life • Questions • What was the earth’s environment life when life arose? • What is life and how did it arise from non-life? • What were the first cells and what characteristics did they have?

4. Origins of Life • Earth formed between 4.5 and 4.6 Bya • First few million years earth too hot to support life • Sometime around 3.8 Bya life began

5. Origins of Life • Which molecule came first? • Proteins or DNA • Proteins do complicated biological tasks but cannot reproduce themselves • DNA cannot do biological functions but replicates itself perfectly • Discovery of catalytic RNA = ribozymes

6. Origins of Life • Ribozymes can do biological work and reproduce themselves • It is now thought that ribozymes were the first life • Ribozymes can simultaneously possess a genotype and a phenotype • Has particular traits and a coding sequence

8. Defining Life • All living organisms possess a phenotype and a genotype • Other characteristics defining life are controversial • Growth • Reproduction • Evolution requires ability to record and make alteration in heritable information with some way of distinguishing valuable changes from detrimental ones • Genotype and phenotype

9. Defining Life • Ribozymes have all of these characteristics and were probably the first life forms • RNA World hypothesis • Catalytic RNA-based life eventually evolved into system we see today • DNA is a more stable molecule for holding genetic information • Avoids problem of having proteins before there was DNA to encode them

10. RNA World • Experimental Evolution of Ribozymes • If ribozymes evolve, they must have a way of changing and recording the change in its genome • RNA evolved faster replication and a shorter genome • In all the experiments, investigators had to add enzymes for ribozymes to replicate themselves • Still self-replication has not been demonstrated

15. RNA World • By recognizing an RNA World, problem of self-replication is pushed back before DNA • RNA World was probably not first self-replicating system • Likelihood of making RNA abiotically is too minute • How did the earliest self-replicating system work?

16. First Self-Replicating System • Information-containing molecules need to be made from simple inorganic compounds • Chemical reactions that constructed larger molecules needed energy source • Building blocks had to self-assemble • Larger biomolecules had to be protected from harsh environments

17. Panspermia Hypothesis • Some argue that it is just as likely that life came from outer space as it is that life was spontaneously created on the earth • Biomolecules have been found on meteroites, asteroids, and comets • Other planets may have more appropriate chemical composition than earth

18. Panspermia Hypothesis • Three versions of panspermia theory • Life arose on another planet in our solar system • Microbes could have been dislodged by meteorite impact and traveled to earth • Mars and Europa may have had water and life • Trip to earth would kill most life unless it was sheltered in carbonate rock • Earth bacteria survived space walk on Apollo 12

19. Panspermia Hypothesis • Three versions of panspermia theory • Life originated in another solar system and traveled to earth • Would need to escape solar system’s gravity and be shielded from radiation • Intelligent extraterrestrials intentionally brought life to earth to seed the galaxy with life • Directed panspermia

20. Where did Stuff of Life Come From? • Meteroites from Australia contained five amino acids • Contained equal proportions of both stereoisomers • All living things on earth have only L-form • If these formed spontaneously in space, could the same thing happen on earth? • Debate about early atmosphere • Reducing or oxidizing

21. Where did Stuff of Life Come From? • Up until 4 Bya earth was bombarded by impacts • If life formed it was probably continually destroyed

22. Where did Stuff of Life Come From? • Oparin-Haldane Model • Miller attempted to re-create the primordial soup and simulate lightning • Biomolecules were spontaneously created in reducing environment • It is now believed that CO2 and N2 instead of methane and ammonia were the dominant gases • Oparin-Haldane model still used as a null model

23. Cellular Life • Search for the most recent common ancestor of all life = cenancestor • Except viruses • All extant life is made up of cells • Descended from cenancestor population of interbreeding individuals • Cenancestors lived at least 2 Bya

25. Cellular Life • Cells allow compartmentalization and organization • Allows genotype and phenotype to be linked • Genotype codes for a product and that product stays within the organism • Does not diffuse to other members of population with other genotypes • Lab mixtures of polyamino acids spontaneously organize themselves into cell-like structures in solution

26. Cellular Life • Examine fossil record for earliest cells • 3.4 bya cells from Swartkoppie S.A. • 3.465 Bya first known organized cells from Apex Chert in Australia • Cells growing in filaments • Probably cyanobacteria • Identification made by extant comparison • Must have been cells before these because cyanobacteriaare relatively advanced

28. Cellular Life • Fossil record spotty before 2.5 Bya to have continuous fossil record • Apex Chert may represent extant or extinct branch of life • No way to tell if those cells were really our ancestors • Because fossil record is inadequate, try to reconstruct a phylogeny of all life for answers on cenancestor

29. Phylogeny of All Living Things • First attempts at a phylogeny of everything were morphological • Doesn’t work for prokaryotes because they don’t have sufficient structural diversity • Reconstructed a phylogeny using molecular characters • Needed to find a gene that all living organisms have that has strong stabilizing selection • Otherwise too much drift would accumulate

30. Phylogeny of All Living Things • Woese selected small-subunit ribosomal RNA (ssu-rRNA) • Is present in all organisms • Strong stabilizing selection because of important function of translating proteins • Surprising results from tree that changed view of life’s phylogeny

32. Phylogeny of All Living Things • Now there are three domains • Bacteria split into Bacteria and Archaea • Archaea more closely related to Eukarya than Bacteria • Protista should be split into many kingdoms • Fungi should be split into two kingdoms • Eukaryotes are minor twigs on tree of life

33. Phylogeny of All Living Things • Where is exact root on tree? • There are no outgroups • Root tree by genetic distance • Most parsimonious solution to root is that there was one most recent common ancestor population that gave rise to all life • Was relatively advanced, similar to modern bacteria

34. Rooting tree of life • Aminoacyl-tRNA synthetase gene family

37. Phylogeny of All Living Things • Estimates of universal phylogeny would be expected to be congruent among genes • This has not been found to be the case • Phylogeny using HMGCoA reductase gene shows very different phylogeny • Because of horizontal (or lateral) gene transfer • Bacterial gene introgressed into Archaea genome

39. Phylogeny of All Living Things • We now know that horizontal gene transfer was rampant in early life and many genes may not belong to the species that have them • Most recent common ancestor could have been a pool of species that exchanged genes

41. Date for Root of Tree of Life • Attempts to use molecular clock to date the root have been unsuccessful • There is not reason to believe that clock was constant over billions of years • Oldest definite eukaryotic fossil is 950 My old • Probable eukaryotic fossil 1.5 By old • Fossil cyanobacteria at least 2 Bya • If Apex Chert is correct, root must be at least 3.5 By old

43. Date for Root of Tree of Life • Earliest possible date 4.4 to 3.85 Bya • Youngest possible date 3.5 to 2 Bya

44. Origin of Organelles • Bacteria and Archaea are similar to fossil common ancestor • Gradual refinement of form • Eukarya are fundamentally different • Complex cells • Organelles • Complex genomes with introns • Some are multicellular

45. Origin of Organelles • Mitochondria and chloroplasts are derived from prokaryotes • Giardia, a protist, lacks complex organelles • Mitochondria and chloroplasts have double membranes and chromosomes • Mitochondria are purple bacteria • Chloroplasts are cyanobacteria

46. Origin of Organelles • Endosymbiont Theory of Lynn Margulis • Both organelles arose by symbiotic relationship with other larger bacteria • Their bacterial relatives are endoparasites • Phylogeny of life including organelles shows that they are bacteria

48. Evolution of All Life • From the root of the tree of life, Bacteria and Archaea changed little • Eukarya evolution involved major innovations • Acquiring mitochondria and chloroplasts is part of what changed them from prokaryotes to eukaryotes