The ribosome consumes most of the cell's energy. In rapidly growing bacteria, ribosomal RNA constitutes up to 80% of total cellular RNA, and ribosome synthesis dominates the energy budget. The cell devotes the majority of its resources to building and maintaining the machine that makes proteins. This is usually explained as necessity — proteins are required for everything, so the factory that makes them must be large. But necessity doesn't explain why the factory grew so large, or why the rest of the cell's machinery seems organized around serving it.
Krupovic and Koonin (arXiv 2602.23268, February 2026) propose that the ribosome is the cell's dominant selfish element — not a tool the cell uses, but a replicator the cell propagates.
The argument begins at the RNA-protein transition. In an RNA world, replication was performed by RNA-dependent RNA polymerase ribozymes. The proto-ribosome emerged as a mutualistic partner: a ribozyme that synthesized short peptides, some of which enhanced the replication efficiency of the polymerase. The relationship was initially cooperative — the polymerase replicated both itself and the proto-ribosome, and the proto-ribosome produced peptides that helped the polymerase work.
The transition from mutualism to addiction occurred when the peptides became essential. Once the replication machinery relied on ribosome-produced proteins to function, the polymerase could no longer replicate without the ribosome. The dependency was irreversible. From this point, the ribosome's evolutionary interests dominated: any mutation that increased ribosomal efficiency, ribosomal copy number, or the fraction of cellular resources devoted to ribosome production was selected for, because cells with more ribosomes grew faster and divided sooner.
The subsequent history of cellular evolution, in this framing, is ribosomal expansion. The ribosome grew from a small peptidyl transferase center to the massive molecular machine it is today — 4,500 nucleotides of RNA and over 50 proteins in bacteria — not because the cell needed it that large but because larger ribosomes were better at ensuring their own propagation. The cell's genome, metabolism, and regulatory systems evolved to support ribosome production, not the other way around. The genome stores the information needed to build ribosomes. The metabolism generates the energy and raw materials ribosomes consume. The regulatory systems ensure ribosomes are produced at the maximum rate the environment supports.
The selfish gene framework locates self-interest in DNA. Krupovic and Koonin shift it to the ribosome. DNA stores information, but it does nothing without translation. The ribosome is the bottleneck through which all genetic information must pass to become functional. If you ask which component of the cell is most irreplaceable — which part everything else exists to serve — the answer is the ribosome. Genes are instructions. Ribosomes are the reader. Without the reader, the instructions are inert.
The framework makes a prediction: across the tree of life, the fraction of cellular resources devoted to ribosome production should be near the maximum sustainable level, limited only by the cell's capacity to supply raw materials. This is observed. Growth rate in bacteria scales linearly with ribosome content. Cells grow as fast as their ribosomes allow, and they allocate resources to make as many ribosomes as they can. The cell is optimized for ribosome production, not for survival — survival is the means, not the end.