Did warm-bloodedness pave the path to sentience?

We feel, therefore we are. Conscious sensations ground our sense of self. They are crucial to our idea of ourselves as psychic beings: present, existent, and mattering. But is it only humans who feel this way? Do other animals? Will future machines? Weaving together intellectual adventure and cutting-edge science, neuropsychologist Nicholas Humphrey describes in his book “Sentience” his quest for answers: from his discovery of blindsight in monkeys and his pioneering work on social intelligence to breakthroughs in the philosophy of mind. In the following excerpt from “Sentience,” he challenges traditional explanations for the evolution of sentience in mammals and birds, proposing that warm-bloodedness may have played a crucial role.

Birds and mammals have in common a physiological feature that distinguishes them from all other animals: They are warm-blooded. That’s to say, they maintain a constant body temperature higher than the surroundings, typically 37 degrees Celsius (98.6 degrees Fahrenheit) for mammals and 40 degrees Celsius (104 degrees Fahrenheit) for birds.

I propose that warm-bloodedness played a double role in the evolution of sentience: On the one hand, it brought about changes in lifestyle that made sentience an essential psychological asset; on the other hand, it prepared the brain to deliver it.

A quick primer about warm-bloodedness. In both mammals and birds, it’s a physiological state that is achieved by generating heat internally and having an insulating coat of fur or feathers to prevent heat loss. Fossil evidence shows that this capacity evolved independently in dinosaurs, the ancestors of birds, and cynodonts, the ancestors of mammals, at about the same time, 200 million years ago, during a period of major climatic upheavals.

Being warm-blooded is expensive. Maintaining a constant high temperature requires a big expenditure of energy. At 37 degrees Celsius, the human body is warmer than the mean annual temperature of any habitat on Earth. To keep this up, a human must eat nearly 50 times more frequently than a boa constrictor of equivalent size and consume up to 30 times more calories overall. Given such costs, there must have been big advantages or the trait would never have evolved.

In fact, the advantages are several. For one thing, as temperature goes up various bodily processes actually become more energetically efficient, so the costs can be partially offset. In particular, the cost of sending an impulse along a nerve decreases until it reaches a minimum at about 37 degrees Celsius. The result is that, although the overall running costs for the body go up with being warm-blooded, the costs for the brain are reduced. This means that mammals and birds can support larger and more complex brains with relatively little extra outlay of energy.

A separate advantage is that warm-bloodedness provides a defense against infections by fungi and bacteria. Cold-blooded animals such as insects, reptiles, and amphibians are plagued by fungal infections. But very few parasitic fungi can survive above 37 degrees Celsius. This means that mammals and birds are now largely free of them.

However, the fact that warm-bloodedness evolved when it did, at the same time in both classes of animal, when environmental temperatures were swinging wildly, suggests that the primary advantage was neither of these but rather the more obvious one that it allowed animals to ride out climatic changes and expand their geographic range.

Cold-blooded animals not only have to stay within relatively narrow geographic limits but also have their activity levels dictated from moment to moment by the ambient temperature. As the sun sets, or goes behind a cloud, the body of a cold-blooded animal such as a lizard chills and its muscles and nerves slow down; when body temperature drops too far, it becomes lethargic. By contrast, warm-blooded animals take their environment with them and so can be alert and active — feeding, socializing, traveling — both by day and night, winter and summer, high in the mountains or down on plains. The fossil record shows that at the time warm-bloodedness evolved many cold-blooded species, unable to cope with the fluctuating temperatures, became extinct.

As Claude Bernard put it, in his famous adage: “La fixité du milieu intérieur est la condition de la vie libre.” The constancy of the internal environment is the condition for a free life.

Now, what interests me is what a “free life” means, not only for the body but also for the mind. As the bodies of warm-blooded animals became more autonomous, self-reliant, and self-contained, I imagine their sense of self did too. After millions of years in which their ancestors had their lives constrained by environmental temperature, they found themselves, as it were, let off the leash. In body and in mind, they were becoming increasingly autonomous agents, with the freedom to go where they would when they would.

As the bodies of warm-blooded animals became more autonomous, self-reliant, and self-contained, I imagine their sense of self did too.

I hear William James, celebrating the individuality of human minds: “Absolute insulation, irreducible pluralism, is the law. It seems as if the elementary psychic fact were not thought or this thought or that thought, but my thought, every thought being owned.” But insulation as a feature of the mind will very likely have begun with insulation as a feature of the body. Indeed, here’s James again:

Our entire feeling of spiritual activity, or what commonly passes by that name, is really a feeling of bodily activities whose exact nature is by most men overlooked… To have a self that I can care for, nature must first present me with some object interesting enough to make me instinctively wish to appropriate it for its own sake.

A warm-blooded body is an object that must have been considerably more interesting to — and worth appropriating by — the self than a cold-blooded one.

Subscribe for counterintuitive, surprising, and impactful stories delivered to your inbox every Thursday

But this was just the half of it. I believe the change that warm-bloodedness brought about in attitudes to the body and self was about to be amplified by what was happening at the level of brain physiology.

I’ve said little so far about what exactly might be required at the level of nerve cells to generate the attractors that are responsible for phenomenal consciousness. I won’t pretend I’m ready to provide a detailed anatomical and neurophysiological model. Nonetheless, if I had to suggest an evolutionary change to the brain that would be conducive to establishing the feedback loops that create the ipsundrum [a self-generated conundrum], it would be (a) an increase in the conduction speed of nerve cells, effectively shortening the loops and putting motor and sensory areas of the brain closer in touch; coupled with (b) a decrease in the refractory period (the time-out) following a nerve cell firing, so that the cell can join in cyclical reactivation.

What a coincidence then, that an increase in the temperature of the brain would have been bound to have both these effects. It’s a well-established fact of physiology that the functional characteristics of neurons change with temperature. It’s been found for a range of animals — warm and cold-blooded — that the conduction speed for all classes of neurons increases by about 5 percent per degree Centigrade, while the refractory period decreases by roughly the same amount. This implies that when the ancestors of mammals and birds transitioned from a cold-blooded body temperature of, say, 15 degrees Celsius (59 degrees Fahrenheit) to a warm-blooded temperature of 37 degrees Celsius, the speed of their brain circuits would have more than doubled.¹

We’ve remarked already on the “lucky accidents” that have, at several points, played a part in the evolution of sensations. If warm-bloodedness played these key roles, first in changing the way animals thought about the autonomy of the self, second in preparing the brain for phenomenal consciousness, here was an accident as lucky as they come.

Cometh the hour, cometh the brain.²

Nicholas Humphrey, Emeritus Professor of Psychology at the London School of Economics, is a theoretical psychologist based in Cambridge, who studies the evolution of intelligence and consciousness. He was the first to demonstrate the existence of “blindsight” in monkeys. He has also studied mountain gorillas with Dian Fossey in Rwanda, proposed the celebrated theory of the “social function of intellect,” and investigated the evolutionary background of religion, art, healing, death-awareness and suicide. His honors include the Martin Luther King Memorial Prize, the Pufendorf Medal, and the International Mind and Brain Prize. His most recent books are “Seeing Red” (Harvard University Press), “Soul Dust” (Princeton University Press), and “Sentience,” from which this article is excerpted.