13. Big Brains, Ancient Molecules

In the late Paleocene and early Eocene, some 55 to 60 million years ago, a lineage of hoofed land mammals waded into shallow water, then swam, then never came back out — the archaeocetes, ancestors of every whale and dolphin now living. Something happened to their brains on that journey that has an equal in only one other lineage on Earth: primates, including, tens of millions of years later, humans.

Comparing brain size across such different animals runs into a problem immediately. A bigger body needs a bigger brain for reasons that have nothing to do with intelligence — more skin to feel, more muscle to command, more distance for a nerve signal to travel. In 1973, Harry Jerison, at UCLA, proposed a fix: the encephalization quotient, or EQ, which divides an animal's actual brain mass by the brain mass statistically "expected" for a body of its size. An EQ of 1 means exactly average for that body size. Higher than 1 means more brain than body size alone would predict.

The formula itself — brain mass divided by 0.12 times body mass raised to the two-thirds power — matters less than what it is built to capture: relative brain size, standardized so that a mouse and a whale can sit on the same scale at all.

On that scale, humans score higher than any other animal measured, with an EQ of roughly 7.4 to 7.8. The sperm whale, holder of the largest brain that has ever existed in any animal — some 8 to 9 kilograms, nearly six times the mass of a human brain's 1.3 to 1.4 kilograms — scores an EQ of only about 0.58, below the mammalian average. Absolute size and relative size point in opposite directions. Gerhard Roth and Ursula Dicke, reviewing the numbers at the University of Bremen in 2005, drew their comparison table from Jerison's original 1973 dataset together with a 1987 anatomical survey by Haug, using EQ as the necessary tool for comparing such different animals while naming its weakness in the same breath: they argued that raw brain size, and especially the number of cortical neurons and the speed signals travel along them, tracks cognitive ability more closely than EQ does. EQ itself is, by the account of researchers who study it closely, an argued and unsettled proxy for "intelligence" — not a settled one.

Restricted to cetaceans alone, the expansion was neither steady nor uniform. Stephen Montgomery and colleagues at University College London reconstructed brain and body mass across a dated cetacean and hippopotamid family tree in 2013, recovering explicit EQ values at points along it: an early branch near 0.371, a later one near 0.702, and an ancestral baleen-whale node near 1.144. The increase happened in episodic jumps within particular branches, not as one smooth climb, and toothed whales and baleen whales do not even follow matching trajectories — an unresolved discrepancy the authors note directly, alongside evidence — drawn from separate work by Barton and Capellini, and by Hager and colleagues, published between 2011 and 2012 — that brain mass and body mass may be shaped by partly separate genes rather than one linked package.

What resulted looks nothing like a primate brain. Lori Marino, then at Emory University, described the cetacean cortex as reaching complexity by multiplication rather than specialization: a thin cortical unit repeated over and over across a highly folded surface, rather than the thicker, more differentiated modules that build a primate cortex. Her 2004 paper drew on four years of fossil work across 66 skulls, cross-checked against data from 144 more. It is her own framework for how cetacean complexity arises mechanically, not a mechanism proven against rival accounts of primate-style complexity — but no rebuttal of it has surfaced since.

Two lineages, one that stayed on land and one that returned to the sea, reached large, complex brains by different anatomical routes, at different paces, under different rules of proportion. What they did not do differently is the chemistry running through the result. The dopamine system built at the root of Vertebrata — the same tyrosine-to-dopamine pathway, the same ancient receptor families — is the one both a primate brain and a cetacean brain inherited before either began building anything new on top of it.

Neither lineage needed to invent a new chemical vocabulary to build something unprecedented. A folded, multiplied cetacean cortex and a thickened, modular primate cortex are different pieces of hardware, arrived at on different schedules. Wired into both is the same handful of ancient signaling molecules, doing the same basic jobs they already did in a fish or an early mammal, just at a larger scale and in a new arrangement. Complexity, on this evidence, looks like a matter of how existing parts get organized and how many of them there are — not of sourcing new parts.

