Rosa Rubicondior: Creationism Refuted - The Transition From Cold-Blooded to Warm-Blooded Mammal Ancestors

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Mystery solved: when mammals’ ancestors became warm-blooded

If the transition from cold-bloodedness to warm-bloodedness is not a change in “kind” in the creationist sense, it is hard to imagine what would qualify. Creationists often try to dismiss major evolutionary transitions as mere “variation within a 'Kind'”, but the shift from ectothermy to endothermy was not some trivial adjustment. It was a profound physiological transformation that allowed animals to maintain a high, stable internal temperature, remain active across a wider range of conditions, and exploit ecological niches closed to their cold-blooded ancestors. Yet, according to creationist mythology, no such transition ever occurred, and there was never a point in time when it began.

Unfortunately for creationists, the evidence says otherwise. An international team of palaeontologists led by Ricardo Araújo of the Universidade de Lisboa, Lisbon, Portugal, Romain David of the Natural History Museum, London, UK, and Kenneth D. Angielczyk of the Field Museum of Natural History, Chicago, USA, believe they have identified when endothermy arose in the mammalian lineage. Their findings were published in Nature in July 2022. The team concluded that mammalian ancestors became warm-blooded about 233 million years ago, roughly 33 million years before the first true mammals appeared, and at about the same time that other recognisably mammalian traits such as fur and whiskers were evolving. The timing is consistent with evolutionary expectations that major innovations can arise in response to changing environmental pressures. [1.1]

They also concluded that this transition was rapid in geological terms, taking less than a million years rather than unfolding gradually over tens of millions of years, as had often been assumed. [2.1]

The researchers reached these conclusions by examining 3D models of the inner ears of dozens of mammalian ancestors, many from South Africa’s fossil-rich Karoo region. Karoo fossils are especially valuable because they preserve an exceptionally detailed record of synapsid evolution across almost 100 million years, documenting the transition from reptile-like therapsids to mammals. What the team focused on was the shape of the semicircular canals of the inner ear, which form part of the balance system. These canals are filled with endolymph, a fluid whose viscosity changes with temperature. As body temperature rose during the evolution of endothermy, the geometry of the canals had to change to keep the balance organ functioning properly. That gave the researchers a way to infer when warm-bloodedness first evolved in the mammalian line. [2.1]

Four of the authors of the paper in Nature also published an article in The Conversation explaining their results and their significance for understanding mammalian evolution. Their article is reproduced here under a Creative Commons licence, reformatted for stylistic consistency.

First, information about the Karoo fossils:

The Karoo Fossil Record. South Africa’s Karoo Basin is one of the world’s most important fossil regions because it preserves an exceptionally long and detailed record of life on land from the late Palaeozoic into the early Mesozoic. Spanning roughly 100 million years, it contains a near-continuous sequence of fossil-bearing rocks that allow palaeontologists to trace evolutionary change through time rather than relying on isolated finds.

The Karoo is especially famous for its wealth of therapsid fossils — the synapsid lineage that eventually gave rise to mammals. These fossils document, in remarkable detail, the transition from more reptile-like ancestors to increasingly mammal-like forms, making the region central to our understanding of mammalian origins.

It is also one of the best places on Earth for studying the end-Permian mass extinction and the ecological recovery that followed. The fossils show not only which animals disappeared, but also how new faunas emerged and diversified in the Triassic world that followed.

Because of this rich and orderly sequence, the Karoo provides one of the clearest records of major evolutionary transitions on land. Far from showing fixed, separately created “kinds”, its fossils reveal a long history of gradual change, extinction, adaptation and descent with modification.

Mystery solved: when mammals’ ancestors became warm‑blooded

Tritylodon, a therapsid, reconstructed as a night dwelling warm blooded animal.
Note the steam coming out of its lungs.

