Origin of Eukaryotes

Oldest record of eukaryotic cells

1.8 billion years ago

Great Oxygenation Event

Cyanobacteria produced oxygen, killing many anaerobic prokaryotes, altering selection pressure

• Rise of oceanic and atmospheric oxygen propelled the evolution of eukaryotic cells and complex, multicellular life forms

Features of eukaryotic cells

• Presence of nucleus and linear chromosomes

• Membrane-enclosed organelles

• Larger size

• Presence of cytoskeleton and cellular dynamics

• Presence of sexual reproduction

DNA Eukaryotic Evolution

DNA’s linear structure along with compact organization of chromosomes set foundation for larger genome size

How do organelles being enclosed by membranes help eukaryotes

Biochemical activities can take place in an isolated environment, free from interference by the contents of the cytoplasm

Cytoskeleton

Network of proteins that facilitate: change of cell shape and cell motility, transpiration of substance within the cell, dynamic membrane for feeding activities

Cellular dynamics

The mechanisms that govern the mechanical movement and interactions of cells over time

Sexual Reproduction and Eukaryotic Evolution

• Promotes genotypic diversity by introducing new variants

• Recombination occurs during meiosis

• Independent assortment of genes that are not linked together

• Random fertilization of gametes

Two main types of sexual life cycles based on complexity

• Unicellular eukaryotes alternate between haploid and diploid life cycles

• Multicellular eukaryotes have distinct organs and stages of cell cycle for gamete production

Symbiosis

A close, long-term relationship between two species in whic at least one species benefits

Endosymbiosis

A relationship between two species in which one organism lives inside the cell or cells of another organism

• typically mutualistic

• common among unicellular organisms

Endosymbiont theory

Mitochondria and plastids originated from small prokaryotes residing within larger host cells

• prokaryotic predecessor likely entered host cell via endocytosis and avoided digestion

• eukaryotic cells likely originated from serial endosymbiosis

• Mitochondria is hypothesized to evolve before plastids in a series of primary endosymbiosis by the ancestral eukaryotes (not all eukaryotes have plastids, but all have mitochondria)

Serial Endosymbiosis

1. Plasma membrane formed end-membranes via infolding, giving rise to structures such as ER and nuclear envelope

2. Endosymbiotic mutualistic relationship was established between host prokaryote and aerobic host prokaryotic symbiont (most likely alpha-proteobacterium, source of energy for host, origin of ancestral heterotrophic eukaryote)

3. Additional endosymbionts (likely photoautotroph, maybe cyanobacterium) were incorporated on one lineage of the ancestral heterotrophic eukaryotes (this lineage could use symbiont to create organic compounds from light and CO2, and led to photosynthetic protists and plants)

Types of endosymbiosis

Primary: prokaryotic cell engulfed by another organism

Secondary: eukaryotic cell engulfed by another eukaryotic cell

Evidence for Endosymbiotic Theory

• Mitochondria and Plastids share structural similarities with prokaryotic bacteria (double membraned wall, homologous inner membranes to plasma membrane, circular DNA, small organelles)

• Phylogenetic comparison show that mitochondrial and plastid genomes are similar to alpha-proteobacteria and cyanobacteria genomes, respectively

• Both plastid and mitochondrial genomes can replicate without interference of nuclear DNA

• Proteins can be synthesized within mitochondria and plastids

Multicellularity Date

First appears 1.5 billion years ago

Evolutionary benefits of multicellularity

• Specialization of cells for distinct functions

• Increased size and complexity, reducing physiological limitation from surface-to-volume ratio

• Life span of organism is extended

• Coordinated activities of multiple cells provide defensive advantage

Phylogeny of basal eukaryotes

• Phylogeny with ribosomal RNA suggests that mitochondria and chloroplasts are placed within the monophyletic bacteria

• Eukaryotes closer to Archaea than to Bacteria

Supporting evidence for the placement of Eukarya

• Nuclear genome in eukaryotes contains genes shared with archaea and bacteria, hinting at a common ancestor

• Nuclear genome in eukaryotes also contains unique genes, separating them as a distinct lineage

Plausible reasons for early evolution of life was more complex than a simple dichotomy

• Evidence is scarce as events took place deep into the evolutionary history

• Horizontal transfer introduced information that is unrelated to common ancestry