Origin of Life on Earth

• Tyrannosaurus rex may have had feathers
• Many dinosaurs were covered in feathers instead of scales

Early Earth Conditions

• Earth's formation occurred approximately 4.6 billion years ago
• Initial conditions involved rock formation and volcanic activity
• The Earth's early atmosphere consisted of:
  • Water vapor
  • Chemicals released by volcanic eruptions
  • Methane (CH4)
  • Nitrogen (N2)
  • Ammonia (NH3)
  • Carbon Dioxide (CO2)
• Oxygen (O2): was not present
• Around 3.8 billion years ago, the Earth's crust cooled
• Oceans began to form as water accumulated on the surface

The Four Main Stages of How Life Started:

• Abiotic Synthesis
• Synthesis of Polymers
• Packaging into Protocells
• Origin of Self-Replicating Molecules

Synthesis and Accumulation of Organic Monomers

• Stanley Miller's 1953 experiment demonstrated the formation of amino acids from raw materials
• Sugars, lipids, and nucleic acids were also formed
• Volcanic activity likely increased amino acid formation

Alternative hypotheses Regarding the synthesis and accumulation of organic monomers:

• Organic monomers may have also formed near submerged volcanoes and deep-sea vents
• Amino acids came from space/meteorites
• A meteorite in Australia was found to contain over 80 amino acids not of Earth origin
• It also contained lipids, simple sugars, and nitrogenous bases

Synthesis of Polymers

• Abiotic synthesis of RNA molecules can occur spontaneously from simple precursor molecules
• Polymers form as amino acids or RNA drip onto hot sand, clay, or rock

Protocells

• Cell reproduction and metabolism is needed
• Self-replicating molecules and early enzymes may have appeared early in protocells
• Protocells have vesicles that are fluid-filled and enclosed by a membrane-like structure

Origin of Self-Replication

• RNA – performs enzyme-like activities
• RNA catalyzes its own reproduction with ribozymes
• An "RNA-world" likely occurred first

Documenting Life: The Fossil Record

• Sedimentary strata are used to determine the relative ages of fossils based on their order in the rock layers
• Index fossils, similar fossils found in the same strata in different locations, help correlate strata across locations
• Radiometric dating is used to determine the absolute age of fossils

Aspects of Radiometric Dating:

• Isotopes of the same element have different numbers of neutrons
• Some isotopes are unstable and 'decay' to form other materials
• Half-life represents the time it takes for half of the material to decay
• Each radioactive isotope has a characteristic half-life unaffected by temperature and pressure
• Plutonium 240 decays to Uranium 236

Carbon-14 Dating

• Carbon-14 has a half-life of 5,730 years
• Example fossils rank from oldest to youngest: A, C, B
  • A has 38.7% 14C
  • B has 77.0% 14C
  • C has 52.1% 14C

Geologic and Fossil Record

• Earth formed approximately 4.6 billion years ago (bya)
• Oldest fossils are 3.8 bya
  • They were discovered in Canada
• Prokaryotes are the first living organisms
• Prokaryotes were anaerobic and heterotrophic
• Stromatolites represent layers of bacteria and sediment
• Stromatolites bind thin films of sediment together

Oldest Microfossils

• Haematite tubes from the NSB hydrothermal vent deposits are the oldest microfossils and evidence of life on Earth
• They were found in Northern Quebec

Geologic and Fossil Record (4.6-2.0 bya)

• Cyanobacteria emerged 3.5 bya
• Photosynthesis began early in cyanobacteria
• Oxygen (O2) reduced iron to iron oxide

Origin of Oxygen

• Initially, there was no O2
• Photosynthesis needed to occur first
• O2 first dissolved into water, then reacted with dissolved iron
• Iron precipitated into iron oxide, which accumulated as sediment
• Excess O2 dissolved in water became saturated, "gassed out," entering the atmosphere
• Visible iron-rich terrestrial rock is from ~2.6 bya

