Eeb 120 - Lecture 3

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Conflicting Evidence for Earth Age

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- Darwin assumed the Earth was very old because of how he understood geological and evolutionary processes to be gradual.

- Lord Kelvin argued Earth was too warm to be more than 20 million years old (He was wrong because this did not account for heat-generating radioactive core).

Front 2

How are elements defined?

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Elements exist as naturally occurring isotopes. An element is defined by the number of protons (6 proton = Carbon). Additional neutrons create isotopes of varying stability.

Front 3

Radiometric Dating

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Unstable isotopes have a predictable rate of decay to more stable forms. If we know the starting point, we can determine the age of a rock by its ratio of stable/unstable isotopes.

Front 4

What does radiometric dating allow us to do?

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It allows us to get precise estimates of the age of geologic formations (Including the Earth at 4.568 billion years old). Duration and boundaries may seem arbitrary, but they tend to coincide with major changes in climate and/or fossil assemblages.

Front 5

What is the half-life of an unstable isotope?

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The time it takes for half of a sample of unstable isotope to decay into stable form

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We don't know the exact ratio of 87Rb/87Sr trapped in a rock sample when it formed. BUT we do know that:

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1. Some minerals in the rock have more 87Rb than others.

2. The ratio of 87Rb/86Sr would have been the same in all minerals of the rock (isochron on graph)

3. The amount of stable 86Sr remains constant over time

As 87Rb decays to 87Sr, the increase in the ratio of 87Sr/86Sr is proportional to the total amount of 87Rb in a given mineral. This increases the slope of the isochron over time, which can be used with known half-life to calculate precise age of rock.

Front 7

Can fossil ages be determined directly?

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No, but rock layers above and below can be precisely dated.

Front 8

Imperfect Record of Fossil Record

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Many gaps exist in the fossil record. This is because fossilization is extremely rare. Scavengers & decomposers destroy living, weathering destroys bones. Quick burial in anoxic sediment increases the odds of fossilization. Even if a fossil forms, there is no guarantee it will ever be exposed.

Front 9

Biases in the Fossil Record

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Front 10

Fossils provide clues about behavior

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Behavior itself has been fossilized (such as live births and feedings). Inferences can be made via comparative anatomy with extant species.

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Fossils provide clues about development...

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- Trilobites were abundant & often fossilized on ocean floor. Comparing different aged trilobites shows it grew isometrically & throughout its life (showed proportions don't change)

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Fossils can preserve cellular structure

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Scanning electron microscopy (SEM) provides evidence of cellular structure. Structure of melanosomes suggests some dinosaurs had striking plumage.

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Fossils provide clues about anatomical function

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CT scans + computerized models helped determine function of hadrosaur crest. Crest connected to nasal cavity (sound generated by blowing air). Ears tuned to this same frequency.

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Occasionally soft tissues fossilize...

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Large rock formation that often contains soft-tissue fossils. Made of burgess shale and is 505 million years old. Contains 65k specimens, 93 species and is one of the most important fossil beds for studying animal life.

Front 15

What is a biomarker?

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A molecule that can only be produced by biological processes.

Front 16

Biomarkers and Early Life

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Front 17

Isotopes & Ecology

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* Organisms that eat other organisms have high levels of C13

Front 18

What is some evidence for the earliest life?

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Front 19

Oldest direct evidence?

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Most claims of oldest life are controversial. Oldest stromatolite (bacteria) fossils date to 3.45 bya (layers of mineralization formed by photosynthetic bacteria mats).

Front 20

Early organisms and the tree of life

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Microbes were the earliest life forms and still most abundant. The earliest Eukarya fossils appeared 1.8 bya. The earliest Archaea fossils are 3.5 bya. Earliest bacteria fossils are 3.45 bya; abundant by 2.6 bya corresponding to rise in oxygen

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Multicellularity

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Oldest fossils of multicellular life are 2.1 billion years old. Unclear where they fit in tree of life

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Earliest eukaryotes

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Earliest fossils of algae date to 1.6 bya (red algae) Green algae dates 750 mya.

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The dawn of animals

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Early animal life resembled sponges. Sessile; no symmetry, organs or tissues. Oldest fossils 650 myo. Biomarkers also demonstrate existence of sponges at least 635 mya.

Front 24

Earliest mobile animal

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Still before cambrian

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Edicaran Fauna

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Front 26

Which of the following was likely a uniquelyimportant driver of diversification in the EarlyCambrian?

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animals began to colonize new ecological niches byburrowing in sediment & swimming in open water

Front 27

Edicaran Fauna

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This was not the rise in multicellularity because this happened before early cambrian

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Cambrian Explosion

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Front 29

Cambrian Explosion Continued...

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Front 30

Cambrian Explosion and Early Chordates (540 mya)

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Rise of animals even includes early chordates (phylum that includes vertabrates) which first appear 515 mya.

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Transition from ocean to land was a bid deal

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Prokaryotes colonized terrestrial environments first– Fossils date to 2.6 bya

• Terrestrial animals, plants, and fungi, appearedmuch later

Front 32

First terrestrial plant and fungal life

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Oldest terrestrial plantfossils are 475 myo

• Large forest ecosystemswithin 100 million years

• Fungi appear ~ 400 myo– Associated with plants

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First terrestrial animal life

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• Invertebrate trackways date to 480 mya– Probably relatives of insects and spiders– Not clear whether they lived on land permanently

• Oldest fossil of fully terrestrial animal dates to 428 mya

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First terrestrial Vertebrates

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• Oldest trackways date to 390 mya

• Oldest fossils of tetrapods date to 370 mya

• Earliest land vertebrates still dependent on water for reproduction

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Recent emergence of familiar life forms

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• Despite Cambrian origins of most phyla, manycurrently existing lineages had yet to evolveuntil ~350 million years ago and later

– Teleosts (bony fishes)

– Mammals

– Birds

– Flowering plants

Front 36

Amniotic Egg

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Front 37

Early Amniotes

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• Living amphibians and amniotes split from acommon ancestor ~350 million years ago

• Early amniotes were more tolerant of dryconditions than the first tetrapods

• The earliest amniotes were small predatorswith sharp teeth and long jaws

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Evolution of mammals

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• Mammals evolved fromsynapsids– Dominant vertebratesaround 280 myo– First mammals emerged 150mya

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Sauropsids (Reptiles + Birds)

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Front 40

Diversification of mammals

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Front 41

Earliest humans

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Oldest human fossil are ~200,000 years old

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Evolution of other major lineages

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• Birds: ~150 mya– Descendants of dinosaurs

• Flowering plants: ~132 mya– Grasses did not diversify until ~20 mya

• Insects: emerged ~400 mya but mostcurrent lineages appear much later

Front 43

Earliest life on Earth

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Front 44

Phanerozoic "Visible life"

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