Origin of Cells ( Protobionts )

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Non-living molecules surrounded by a membrane had some properties of life: simple reproduction & metabolism, separation of internal environment from surroundings

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Bubbles … Tiny bubbles … Could have formed spontaneously from organic compounds

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Phospholipid (contain carbon

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organic!) bilayers can form when lipids are placed in water

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May have taken up additional organic molecules from environment AP Biology

RNA World Hypothesis : Origin of Genetics

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RNA is likely first genetic material

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Able to self-replicate and store protobionts ’ genetic information

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multi-functional

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codes information

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makes inheritance possible

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Dawn of natural selection & evolution

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enzyme functions (catalyst)

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Ribozymes

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RNA enzymes that can make short pieces of RNA

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Involved in cell replication

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Involved in protein synthesis Likely lead to a “DNA World”

Geological Evidence Supports the Models for the Origin of Life

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Fossils found in sedimentary rocks tell us which organisms lived first

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Rocks occur in “strata”, or layers

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Younger sediments are closer to surface than older ones

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Method for determining age of fossils:

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Radiometric dating

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Involves analyzing amount of certain radioactive isotopes remaining

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Each isotope has a unique half-life : # of years it takes for 50% of the original sample to decay AP Biology

Key Events in Origin of Life

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Key events in evolutionary history of life on Earth

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Earth formed approximately 4.6 bya Environment became suitable for life 3.9 bya

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Earliest fossils are from 3.5 bya … provides evidence for when life could have originated (probably 3.5- 4.0

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Prokaryotes

Prokaryotes dominated life on Earth from 3.5

–2.0 bya 3.5 billion year old fossil of bacteria modern bacteria chains of one-celled cyanobacteria AP Biology

First prokarotes: Stromatolites

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Rocklike structures composed of layers of prokaryotes & sediment Oldest known fossils Existed 3.5 bya & formed complex communities

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So life on earth must have originated earlier than that Lynn Margulis AP Biology

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Prokaryotes were the first life forms on earth.

AP Biology

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Protobionts were replaced by autotrophs – organisms that can produce all their needed compounds from molecules in the environment

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Often use light as an energy source

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Autotrophs likely led to heterotrophs – organisms which live on products excreted by autotrophs, or on autotrophs themselves

Oxygen atmosphere

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Oxygen began to accumulate 2.7 bya

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reducing

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oxidizing atmosphere

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Produced via photosynthesis

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Photosynthetic prokaryotes called cyanobacteria

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makes aerobic respiration possible AP Biology

First Eukaryotes

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Development of internal membranes ~2 bya

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create internal micro-environments

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advantage: specialization = increase efficiency

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infolding of the plasma membrane natural selection! plasma membrane endoplasmic reticulum (ER) nuclear envelope nucleus DNA Prokaryotic cell AP Biology cell wall Prokaryotic ancestor of eukaryotic cells plasma membrane Eukaryotic cell

Endosymbiosis

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Process explaining the origin of mitochondria and chloroplasts

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Mitochondria & chloroplasts were formerly small prokaryotes living within larger cells

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Evolution of eukaryotes

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Mitochondria & chloroplasts became a single, interdependent organism w/ their host internal membrane system aerobic bacterium mitochondrion Ancestral eukaryotic cell Endosymbiosis Eukaryotic cell with mitochondrion

Endosymbiosis: Origin of Mitochondria

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Proposed ancestors of mitochondria: aerobic (oxygen-using) heterotrophic prokaryotes Cells engulfed aerobic bacteria, but did not digest them

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mutually beneficial relationship: aerobic cells could benefit from having a structure that itself utilized oxygen

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natural selection AP Biology

Endosymbiosis: Origin of Chloroplasts

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Proposed ancestor: photosynthetic prokaryotes

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Cells engulfed photosynthetic bacteria, but did not digest them

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mutually beneficial relationship: hetertrophic “host” could use nutrients released from photosynthesis

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natural selection! photosynthetic bacterium Eukaryotic cell with mitochondrion chloroplast Endosymbiosis Eukaryotic cell with mitochondrion

Evidence of Endosymbiosis

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structural

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mitochondria & chloroplasts resemble bacterial structure

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genetic

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mitochondria & chloroplasts have their own circular DNA, like bacteria

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functional

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mitochondria & chloroplasts move freely within the cell

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mitochondria & chloroplasts reproduce independently from the cell via binary fission (the same process that some prokaryotes use) AP Biology

The Origin of Multicellularity

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The evolution of eukaryotic cells allowed for a greater range of unicellular forms (what we call protists today)

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A second wave of diversification occurred when multicellularity evolved and gave rise to algae, plants, fungi, and animals

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The oldest known fossils of multicellular eukaryotes are of small algae that lived about 1.2 billion years ago AP Biology