A third lineage makes the same point with a stranger anatomy still. Birds evolved cortex-equivalent processing using a structure organized nothing like the mammalian neocortex: a "nuclear" pallium made of clustered neuron groups, instead of the mammalian arrangement in layers. For roughly a century, the relevant bird-brain structures were named and treated as a primitive, reptile-like "striatum," as though the avian forebrain were mostly reflexive tissue with nothing resembling a cortex in it at all — until an international group, the Avian Brain Nomenclature Consortium, led by Erich Jarvis and Anton Reiner, used molecular markers, connectivity tracing, and electrophysiology to show these structures are true pallium, the evolutionary counterpart of cortex, and renamed them accordingly in 2005. One region, the nidopallium caudolaterale, is now treated as functionally equivalent to a mammalian prefrontal cortex. Birds and mammals last shared a common ancestor conventionally dated to around 320 million years ago, in the Carboniferous, at the split between the lineages leading to reptiles and birds on one side and to mammals on the other — meaning bird and mammal brains built convergent higher-order processing from independent starting anatomy, over comparably vast stretches of geological time.

Even the messenger molecule itself can be pressed into a genuinely new structural role. In the octopus visual system, researchers have identified a dopamine receptor with no equivalent in vertebrate biology: an ionotropic receptor, a fast physical channel that opens on direct contact, where every vertebrate dopamine receptor instead works through the slower, cascade-based metabotropic route. The finding, which named the new receptor DopC1, was posted as a preprint in April 2025 and has not yet completed peer review — a caveat worth carrying alongside it. Cephalopods and vertebrates split at the same protostome-deuterostome divide discussed for dopamine's chemical origin, meaning the molecule itself is inherited, but this particular way of reading it is a separate, convergent invention layered on afterward — fitting company for an animal already described in the cephalopod-neuroscience literature as the most behaviorally advanced invertebrate known.

Ancestry and Reinvention

Reviews synthesizing work across several phylogenomics groups between 2013 and 2021 draw a clean line through all of this. The basic neural parts list — neurons, synapses, neuropeptides, and the front-to-back patterning program that lays out a nervous system's plan before it is built — was already present in the common ancestor shared by cnidarians and every bilaterally symmetric animal. Cnidarians, such as jellyfish and sea anemones, have only a diffuse nerve net and no brain at all, and they sit entirely outside the group containing every animal with left-right symmetry, which places this shared toolkit above 600 million years old.

Every large, complex brain built from those parts since — in ribbon worms, in annelids, in cephalopods, in arthropods, in vertebrates — is now read as an independent construction project, not a shared inheritance of "brain" as such: an example of homoplasy, the term for a trait that evolved more than once rather than being passed down from one common possessor. That reading replaced an older view in which shared complexity across these groups was itself taken as evidence of one deep common origin. The newer position, built on fossil and developmental gene-mapping evidence gathered through 2021, keeps the molecular toolkit's single origin while giving up the idea that complex brains themselves share that same single origin.

That is the honest shape of the answer here. Every dopamine molecule moving through a human prefrontal cortex, a dolphin's folded neocortex, and a bird's nidopallium caudolaterale descends from the same billion-year-old chemistry, inherited once, from one source. The organs doing the thinking around that chemistry do not share that single ancestry. They were built, independently, at least three or four separate times over, by lineages that never had anything to work with but old molecules and a very long stretch of time. The molecule is exactly as old in a bird's brain as it is in a human one; only the architecture wrapped around it is new. Trace any of these borrowed molecules back far enough, past every brain they were ever built into, and they converge on something older than any brain at all: the single ancestral population that first wrote the alphabet they are made of.

Overturned if: cetacean and primate brain enlargement were shown to trace to a shared genetic driver already present in their last common ancestor — a Mesozoic mammal with neither trait — rather than to independent mutations arising separately in each lineage, converting a twice-told story of convergence into one shared inheritance.

Sources

• 1973 · Jerison, H.J., Evolution of the Brain and Intelligence — ScienceDirect topic overview
• 2004 · Marino, L., "Cetacean Brain Evolution: Multiplication Generates Complexity" — eScholarship
• 2013 · Montgomery, S.H. et al., "The Evolutionary History of Cetacean Brain and Body Size" — Evolution 67:3339
• 2005 · Roth, G., Dicke, U., "Evolution of the Brain and Intelligence" — Trends in Cognitive Sciences 9:250
• 2005 · Jarvis, E.D., Reiner, A. et al., "Avian Brains and a New Understanding of Vertebrate Brain Evolution" — Nature Reviews Neuroscience 6:151
• 2025 · "Novel dopaminergic neurotransmission in the Octopus visual system" — bioRxiv