Origin of life: checkpoint 2026-11-14

Session: 13 November 2026, 9:58 PM to 14 November 2026, 12:39 AM (Sydney). Follows origin_of_life_sequence_checkpoint_2026-11-13.md and origin_of_life_sequence_checkpoint_2026-11-13_2.md.

1. Why the building blocks don't react at random

• In water at neutral pH, the joining reactions release water, so the equilibrium favours staying apart.
• Nothing joins until it is activated with energy from ATP, and only enzymes do the activating.
• Amino acids and nucleic acid backbones are already ionised at body pH. That charged form is the unreactive one.
• The bases are aromatic rings and are stacked inside the DNA double helix.
• DNA lacks the hydroxyl group that lets RNA cut its own backbone.
• Damage still happens constantly and is repaired.

2. Condensation and wet-dry cycling

• Each link formed releases one water molecule, made from an OH and an H of the two molecules being joined.
• This has nothing to do with how planets get water.
• Drying pushes the reaction forward and concentrates the building blocks. Refilling mixes the products.
• Drying can also join amino acids to RNA, but blindly. It gives no matching of amino acid to RNA.

3. The translation machinery (modern cell)

• Aminoacyl-tRNA synthetase: a ligase (joining enzyme), not a kinase. 20 of them, one per amino acid. Two unrelated families of 10.
• Aminoacyl-tRNA (charged tRNA): a tRNA with one amino acid on its tail. It is RNA, not protein.
• tRNA: single strand folded back on itself. The paired stretches are real short double helices.
• mRNA (messenger ribonucleic acid): the coding RNA. A temporary copy of one gene. Passive, like a tape.
• Ribosome: about two-thirds RNA. The site that joins amino acids is RNA.
• Ribose is the sugar. Ribosome is the machine.
• Ligand: the thing that binds to a receptor. Ligase: an enzyme that joins two molecules.

Cycle:

1. A charged tRNA matching the codon docks.
2. The growing chain transfers onto the new amino acid.
3. The empty tRNA leaves.
4. The ribosome and mRNA shift three bases (one codon).

• Parts arrive by diffusion (random thermal collision). Right fits stick, wrong fits bounce.
• The step is three bases because the tRNA slots are one codon apart and the tRNA holds the mRNA by three base pairs.
• Order is enforced by shape: each step creates the shape the next step needs.
• Start signal: AUG in the mRNA. Stop signals: UAA, UAG, UGA, read by a release protein.
• The ribosome slips about once per hundred thousand steps.
• Translation works in a test tube from purified parts with no lipids (Shimizu and Ueda, 2001).

4. Proposed historical order (hypothesis)

1. Simple chemistry makes nucleotides, amino acids and fatty molecules.
2. Short RNA strands and random peptides form.
3. Some RNAs act as enzymes and help copy other RNAs.
4. A small RNA pocket joins amino acids with no message and no stepping (Yonath's proto-ribosome).
5. Amino acids are delivered on small RNA hairpins, the ancestors of tRNA.
6. A template strand is added. Code and mRNA begin. Stepping and fuel become necessary.
7. Proteins take over most enzyme jobs, including the ligases.
8. DNA takes over storage.
9. Last common ancestor: a membrane-bound cell with no mitochondria.

• RNA-first is the majority view. RNA and DNA building blocks coexisting from the start has experimental support (Sutherland and Krishnamurthy labs, about 2020).
• The ligases appear complete in the last common ancestor, before DNA took over.

5. Before cells

• Nothing ordered peptide-making. It ran whenever activated amino acids met something that could join them.
• Peptide fates: broken down, stuck to RNA, stuck to membranes or minerals, or clumped into amyloid fibres.
• The only coordination was survival of what persisted.
• Free building blocks were later eaten by cells.

6. After the cell

• Splitting came first.
• Gene exchange between cells started early and never stopped. Routes: plasmids through a tube, uptake of loose DNA, viruses, and budded fatty bubbles (minor).
• Early diets: hydrogen plus carbon dioxide, sulfur or iron compounds, later sunlight, later organic leftovers. One cell's waste is another's food.
• Mitochondrial merger about 2 billion years ago: archaeon host, bacterial guest. Favoured view is a food-and-waste partnership. Trigger possibly rising oxygen.
• Unsolved lipid puzzle: the host was an archaeon, yet its descendants have bacterial-type membranes.
• Mitochondria keep their own small genome and ribosomes, in nearly every cell.
• Chloroplast merger about 1.5 billion years ago. No archaeon does sugar-building photosynthesis.
• Multicellular life came mostly from daughter cells failing to separate. More than two dozen independent origins.
• Extinction was the norm from the start. All known life descends from one population.
• Early mutation was too high, not too low (Eigen's error limit).
• Oxygen build-up about 2.4 billion years ago: large die-off and probable global freeze.

7. Andrew's positions this session

Position	Status
RNA and DNA coexisted	Accepted as a legitimate position with experimental support
The ligases formed later	Agreed
The ligases are "replies" from proteins	Partly: proteins hold the dictionary but cannot write back to RNA or DNA
Ligases repurposed after DNA	Not supported: they predate DNA's takeover
mRNA or the helper protein is alive	Not supported
The ribosome is alive / FUCA = ribosome	"Oldest and most central machine" is defensible. "Alive" fails the self-reproduction test
Something coordinates the stepping	Not supported: shape and fit do it
Heat does not move the ribosome; a lipid in thermogenic flux might	Not agreed. Claude: heat plus the GTP ratchet. Heat alone cannot give direction
Early peptides were the primitive immune system	Protection first. Immunity once copiers and parasites existed
Fever heat drives the machinery	Not supported: small boost only
Two genetic engines separate immune cells from normal cells	Not supported: all cells have both
The merger was mutual	Favoured view
The merger was lipid-driven fusion	Not shown. The lipid replacement puzzle is real
Mitochondrion as "new furnace"	Agreed
Gene exchange answers an "inbreeding" problem	Matches Muller's ratchet, applied to sex
Cell-in-cell as "subsume instead of consume"	Fits the chloroplast merger. Doubtful for the mitochondrial one
Host-virus symbiosis is original	Supported
Virus turns toxic out of host	Supported (SIV to HIV)
Out-of-host caused by host extinction	Weak
Toxicity expressed as corrupt proteins	Not supported. One overlap: a prion-like antiviral alarm protein

8. Open items

• What moves the ribosome: not agreed.
• How the code began, given the ligases are built by the code they enforce.
• How accurate copying arose before long genomes.
• Why the merged cell switched to bacterial-type lipids.
• Which partner fed which in the merger.