Life on earth 1 - diversity of eukaryotes

The Evolution and Diversity of Eukaryotes

Three Domains of Life:

  • Bacteria

  • Archaea

  • Eukarya

Last Eukaryotic Common Ancestor (LECA):

  • Already a complex cell with numerous compartments and organelles.

  • Possessed at least 3000 new gene families.

Hypotheses for the origin of eukaryotes:

Serial Endosymbiosis Theory

  • Multiple different endosymbiotic events gave rise to eukaryotes with multiple different endosymbionts.

Autogenous Evolution

  • Eukaryotes arose autogenously via the loss of the cell wall.

  • Acquisition of mitochondria occurred late and was not qualitatively important.

Hydrogen Hypothesis

  • Focuses on the mechanism and driving force of endosymbiosis. In this hypothesis, an anaerobic hydrogen-dependent bacterium and a facultatively anaerobic archaeon that produced hydrogen as a byproduct merged, driven by the dependence of the archaeon for hydrogen. The bacteria (possibly an alpha-proteobacterium) was internalized by the archaeon and became the mitochondrion.

The Paradox of Eukaryotic Evolution

  • All complex life is eukaryotic; the eukaryotic cell arose only once.

  • Eukaryotes share universal traits.

  • Prokaryotes show no tendency to evolve morphological complexity or any eukaryotic traits.

  • If each trait evolved step by step with selective advantage, why did none evolve in prokaryotes?

Eukaryotic Supergroups

  • There are 5 or 6 eukaryotic supergroups.

  • Unicellular groups contain most eukaryotic genetic diversity.

  • Branch lengths separating groups are much shorter than branches within the groups.

  • This suggests a "big bang" radiation after the evolution of LECA.

The "Black Hole" at the Heart of Biology

  • All eukaryotes share traits essentially absent from prokaryotes.

    • Examples: nucleus, nuclear membrane, phagocytosis, transcription/translation mechanisms.

Selective Bottleneck as a Possible Explanation

Selective Pressure

  • Only the best 'pre-adapted' groups made it through.

  • This freed niches and led to the fast evolution/explosive radiation of new forms.

Possible Selective Bottlenecks

  • Great Oxidation Event: wiped out all anaerobic cells.

  • Snowball Earth.

  • Groups that could respire aerobically flourished.

Predictions of a Selective Bottleneck

  • If the bottleneck was selective and related to environmental constraints, and biologically 'easy' given the right conditions, there should have been a polyphyletic radiation.

    • Different groups of bacteria should have given rise to different forms of complex life independently.

  • Forms arising later must always have been outcompeted to extinction by fully fledged eukaryotic cells.

Archezoa Argument

  • Archezoa argue against a selective bottleneck.

  • They possess organelles derived from mitochondria.

  • They represent ecological intermediates, having undergone reductive evolution from more complex ancestors.

Restrictive Bottleneck

  • Relates to biological constraints.

  • All Archezoa once had mitochondria and lost them through reductive evolution.

The Ring of Life

  • Suggests the eukaryotic genome is a chimera of many lineages, both archaeal and bacterial, as a result of lateral gene transfer between at least one archaea and one bacterium (possibly an alpha-proteobacterium, an ancestor of mitochondrion)

  • Focuses on the genetic composition of eukaryote and evidence for hybrid genome.

Last Universal Common Ancestor (LUCA)

  • LUCA

  • Host cell for the eukaryotes was an Archaeon

Reductive Evolution of Endosymbiont Genomes

  • Genome size shrinks in symbiotic bacteria.

Why Endosymbiosis is Necessary

  • Tiny mitochondrial genomes support very large host genomes.