Ch 26

• Progenitor for all life

  Arose in the Archean eon
• Last universal common ancestor (LUCA)


  Hypothesized to exist
• Descendants of LUCA

  • Eubacteria
  • Archaea
  • Eukarya
• Relationships between the three domains are still contentious, although the most common model has Eukarya branching from within Archaea
• Asgard


  A group of deep-sea Archaean that appear to be the closest prokaryotic relatives to modern eukaryotes
• Five supergroups within the domain Eukarya


  • Excavata
  • SAR (Stramenopila, Alveolata, Rhizaria)
  • Archaeplastida
  • Amoebozoa
  • Opisthokonta
• The SAR group combines the older supergroup called Chromalveolata (Stramenopiles and Alveolates) with Rhizaria
• For at least 1 billion years, bacteria and archaea ruled the Earth
• Archaea may be found in extreme environments, including environments characterized by high temperature, high pressure, or high salt
• Bacteria and archaea have some functional compartments, but eukaryotes are characterized by extensive compartmentalization
• Eukaryotes appear in the fossil record only about 1.5 bya
• Despite sharing basic cellular metabolism with prokaryotes, eukaryotic cells evolved structures and functions that enabled them to be larger, and eventually, allowed the evolution of multicellular life
• Endomembrane system


  Subdivides the eukaryotic cell into functional compartments, including the nucleus
• Nuclear envelope


  Segregated genetic material into its own compartment, causing the process of gene expression to be separated in time and space
• Golgi apparatus and endoplasmic reticulum

  Key innovations that facilitate intracellular transport and the localization of proteins to specific regions of the cell
• The nuclear membrane, as well as other membrane systems, arose through infolding of the plasma membrane
• Endomembrane system


  Subdivides the cell into functional compartments, including the nucleus
• Transcription in the nucleus followed by translation in the cytoplasm allows additional control over gene expression
• Not all cellular compartments are derived from the endomembrane system
• Endosymbiosis


  A cell engulfed by endocytosis becomes part of the engulfing cell
• Mitochondria


  Derived from the parasitic bacteria Rickettsia, being incorporated into eukaryotic cells early in their history
• Chloroplasts


  Derived from cyanobacteria, acquired by red and green algae through direct incorporation, and by brown algae through engulfing red algae
• Molecular phylogenetic data indicate that eukaryotes arose early in the Proterozoic and diversified later in that eon
• Microfossil evidence from the Proterozoic supports the existence of eukaryotes as early as 1.5 bya
• Multicellularity


  Allowed organisms to deal with their environments in novel ways through differentiation of cell types into tissues and organs
• True multicellularity, in which the activities of individual cells are coordinated and the cells themselves are in contact, occurs only in eukaryotes
• Bacteria and many single-celled eukaryotes form colonial aggregates of many cells, but the cells in the aggregates have little differentiation or integration of function
• Multicellularity has arisen independently in different eukaryotic supergroups, such as the red, brown, and green algae
• One lineage of multicellular green algae was the ancestor of the plants
• A different unicellular ancestor in the Opisthokonts gave rise to all multicellular animals
• Multicellularity required that cells connect to each other and communicate
• Although each cell has identical genetic information, gene expression varies among cells to allow specialization
• Mechanisms for coordinating gene expression and cell differentiation evolved
• Sexual reproduction


  Allows greater genetic diversity through the processes of meiosis and crossing over
• The first eukaryotes were probably haploid, and diploids seem to have arisen on a number of separate occasions by the fusion of haploid cells
• Rapid diversification occurred during the Cambrian period, which lasted from 542 to 488 mya
• The Cambrian explosion was a period of extremely rapid expansion of life, with the hard and soft parts of organisms preserved in the fossil record
• For 3 billion years, life had been primarily unicellular, with the exception of a few groups of algae
• In the period leading up to the Cambrian radiation, the first multicellular animals appeared
• During the 50 million years that followed the Cambrian, ancestors of almost every group of animals evolved