BIOL214 - History of the Earth and Life (lec 4+5)

Abigail Allwood

• studies fossil stromatolites (mats of ancient bacteria)

• first female and first Australian PI on a NASA Mars mission

Darwin’s Background in Geology

• studied geological formations during the HMS Beagle

• Concluded Earth must be much older than a few thousand years

• Used depositional rates to conclude Earth took 300 million years to form → Earth must be way older

Lord Kelvin’s Counterargument

• Earth is 20 million years old max

• claimed that newly formed earth was HOT and is cooling to the temperature of space

• calculated the rate that earth cools and used it to determine earth was 20 million years old at maximum

• he is wrong because earth is not a rigid sphere

Discovery of Atomic Structure led to Radiometric Dating

• unstable isotopes (radioisotopes) have fixed probabilities of decaying → transforming into a more stable form

• ex. 14^C (unstable) → 14^N (stable)

• can be used as “clocks” to provide absolute dates of rocks

• decay rates measured as half-lives

• first published by Bertram Boltwood

• estimates the age of earth at 4.56 billion years old

β−decay

• beta particle (fast energetic electron) is emitted from an atomic nucleus

α-decay

• atomic nucleus emits an alpha particle (helium nucleus) and transforms/decays into a different atomic nucleus

Age Equation

• relates radioactive decay to geologic time

• D* = D₀ + N_(t)(e^λt − 1)

• t - age of sample

• D* - number of atoms of radiogenic daughter isotope

• D₀ - number of atoms of daughter isotope in orginal composition

• N_(t) - number of atoms of the parent isotope in the sample at time t

• λ - decay constant (equal to inverse of the radioactive half-life of parent isotope * ln(2) )

Isochron

• line on a graph, connecting points of which an event occurs simultaneously

Most Organisms Don’t Fossilize

• scavengers

• weathering

• mechanically broken or lost

Gaps in the Fossil Record

• common because fossilization is rare

• some fossils are inaccessible

Fossils Provide Clues about…

• behavior

  • mating behavior in turtles

  • live birth of ichthyosaurs

  • feeding behavior of fish

• development

  • trilobites grew slowly and steadily despite molting

Countershading

• evidence from scanning electron microscopy and x-ray spectroscopy

• parts in shadow are light

• parts exposed to the sky are dark

• Borealoptela had more melanin on its back than on its underside

Nanostructures

• used to reconstruct colors in dinosaurs

• Anchiornis

CT Scans

• determined function of hadrosaur crest

• connected to nasal cavity and generated sound by blowing air

• ears were tuned to frequency

Lagerstatte

• a site with an abundant supply of unusually well-preserved fossils from the same time period

• example is the Burgess Shale

Biomarkers

• distinctive molecule only produced through biological activity

• can serve as evidence that a species existed

• ex. okenane (carotenoid) reveals growth of purple sulfur bacteria 1.64 billion years ago

Carbon Isotopes used to Infer Early Hominin Diet

• C₄ plants have less ¹³C than C₃ plants

• ¹²C:¹³C ratios infer plants eaten

• Early hominins fed more on C₃ plants (e.g., shrubs/trees), while later hominin shifted to C₄ plants (e.g., grasses)

Earth’s Beginning

• coalesced from the primordial solar disk 4.568 billion years ag

• cooled over millions of years, while gases and H2O vapor were released by rocks

• H2O condensed and rainwater filled Earth’s basins

• A collision dislodged material that formed the moon (~4.4 billion years ago)

Zircons

• carbon was preserved in minerals called zircons 4.4 billion years ago

• isotopic signatures recovered from zircons indicate life

Earliest signs of life

• evidence of stromatolites dating to 3.5-4 bya

• not confirmed

Tree of Life

• Bacteria

  • single-celled organisms

  • peptidoglycan in membranes

  • earliest fossils 3.5 bya; abundant by 2.6 bya

• Archaea

  • single-celled organisms

  • no photosynthetic species

  • distinct set of proteins that transcribe DNA → RNA

• Eukarya

  • everything that isnt’t bacteria or archaea

  • many are single-celled

  • many structures within cells

Origin of Multicellularity

• may have evolved independently in different lineages

• bacteria can form multicellular aggregates (ex. biofilms)

