The Quiet Galaxy

Explanations · They were never there

Complex cells are rare: life may start easily but then stay simple forever, because the one step that made big cells happened only once

Proposed by Nick Lane and William Martin, 2010 fi smaller

Life on Earth was single-celled and simple for about two billion years before anything more complicated appeared, and everything complicated since, every plant, animal and fungus, descends from one event in which one simple cell took up residence inside another. That merger gave cells a power supply big enough to run large genomes and build large bodies. It happened once in four billion years, and there is no sign it was easy. If the same bottleneck holds elsewhere, most living planets are covered in microbes and never get further, and the galaxy is alive but has nobody in it to talk to.

The two oldest kinds of life on Earth are bacteria and a second group of single cells that look much like them; between them they ran the planet alone for roughly half its history. Everything else, from yeast to whales, is built from a third kind of cell that is far larger and more complicated, with an internal skeleton, compartments and, above all, many small power stations called mitochondria. Those power stations were once free-living bacteria. At some point, probably between about 1.2 and 2.1 billion years ago (estimates differ), one cell swallowed another and failed to digest it, and the two became a single organism. Every complex cell alive carries the descendants of that captured bacterium.

The biochemist Nick Lane and the microbiologist William Martin argued in 2010 that this merger was not just one event among many but the event. A simple cell makes its energy on its outer membrane, and a cell's surface grows more slowly than its volume, so the energy it can make is a poor match for the genes and bulk it would have to support. A cell with hundreds of internal power stations carries a far larger area of energy-making membrane inside it, and Lane and Martin estimated that the merger allowed a 200,000-fold expansion in the number of genes a cell could use. That is what could pay for the large genomes from which complex bodies were later built. On this view the long stall in Earth's history was not bad luck but a wall, and the merger was the one way through it. The wall is sometimes called the eukaryote bottleneck, after the technical name for complex cells.

The bearing on the Drake Equation is on the share of living planets where intelligence appears, since a mind needs a body and a body needs complex cells. It leaves the chance that life starts untouched, and so sits comfortably beside the view that microbes are common. It is one of the strongest candidates for a step that is both necessary and hard, which makes it a favourite location for the Great Filter, the idea that some step on the road from dead matter to a galaxy-spanning civilisation is very unlikely.

The main objection is again the sample of one. That the merger happened once on Earth may mean it is hard, or may mean that once it happened the winners took every available niche and left no room for a second attempt, which is the usual pattern when a new kind of life appears. Some biologists also question whether the energy argument is as tight as Lane and Martin make it. Michael Lynch and Georgi Marinov argued in 2015 that the energy cost of each gene does not rise, and generally falls, in larger cells of both bacteria and complex cells, and in 2017 that the area of a cell's membrane does not look like the limit on its energy supply. Lane and Martin themselves wrote that even giant bacteria carrying many copies of their genome cannot pay the energy price of complexity. Others note that even if the step is hard, two billion years is a long time and most planets that can carry life get at least that. The argument stands or falls on whether there are other ways to build a complex cell, and nobody knows.

What it says about the equation

Every explanation is a claim about one or more of the seven pieces that are multiplied to give the number of detectable civilisations. This one bears on 1 piece:

The test: what would strengthen or weaken it

Would strengthen it: Discovery of life elsewhere that is simple and has stayed simple for billions of years; further evidence that no second lineage of complex cells ever arose on Earth; a theoretical result showing that the energy limit on simple cells cannot be got around any other way.

Would weaken it: Discovery of a second, independent origin of complex cells on Earth, living or fossil; a laboratory or natural example of a simple cell growing a large genome without captured power stations; complex life found on another world.

These two lines are what the record is read against when a new paper or discovery is logged. A result of the kind named here, published with its evidence, is what would settle it; a press claim on its own settles nothing.

The last reading

Once a week the records are read against the news, each against the test above. Open a line to see what that week held. Every reading ever made of this record is on the reading page.

  • 2026-10-16 Logged, no change By hand

    No second origin of complex cells, living or fossil. Bernabeu and others (Nature, 2026-06-10) reconstruct the ancestor of all complex cells and find its genes came from at least four microbial partners plus giant viruses over a long period, with the mitochondrion acquired late: the step still happened once, but it looks like a drawn-out association rather than a single unlikely capture, which bears on the argument that the one step was a freak. Logged.

    Bernabeu, 2026 source

How seriously it is taken

Respected among cell biologists and widely cited in writing about the Fermi question; whether the merger was a hard step or simply a first-come-first-served event is openly argued.

The papers that bear on it

Books, films and other work that take it up

Nothing in the library illustrates this explanation yet.

Read next

Notes

Checked 2026-10-16. Borne out: Lane and Martin, 'The energetics of genome complexity', Nature 467, 929-934 (2010); the abstract says prokaryotic genome size is constrained by bioenergetics and that the mitochondrial merger allowed a 200,000-fold expansion in the number of genes expressed (https://doi.org/10.1038/nature09486). The surface-area point is how Lynch and Marinov, eLife 6, e20437 (2017), report Lane and Martin's case: energy-making machinery is confined to the cell membrane in simple cells, and mitochondria give a large internal membrane area (https://elifesciences.org/articles/20437); the Nature paper itself could not be read tonight, so the 'surface against volume' wording is that report's. Corrected: the merger date was given as 1.5 to 2 billion years ago, but a 2018 molecular-clock study dates the origin of mitochondria to between 2.053 and 1.21 billion years ago (Betts and colleagues, Nature Ecology and Evolution 2, 1556-1562, https://doi.org/10.1038/s41559-018-0644-x), so the page now says about 1.2 to 2.1 billion years and that estimates differ. The objection from giant bacteria and 'simple cells with internal compartments' was not found as a published critique: the specific critiques found are Lynch and Marinov, 'The bioenergetic costs of a gene', Proceedings of the National Academy of Sciences 112, 15690-15695 (2015), and the 2017 eLife paper, which the body now cites; giant bacteria with many genome copies are discussed by Lane and Martin themselves (quoted in the 2017 eLife paper), and the internal-compartments half of the claim was removed because no source was found.

Think this page gets the explanation wrong, or misses an objection? Say so, or read how disagreement gets folded in. The record itself is data/explanations/complex-cells-are-rare.toml in the public repository. This page's address will never change; if the explanation is ever retired, the address will say what replaced it.