Tuna Evolution Took 50 Million Years, Not One Asteroid Strike

Somewhere in the Eocene, a fish began keeping the heat its own swimming muscles made. Instead of bleeding that warmth out through the gills into cold water, as nearly every fish that has ever lived does, it held on. The trick would eventually let its descendants chase prey into water that should have slowed them to a crawl. What it did not come with, though, was size.

The bulk arrived later. Much later, in fact, by something like twenty million years, and that gap is the reason a Yale team has pulled apart one of the tidier stories in marine evolution.

The tidy version runs like this. An asteroid strikes the Yucatán 66 million years ago and takes the non-avian dinosaurs with it. It also takes the enormous predatory fishes that had been patrolling the open ocean for most of the Mesozoic: the ichthyodectiforms, the crossognathiforms, animals like Xiphactinus audax, which was five metres of teeth and appetite. The seas are emptied of their top hunters. Into that vacancy swim the ancestors of tunas and mackerels, which promptly get big, get fast, and get warm, in a neat marine echo of mammals inheriting the land. Very teachable. Rather too teachable, as it turns out.

Chase Brownstein, a graduate student in ecology and evolutionary biology at Yale and lead author on the study, is clear about where that leaves things. “Our results demonstrate the K-Pg extinction did not trigger the evolution of tunas and related large, endothermic predators,” he said.

Reading the Clock More Carefully

Getting to that conclusion meant building a better clock. The team sequenced 1,001 ultraconserved element loci across the family Scombridae, adding mitochondrial genes to fill taxonomic holes, until they had data covering 50 species and fourteen of the fifteen genera. Fossil calibrations at fourteen nodes anchored the whole thing to real time. The result is the most complete time-calibrated tree the group has ever had.

And the tree does place the origin of Scombridae right where the old story wants it, at roughly 68 million years ago, straddling the boundary. The major lineages all split within twenty million years of the impact. So far, so orthodox.

Then the traits refuse to cooperate. Endothermy shows up three separate times in the family, by three different anatomical routes: systemic warming throughout the body in the ancestor of skipjack, the frigate tunas, the little tunnies and Thunnus; a heated eye muscle in the slender tuna Allothunnus fallai; and a peculiar modified lateral rectus in the butterfly kingfish, Gasterochisma melampus. At least two of those three arrived 10 to 15 million years after the asteroid, which is a long time to wait for an opportunity you were supposedly seizing. Large body size, meanwhile, evolved multiple times independently, and every instance falls well after the extinction, with several lineages crossing the two-metre mark only in the last 25 million years or so. The single big jump in body size that the team’s Bayesian models flag with confidence sits at the base of Thunnus, the market tunas, and that node is around 10.6 million years old.

Two Traits, Two Different Stories

Which means the warm body and the big body are not the same story at all. Brownstein put it plainly: “We show that the body plans of these predators evolved over tens of millions of years and that there is no connection between the origins of endothermy and large body sizes in these lineages.” The statistics back him up; regressions found no significant link between endothermy, body size, and life history traits like longevity, growth rate or spawning interval.

The best single piece of evidence is a fish most people have never heard of. Allothunnus fallai, the slender tuna, is a regional endotherm, warming the muscle behind its eye, and it is smaller than most of its relatives. Warm and unremarkable in size. If heat and bulk were a package deal, it should not exist.

All three endothermy origins sit on very long, lonely branches of the tree. The butterfly kingfish’s branch runs about 69 million years; its heater could have evolved essentially anywhere along that stretch, and no amount of statistical machinery will narrow it down. That limitation cuts both ways. It weakens the rival hypothesis that fish endothermy was a response to Eocene climate swings, or to the rise of competing warm-blooded whales, but it also means the paper is better at ruling things out than in. Notably, two of the longest branches in the whole family lead to endothermic species, which is not what you would expect if warm-bloodedness were the sort of key innovation that reliably sets off a burst of new species.

That is a lesson about method as much as about tunas, and Brownstein says so, noting that the work “highlights the need to be cautious when interpreting the evolution of species’ body plans directly from evolutionary trees.” A branching diagram tells you when lineages split. It does not, on its own, tell you when they became what they are now.

Thomas Near, the study’s senior author and curator of ichthyology at the Yale Peabody Museum, thinks the multiple independent origins are interesting for reasons that have nothing to do with fish. “Understanding that endothermy independently evolved multiple times in tunas and mackerels provides insight into the fundamental machinery underlying metabolism and thermoregulation,” he said. He is careful about how far to push it, adding that these are systems central to human conditions including obesity and diabetes, while stating there is no explicit connection, only the value of watching how biodiversity has handled similar problems across deep time. Near also points out the more immediate stake: Atlantic bluefin populations have fallen sharply over recent decades under fishing pressure, and knowing how these animals were assembled is not irrelevant to keeping them around.

Fifty million years of tinkering, then, spread across separate lineages, on separate schedules, for reasons the tree cannot fully disclose. The bluefin cruising off Nova Scotia at 30 degrees Celsius in water half that temperature is not a survivor of an ancient catastrophe so much as the accumulated result of a great many unrelated evolutionary accidents that happened to end up in the same fish. Whether the same protracted, decoupled pattern holds for billfishes and opahs, the other warm-blooded lineages in the open ocean, is the obvious next question. The team suspects it does.

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