• date back to 2.1 bya

Dawn of Animals

• early animal life resembles sponged

• date back to 600 mya

• biomarkers of sponges 635 bya

Ediacaran Period

• youngest period of the three that make up the Neoproterozoic Era

  • youngest era in the Proterozoic Eon

• Ediacaran is sandwiched between Cryogenian Period and Cambrian Period

• diverse and unique animals dominated oceans 570-540 mya

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Cambrian Period

• 541 mya

• most major animal groups appear in fossil record

• animals diversified, some grazed on microbial mats, predator-prey interactions evolved

• chordates (have notochord)

• large predaceous fish

Transition from Ocean → Land

• prokaryotes colonized terrestrial environments first

• fossils date 2.6 bya

First Terrestrial Plant and Fungal Life

• oldest terrestrial plant fossils are 475 mya and resembled liverworts and mosses

• large forest ecosystem within 100 million years

• fungi appear 400 mya

First Terrestrial Animal Life

• invertebrate trackways date 480 mya

  • probably relatives of insects and spiders

  • not clear if they were strictly terrestrial

• oldest fossil of fully terrestrial animal dates 428 mya (pnemodesmus)

First Terrestrial Vertebrates

• oldest tracksways date to 390 mya

• oldest fossils of tetrapods date 370 mya

Currently Existing Lineages

• 350 mya many currently existing lineages had not yet evolved

• Teleost fish (bony fish)

• Mammals

• Birds

• Flowering Plants

First Insects

• evolved 400 mya

• first flies evolved 250 mya

Evolution of Mammals

• synapsids evolved 320 mya

  • dominant vertebrates around 280 mya

• First mammals evolved from synapsids 200 mya

• Current mammal lineages evolved 160 mya

Diversification of Mammals

• mammals diversified after dinosaurs went extinct (~65 mya)

• Whales, bats, primates emerged around 50 mya

• oldest human fossils dates to 300,000 years ago

Evolution of other major lineages

• dinosaurs: ~ 240 mya

• birds: ~ 160 mya (dinosaur descendents)

• flowering plants: ~ 136 mya

  • grasses not until ~ 20 mya

• insects: ~ 400 mya

Natural selection

• the process by which organisms best adapted to their environment survive and reproduce while those less well-adapted are eliminated

• tends to lead to an increase in the frequency of the allele that confers the highest fitness in a given environment

• selection acts to maintain a number of alleles in a population if the relative fitness of different alleles changes on a spatial or temporal scale

Directional Selection

• favors individuals at one extreme of a phenotypic distribution that have greater reproductive success in a particular environment

• happens when a new allele is introduced into a population by mutation

• new allele may grant higher fitness

Stabilizing Selection

• favors survival of individuals with intermediate phenotypes

• extreme values are selected against

• ex. eggs in a clutch (too many or too little are bad)

Balancing Selection

• maintains genetic diversity in a population

• two or more alleles are kept in balance and are maintained

• does not favor one particular allele

• heterozygote advantage

• frequency dependent selection

Heterozygote advantage

• sickle cell allele of the human β-globin gene

• homozygous individual with two copies of this allele has sickle-cell disease (low fitness)

• however, heterozygote with one copy of this allele has resistance to malaria (highest fitness)

Frequency-dependent selection

• Positive frequency-dependent selection

  • common phenotypes have an advantage

  • ex. prey are warningly colored to advertise bad taste/toxicity

  • prey gain most benefit

• Negative frequency-dependent selection

  • rare phenotypes are favored

Disruptive Selection

• favors survival of individuals at both extremes, rather than intermediate

• over time, this may result in speciation

Speciation

• occurs where genetically distinct groups separate into species

Biological Species Concept

• “groups of populations that can actually or potentially exchange genes with one another and that are reproductively isolated from other such groups”

• Disadvantages

  • For many species with widely separate ranges, we have no idea if the reproductive isolation is by distance only or whether there is some species-isolating mechanism.

