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Ocean Mysteries: Did Life begin in the Ocean?2021-05-02T08:38:07-05:00

Ocean Mysteries: The Origin of Life

Did Life begin in the Ocean?

Anomalocaris fossil

Image of the first complete anomalocaris fossil found, residing in the Royal Ontario Museum, Toronto. By Keith Schengili-Roberts

Let’s begin at the beginning…

Origins of the Universe

  • Big Bang roughly 10-18 billion years ago
  • Formulation of Carbon and higher elements in the first generation of stars
    • Hydrogen, Helium main elements in the early universe

Formation of the Earth and Solar system

  • Earth roughly 4.7 billion years old
  • Earth’s crust becomes stable by 3.9 billion years ago
  • Life appears around 3.6-3.7 billion years ago

Reducing versus oxidizing atmosphere

  • Current atmosphere is oxygen rich (Oxidizing)
  • Breaks down Organic molecules
    • One manifestation of this is fire
  • Early Earth’s atmosphere was slightly reducing
    • Organic molecules are much more stable
    • little free O2

The Appearance of Life

Beginnings of Life: Khan Academy

The timetable

  • 3.6-3.7 billion years ago: appearance of life
  • 2.5 billion years ago oxygen-forming photosynthesis
  • ~2.2 billion years ago: aerobic respiration
  • ~1.5 billion years ago: first evidence of fossil eukaryotes

The appearance of Life: anaerobic heterotrophes

  • 3.6-3.7 billion years ago: appearance of life
  • Most likely first cells were anaerobic, heterotrophic bacteria
    • anaerobic = does not require free oxygen
    • heterotrophic = does not make its own food

The next step: anaerobic autotrophs

  • Were able to fix CO2
    • turning CO2 + H into organic molecules
  • Hydrogen donors initially were H2, H2S

Energy sources for autotrophics

  • First used chemical energy from elements in surrounding medium
    • chemoautotrophs (deep-sea vents)
  • As this energy ran low, evolved ability to capture energy from light
    • Photoautotrophs

Life’s first major crisis

  • Easy hydrogen donors (H2, H2S) used up quickly
  • Key innovation around 2.5 billion years
    • oxygen-forming photosynthesis (cyanobacteria)
    • Use of H2O as a hydrogen donor

Life’s second major crisis

  • Huge amounts of toxic O2 released
  • Most of the initial O2 was locked up by iron in the oceans and soils (Banded iron formations) = rust
  • More O2 from water keep coming, leading to an O2 rich atmosphere

Life’s next major innovation

  • Aerobic respiration
    • much more efficient than anaerobic respiration
    • Allowed larger cells and the future potential of multicellular organisms

The Fossil Record relating to the Origins of Life

Map of important fossil locations

Fossil bacterial series showing evidence of cell division

Early cells arranged in a filament (Warawoona, 3.5 BYA)

Stromatolites (mounds of photosynthetic cyanobacteria) 2.7 billion years old.

  • Stromatolites fossil from Warrawoona

  • Intact stromatolites present today in Shark’s Bay

Experimental studies of the origins of life

  • Early thinking on the origin of life (i.e., the first cell)
    • Spontaneous generation
      • Pasteur’s experiments (1860’s)
    • Oparin, Haldane (1920’s)
    • Notion of a primeval soup
  • Key steps in the origins of life
    • Formation of complex organic molecules
    • Self-replicating systems
    • Protein synthesis
      • DNA is the genetic material, but it requires proteins to replicate
    • Compartmentalization: the first cell
  • Origins of complex organic molecules
    • nucleosynthesis in stars to form complex molecules
    • molecular clouds
    • A very significant fraction of the Earth’s carbon came from extensive cometary bombardment on the primitive Earth
  • Model systems for prebiotic evolution
    • Miller-Urey experiment
    • Fox’s microspheres
    • Cech’s Catalytic RNA
  • The Miller-Urey experiment (1953)
    • Showed that complex organic molecules (amino acids) can be built up from very simple organic molecules (such as methane)

    Compartmentalization: Fox’s microspheres

    • In the 1970’s, Fox showed that by heating certain proteins, microspheres form spontaneously

    Catalytic RNAs

    • Self-cleaving rRNA
    • RNA can both cleave itself as well as polymerase itself
    • the solution to the chicken versus egg problem
      • don’t need proteins as RNA can act as an enzyme
  • The first cells may have had RNA genomes
    • DNA synthesis requires RNA primer
    • RNA, not DNA used in protein synthesis
    • Reverse transcriptase RNA –> DNA

Recent Research

Group I Self-Splicing Introns
“The question of life’s origins is one of the oldest and most difficult in biology. The answer, if ever known, will not be a single statement of fact but rather an extended chronology, beginning with the formation of the Earth and ending with the appearance of cellular organisms. This problem is confounded because there is little direct evidence of the events that occurred during roughly the first thousand million years of Earth’s history. The oldest rocks that provide clues regarding life’s origins are 3.6 x 109 years old, and by that time cellular life seems already to have been well established. However, remnants of ancient organisms may be found in the form of “molecular fossils” within the genomes of modern organisms. One such candidate might be self-catalytic RNA molecules which can serve as hereditary molecules as well as exhibit some properties of proteins. The discovery of self-catalytic RNA of molecules has lead to a revival of interest in the idea that there was a time, before the origin of protein synthesis, when life was based entirely on RNA. One such class of catalytic molecules are the Group I self-splicing introns.” – Sandra A. Nierzwicki-Bauer

Studies on the Origins of Life: The Formation of the RNA World
“The discovery of ribozymes suggests that RNA was the most important biopolymer in the first life since it can both store genetic information and catalyze reactions of other RNA molecules. One scenario for the origins of life is the formation of an RNA world from prebiotic molecules. It is postulated that RNA served as both the catalyst and the site of information storage in this RNA world which eventually evolved into the contemporary DNA and protein world. In this scenario, RNA formed spontaneously from the monomers produced by prebiotic synthesis and this RNA had the ability to catalyze its own replication.

A likely route to RNA from compounds formed spontaneously on the primitive Earth, is by their selective adsorption on a mineral which catalyzes their condensation to polymers. At Rensselaer we discovered that montmorillonite clay catalyzes the conversion of activated RNA monomers to oligomers. The role of monomer structure, phosphate activating group and mineral catalysis on the formation of RNA under prebiotic and conditions is being explored.” – James P. Ferris

Photochemistry of Planetary Atmospheres and the Origins of Life
“Photochemical reactions, driven by solar UV, are believed to be the principal source of complex molecules observed in most planetary and lunar atmospheres. For example, photochemical transformations of the simple components of the atmospheres of Jupiter and Titan result in the formation of more complex organic molecules. It is proposed that knowledge of the photochemical routes by which organics are formed in other planetary atmospheres provides insight into photoproducts that were formed in the atmosphere of the primitive Earth.” – James P. Ferris

References
The New York Center for Studies of the Origin of Life
Department of Ecology and Evolutionary Biology at the University of Arizona
W. F. Loomis, 1988. Four billion years. Sinauer. — A biochemical viewpoint
W. Day. 1984. Genesis on planet earth, 2nd ed. Yale. — A more general treatment
R. Cowen. 1990. History of life. Blackwell.

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