If there’s a single certainty in science, it’s that debate never stops. Scientists of all stripes have specific opinions of what they think happened, when it happened and how it happened, and they’re nothing if not passionate — at least until the evidence comes in.
This is where (until recently) research found itself with a tricky question: How did warm-blooded animals evolve? While we’ve been around for a while, exactly how long we’ve been strutting our hot stuff has been up for debate. Now, thanks to some clever thinking on the part of researchers, it’s possible to pinpoint the preponderance of endotherms on our planet within a million years or so.
Spoiler alert: It’s about 20 million years later than previously thought.
A Tale of Two Temperatures
When it comes to staying comfortable, animals fall into two broad categories: endotherms and ectotherms.
- Endotherms, also known as warm-blooded animals, include mammals and birds. As noted by the Ecology Center, endothermic animals use internal biological structures and processes to regulate their body heat and keep it consistent, or in a state of homeostasis. This allows endotherms to survive in a variety of climates since their internal body temperature remains the same. The caveat is the proportional need for energy — maintaining consistent body temperature requires a substantial amount of food as climates become more extreme.
- Ectotherms, or cold-blooded animals, take on the temperature of their environment. Reptiles and insects are good examples — lizards may use hot rocks in the sun to increase their temperature, while colonies of termites or bees may rely on the proximity of other insects to create a consistent climate. As a result, ectotherms are typically built to survive in narrow climate bands. Moving outside their desired temperature range may be possible but is rarely preferable.
It’s worth noting that there are exceptions to the rule. For example, ectotherms such as bumblebees may temporarily increase their internal body temperature by shivering their flight muscles, while endotherms like bears may reduce their temperature for a limited period of time during hibernation to cut the energy costs associated with core temperature regulation.
The Ears Have It
How did warm-blooded animals evolve? The answer was nearby the whole time: as noted by ScienceDaily, it’s all about tubes — three, tiny fluid-flowing tubes in animals’ inner ears. While these tubular travel trails have long been used to help understand the locomotive behaviors of fossil organisms, researchers realized that they could play another role: pinpointing the uptake of endotherms.
According to a paper published by Nature, ear canal fluid plays an essential part in balance. This fluid is why you can maintain your balance on a swaying ship or an uneven surface. When your body changes position, your ear fluid remains steady.
Both endotherms and ectotherms use ear fluid to find balance. While cold-blooded animals have more vicious fluid, the fluid of their warm-blooded counterparts is runnier. This difference set the stage for discovery. Given the high viscosity of ectotherms’ inner ear fluid, their ear canals were larger to accommodate the proper flow. The higher temperature and lower viscosity of endotherms fluid, meanwhile, meant smaller ear tubes were sufficient.
By measuring the sizes of both living and fossilized ear canals, researchers were able to reliably determine when the first ectotherms appeared on Earth.
Put It in Park(a)
After comparing the ear canals of 243 living and 64 extinct species, scientists had their answers: ectotherms arrived approximately 233 million years ago, or 20 million years later than previously thought. What’s more, this move to homeostatic temperature regulation occurred remarkably fast from an evolutionary perspective. Unlike previous postulations that endothermy appeared slowly over tens of millions of years, new data suggests it took less than a million years for endothermy to make the move from environmental outlier to ubiquitous characteristic.
When considered with other evolutionary changes from around that same time period, including the development of fur, whiskers and specialized backbones, the shift to internal heat generation makes sense, since fur and whiskers would help trap heat and regulate overall temperature. The scope of these changes may have been part of a larger adaption driven by the Late Cenozoic Ice Age, or Antarctic Glaciation, which began 33.9 million years ago and remains ongoing.
Here Come the Hot Steppers
The era of the endotherms has finally been identified. While it’s later than expected, it also shows a rapid uptake of internal temperature techniques that helped humankind adapt to the changing conditions of a hot-and-cold world.
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