  • plant hybrids

  • cannot be applied to asexually reproducing species

Phylogenetic Species Concept

• a phylogenetic species is a basal grouping of organisms distinct from other such groupings, within which there is a parental pattern of ancestry and descent

• members of a single species have a unique combination of characters

• taxonomic and molecular view of species

• Disadvantage: determining how much difference between populations is enough to call them species

Evolutionary Species Concept

• George Gaylord Simpson (1961)

• species is distinct from other lineages if it has its own evolutionary tendencies and historical fate

Ecological Species Concept

• Leigh Van Valen (1976)

• a species occupies a distinct ecological niche, a unique set of habitat requirements

• competition results in each individual species occupying a unique niche

• useful in distinguishing asexually reproducing and morphologically similar species such as bacteria

Allopatric Speciation

• spatial separation of populations by a geographical barrier

Sympatric Speciation

• members of a species that initially occupied the same habitat within the same range diverge into two or more different species

Artificial Speciation

• Rice and Salt (1990) proved it was possible to obtain reproductive isolation in 35 generations of fruit flies

• Dodd (1989) did it in 8 generations

History of Earth

• 4.5 bya Earth was formed by a coalescence if material from the solar nebula

• planet was so hot, the iron melted and sunk to the center

• water not present in free form, but instead bound to hydrated minerals such as mica in earth’s crust

Early Life (1)

• water released from rocks via volcanic explosions condense → form hydrosphere

• atmosphere rich in carbon dioxide; little oxygen until 2.5 bya

• oxygen in atmosphere when photosynthetic organisms evolved 3 bya

Early Life (2)

• formation of replicating DNA → origin of life

• first fossil record of life ~ 3.5 bya

  • unicellular prokaryotic lfie forms - cyanobacteria

• atmospheric conditions were anaerobic and fermentation provided most of the energy

Early Life (3)

• 2 bya eukaryotes appeared

  • chromosomes, meiosis, and sexual reproduction evolved

• 2 billion year gap between origin of life and appearance of multicellular life

  • time required to build up oxygen layer through photosynthesis

• metazoans - multicellular aerobically respiring animals

• build up of oxygen → ozone layer (shields life from radiation)

  • led to the demise of many early anaerobic organisms

Precambrian Era

• 3.9 bya

  • no atmospheric oxygen

• 3 bya

  • moon closer to earth causing larger tides

  • photosynthesizing cyanobacteria evolve

  • oxygen is toxic for many bacteria

Cambrian Period

• 530 mya, Cambrian explosion marked appearance of most of our current marine invertebrate phyla

  • sponges, cnidarians, annelids, mollusks, crustaceans, echinoderms, tunicates

• ozone layer formed

Paleozoic Era

• Ordovician Period- first chordates; terrestrial life

• Silurian Period- first jawed fish, warm wet climate

• Devonian Period- age of fishes

• Carboniferous Period- extensive forests

• Permian Period- mass extinction of plant life

Ordovician Period

• 488-444 mya

• mostly southern/equatorial land masses

• Gondwanaland over South pole

• first chordates, jawless fish, echinoderms

• terrestrial life as primitive plants and fungi

• organic soils formed

• at the end of this period, climate cooled, sea level dropped, mass extinction event

Silurion Period

• 444-416 mya

• first jawed fish, arthropods, vascular plants

• two large continents

• warm and wet climate

• sea levels rose

Devonian Period

• 416-359 mya

• age of fishes

• marine invertebrates (trilobites and corals) diversified

• first bony fishes in fossil record

• at the end of the Devonian, amphibians

• land plants caused CO2 concentrations to decline and oxygen to increase

• major glaciation and mass extinction late into period

Carboniferous Period

• 359-299 mya

• insects radiated because of 30%+ oxygen levels

• reptiles arose and amphibians radiated

• extensive forests provided today’s rich coal beds and increase oxygen levels

• vascular plants were the first to use lignin for support

• lack of CO2 cooled planet → ice caps and glaciers

• forest fires frequent, but effects dampened by swamp conditions

Permian Period

• 299-251 mya

• continents aggregated into one land mass → Pangaea

• reptiles and insects radiated

• extinctions in amphibia

• extinction of marine invertebrates including trilobites, plankton, corals, and benthic invertebrates

• at the end of the period, oxygen levels dropped, CO2 rose, temps rose → hot and dry conditions

• seed plants arose, plant life was scarce

Mesozoic Era

• 251-200 mya

• beginning:

  • Pangaea → Gondwana (south) + Laurasia (north)

• end:

