Evolution Does Not Promise Complexity, So Why Did Complexity Happen?

Evolution has no built-in ladder. There is no law saying bacteria must become amoebas, amoebas must become animals, or primates must become poets. Szathmáry and Maynard Smith begin from that bracing point: there is neither a theoretical necessity nor a clean empirical rule that all lineages increase in complexity over time.

And yet, here we are.

Eukaryotic cells are more internally elaborate than prokaryotes. Animals and plants are more complex than single-celled protists. Human societies transmit information in ways no bacterium ever dreamed of, assuming bacteria dream in plasmids.

The authors’ central proposal is that complexity increased in some lineages because evolution passed through a small number of “major transitions.” Each transition changed not merely what organisms looked like, but how biological information was stored, replicated, transmitted, and organized.

The major transitions

The article’s Table 1 lists the great evolutionary handoffs:

1. Replicating molecules became populations of molecules inside compartments.
2. Unlinked replicators became chromosomes.
3. RNA, once both gene and enzyme, gave way to DNA plus protein, via the genetic code.
4. Prokaryotes became eukaryotes.
5. Asexual clones became sexual populations.
6. Protists became animals, plants, and fungi through cell differentiation.
7. Solitary individuals became colonies with non-reproductive castes.
8. Primate societies became human societies through language.

At each step, previously independent units became locked into a larger evolutionary unit. Free-living bacteria became organelles. Individual cells became parts of multicellular bodies. Individual insects became components of colonies. Words and gestures became grammar-bearing language.

Complexity, but how do we measure it?

The article is cautious about complexity. There is no universally accepted biological complexity-meter, no little dashboard reading “complexity: 87%.”

The authors discuss two rough measures.

First, genome size and coding DNA. Table 2 compares organisms such as E. coli, yeast, nematodes, fruit flies, newts, humans, lungfish, and flowering plants. The general pattern is that eukaryotes have larger coding genomes than prokaryotes, and animals and plants often have more genetic material than protists. But genome size is a slippery clue. Lungfish and some plants have enormous genomes without being obviously “more complex” than humans.

Second, behavioral and morphological richness. A bacterium does many impressive things, but it does not phagocytose prey with a cytoskeleton, compose music, or build a bee colony. Cell types, behaviors, and developmental possibilities may better capture the intuitive sense of complexity.

The article’s deeper point is not simply that complexity increased. It asks: by what mechanisms could the amount and organization of information increase?

The three engines of added information

Figure 1 gives three major routes:

Duplication and divergence. A gene is copied. One copy keeps the old job, while the other is free to mutate into a new role. This is the classic “photocopy, then improvise” engine of genetic innovation.

Symbiosis. Separate replicators or organisms join into a cooperative unit. The figure moves from independent replicators, to a hypercycle, to enclosure in a compartment, to physical linkage. This is the visual seed of mitochondria, chloroplasts, and other once-independent entities becoming parts of a larger whole.

Epigenesis. Genes do not merely exist as sequences. They can be switched on or off in heritable states. Figure 1 shows genes A, B, and C with activity states passed through cell division. This foreshadows the evolution of differentiated cell types in multicellular organisms.

The series thesis

The rest of the article keeps circling a single question with different masks:

How can evolution make a new individual out of old individuals?

That question applies to genes on chromosomes, organelles inside cells, cells inside bodies, insects inside colonies, and minds inside language communities. The answer is not sentimental cooperation. It is a rugged evolutionary bargain: cooperation can evolve when conflicts are suppressed, relatedness is high, division of labour pays, and information transmission becomes more powerful.