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 Archaean Proterozoic

PaleozoicMezozoic

Cenozoic

Origin of earth

4.6 billion year history of earth

First life Oxygen in atmosphere

Eukaryotes

543 mya

macroscopic fossils

Geologic time: Periods

0.543 billion year fossil history of earth (roughly)

Pre-Cambrian

CambrianOrdovician

SilurianDevonian

Miss.Penn.

PermianTriassic

JurassicCretaceous

TertiaryQuaternary

Paleozoic Mezozoic Cenozoic

Mnemonic: “Please, come over soon, dear,” Mary pleaded prettily. Tragically, John came too quickly.

543 495 439 408 354 290 323 251 206 144 65 1.8

Geologic time: PeriodsPaleozoic Mezozoic Cenozoic

Cambrian “explosion”

0.534 billion year fossil history of earth

First plants on land

First insects

Mass exctinction

Pre-Cambrian

CambrianOrdovician

SilurianDevonian

Miss.Penn.

PermianTriassic

JurassicCretaceous

TertiaryQuaternary

Mass extinction

First apesFirst mammals

543 495 439 408 354 290 323 251 206 144 65 1.8

Burgess Shale, BC

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?

A

B

C

D

EF

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

Decay constant for 40-K: 5.34 x 10 -10

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?)

Wiwaxia

Anomalocaris

Pikaia

Burgess Shale, BC

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

Deuterostomes

Bilateria

Diversity in time: number of genera of marine invertebrates

Paleozoic diversity plateau

Post-paleozoic diversity increase

Time (mya)

Num

ber o

f gen

era

Correcting for bias: genera per fossil collected

Time (mya)

Num

ber o

f gen

era

IV. What do fossils tell us about evolution?

Case study: evolution of the horse

Heiracotherium

Eocene

Miohippus

Oligocene

Merychippus

Miocene

Equus (horse)

to present

Florida Museum of Natural History

Oligo

PaleoceneEocene

OligocenePliocene

Pleistocene65 mya present

A more complete view

Neohipparion: most common fossil

The pace of evolutionHow do traits change over time?

stasis / puncuated equilibrium (“punc eq”):

gradualism

Stasis vs. gradualism: which is the typical pattern?

Challenges to testing:

Stasis vs. gradualism: evidence

Bryozoans (colonial organisms, similar to coral)

Stasis vs. gradualism: evidenceBivalves (Mollusca) in Pliocene: 3 species, 24 characters

Trends in the history of life?

• Complexity?– amount of DNA– number of cell types

Species Genome Size (picograms)*

Escherichia coli (bacteria) 0.005Saccharomyces cerevisiae (yeast) 0.009Drosophila melanogaster 0.18Arabidopsis thaliana (a weed) 0.2Homo sapiens 3.5Triturus cristatus (a newt) 19Fritillaria assyriaca (a monocot plant) 127

Protopterus aethiopicus (a lungfish) 142

*Haploid genome size. 1 pg = ~109 base pairs

Genome size: phylogenetic context

Complexity: cell typesN

umbe

r of c

ell t

ypes

Overall patterns of life on earth?

Fossils: summary

Fossils: referencesCaroll, 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: questions1. (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: questions8. 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?