Did the asteroid that killed the dinosaurs create the tuna? The reality is much more complex

The Mystery of Warm-Blooded Fish and Their Presumed Role in the Mass Extinction

When we think of a warm-blooded animal, the first images that come to mind are usually those of a cow, a dog, or an elephant. However, the tuna belongs to an extremely rare category in the marine world. Endothermy—that is, the ability to maintain a high body temperature through heat generated by muscles—remains a rare trait among the 33,000 species of fish recorded on the planet. This ability gives tuna superior muscular efficiency and allows them to maintain a high cruising speed over immense distances.

Tuna belong to the Scombridae family, a group of predatory fish that also includes mackerel and bonito. The Scombridae account for about half of all extant endothermic ray-finned fish. According to a long-established theory, these fish began to emerge and thrive around the same time as the extinction of the dinosaurs, approximately 66 million years ago. The conventional story held that their ancestors became warm-blooded and grew larger to fill the ecological niches left vacant by the large Cretaceous fish and marine reptiles decimated by the asteroid impact.

However, as reported by the science media outlet IFLScience, new research is now challenging this historical narrative, which is considered too simplistic. The evolutionary transition toward the ability to regulate body temperature and reach gigantic sizes turns out to be far more complex than a simple reaction to a planetary cataclysm.

An evolutionary timeline spanning tens of millions of years

A recent study published in the journal Proceedings of the Royal Society B suggests that the previously accepted timeline does not align with actual biological data. According to Chase Brownstein, a researcher at Yale University, science too often tends to attribute direct, narrative explanations to the emergence of complex traits. "In our eagerness to explain the evolution of every biological trait, we biologists have relied on simplistic stories about how they arose," he explains to IFLScience.

To test this hypothesis, the team led by Chase Brownstein analyzed genetic sequencing data from 50 species in the Scombridae family. By cross-referencing this molecular information with fossil data, the researchers reconstructed the evolutionary family tree of tunas, mackerels, and bonito. While their results confirm that the Scombridae family did indeed emerge about 68 million years ago, toward the end of the Cretaceous period, they contradict the idea of rapid diversification immediately following the asteroid impact.

The phylogenetic tree actually reveals that the family diversified very gradually over a long period of 40 million years. In particular, the study shows that endothermy evolved independently on three distinct occasions within this family, and that at least two of these events occurred 10 to 15 million years after the extinction of the dinosaurs. Similarly, the increase in body size occurred sporadically over the last 50 million years, with tuna only reaching their current gigantic size in the last 10 million years or so.

The phenomenon of exaptation and the decoupling of biological traits

The findings presented by Chase Brownstein demonstrate that the evolution of large body size and the development of endothermy are phenomena that are not only decoupled from one another but also independent of the asteroid impact. Although the Scombridae clearly thrived by capitalizing on the extinction of Cretaceous fish, this cosmic event was not the direct driver that led them to become large endothermic predators.

The researcher recalls a fundamental rule of evolutionary biology: “Just because a trait fulfills a particular function does not mean it evolved because of that function. Evolution is blind, so traits come together in many different ways and can then be repurposed for new functions.” This is what scientists call the principle of exaptation, when a characteristic initially arises for one role before being repurposed to serve another advantageous function.

In the case of modern tuna, their large size and warm-bloodedness now work in synergy to enhance their ability to undertake long migrations, dive to great depths, and hunt more efficiently. However, these formidable abilities are thought to be the result of a secondary rearrangement of traits that emerged separately over time, rather than the result of an immediate adaptation to the consequences of the asteroid impact.

The hypothesis of competition with cetaceans sparks debate

The asteroid impact is not the only explanation put forward by the scientific community to account for the emergence of warm-bloodedness in the Scombridae. Another major hypothesis suggests that this thermal adaptation resulted from evolutionary competition with the ancestors of whales and dolphins—known as cetaceans—or from the need to escape these new marine predators, which were also beginning to proliferate at that time.

This theory is, however, strongly contested by Chase Brownstein, who bluntly calls it an unsubstantiated fabrication: “Unless I’m missing something, no biologist has a time machine to go back 56 million years and observe just how much tuna were outmatched by whales. You simply cannot substantiate claims about these ecosystems without an extremely comprehensive fossil record—and even then, it’s difficult.”

In the face of this criticism, proponents of the cetacean hypothesis firmly maintain their position. Dahiana Arcila, a researcher at the University of California, San Diego, points out that the Yale team’s findings still place the origin of endothermy during the Eocene, a period that corresponds precisely to when cetaceans were diversifying. According to her, the timelines remain perfectly consistent.

Macroevolutionary Methods in Light of Scientific Evidence

To support the hypothesis of an interaction between fish and cetaceans, Dahiana Arcila’s team conducted a comprehensive study involving more than 1,051 fish species belonging to numerous families. Published in the journal Science Advances, this research combines fossil records, dietary analyses, and the geographic distribution of species during the Eocene, all evaluated using rigorous statistical models. Dahiana Arcila therefore believes that dismissing this theory as purely fictional does not constitute a valid scientific refutation.

“This is a hypothesis supported by multiple lines of evidence, a well-established line of reasoning, and one that is consistent with the chronology,” says Dahiana Arcila. “The relevant question is not whether the inference is experimental, but whether it is supported by evidence. Ours is.” To support their approach, she and her colleague Fernando Melendez Vazquez cite the example of the emergence of warm-bloodedness in the ancestors of mammals during the Triassic, driven by an evolutionary arms race against the ancestors of dinosaurs.

Finally, Dahiana Arcila highlights the unique nature of research on macroevolution: “Macroevolution does not lend itself to experimental manipulation, so every claim here is based on inferences. It works both ways.” She concludes that while a coincidence of dates is not enough to prove the impact of cetaceans, a simple chronological decoupling observed within a single recent family is also not enough to definitively rule out this possibility.

Source: iflscience.com

Did the asteroid that killed the dinosaurs create the tuna? The reality is much more complex