LUCA Evidence
All lifeforms on Earth share certain characteristics, suggesting that all extant organisms are derived from a common source
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The last universal common ancestor (LUCA) is the most recent population from which all organisms on Earth share a common descent
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The LUCA would represent the immediate antecessor of the three current domains of life (bacteria, archaea and eukaryotes)
Evidence for the existence of a last universal common ancestor include the fact that:
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The genetic code is universal and all life shares a common mechanism of transcription and translation
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Certain genes are broadly distributed across all cellular organisms (e.g. instructions for DNA replication)
The LUCA does not represent the first cellular organism and it is probable that other lifeforms coexisted at the time of the LUCA
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Due to horizontal gene transfer (e.g. viral integration), the genome of the LUCA is likely representative an ancient community of organisms that evolved independently but became extinct due to competition from the LUCA and its descendants
Proposed Tree of Life
Estimating Dates
Because microbes do not typically leave discernable fossils, major uncertainties exist regarding the dates of key events that led to the evolution of life on Earth
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Scientists can use biochemical evidence or phylogenetic comparisons to estimate dates of the first living cells and the LUCA
Biochemical Evidence
Biosignatures are the chemicals produced by cellular processes that provide evidence of past or present life
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The earliest evidence of life dates to 3.7 billion years ago in the form of stromatolites – a layer of sediment laid down by microbes
Phylogenetic Comparisons
Scientists can also compare the genome of different species in order to develop a timeline for evolutionary divergence
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The more differences between the genomic sequences of two species, the longer the time period since the two species shared a common ancestor
To establish the original genome of the LUCA, scientists searched for genes present in both prokaryotic domains (bacteria and archaea)
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Genes that were not highly conserved within a domain were disregarded (these could have been shared by horizontal gene transfer and not been present in the LUCA)
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This led to the identification of 355 genes that are proposed to have originated directly from the LUCA
Molecular Clock
The specific timing of evolutionary divergence can be estimated from genomic comparisons by using the molecular clock technique
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This technique uses the mutation rate of biomolecules (DNA, RNA or proteins) to deduce the time since two or more life forms diverged
The molecular clock technique is predicated on the assumption that there is a direct correlation between number of sequence differences and the time since two species diverged
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Example: If a gene mutates at a rate of 1 bp per 100,000 years and two species have 6 differences in their gene sequences, then divergence occurred 600,000 years ago
There are a number of limitations with using the molecular clock technique to estimate dates of evolutionary events:
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The mutation rate is not constant across all genes and all species
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Future mutations may reverse earlier mutations, obscuring the specific number of mutations that have occurred
Based on phylogenetic comparisons and the molecular clock technique, the LUCA’s evolution into archaea and bacteria could have occurred at any point between 2 to 4 billion years ago
Hydrothermal Vents
Current evidence suggests that the last universal common ancestor likely existed in the vicinity of hydrothermal vents
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Hydrothermal vents can develop wherever a heat source comes into close contact with fluid system from within the Earth’s crust, resulting in the convective flow of fluid to the seafloor
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This fluid is rich in minerals that precipitate upon reaching the seafloor, creating chimney-like structures or vents
Hydrothermal Vent Formation
Fossil Evidence
The precipitation of minerals from vent fluids also occurs on biological structures, resulting in their subsequent mineralisation and preservation
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Fossilized evidence of bacteria have been discovered in ancient seafloor hydrothermal vent precipitates that are dated as being ~3.7 billion years old
Genomic Evidence
Genomic analysis also supports the idea the last universal common ancestor developed in close proximity to hydrothermal vents
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Genes proposed to belong to LUCA are involved in the use of molecular hydrogen as an energy source (hydrothermal vents are rich in hydrogen)
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LUCA is also thought to have possessed a gene that is currently found in extremophiles existing in high temperature environments (like hydrothermal vents)