Fossil lecture outline Fossils preservation and bias Establishing

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 • Fossil: any trace left by an organism from the past

Implications

Some geological basics: types of rock • Igneous: • Sedimentary: • Metamorphic:

Preservation of fossils: geologic cycle • Ocean sediments: • Continents:

Bias in fossils • Species? • Habitat? • Structures? • Rarity?

II. Dating fossils: methods • Relative dating: stratigraphic analysis – older layers below younger – layers start out horizontal – cross-cutting features are younger

Geologic time: Eons Hadean Origin of earth Proterozoic Archaean First life Paleozoic Mezozoic Cenozoic Oxygen in atmosphere Eukaryotes 543 mya 4. 6 billion year history of earth macroscopic fossils

Geologic time: Periods Pre. Cambrian 543 495 439 408 354 323 290 251 206 144 65 1. 8 Tertiary Silurian Miss. Permian Jurassic Cambrian Cretaceous Quaternary Ordovician Devonian Penn. Triassic Paleozoic Mezozoic Cenozoic Mnemonic: “Please, come over soon, dear, ” Mary pleaded prettily. Tragically, John came too quickly. 0. 543 billion year fossil history of earth (roughly)

Geologic time: Periods Pre. Cambrian 543 495 Paleozoic 439 408 Mezozoic 354 323 290 251 206 144 Cenozoic 65 1. 8 Tertiary Silurian Miss. Permian Jurassic Cambrian Cretaceous Quaternary Ordovician Devonian Penn. Triassic Burgess Shale, BC First insects Cambrian “explosion” First plants on land First mammals Mass extinction 0. 534 billion year fossil history of earth First apes Mass exctinction

Dating methods: absolute • Radio-isotope dating (box 2. 3) – potassium – argon (K-40 to Ar-40) – Carbon 14 (C-14 N-14 + ) Figure 2. 19

Radioisotope dating • Radioactive decay does not depend on pressure or temperature X(t) = X(0)e-Lt Note: half life = 0. 693 / L

Key assumptions

Key practices • Potassium – Argon • Carbon 14

Utility of radioisotope dating

• Problem: C-14 levels vary

Carbon-14 calibration • Recent time: dendrochronology (10, 000 years) • Ancient: calibrate using other isotopes,

Radioisotope verification • How do we know that the method works? • Early: • Coral clocks Annual rings in coral (bar is 0. 5 mm) NOAA

Coral clocks • Length of earth orbit: 24 hours * 365. 25 days = 8766 hours / yr • Change in speed of rotation due to friction: 20 seconds / million years • Day length = • Days per year =

What is the range of ages for the fish skeletons found in layer B? Decay constant for 40 -K: 5. 34 x 10 -10 A B C D E F A: Ash layer: crystals contain 99. 85% 40 K and 0. 15% 40 Ar B: Sandstone with fossil fish C: Limestone with fossil shells D: Mudstone containing pollen E: Mudstone layer F: Granitic intrusion: crystals contain 96% 40 K AND 4% 40 Ar

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?

Life: the big picture

Organelles

III. History of Life: Before the Cambrian Explosion Anemones

Before the Cambrian Explosion Medusa

Rarely bilateral Kimberella

Cambrian Explosion: every known animal phylum (and more? ) Pikaia Wiwaxia Burgess Shale, BC Anomalocaris

What caused the Cambrian Explosion? Halucigenia

When did animal diversity originate? Estimates of splits between arthropods and vertebrates (Agnatha): 833 – 953 mya

Factors in Cambrian explosion: gene duplication?

Hox gene evolution Protostomes Bilateria Deuterostomes

Number of genera Diversity in time: number of genera of marine invertebrates Post-paleozoic diversity increase Paleozoic diversity plateau Time (mya)

Number of genera Correcting for bias: genera per fossil collected Time (mya)

IV. What do fossils tell us about evolution?

65 mya Eocene Paleocene Pliocene present Pleistocene Oligo 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 Neohipparion: most common fossil

The pace of evolution How do traits change over time? gradualism stasis / puncuated equilibrium (“punc eq”):

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

Species Escherichia coli (bacteria) Saccharomyces cerevisiae (yeast) Drosophila melanogaster Arabidopsis thaliana (a weed) Homo sapiens Triturus cristatus (a newt) Fritillaria assyriaca (a monocot plant) Protopterus aethiopicus (a lungfish) Genome Size (picograms)* 0. 005 0. 009 0. 18 0. 2 3. 5 19 127 142 9 *Haploid genome size. 1 pg = ~10 base pairs

Genome size: phylogenetic context

Number of cell types Complexity: cell types

Overall patterns of life on earth?

Fossils: summary

Fossils: references Caroll, S. B. 2005. Endless forms most beautiful: the new science of evo devo. Very readable description of insights into evolution from developmental genetics. Gould, S. J. 1990. 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. 2002. 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?