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Origins of Life and Precambrian Evolution - Coggle Diagram
Origins of Life and Precambrian Evolution
Timeframe
Life existed at least 3.85 billion ya
Oldest known sedimentary rocks contain oldest known life
Study and knowledge enabled by stable carbon isotopes C12 and C13
Start of Life
The first living thing presumably had several descendant lineages, all but one of which died out. The phylogeny is hypothetical and subject to revision
Major Hypotheses:
Extra-terrestrial origin hypothesis
Young E experienced heavy bombardment by meteors and comets
Murchison Meteorites
Analyses revealed organic compounds in the interior of the rocks, including the amino acids glycine, alanine, glutamic acid, valine, and proline
The present organic compounds are used by modern organisms to make proteins.
Significant because...
Biomolecules of life and their likely precursors require some elements in large amounts (C, H, O, N, S, P), as well as trace quantities of other elements
These elements must be in a chemical form that allows them to be used in the construction of biological building blocks, such as amino acids, sugars, and carbohydrates
If these were not synthesized on early earth, extraterrestrial sources such as meteors could account for their presence.
in 1969, a meteor entered earth's atmosphere over Murchison, Australia, scattering meteorites across 5 square miles. Scientists collected some for chemical study.
The simple organic molecules from which life was built may have formed in space and then fallen to earth
It is possible, at least in principle, that the stuff of life was delivered to E from elsewhere
Problem with terrestrial sources
Some geochemical evidence points to an atmosphere unfavorable for the generation of biologically-important molecules 4 billion ya
Three main possibilities exist for what E's environment was like 4 billion ya...
Primarily oxidizing, with high abundance of O2 and CO2
Primarily reducing, with high concentrations of H2, CH4, NH3
Intermediate in oxidizing activity
Oparin-Haldane Model
Suggests that life arose gradually
Formation of biological building blocks from existing inorganic material
Assembly of polymers to form biological macromolecules
Macromolecules then directed the formation of other biological structures
Basic Building Blocks
Experiments to create basic building blocks of life have been conducted. Their processes could have happened on earth in a highly reducing atmosphere but are chemically less likely in an oxidizing atmosphere.
Since 1953, scientists have inorganically synthesized a wide range of organic molecules (amino acids, nucleotides, sugars)
in 1953, Stanley Miller synthesized biologically active molecules (glycine and alanine) by boiling methane, ammonia, and hydrogen alongside electricity
RNA World Hypothesis
Proposes that RNA preceded DNA (and proteins)
Cenancestor (LUCA) could have been DNA based, but the first living things were RNA-based
Altman and Cech discovered ribozymes
changed how biologists view the operations of the cell
changed the view of the initial Darwinian ancestor (IDA)
RNA has a capacity for information storage, transmission, and ability to perform biological work
Probably not the first self-replicating system
abiotic RNA production is very unlikely
RNA may be derived from a more primitive chemical system
IDA was perhaps not made of RNA, but of something else that preceded RNA
All living things possess:
Phenotype
Genotype
Ability to evolve
Last Common Ancestor of All Extant Organisms
Once self-replicating systems evolved on E, at least one of them adapted to the use of DNA to store heritable information and the use of proteins to express that information. This system eventually gave rise to all lineages of life on the planet today.
Had a cellular form. How was this gained?
Sidney Fox and colleagues found that mixtures of polyamino acids in water or salt solution spontaneously organize themselves into microspheres with properties reminiscent of living cells
Fatty acids and related molecules form vesicle-enclosing bilayers and can serve as containers for nucleic acids
Vesicles can grow by absorbing more of their constituent molecules and will divide if squeezed through a small hole
Fatty-acid based vesicles are permeable to nucleic acids
Known as LUCA or Cenancestor
Not necessarily closely similar to first living things
Inferring Traits
Mapping character-state changes onto phylogenies
The most conserved pieces of machinery inside cells function in the translation of genetic information from nucleic acids into proteins
There are 60 genes that occur in the genomes of all cellular organisms in all domains
30 are ribosomal proteins
15 are aminoacyl-tRNA synthetases
Phylogeny of All Living Things
A method of studying the ancestral lineage is by reconstructing the phylogeny of all living things.
First attempts were based on morphological data, which was effective for eukaryotes but not prokaryotes
DNA and RNA sequence data
Uses relative similarity of sequences to infer phylogeny
Challenge: finding a gene that works
Shows recognizable sequence similarity even between distantly related species
Gene has same function across all organisms
Subject to strong stabilizing selection
Small Subunit Ribosomal RNA
in all organisms...
is present
has similar composition
has same function (translation)
Chosen by Carl R. Woese, a pioneer of the use of molecular sequences in estimating the universal phylogeny
Remains an informative resource for whole-life phylogenies
How the ancestral community gave rise to the bacteria, archaeans, and eukaryotes that populate the earth: Four Hypotheses
Universal Gene Exchange Pool
proposes a time when genomes were modular and when organisms assembled their genomes from a common pool
LUCA was not a single species but rather a pool of readily exchanged and largely independent genes
not conducive to evolution by natural selection
Gradually, as proteins became more independent, the modularity of genomes gave way to a more integrated and stable format. Individual genes could no longer move so easily among genomes.
The Ring-of-Life
Proposes that the first eukaryote arose when a bacterium fused with an archaean
Eukaryotic genes involved in...
...transcription and translation tend to be more similar to archaean genes
...metabolic processes tend to be more similar to bacterial genes
The Chronocyte
Proposes that the tree of life separates a lineage that will become the bacteria and the Archaea from the lineage that will become the Eukarya
a. The chronocyte lineage (an early eukaryote) evolved the ability to eat other microbes by phagocytosis
b. A chronocyte then ate an archaean that resisted digestion and became an endosymbiont
c. This endosymbiont eventually evolved into an organelle: the nucleus
Three Viruses, Three Domains
Proposes that each cellular domain originated independently from the fusion of an RNA cell and a large DNA virus
a. Viruses infecting RNA-based cells first evolved DNA to counter their host's defenses
b. DNA was then transferred to cellular life when DNA-based viruses took up permanent residence inside their hosts
