Our ancestors became warm-blooded later than we thought | Science

When paleontologist Romain David first laid eyes on a set of penny-size tubes of fossil bone at the National Museum of Natural History in Paris in 2008, he was struck by the unusual shape and varying sizes of these inner ear structures, called semicircular canals. Paleontologists have long used them to infer ancient animals’ movement patterns, but David and his colleagues today apply them to another mystery: They argue that warm-bloodedness first arose in mammal ancestors likely about 233 million years ago in the Late Triassic, 20 million years later than previously thought. 

“It’s quite exciting,” said Jasmina Weimann, a molecular paleontologist at the California Institute of Technology who wasn’t involved with the study. “It is the first time that any relationship between canal shape and temperature has been proposed,” she says, adding that the new method also offers a promising way to trace the evolution of long-extinct animals.

Warm-bloodedness, or endothermy, is a trademark of living mammals and birds—and apparently some dinosaurs. It allows a more constant body temperature in the face of outside temperature fluctuation. This requires more energy, but it allows animals to move at night and in the cold, which cold-blooded (ectothermic) animals cannot do. Paleontologists agree that early vertebrates were all cold-blooded, but they’ve debated when warm-bloodedness arose.

Warm-blooded animals usually have faster growing bones and sport fur or feathers for insulation, so researchers have studied those features to estimate when mammals’ journey to endothermy started. But those measures aren’t exact proxies and could have first evolved for other reasons.

Enter David, now a paleontologist at the Natural History Museum in London, with a new idea about semicircular canals: After doing his Ph.D. on the structures, he noted that mammals have smaller semicircular canals for their body size than other vertebrates. “The whale shark is actually the animal with the largest semicircular canals on Earth, much larger than the canals in whales,” David says. “I thought, ‘Maybe this has to do with body temperature.’” 

He was also struck by the importance of the fluid inside the canals, endolymph, which changes in viscosity depending on temperature. Like oil in a hot pan, it becomes thinner as it heats up and thicker as it cools. David suspected that endolymph viscosity and semicircular canal size were related, and that both could reflect endothermy. 

To test that hypothesis, David and his team examined the ear canals of 277 living animal species including alpacas, turkeys, and lizards. Their results confirmed David’s suspicions: The warm-blooded animals had less viscous endolymph and smaller and thinner canal shapes. Cold-blooded animals had thicker endolymph and larger, thicker canals.  

The team then applied this knowledge back to fossil specimens to figure out when endothermy evolved. Although the ear canal, because it’s made of soft tissue, doesn’t often fossilize, the bony tubes that encompass these canals do, and can be used to infer canal shape. The researchers examined 64 extinct species, including mammals, mammallike ancestors from more than 230 million years ago called mammaliamorphs, and much older ancestors known as nonmammalian synapsids. The team was able to trace when animals with smaller, thinner ear canals appear: in the Late Triassic, about 233 million years ago—just about the time when mammaliamorphs evolved from nonmammalian synapsids, the researchers report today in Nature. This change happened relatively suddenly, over less than 1 million years, says co-author Ricardo Araújo, vertebrate paleobiologist at the Institute for Plasmas and Nuclear Physics of the University of Lisbon’s Higher Technical Institute. “We are not only saying when endothermy starts, we can also say when ectothermy [in mammalian ancestors] ends.”  

Endothermy starting with mammaliamorphs makes sense, David says: The beginning of mammaliamorphs also corresponds with the evolution of traditionally mammalian features, such as whiskers and backbone flexibility for grooming fur. “It makes sense that [endothermy] happens at more or less the same time when mammal ancestors become more and more mammallike.”  

Some researchers had previously speculated that nonmammalian synapsids, which include the famous sail-backed behemoth Dimetrodon, were also warm-blooded. The new finding puts the “nail in the coffin” of that idea, Araújo says.

However, the study is one piece of a larger unfinished puzzle, cautions Hans Straka, a neurobiologist at the Ludwig Maximilian University of Munich. Endothermy did not just come out of nowhere, he notes, and more fieldwork needs to be done to prove the suddenness of the change. If a new fossil mammaliamorph were to be discovered from a different time period, for instance, the beginning of endothermy would change as well. “Endothermy, it’s not all or nothing,” he says. “There is no black and white transition, it’s a gradual thing.” 

But Straka agrees that ear canal bones become more prevalent in future endothermy studies, for example in the debate over dinosaur warm-bloodedness that has lasted decades. “It’s refueled the discussion,” Straka says. “And that’s good.” 