  • Gondwana → South America, Africa, Australia, Antarctica, India

  • Laurasia → Eurasia and North America

Age of Reptiles

• Triassic Period

• Jurassic Period

• Cretaceous Period

Triassic Period

• 250-200 mya

• Pangaea started drifting apart

• climate was hot and wet

• sea levels dropped significantly

• Gymnosperms became dominant

• Reptiles increase, first dinosaurs appeared

• mammal appeared

• mass extinction at end

Jurassic Period

• 250-146 mya

• dinosaurs dominated the terrestrial vertebrate fossil records

• first birds

• oxygen levels dropped to 13%

• Diptera (flies) and Hymenoptera (wasps) evolved in conjunction with angiosperms (flowering plants)

Cretaceous Period

• 146-65 mya

• angiosperms dominated over gymnosperms

• dinosaurs became extinct at the end

Cenozoic Era

• Paleogene Period

• Neogene

• Quarternary Period (TODAY)

Paleogene Period

• 65-23 mya

• continents at their positions they are today

• atmospheric oxygen reaches today’s levels 21%

• angiosperms dominate forests

• rapid diversification of mammals and birds

Neogene Period

• 23-2.5 mya

• climate became more seasonal

• ice caps on poles grow

• continents generally in their modern positions

• Isthmus of Panama arose (cut off between Atlantic and Pacific Ocean → Gulf Stream)

• Mammals adapted to cold conditions

• Ended with an Ice Age

Quaternary Period

• 2.5 mya

• punctuated by a series of repeated glaciations

• Evolution of Homo

• extinctions of large mammals

Continental Drift

• slow movement of the Earth’s surface plates

• first proposed by Alfred A. Wegener

• present day Earth consists of a molten mass with a 100km thick crust

• crust is made of 14 irregular pieces called tectonic plates

Continental Drift and Fossils

• fossil remains spread across Gondwana → South America, Africa, India, Antarctica, and Australia

• Nothofagus found on multiple continents that were once part of Gondwana

Biogeographical Realms

• Alfred Russel Wallace realized that certain plant and animal taxa were restricted to certain geographic areas of the earth

• describe areas bounded by major barriers to dispersal

Six Major Realms

1. Nearctic- North America + Greenland

2. Neotropical- Southern Mexico + South America

3. Ethiopian- Africa

4. Palearctic- North Tip of Africa + Europe + Russia

5. Oriental- India + China + South Asia

6. Australian- Australia + Pacific Islands

Convergent Evolution

• areas of similar climates are often inhabited by species of similar appearance but from different taxonomic groups

Red Queen Hypothesis

• Leigh Van Valen (1973)

• evolutionary history of life is a continual race with no winners, only losers

• species have to evolve just to keep pace with environmental change and to avoid extinction

• species become extinct when all individuals die without producing progeny

Pseudoextinction

• species can disappear when a lineage is transformed over evolutionary time or divides into multiple lineages

Gradualism

• theory that new species evolve continuously over long periods of time

Punctuated Equilibrium

• theory that the tempo of evolution is sporadic and not continuous

Permian Extinction

• largest recorded extinction for fishes and tetrapods

• geologically rapid changes in climate, continental drift, and volcanic activity

Triassic and Devonian Extinctions

• causes are not well known

Cretaceous Extinction

• Luis Alvarez suggested that cause may have been a single catastrophic event such as a meteor strike

• 75% of tetrapod species in fossil record disappeared at that time

Extinction Rates on Islands vs. Mainland

• islands often have greater extinctions

• lower number of species on islands → higher percentages of extinct taxa

• many island species contain single populations

Extinction Causes

• introduced species and habitat destruction by humans

• hunting and overcollecting

Extinctions Due to Introduced Species

• competition

• predation

• disease and parasitism

• competition may exterminate local populations, but not shown to eradicate entire populations of rare species

Endangered Species

• Ecuador and United States have highest number of endangered species

• majority of threatened mammals are in tropical countries

Vulnerability to Extinction

• rarity

• dispersal ability

• degree of specialization

• population variability

• feeding level

• life span

• reproductive ability

Conserving Endangered Species

• decrease habitat destruction

• reduce hunting

• prevent release of exotic species

• identify species at risk for extinction before they reach endangered status

• understand aspects of their biology

• understand behavior can affect its reproductive ability and therefore susceptibility to extinction