BIO153 Lecture 10 rev1

Early Life on Earth and Rise of Eukaryotes

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• Phylogenetic trees on slides 2, 11, 19, 20, and 21: The sister group of cyanobacteria is Gram positives, not Gram negatives.

Eukaryotes

• Eukaryotes include four major groups:

  • Unikonta: Contains fungi and animals.

  • SAR: Consists of Stramenopiles, Alveolates, and Rhizarians.

  • Archaeplastida: Includes plants.

  • Excavata: Contains various protists.

• Phylogenetic relationships among some eukaryotes remain unresolved.

Protists

• Definition: A polyphyletic group of eukaryotic organisms that are not classified as plants, fungi, or animals.

• Historically considered a Kingdom of Eukaryotes, this classification is not used anymore.

• Protists are found across all four supergroups of eukaryotes.

• Some protists are closer to plants, while others are more related to fungi and animals.

• Nutritional modes:

  • Many are chemoheterotrophs.

  • Others are photoautotrophs.

Rise of Eukaryotes

• Exploration of how eukaryotes acquired their organelles (e.g., through endosymbiosis).

• Investigation into how ancestral protists gained the ability for photoautotrophy.

History of Life on Earth

• Stromatolites: Earliest fossils of life dating back around 3.5 billion years ago.

  • Ancient Greek term "strôma-lithos" means layered rocks.

  • First life forms were prokaryotes, responsible for building stromatolites through mineralized layers.

• Stromatolites are mainly constructed by cyanobacteria that perform oxidative photosynthesis. ![Fossilized stromatolite image]

Oxygen Revolution

• The Earth's early atmosphere had very low levels of oxygen (O2).

• The first cellular life forms were anaerobic.

• Oxygen is mostly produced by biological activity via oxygenic photosynthesis, predominantly from cyanobacteria.

• Earliest oxygen production observed at about 3.5 billion years ago.

• Oxygen saturation of water and atmosphere significantly increased around 2.7 billion years ago.

• Impact on existing protists, causing some anaerobic organisms to become extinct while others adapted to survive in anaerobic niches. This led to the development of aerobic respiration.

Eukaryogenesis and Mitochondrial Endosymbiosis

• Eukaryotes likely descended from an ancestral, possibly anaerobic, Archaeon.

• An aerobic bacterium was engulfed by an ancestral eukaryote leading to a mutualistic relationship:

  • The symbiont provides aerobic respiration, while the host offers nutrients and protection.

  • This fusion led to the evolution of mitochondria from the engulfed aerobic bacterium.

Mitochondria

• Mitochondria serve as the organelle for aerobic respiration.

• The TCA cycle occurs in the mitochondrial cytosol, while the Electron Transport Chain (ETC) is located in the inner mitochondrial membrane.

Evidence for the Bacterial Origin of Mitochondria

• Mitochondria are comparable to Gram-negative bacteria:

  • Presence of two membranes (inner and outer).

  • Circular genomes (mitochondrial DNA vs. bacterial chromosome).

  • Presence of ribosomes for protein translation.

  • Presence of homologous proteins on the inner membrane.

Molecular Analyses

• Eukaryotic mitochondrial DNA is classified within the Domain Bacteria.

• Mitochondria are closely related to alpha-proteobacteria, based on molecular phylogenetic studies.

• The bacterial origin of mitochondria is well-supported by morphological and genetic evidence.

Primary Endosymbiosis and Chloroplasts

• Primary Endosymbiosis: Resulted when an ancestral heterotrophic eukaryote engulfed a cyanobacterium.

  • This transition led to the establishment of chloroplasts and allowed the host to perform oxygenic photosynthesis.

  • Occurred approximately 1 to 1.5 billion years ago.

  • Gave rise to Archaeplastida, which includes land plants, red algae, and green algae.

Evidence for the Cyanobacterial Origin of Chloroplasts

• Similar evidence supporting cyanobacterial origin as that of mitochondria.

• Shared morphological and genetic characteristics between cyanobacteria and chloroplasts in terms of photosynthesis and protein translation systems.

Secondary Endosymbiosis

• Occurred when red algae and green algae were engulfed by different eukaryotes multiple times, thus spreading photosynthesis to other clades.

  • Examples include the spread from red algae to Stramenopiles, Alveolates, and green algae to Excavata and Rhizarians.

Evidence for Secondary Endosymbiosis

• Living examples of algae-protist symbiosis observed today, such as Paramecium hosting green algae in a symbiotic relationship.

• Distinct traces of endosymbiotic events noted in cryptophytes, which retain a nucleomorph from red algae.

• Cryptomonads harbor four different genomes representing multiple endosymbiotic events.