August 2, 2022 | Jerry Bergman

Warm-Bloodedness Not Explained by Evolution

How can you determine when something evolved
if you have no evidence that it evolved?


A new published study in Nature assumed that active thermoregulation
evolved, and then attempted to determine when it evolved.

by Jerry Bergman, PhD

Attempts to determine when warm-bloodedness in mammals evolved first require showing how the cold-blooded system could have evolved into the much more complex warm-blooded temperature regulating system. The contrast between warm- and cold-blooded animals is enormous, and all attempts to bridge it by some theoretical evolutionary scenario have so far failed. Temperature regulation is critical for life’s biochemistry for many reasons, especially for enzymes to function. Too cold, and enzymes work poorly if at all; too warm, and they can denature, rendering them nonfunctional. The loss of key enzyme function is lethal.[1]

Cold-blooded animals are largely incapable of regulating their internal body temperature except by moving to a warmer or cooler environment. Thus, the body temperature of cold-blooded animals fluctuates as they move within different temperature environments. As a result, cold-blooded animals will not survive in extreme temperatures, and for this reason must live in Earth’s temperate and tropical zones.

Examples of cold-blooded life-forms include insects, fish, reptiles, amphibians, and most invertebrates. Cold-blooded animals rely on three basic thermoregulation mechanisms: poikilothermy, ectothermy, and heterothermy.

  • Poikilothermy is where the animal’s internal temperature can vary, but its core temperature remains close to the same as the ambient (surrounding) temperature of its immediate environment.[2]
  • Ectothermy is where an animal can utilize external means, such as basking in the sunlight, to help control their body’s temperature.[3] Some crocodiles and other lizards live in the water during hot seasons. Then during the colder seasons migrate towards the land to burrow pits by the shore to keep warm.
  • Heterothermy is where the body temperature changes as the animal moves from one environment to another. Most cold-blooded animals utilize a combination of the above thermoregulation mechanisms.

Although it is commonly assumed that warm-blooded animals evolved from their cold-blooded cousins, some poikilotherms have more complex metabolisms than their warm-blooded counterparts. To cause a single chemical reaction to occur, some poikilotherms require up to ten different enzyme systems instead of one to operate in different temperatures.4 This system thus has a more complex genomic structure than warm-blooded animals occupying the same ecological niche. Thus the simple cold-to-warm evolutionary scenario is problematic from step one.[4]

Warm-Blooded Animals

Warm-blooded animals, also called homeothermic animals, include birds and mammals. They are capable of maintaining a nearly constant internal body temperature, usually from 35-40 °C, irrespective of the environmental temperature. Their body temperature  remains close to the same level even if the animal moves to another environment.

Internal mechanisms that help warm-blooded animals stay warm in colder areas, and cool in warmer areas, include metabolic regulation. Warm-blooded animals use both endothermy (physiological generation and regulation of body temperature by internal means, such as burning fat and panting) and homeothermy (relatively uniform body temperature maintained by sweating and shivering) for thermoregulation. Added to that, the ability to grow thick fur during winter and shed it during summer helps some mammals maintain a constant internal temperature in varying external temperatures.

No cold-blooded animal has sweat glands; only warm-blooded animals do. Humans have sweat glands throughout the body. Some mammals have localized sweat glands; dogs have sweat glands on their paws. They can lower their body temperature by panting to cool their tongue with evaporative cooling. During winter some mammals like bears maintain metabolic activities during hypothermia via a process called hibernation, or estivation during arid conditions.

Evolution of Thermoregulation

In an extensive literature review I was unable to find any credible evidence for the evolution of cold-bloodedness into warm-bloodedness. I did locate papers that described why warm-bloodedness is beneficial, and why it is important. But how could it evolve from a cold-blooded system?

Although “Endothermy underpins the ecological dominance of mammals and birds in diverse environmental settings….  it is unclear when this crucial feature emerged during mammalian evolutionary history, as most fossil evidence is ambiguous.”[5] The evolution from cold-bloodedness into warm-bloodedness requires significant modifications in both anatomy and physiology, a change that “is one of the most important in evolution.”[6]

The leading theory for why it evolved is that in “vertebrates at least, cold-blooded animals are aquatic and warm-blooded animals are primarily terrestrial.” The temperature differences in ocean and lake water is fairly consistent from hour to hour, but on land the temperature can change as much as 30 degrees Fahrenheit within a few hours. This explains why the alteration of the thermoregulation design is important for each type of animal to live in water or on land, but it does not explain how the warm-blooded trait could have evolved. A better explanation is that warm-blooded vertebrates were created to live in a terrestrial world, and cold-blooded vertebrates were created to live primarily in a water world as part of their original design.

An early evolutionary explanation was put forth by William A. White in 1891. He recognized that the movement from water to land would require enormous amounts of time to allow for warm-bloodedness to evolve. As evolution postulates, life first evolved in water, therefore was originally cold-blooded. For life to live on land, the warm-blooded design was required. Until this system was functional, terrestrial life would be impossible.

