The article’s most ambitious section follows heredity itself through a series of upgrades.
Heredity means like begets like. But there are different kinds of “like,” and different systems for transmitting information.
The authors distinguish between limited heredity, where only a few states can be transmitted, and unlimited heredity, where an indefinitely large number of messages can be transmitted.
This distinction links genes, epigenetic marks, and language in one shimmering information-thread. 🧵
Stage 1: Simple autocatalytic systems
Autocatalysis means a molecule helps produce more molecules of the same kind. This is essential for growth, but not enough for true heredity. Heredity requires that if the original molecule changes, the system reproduces the changed type.
Some autocatalytic networks may have shown limited heredity, but only among a small number of molecular states.
Stage 2: Polynucleotide-like molecules and unlimited heredity
The origin of polynucleotide-like molecules was a decisive shift because they could encode open-ended digital information.
But the article emphasizes that this transition is hard. Problems include enantiomeric cross-inhibition, where mirror-image building blocks interfere with chain formation, and failure of template and copy to separate because they bind too strongly.
Short oligonucleotides may have been intermediate. Their shorter length allows spontaneous separation, but their growth dynamics can be parabolic rather than exponential. That produces “survival of everybody” rather than sharp survival of the fittest.
For full Darwinian competition, replicators need something closer to exponential growth.
Stage 3: The genetic code before translation
The article suggests that the genetic code may have begun before full translation. The key idea is that amino acids became attached to specific oligonucleotide handles.
The authors favor a scenario in which amino acids acted as coenzymes for ribozymes. Each amino acid had a trinucleotide “handle” allowing it to bind by base pairing. This could let different ribozymes recruit the same amino acid, gradually building the logic later used in the genetic code.
This avoids the “all at once” problem. Translation does not need to appear fully formed, wearing a tuxedo and carrying a ribosome.
Stage 4: Encoded protein synthesis
The origin of translation and encoded protein synthesis is treated briefly in the article, with details deferred to the authors’ larger book. But in the series of transitions, this is enormous: proteins become the main catalytic workforce, while nucleic acids specialize in information storage.
Stage 5: DNA replaces RNA
The article argues that DNA may have replaced RNA because DNA is chemically more stable. Thymine is more stable than uracil, and deoxyribose more stable than ribose.
The authors challenge a common argument that RNA lacks repair systems. In principle, damage repair could be chemically feasible in double-stranded RNA. Stability itself may have been the main advantage.
Stage 6: Epigenetic heredity
The authors then turn to heritable regulatory states. In prokaryotes and simple eukaryotes, methylation patterns can be transmitted through cell division. That means inheritance can depend not only on DNA sequence but also on gene-activity states.
This is central for development. Multicellular organisms need cells with the same genome to behave differently. A neuron and a liver cell are not different because they have different genes, but because they maintain different gene-expression states.
Figure 1c illustrates this idea with genes A, B, and C carrying heritable activity states, marked by asterisks.
Stage 7: Multicellular heredity
Animals, plants, and fungi evolved epigenetic inheritance systems with enough richness to support many differentiated cell types. The authors note that this happened three times, suggesting the transition may not have been extraordinarily difficult once the relevant epigenetic machinery existed.
Stage 8 and 9: Protolanguage and true language
The final heredity transition is cultural.
Proto-language in Homo erectus may have allowed limited communication without grammar. Human language, by contrast, has grammar and unlimited semantic representation. With finite vocabulary and rules, humans can generate indefinitely many meanings.
The authors compare this directly to the genetic code: finite components, infinite combinatorial potential.
They accept Chomsky’s argument that grammar is uniquely human and specific to language, but they criticize reluctance to think evolutionarily about grammar. They argue that intermediate forms are possible. A partial grammar can still be useful, just as a light-sensitive patch can be useful before a full eye evolves.
The article even discusses hereditary variation in linguistic competence, including a family with inherited difficulty automatically generating plurals and past tense. This suggests that grammar can be biologically dissected, much as development can.
The grand move here is bold: heredity includes genes, epigenetic states, and culture. Evolutionary transitions are information revolutions.
No comments:
Post a Comment