Illustrated by Luzia Soares

Julien Benoit, University of the Witwatersrand; Kenneth D. Angielczyk, University of Chicago; Ricardo Miguel Nóbrega Araújo, Universidade de Lisboa , and Romain David, Natural History Museum

Mammals and birds produce their own body heat and control their body temperatures. This process is known as endothermy, or warm-bloodedness, and it may be one of the reasons why mammals tend to dominate almost every global ecosystem. Warm-blooded animals are more active during both days and nights than their cold-blooded counterparts and they reproduce faster.

But until now it hasn’t been known exactly when endothermy originated in mammalian ancestry. Our new study, just published in Nature, changes that. A combination of scientists’ intuition, fossils from South Africa’s Karoo region and cutting-edge technology has provided the answer: endothermy developed in mammalian ancestors about 233 million years ago during the Late Triassic period.

The origin of mammalian endothermy has been one of the great unsolved mysteries of palaeontology. Many different approaches have been used to try to pinpoint the answer but they have often given vague or conflicting results. We think our method shows real promise because it has been validated using a very large number of modern species. It suggests that endothermy evolved at a time when many other features of the mammalian body plan were also falling into place.

Warm-bloodedness is the key to what makes mammals what they are today. Endothermy was likely the starting point where mammalness evolved: the acquisition of an insulating fur coat; the evolution of a larger brain, supplied with warmer blood; a faster reproduction rate; and a more active life are all defining mammalian traits that evolved because of warm-bloodedness.

Until now, most scientists had speculated that the transition to endothermy was was a gradual, slow process over tens of millions of years beginning near the Permo-Triassic boundary, although some suggested it happened closer to the origin of mammals, about 200 million years ago.

In contrast, our results suggest that it appeared in mammalian ancestors some 33 million years prior to the origin of mammals. The new date is consistent with recent findings that many of the traits usually associated with “mammalness”, such as whiskers and fur, also evolved earlier than previously expected. And according to our results, endothermy evolved very quickly in geological terms, in less than a million years. We suggest that the process may have been triggered by novel mammal-like metabolic pathways and the origin of fur.

Scientists’ intuition

Our research began with Dr Araújo and Dr David’s intuition about the inner ear. It is more than the organ of hearing: it also houses the organ of balance, the semicircular canals.

The three semicircular canals of the inner ear are oriented in the three dimensions of space. They’re filled with a fluid that flows in the canals as the head moves and activates receptors to tell the brain the exact three-dimensional position of the head and body. The viscosity, or runniness, of this fluid (called the endolymph) is critical to the balance organ’s ability to efficiently detect head rotation and aid balance.

The brain (pink) and inner ear (green) of a modern mammal, a primate, reconstructed in 3D.

Julien Benoit

In the same way as a piece of butter turns from solid to liquid in a warm pan, or honey becomes thicker when it is cold, the viscosity of the endolymph changes with body temperature. That means the endolymph’s viscosity would normally be altered by the evolution of a higher body temperature. But the body has to adapt because changing viscosity would prevent the semicircular canals from working properly. In mammals, the canals adapt to higher body temperature by changing their geometry.

The researchers realised that this change in the semicircular canals’ shape would be easy to trace through geological time using fossils. Pinpointing the species in which the change of geometry occurred would, they reasoned, provide an accurate guide to when endothermy evolved.

They needed fossils to test their hypothesis – and that’s where South Africa’s wealth of fossils from the Karoo region came in.

Reconstruction and study

The arid Karoo region preserves a treasure trove of fossils, many of them belonging to mammalian ancestors. These fossils offer an unbroken record of the evolution of life over a period of almost 100 million years. They document the transformation from reptilian-like animals (therapsids) to mammals in exquisite detail.

Using cutting edge CT-scanning techniques and 3D modelling, we were able to reconstruct the inner ear of dozens of mammalian ancestors from the South African Karoo and elsewhere in the world. From there we could point out exactly which species had an inner ear anatomy consistent with a warmer body temperature, and which ones did not.

What fossils reveal about the hairy history of mammals’ ancestors

One thing we had to take into consideration was the geographical position of the Karoo at the time when these animals lived. It was situated closer to the South Pole than it is now as a result of continental drift. That means the warmer body temperature suggested by the geometry of the inner ear cannot be due to an overall warmer climate. As the South African climate was colder on average, the change in inner ear fluid viscosity can only have been caused by a generally warmer body temperature in mammalian ancestors.