When paleontologist Romain David first laid eyes on a set of penny-size tubes of fossil bone at the National Museum of Natural History in Paris in 2008, he was struck by the unusual shape and varying sizes of these inner ear structures, called semicircular canals. Paleontologists have long used them to infer ancient animals’ movement patterns, but David and his colleagues today apply them to another mystery: They argue that warm-bloodedness first arose in mammal ancestors likely about 233 million years ago in the Late Triassic, 20 million years later than previously thought. 

“It’s quite exciting,” said Jasmina Weimann, a molecular paleontologist at the California Institute of Technology who wasn’t involved with the study. “It is the first time that any relationship between canal shape and temperature has been proposed,” she says, adding that the new method also offers a promising way to trace the evolution of long-extinct animals.

Warm-bloodedness, or endothermy, is a trademark of living mammals and birds—and apparently some dinosaurs. It allows a more constant body temperature in the face of outside temperature fluctuation. This requires more energy, but it allows animals to move at night and in the cold, which cold-blooded (ectothermic) animals cannot do. Paleontologists agree that early vertebrates were all cold-blooded, but they’ve debated when warm-bloodedness arose.

Warm-blooded animals usually have faster growing bones and sport fur or feathers for insulation, so researchers have studied those features to estimate when mammals’ journey to endothermy started. But those measures aren’t exact proxies and could have first evolved for other reasons.

Enter David, now a paleontologist at the Natural History Museum in London, with a new idea about semicircular canals: After doing his Ph.D. on the structures, he noted that mammals have smaller semicircular canals for their body size than other vertebrates. “The whale shark is actually the animal with the largest semicircular canals on Earth, much larger than the canals in whales,” David says. “I thought, ‘Maybe this has to do with body temperature.’” 

He was also struck by the importance of the fluid inside the canals, endolymph, which changes in viscosity depending on temperature. Like oil in a hot pan, it becomes thinner as it heats up and thicker as it cools. David suspected that endolymph viscosity and semicircular canal size were related, and that both could reflect endothermy. 

To test that hypothesis, David and his team examined the ear canals of 277 living animal species including alpacas, turkeys, and lizards. Their results confirmed David’s suspicions: The warm-blooded animals had less viscous endolymph and smaller and thinner canal shapes. Cold-blooded animals had thicker endolymph and larger, thicker canals.  

The team then applied this knowledge back to fossil specimens to figure out when endothermy evolved. Although the ear canal, because it’s made of soft tissue, doesn’t often fossilize, the bony tubes that encompass these canals do, and can be used to infer canal shape. The researchers examined 64 extinct species, including mammals, mammallike ancestors from more than 230 million years ago called mammaliamorphs, and much older ancestors known as nonmammalian synapsids. The team was able to trace when animals with smaller, thinner ear canals appear: in the Late Triassic, about 233 million years ago—just about the time when mammaliamorphs evolved from nonmammalian synapsids, the researchers report today in Nature. This change happened relatively suddenly, over less than 1 million years, says co-author Ricardo Araújo, vertebrate paleobiologist at the Institute for Plasmas and Nuclear Physics of the University of Lisbon’s Higher Technical Institute. “We are not only saying when endothermy starts, we can also say when ectothermy [in mammalian ancestors] ends.”  

Endothermy starting with mammaliamorphs makes sense, David says: The beginning of mammaliamorphs also corresponds with the evolution of traditionally mammalian features, such as whiskers and backbone flexibility for grooming fur. “It makes sense that [endothermy] happens at more or less the same time when mammal ancestors become more and more mammallike.”  

Some researchers had previously speculated that nonmammalian synapsids, which include the famous sail-backed behemoth Dimetrodon, were also warm-blooded. The new finding puts the “nail in the coffin” of that idea, Araújo says.

However, the study is one piece of a larger unfinished puzzle, cautions Hans Straka, a neurobiologist at the Ludwig Maximilian University of Munich. Endothermy did not just come out of nowhere, he notes, and more fieldwork needs to be done to prove the suddenness of the change. If a new fossil mammaliamorph were to be discovered from a different time period, for instance, the beginning of endothermy would change as well. “Endothermy, it’s not all or nothing,” he says. “There is no black and white transition, it’s a gradual thing.” 

But Straka agrees that ear canal bones become more prevalent in future endothermy studies, for example in the debate over dinosaur warm-bloodedness that has lasted decades. “It’s refueled the discussion,” Straka says. “And that’s good.”