Eukaryotic Evolution

• Organisms are classified into three domains: Archaea, Bacteria, and Eukarya.
• Archaea and Bacteria include all prokaryotic cells, while Eukarya contains all eukaryotes.

Fossil Record

• A sparse fossil record makes it difficult to determine the appearance of the first members of each lineage.
• The earliest fossils found belong to domain Bacteria, dating back 3.5 to 3.8 billion years.
• Fossil stromatolites suggest some prokaryotes lived in interactive communities.
• Structures resembling fossilized remains of early eukaryotes appear in geological deposits dating to about 2.1 billion years ago.
• Comparative biology and the fossil record offer insights into the evolution of Eukarya.
• Living eukaryotes are descendants of a single common ancestor.

Endosymbiosis

• All extant eukaryotes are likely the descendants of a chimera-like organism.
• The chimeric organism was a composite of a host cell and an alpha-proteobacterium.
• Endosymbiosis is a major theme in the origin of eukaryotes, where one cell engulfs another.
• The engulfed cell survives within the host and both benefit.
• Endosymbiotic events contributed to the origin of the last common ancestor of today's eukaryotes and later diversification.

Metabolic Processes

• Many important metabolic processes arose in prokaryotes, but some, like nitrogen fixation, are not found in eukaryotes.
• Aerobic respiration, localized in the mitochondria, is present in all major lineages of eukaryotes.
• Molecular oxygen (O2) was originally lacking in the Earth's atmosphere.
• Some prokaryotes evolved the ability to photosynthesize between 3.2 and 3.5 billion years ago.
• Cyanobacteria used water as a hydrogen source and released O2 as a waste product around 2.2 billion years ago.
• Aerobic respiration allowed organisms to thrive in oxygen-rich environments and generate high levels of ATP.

Mitochondria

• Observed in the late 1800s as worm-shaped structures moving in the cell.
• Lynn Margulis developed the endosymbiotic theory, stating eukaryotes were produced from one cell engulfing another.
• The engulfed cell lives within another and coevolves over time.
• Mitochondria are the organelles responsible for producing ATP using aerobic respiration.
• Nuclear genes and molecular machinery for replication and expression are closely related to those in Archaea.
• Eukaryotic cells have from one to thousands of mitochondria, being most abundant in the liver and skeletal muscles in humans.
• Mitochondria cannot survive and reproduce outside the cell.
• Mitochondria have their own genomes, with a circular chromosome stabilized by attachments to the inner membrane, ribosomes, and tRNAs that resemble prokaryotes.
• Genes for respiratory proteins are now relocated in the nucleus.

Mitochondrial Structure

• Shaped like alpha-proteobacteria and have two membranes.
• The outer membrane is eukaryotic.
• The inner membrane is bacterial in nature.
• Mitochondrial inner membrane has infoldings called cristae.

Mitochondrial Reproduction

• Divide independently through a process resembling binary fission in prokaryotes.
• Mitochondria arise only from previous mitochondria rather than being built from scratch.
• They reproduce within their enclosing cell and are distributed with the cytoplasm when a cell divides or two cells fuse.

Anaerobic Eukaryotes

• Some living eukaryotes are anaerobic and cannot survive with too much oxygen.
• Reduced organelles are found in most anaerobic eukaryotes, and virtually all eukaryotes carry some genes in their nuclei that are of mitochondrial origin.
• Mitochondria also generate clusters of iron and sulfur that are important cofactors of many enzymes.
• The protist Monocercomonoides has no mitochondria, derived genes, or related nuclear genes, representing an end-point in mitochondrial reduction.

Plastids

• Some groups of eukaryotes are photosynthetic with cells containing plastids.
• Plastids contain chlorophyll a and accessory pigments for harvesting light energy.
• Photosynthetic plastids are called chloroplasts.
• Plastids have an endosymbiotic origin and are derived from cyanobacteria that lived inside the cells of an ancestral, aerobic, heterotrophic eukaryote.
• Plastids of primary origin are surrounded by two membranes.

Primary Endosymbiosis

• The common ancestor of Archaeplastida took on a cyanobacterial endosymbiont.
• The ancestor of rhizarian taxon Paulinella also took on a cyanobacterial endosymbiont.

Cyanobacteria

• Gram-negative bacteria with internal, membrane-bound sacs called thylakoids.
• Chlorophyll is a component of thylakoid membranes.
• Cyanobacteria have peptidoglycan walls and lipopolysaccharide layers.
• Chloroplasts have thylakoids, circular DNA chromosomes, and ribosomes similar to cyanobacteria.
• Outer membrane is derived from the host’s enclosing vacuole.
• The inner membrane is derived from the plasma membrane of the cyanobacterial endosymbiont.

Plastid Evolution

• In glaucophytes and Paulinella, a thin peptidoglycan layer is still present between the outer and inner plastid membranes.
• Genes of the endosymbiont were transferred to the nucleus.
• Plastids cannot live independently outside the host.
• Plastids come from division of other plastids and are never built from scratch
• Archaeplastida occurred 1 to 1.5 billion years ago.

Secondary Endosymbiosis

• Some groups of algae became photosynthetic by taking in green or red algae as endosymbionts.
• Secondary plastids are surrounded by three or more membranes.
• Some secondary plastids have nucleomorphs of endosymbiotic algae.
• Tertiary or higher-order endosymbiotic events explain the features of some eukaryotic plastids.

Chlorarachniophytes

• Molecular and morphological evidence suggest that chlorarachniophyte protists are derived from a secondary endosymbiotic event.
• Chlorarachniophytes are reticulose amoebae found in tropical seas and sand.
• A eukaryote engulfed a green alga that had an endosymbiotic relationship with a photosynthetic cyanobacterium.
• Chloroplasts in chlorarachniophytes are still capable of photosynthesis.
• The green algal endosymbiont exhibits a vestigial nucleus.
• Plastids of chlorarachniophytes are surrounded by four membranes.

Plastid Membranes

• The first two membranes correspond to the inner and outer membranes of the photosynthetic cyanobacterium.
• The third membrane represents the plasma membrane of the green alga.
• The fourth membrane is the vacuole that surrounded the green alga when engulfed by the chlorarachniophyte ancestor.
• Secondary plastids are also found in the Excavates and the Chromalveolates.
• Secondary endosymbiosis of green algae led to euglenid protists in the Excavates.
• Secondary endosymbiosis of red algae led to the evolution of plastids in dinoflagellates, apicomplexans, and stramenopiles in the Chromalveolates.

Explore the origin and evolution of eukaryotes, including endosymbiosis and the three-domain system. Test your knowledge of eukaryotic cell characteristics, fossil records, and the distinctions between Archaea, Bacteria, and Eukarya. Review the timeline of early life and the processes of endosymbiosis.