The attempted explanation is that cold-bloodedness sufficed on land until it gradually evolved into the warm-blooded system. This explanation for the transition has suffered from so many problems that it is now rarely mentioned. The main attempt to support evolution is to show that the more warm-blooded the animal is, the higher it is on the evolutionary scale. Reptiles are the least warm-blooded, man the most. The problem with this reasoning is that a sharp dichotomy exists between cold-blooded and warm-blooded systems—not a gradual progression.

Do Scientists Know the Exact Moment When Mammals Became Warm-Blooded?

Mammals maintain a constant internal temperature, called endothermy.

This brief review of the contrast between warm- and cold-bloodedness shows the lack of even a plausible scenario to explain how warm-bloodedness could have evolved from the very different cold-bloodedness design. Thus, a news article about the Nature paper on Live Science that claims scientists have pinpointed “the exact moment” in evolutionary time when mammals became warm-blooded is irresponsible hype.[7]

Other news articles about the paper were more cautious; one stating “Warm-Bloodedness in Mammals May Have Arisen in Late Triassic.”[8] The actual study published in Nature was far less confident about the results, stating that their research only “suggests that endothermy evolved abruptly during the Late Jurassic.”[9]  The review added: “it happened much more quickly than scientists expected.”

The Research Published in Nature

Araújo and colleagues used an X-ray scanning technique called microtomography to analyze the ear canal morphologies of hundreds of both modern and extinct vertebrates. They found that mammals, all which are warm-blooded, have smaller, thinner, and more circular ear canals compared to cold-blooded animals.[10] They then used these findings to evaluate the endolymph-filled semicircular duct shape of 56 extinct synapsid species and correlated this to body temperature. The conclusion was that inner ear biomechanics reveal a late Triassic origin for warm-blooded animals.

The results rely on the assumption that the ectotherm-endotherm transition is correlated with the changes they found for the circular ear canals. This change includes a decrease in semicircular endolymph viscosity. They reasoned that, because the endolymph fluid is more viscous in warm-blooded animals, their canal structures were very different than that employed in ectotherms. In view of the fact that mammals have very unique inner ears, the researchers attempted to retrodict by this indirect method the stage when the mammals’ ancestors first became endothermic. Supposedly this indicated when they switched from relying on external heat (cold-bloodedness) to regulating their body temperatures, generating their own heat as is employed today in warm-blooded (endothermic) animals.[11]


The chasm between cold-blooded systems and the warm-blooded system is enormous and has not been even theoretically bridged by evolutionists, even by “just-so” stories. Nonetheless, attempts have been made to determine when this great evolutionary leap occurred. The research reviewed in this paper suggested that changes in the inner-ear-canal morphology suggests that mammalian ancestors evolved warm-bloodedness about 233 million years ago during the late Triassic period. This evolutionary theory has many problems, including the fact that it does not apply to birds, which evolutionists believe evolved warm-bloodedness independently from mammals. That theory also lacks fossil or other evidence.


Learn amazing design facts about animals in this book by the author.

[1] Osilla, Eva V., et al. Physiology, Temperature Regulation. National Library of Medicine., 8 May 2022.

[2] Blattels, Clark. Physiology and Pathophysiology of Temperature Regulation. World Scientific, Hackensack, New Jersey, 2001.

[3] Cloudsley-Thompson, J.L. Hot blood or cold? Thermoregulation in terrestrial poikilotherms. Science Progress 56(224):499-509, October 1968.

[4] Richards, 1973.

[5] Araujo, Ricardo, et al. Inner ear biomechanics reveals a late Triassic origin for mammalian endothermy. Nature, 2022, p. 1.

[6] White, William Hale. A Theory to Explain the Evolution of Warm-Blooded Vertebrates. Journal of Anatomy and Physiology 25:374-385, 1891, p. 374.

[7] Turner, Ben. Scientists pinpoint the exact moment in evolutionary time when mammals became warm-blooded. Live Science., 2022.

[8] Carstens, Andy. Warm-Bloodedness in Mammals May Have Arisen in Late Triassic., 2022.

[9] Araujo, Ricardo, et al. Inner ear biomechanics reveals a late Triassic origin for mammalian endothermy. Nature, 2022, p. 1.

[10] Araujo, 2022.

[11] Araujo, 2022.

Dr. Jerry Bergman has taught biology, genetics, chemistry, biochemistry, anthropology, geology, and microbiology for over 40 years at several colleges and universities including Bowling Green State University, Medical College of Ohio where he was a research associate in experimental pathology, and The University of Toledo. He is a graduate of the Medical College of Ohio, Wayne State University in Detroit, the University of Toledo, and Bowling Green State University. He has over 1,300 publications in 12 languages and 40 books and monographs. His books and textbooks that include chapters that he authored are in over 1,500 college libraries in 27 countries. So far over 80,000 copies of the 40 books and monographs that he has authored or co-authored are in print. For more articles by Dr Bergman, see his Author Profile.

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