An exciting time

This is an exciting time for our field. Until now, to reconstruct the evolution of endothermy, scientists only had access to skeletal features that questionably correlated with warm-bloodedness. Every attempt was a long shot to get any accurate results. The inner ear, as this research shows, changes this. We believe it may be the key to unlocking more knowledge about mammalian ancestors in future.
Julien Benoit, Senior Researcher in Vertebrate Palaeontology, University of the Witwatersrand; Kenneth D. Angielczyk, Lecturer, University of Chicago; Ricardo Miguel Nóbrega Araújo, Junior Researcher, Universidade de Lisboa , and Romain David, Postdoctoral Researcher, Natural History Museum

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Published by The Conversation.
Open access. (CC BY 4.0)

Abstract
Endothermy underpins the ecological dominance of mammals and birds in diverse environmental settings^1,2. However, it is unclear when this crucial feature emerged during mammalian evolutionary history, as most of the fossil evidence is ambiguous^{3,4,5,6,7,8,9,10,11,12,13,14,15,16,17}. Here we show that this key evolutionary transition can be investigated using the morphology of the endolymph-filled semicircular ducts of the inner ear, which monitor head rotations and are essential for motor coordination, navigation and spatial awareness^{18,19,20,21,22}. Increased body temperatures during the ectotherm–endotherm transition of mammal ancestors would decrease endolymph viscosity, negatively affecting semicircular duct biomechanics^23,24, while simultaneously increasing behavioural activity^25,26 probably required improved performance²⁷. Morphological changes to the membranous ducts and enclosing bony canals would have been necessary to maintain optimal functionality during this transition. To track these morphofunctional changes in 56 extinct synapsid species, we developed the thermo-motility index, a proxy based on bony canal morphology. The results suggest that endothermy evolved abruptly during the Late Triassic period in Mammaliamorpha, correlated with a sharp increase in body temperature (5–9 °C) and an expansion of aerobic and anaerobic capacities. Contrary to previous suggestions^{3,4,5,6,7,8,9,10,11,12,13,14}, all stem mammaliamorphs were most probably ectotherms. Endothermy, as a crucial physiological characteristic, joins other distinctive mammalian features that arose during this period of climatic instability²⁸.

Araújo, R., David, R., Benoit, J. et al.
Inner ear biomechanics reveals a Late Triassic origin for mammalian endothermy. Nature 607, 726–731 (2022). https://doi.org/10.1038/s41586-022-04963-z

© 2026 Springer Nature Ltd.
Reprinted under the terms of s60 of the Copyright, Designs and Patents Act 1988.

What makes this research so awkward for creationism is that it captures a major evolutionary transition not as a vague just-so story, but as a datable, testable change in physiology recorded in a succession of fossils. Warm-bloodedness did not appear by magic, fully formed and detached from any ancestry; it emerged in a lineage that was already accumulating other mammalian features, such as fur and whiskers, under changing environmental pressures. That is exactly what evolutionary theory predicts: not sudden creation of fixed “kinds”, but descent with modification as populations adapt to new conditions.

The Karoo fossils are especially damaging to creationist claims because they preserve this transition in context. Instead of sharp boundaries between separately created forms, they reveal a long sequence of intermediate synapsids showing the gradual assembly of the mammalian body plan. The rise of endothermy was one more step in that process, and a profoundly important one, because it opened up new ecological opportunities and helped set the stage for the later success of mammals.

So, once again, the evidence points not to immutable “kinds” created in a single week a few thousand years ago, but to a deep history of evolutionary change stretching back hundreds of millions of years. The fossil record, comparative anatomy and modern imaging techniques all converge on the same conclusion: mammalian characteristics evolved piecemeal, over immense spans of time, in complete contradiction of creationist mythology and entirely in accord with the Theory of Evolution.

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