Theoretical origin of life; horizontal gene transfer (Introduction)

by David Turell @, Monday, September 11, 2017, 22:33 (9 days ago) @ David Turell

A suggestion that HGT played a major role:

"The Last Universal Common Ancestor is dated to be about 3.8 billion years ago. The earth is 4.6 billion years old. Life went from zero to essentially the complexity of the modern cell in less than a billion years. In fact, probably a lot less: Since then, relatively little has happened in terms of the evolution of cellular architecture. So evolution was slow for the last 3.5 billion years, but very fast initially. Why did life evolve so fast?

"[The late biophysicist] Carl Woese and I felt that it was because it evolved in a different way. The way life evolves in the present era is through vertical descent: You give your genes to your children, they give their genes to your grandchildren, and so on. Horizontal gene transfer gives genes to an organism that’s not related to you. It happens today in bacteria and other organisms, with genes that aren’t really so essential to the structure of the cell.


"Life could only have evolved as rapidly and optimally as it did if we assume this early network effect, rather than a [family] tree. We discovered about 10 years ago that this was the case with the genetic code, the rules that tell the cell which amino acids to use to make protein. Every organism on the planet has the same genetic code, with very minor perturbations. In the 1960s Carl was the first to have the idea that the genetic code we have is about as good as it could possibly be for minimizing errors. Even if you get the wrong amino acid — through a mutation, or because the cell’s translational machinery made a mistake — the genetic code specifies an amino acid that’s probably similar to the one you should have gotten. In that way, you’ve still got a chance that the protein you make will function, so the organism won’t die. David Haig [at Harvard University] and Laurence Hurst [at the University of Bath] were the first to show that this idea could be made quantitative through Monte Carlo simulation — they looked for which genetic code is most resilient against these kinds of errors. And the answer is: the one that we have. It’s really amazing, and not as well known as it should be.

"Later, Carl and I, together with Kalin Vetsigian [at the University of Wisconsin-Madison], did a digital life simulation of communities of organisms with many synthetic, hypothetical genetic codes. We made computer virus models that mimicked living systems: They had a genome, expressed proteins, could replicate, experienced selection, and their fitness was a function of the proteins that they had. We found that it was not just their genomes that evolved. Their genetic code evolved, too. If you just have vertical evolution [between generations], the genetic code never becomes unique or optimal. But if you have this collective network effect, then the genetic code evolves rapidly and to a unique, optimal state, as we observe today.

"So those findings, and the questions about how life could get this error-minimizing genetic code so quickly, suggest that we should see signatures of horizontal gene transfer earlier than the Last Universal Common Ancestor, for example. Sure enough, some of the enzymes that are associated with the cell’s translation machineries and gene expression show strong evidence of early horizontal gene transfers.

"Tommaso Biancalani [now at the Massachusetts Institute of Technology] and I discovered in the last year or so — and our paper on this has been accepted for publication — that life automatically shuts off the horizontal gene transfer once it has evolved enough complexity. When we simulate it, it basically shuts itself off on its own. It’s still trying to do horizontal gene transfer, but almost nothing sticks. Then the only evolutionary mechanism that dominates is vertical evolution, which was always present. We’re now trying to do experiments to see whether all the core cellular machinery has gone through this transition from horizontal to vertical transmission."

Comment: this theory doesn't tell us how life started, but it gives us a reason to explain how different forms of bacteria advanced so early and so rapidly. It also suggests why more advanced forms of life do not use horizontal gene transfer as bacteria still do. It also fits the research on zircons suggesting a very early origin of life presented today.

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