Time-frame of Geologic and Fossil Record

• 2.7 bya - O2 began accumulating in the atmosphere
• 2.1 bya - the first eukaryote cells appeared

Endosymbiotic Hypothesis

• Endosymbiosis explains the origin of certain eukaryotic organelles
• Plasma membrane, DNA, & ribosomes might be expected to be contained in the organelles per the hypothesis

Analogy of a Clock

• Used to place major events in Earth’s history within the context of the geological record

Fossil Record (2.0 bya to present)

• Oldest fossils of multicellular eukaryotes are 1.2 bya
• Molecular clock data suggests that these organisms are 1.5 bya
• Colonial forms existed
• Complex multicellular organisms appeared 1.0 bya – 700 mya
• Animal phyla started to diverge around this time
• Oldest animal fossil is 570 mya

The Cambrian Explosion (540-520 mya)

• Most major animal phyla appeared

• The event occurred suddenly, during the first 20 million years of the Cambrian period

• Cnidaria and Porifera phyla are somewhat older dating from the late Proterozoic

• Around 500 mya colonization of land by fungi, plants, and animals occurred

Snowball Earth Hypothesis

• The Cambrian Explosion may have been due to the Snowball Earth
• Earth may have been covered by ice and snow between 750-550 mya
• This was attributed to 3 separate events
• Each ice age event took from 4-50 million years to reverse

Large-Scale Processes

• Processes that influenced the rise and fall of groups of organisms:
  • Plate Tectonics (Continental Drift)
  • Mass Extinctions
  • Adaptive Radiation

Plate Tectonics

• Continental plates slide along the boundary of other plates
• Plates pull or push against each other

Continental Drift

• Late Paleozoic Era (250 mya)
• Formation of the supercontinent Pangaea
• Mesozoic Era (250 – 65 mya)
• Modern-day arrangement breakup
• 241 mya Pangaea existed
• 135 mya Laurasia and gondwana existed

Mass Extinctions

• Evidence in the fossil record
• Marked by cycles of ice ages
• Used to mark beginning/end of Periods

Mass Extinctions-The Permian and the Cretaceous

• Approximately 96% of marine animal species were lost (8/27 orders of insects) during the Permian extinction
• Caused by enormous volcanic eruptions
• The Cretaceous extinction likely resulted from a large meteor impact

Adaptive Radiations

• Evolution of diversely adapted species from a common ancestor
• Results in increased diversity
• Follow mass extinctions
• Follow colonization of new regions
• Adaptation to different environments using alleles present in the population's gene pool
• Mammals had an adaptive radiation after the extinction of terrestrial dinosaurs
  • Prior to the event, mammals were small, nocturnal “prey”, poor competitors
  • After the event, mammals were large, diverse, and filled niches left by dinosaurs
• Other notable radiations include land plants and insects

Hawaiian Islands

• Considered earth's showcase of adaptive radiation

Evo-Devo

• Research at the interface of evolutionary biology and development
• Used to understand biological mechanisms underlying fossil record
• Genetic mechanisms of change are examined

Evolution of Development

• Includes the rate of development of body parts
• Allometric growth involves the proportioning of body parts
  • e.g - human development
  • Different shapes of skulls
• Heterochrony is a change in the timing or rate of development

Development Examples

• Relative skull growth rates
• Human lineage is an example of heterochrony
• Mutations slow head growth relative to other parts
• Adult head resembles chimp infant

Paedomorphosis

• Paedomorphosis indicates the retention of juvenile structures in the adult form
• This includes features found in ancestral juveniles
• Gills retained in adult salamander is an example

Changes in Genetic COntrol of Development

• Can result from placement and arrangement of body parts
• Homeotic genes determine basic features of the body
• A bird may develop a pair of wings and a pair of legs due to these genes
• Also controls how flower parts are arranged

Hox Genes

• Homeotic genes that provide positional information during the development of fins in fish and limbs in tetrapods
• Hox6 controls limbless regions