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Authors: Adam Rutherford
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history of life using DNA as our guide may only work convincingly from the point where descent becomes vertical. The tree of life only begins to resemble a branching thing that deserves to be named a tree
after
the emergence of complex life. From that species on, when an archaea swallowed a bacteria, the overwhelming majority of inheritance was from parent to offspring: descent with modification. UCL biochemist Nick Lane calls this the genetic event horizon: comparing genes will take us all the way back to the point where it looks like a nice, branching tree, but our vision grows blurred before that point. It just becomes far too messy to unpick the deepest past.
Therefore, while we can sensibly use DNA to infer that humans and chimps had a common ancestor around six or seven million years ago, and humans and the meager sea worm amphioxus had a common ancestor around five hundred million years ago, we cannot reliably use this to date Luca. The branches of the tree become tangled and scrambled before complex life, and tracing the changing patterns of DNA as species evolve is impossible. We are left, then, with very little genetic evidence of what Luca actually was at all.
The Creation of Luca
We can figure out some things about Luca, though. Calculations and logic predict that the last universal common ancestor had characteristics that are shared between archaea and bacteria, and that means genes, proteins, and the cellular mechanics largely similar to what we see today. That means we can use comparisons of these molecules to understand some things about Luca, even if we cannot apply an accurate date. A study by Douglas Theobald (Brandeis University, MA) in 2010 applied hard statistical analysis to the domains of bacteria, archaea, and complex life. He looked carefully at the construction of twenty-three proteins that are present in each of these domains with seemingly common descent, like words that sound and mean similar things in different languages. Based on the similarities of the sequence of amino acids that make up these proteins, Theobald calculated that the odds of their having arisen independently was 1 in 10^2,860 (i.e., a 1 with 2,860 zeroes after it). 13
Another clue to Lucaâs singular origin concerns the most fundamental cellular machineryâthe ribosome. It exists in all cells as a processing plant organized in minuscule blocks of molecules. Its role is universal, and as such, again supports the single-origin idea. The ribosome reads the genetic code and translates it into protein. The intricacies of this exquisite machine are explored later in the book (and particularly in the afterword, p. 240), but in essence its job is to read the genetic code (already transcribed into an RNA version) and translate each three-letter section into an amino acid. The ribosome strings together the amino acids in accordance with the messenger RNA, and a protein feeds out like a ticker tape.
We look to the ribosome as a useful indicator of relatedness because it is fundamentalâwithout its manufactory we have no proteins, and none of the life we are aware of can exist. We can understand the idea that mitochondria in complex cells are derived from annexed bacteria because mitochondria have their own ribosomes, separate from their host cell, and these ribosomes are much more similar to bacterial ones than animal ones. We can compare the sequences of the genes that encode parts of the ribosome in all species, and trace backward the changes that have occurred over time to predict what Lucaâs ribosome looked like.
This is a fruitful endeavor, but the results are contested. For example, by looking at the particular sequence of ribosome parts across many species, you can reasonably infer the temperature at which that host organism thrives. Several parts of the ribosome are built from neatly folded RNA molecules, themselves made from letters of genetic code
A, C, G,
and
U
. As in the double helix of DNA,
C
pairs
after
the emergence of complex life. From that species on, when an archaea swallowed a bacteria, the overwhelming majority of inheritance was from parent to offspring: descent with modification. UCL biochemist Nick Lane calls this the genetic event horizon: comparing genes will take us all the way back to the point where it looks like a nice, branching tree, but our vision grows blurred before that point. It just becomes far too messy to unpick the deepest past.
Therefore, while we can sensibly use DNA to infer that humans and chimps had a common ancestor around six or seven million years ago, and humans and the meager sea worm amphioxus had a common ancestor around five hundred million years ago, we cannot reliably use this to date Luca. The branches of the tree become tangled and scrambled before complex life, and tracing the changing patterns of DNA as species evolve is impossible. We are left, then, with very little genetic evidence of what Luca actually was at all.
The Creation of Luca
We can figure out some things about Luca, though. Calculations and logic predict that the last universal common ancestor had characteristics that are shared between archaea and bacteria, and that means genes, proteins, and the cellular mechanics largely similar to what we see today. That means we can use comparisons of these molecules to understand some things about Luca, even if we cannot apply an accurate date. A study by Douglas Theobald (Brandeis University, MA) in 2010 applied hard statistical analysis to the domains of bacteria, archaea, and complex life. He looked carefully at the construction of twenty-three proteins that are present in each of these domains with seemingly common descent, like words that sound and mean similar things in different languages. Based on the similarities of the sequence of amino acids that make up these proteins, Theobald calculated that the odds of their having arisen independently was 1 in 10^2,860 (i.e., a 1 with 2,860 zeroes after it). 13
Another clue to Lucaâs singular origin concerns the most fundamental cellular machineryâthe ribosome. It exists in all cells as a processing plant organized in minuscule blocks of molecules. Its role is universal, and as such, again supports the single-origin idea. The ribosome reads the genetic code and translates it into protein. The intricacies of this exquisite machine are explored later in the book (and particularly in the afterword, p. 240), but in essence its job is to read the genetic code (already transcribed into an RNA version) and translate each three-letter section into an amino acid. The ribosome strings together the amino acids in accordance with the messenger RNA, and a protein feeds out like a ticker tape.
We look to the ribosome as a useful indicator of relatedness because it is fundamentalâwithout its manufactory we have no proteins, and none of the life we are aware of can exist. We can understand the idea that mitochondria in complex cells are derived from annexed bacteria because mitochondria have their own ribosomes, separate from their host cell, and these ribosomes are much more similar to bacterial ones than animal ones. We can compare the sequences of the genes that encode parts of the ribosome in all species, and trace backward the changes that have occurred over time to predict what Lucaâs ribosome looked like.
This is a fruitful endeavor, but the results are contested. For example, by looking at the particular sequence of ribosome parts across many species, you can reasonably infer the temperature at which that host organism thrives. Several parts of the ribosome are built from neatly folded RNA molecules, themselves made from letters of genetic code
A, C, G,
and
U
. As in the double helix of DNA,
C
pairs
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