For years, many scientists believed that the asteroid impact that wiped out the non-avian dinosaurs 66 million years ago also helped shape the evolution of tuna.
The idea was that, after many large land animals and marine predators disappeared, tunas quickly evolved to fill the empty role of fast, powerful hunters in the oceans.
However, a new study suggests the story is much more complicated.
Researchers have found that while the ancestors of tunas appeared around the time of the asteroid impact, the features that make modern tunas such successful predators developed much later.
Their findings were published in the journal Proceedings of the Royal Society B.
The research was led by scientists at Yale University, who combined DNA data from living fish with information from fossil specimens.
Using these data, they created the most detailed evolutionary family tree yet for the fish family known as Scombridae. This group includes tunas, mackerels, bonitos, and several other related species.
Scientists have long wondered whether the asteroid that caused the mass extinction at the end of the Cretaceous Period also triggered the rise of large, fast-swimming fish predators.
The extinction wiped out many giant marine reptiles and other top predators, leading some researchers to believe that tunas rapidly evolved to take over these vacant ecological roles.
The new study found that this theory does not match the evidence.
Although the Scombridae family first appeared around the same time as the asteroid impact, the researchers discovered that modern tuna characteristics evolved gradually over tens of millions of years rather than appearing soon after the extinction.
One of the most unusual features of tunas is their ability to keep parts of their bodies warmer than the surrounding water. This ability, known as endothermy, allows them to swim quickly and remain active even in cold ocean waters. Unlike most fish, which depend entirely on the temperature of the surrounding water, tunas can generate and retain heat, giving them an advantage when hunting.
The researchers found that this warm-bodied ability evolved independently three separate times within the Scombridae family. At least two of these evolutionary events occurred between 10 and 15 million years after the asteroid impact, far too late to have been directly caused by the extinction event itself.
The study also challenges another common assumption. Scientists had previously believed that warm-bodied fish naturally evolved larger body sizes at the same time.
Instead, the researchers found very little evidence linking the two traits. Large body size appeared at different times throughout the group’s history, suggesting that becoming bigger and becoming warm-bodied followed separate evolutionary paths.
Overall, the body shapes and abilities seen in today’s tunas and their close relatives developed gradually over roughly 50 million years. Rather than one sudden evolutionary burst after the asteroid impact, the evidence points to a long and complex process driven by many different environmental and biological factors.
The findings also have practical importance. Tunas are among the world’s most valuable commercial fish and provide an important source of food for millions of people.
Better understanding their evolutionary history can help scientists develop improved conservation strategies, especially for species such as the Atlantic bluefin tuna, whose populations have fallen sharply because of overfishing.
The researchers also note that studying how warm-bodied fish evolved may provide useful clues about metabolism and temperature regulation.
These biological systems play important roles in human health and are linked to conditions such as obesity, diabetes, and metabolic syndrome. While the study does not suggest a direct medical connection, understanding how different animals evolved similar metabolic abilities could help scientists better understand these processes over evolutionary time.
The new research shows that one of the most famous events in Earth’s history did not directly create modern tunas as previously believed.
Instead, these remarkable fish became the fast, powerful ocean predators we know today through millions of years of gradual evolution, highlighting that nature’s biggest changes often unfold far more slowly than they first appear.
