Key questions to consider. Fossil lecture outline. I. Fossil preservation. Implications

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1 Fossil lecture outline Fossils, preservation, and bias Establishing dates for fossils How do these challenges affect our ability to address patterns of diversity and evolution? Fossils and the history of life: diversity Fossils and patterns of evolution: stasis and gradualism Key questions to consider How old is life on earth? How do we know? How did the earliest life begin? What was it like? How often do mass extinctions happen, and why? Why do some species survive and not others? What are the major patterns in the history of life? I. Fossil preservation Implications Fossil: any trace left by an organism from the past 1

2 Some geological basics: types of rock Igneous: Preservation of fossils: geologic cycle Ocean sediments: Sedimentary: Continents: Metamorphic: Bias in fossils II. Dating fossils: methods Species? Habitat? Relative dating: stratigraphic analysis older layers below younger layers start out horizontal cross-cutting features are younger Structures? Rarity? 2

3 Origin of earth Geologic time: Eons Hadean Archaean Proterozoic First life 4.6 billion year history of earth Oxygen in atmosphere Eukaryotes Paleozoic Mezozoic Cenozoic 543 mya macroscopic fossils Geologic time: Periods Pre- Cambrian Cambrian Silurian Miss. Permian Jurassic Tertiary Ordovician Devonian Penn. Triassic Cretaceous Quaternary Paleozoic Mezozoic Cenozoic Mnemonic: Please, come over soon, dear, Mary pleaded prettily. Tragically, John came too quickly billion year fossil history of earth (roughly) Geologic time: Periods Pre- Cambrian Paleozoic Mezozoic Cenozoic Cambrian Silurian Miss. Permian Jurassic Tertiary Ordovician Devonian Penn. Triassic Cretaceous Quaternary Dating methods: absolute Radio-isotope dating (box 2.3) potassium argon (K-40 to Ar-40) Carbon 14 (C-14 N-14 + β) Burgess Shale, BC Cambrian explosion First insects First plants on land First mammals Mass extinction First apes Mass exctinction Figure billion year fossil history of earth 3

4 Radioisotope dating Key assumptions Radioactive decay does not depend on pressure or temperature X(t) = X(0)e -Lt Note: half life = / L Key practices Utility of radioisotope dating Potassium Argon Carbon 14 4

5 Carbon-14 calibration Problem: C-14 levels vary Recent time: dendrochronology (10,000 years) Ancient: calibrate using other isotopes, Radioisotope verification How do we know that the method works? Early: Coral clocks Coral clocks Length of earth orbit: 24 hours * days = 8766 hours / yr Change in speed of rotation due to friction: 20 seconds / million years Annual rings in coral (bar is 0.5 mm) NOAA Day length = Days per year = 5

6 What is the range of ages for the fish skeletons found in layer B? Decay constant for 40-K: 5.34 x A B C D E F A: Ash layer: crystals contain 99.85% 40K and 0.15% 40Ar B: Sandstone with fossil fish C: Limestone with fossil shells D: Mudstone containing pollen E: Mudstone layer F: Granitic intrusion: crystals contain 96% 40K AND 4% 40Ar Origins of Life What is known about the common ancestor of all life? Is this true of the earliest life on earth? Miller: prebiotic soup Chicken and egg problem Proteins: DNA: So? 6

7 Life: the big picture Organelles III. History of Life: Before the Cambrian Explosion Before the Cambrian Explosion Medusa Anemones 7

8 Rarely bilateral Cambrian Explosion: every known animal phylum (and more?) Pikaia Kimberella Wiwaxia Burgess Shale, BC Anomalocaris What caused the Cambrian Explosion? When did animal diversity originate? Halucigenia Estimates of splits between arthropods and vertebrates (Agnatha): mya 8

9 Factors in Cambrian explosion: gene duplication? Hox gene evolution Protostomes Bilateria Deuterostomes Diversity in time: number of genera of marine invertebrates Correcting for bias: genera per fossil collected Number of genera Post-paleozoic diversity increase Paleozoic diversity plateau Number of genera Time (mya) Time (mya) 9

10 IV. What do fossils tell us about evolution? 65 mya Eocene Oligo Pliocene present Paleocene Oligocene Pleistocene Case study: evolution of the horse Heiracotherium Miohippus Merychippus Equus (horse) Eocene Oligocene Miocene to present Florida Museum of Natural History A more complete view The pace of evolution How do traits change over time? stasis / puncuated equilibrium ( punc eq ): gradualism Neohipparion: most common fossil 10

11 Stasis vs. gradualism: which is the typical pattern? Challenges to testing: Stasis vs. gradualism: evidence Bryozoans (colonial organisms, similar to coral) Stasis vs. gradualism: evidence Bivalves (Mollusca) in Pliocene: 3 species, 24 characters Trends in the history of life? Complexity? amount of DNA number of cell types 11

12 Species Genome Size (picograms)* Escherichia coli (bacteria) Saccharomyces cerevisiae (yeast) Drosophila melanogaster 0.18 Arabidopsis thaliana (a weed) 0.2 Homo sapiens 3.5 Triturus cristatus (a newt) 19 Fritillaria assyriaca (a monocot plant) 127 Protopterus aethiopicus (a lungfish) 142 Genome size: phylogenetic context *Haploid genome size. 1 pg = ~10 9 base pairs Complexity: cell types Overall patterns of life on earth? Number of cell types 12

13 Fossils: summary Fossils: references Caroll, S. B Endless forms most beautiful: the new science of evo devo. Very readable description of insights into evolution from developmental genetics. Gould, S. J Wonderful life: Burgess shale and the nature of history. Account of the discovery of the Burgess shale fossils and their early interpretation. Many of Gould s ideas were shown to be incorrect soon after this book was published in particular, it appears that most of the fossils can be assigned to contemporary phyla. Still an enjoyable and informative read. Lane, Nick Oxygen: the molecule that made the world. Oxford University Press. Does a very nice job of recounting the evidence for oxygen levels on earth in the early history, and the interaction of the atmosphere with early life. Fossils: questions 1. (from text) Explain why each of these are relatively common in the fossil record. -burrowing species -marine species -recent organisms - pollen grains 2. (from text) Explain why each of these are relatively rare in the fossil record. - desert dwelling species - species capable of flight - organisms that lived over 3 billion years ago - flowers 3. Suppose a species occurs in the fossil record 190 million years ago. Why is it logical to argue that it actually existed prior to this date? 4. How might rising oxygen levels help explain the Cambrian explosion occurred? 5. Explain why using radioisotope dating on crystals from sedimentary rocks give incorrect ages for those rocks. 6. Why does the amount of C-14 in the atmosphere vary over time? Explain how carbon-14 dating can be calibrated despite this. 7. Why might fossils show no morphological change for long periods of time? Fossils: questions 8. Order these events: first dinosaurs, first flowering plants, oxygen in atmosphere, eukaryotes, apes, Cambrian explosion, K/T boundary, horses. 9. Why are fossils so scarce prior to the Cambrian? 10. a) We can tell much about the last common ancestor of all life by comparing today's organisms. What can we infer about its genetics and biochemistry? b) Proponents of an "RNA world" suggest that RNA was the first genetic material. Given that all present day organisms encode their genomes with DNA (except for a few derived retroviruses), what evidence do we have of the RNA world? 11. How do we know that mitochondria and chloroplasts were once free living bacteria? In what ways have they changed since endosymbiosis? 13