As mentioned, multicellular organisms came about during the Cambrian period some 500 million years ago (Bronfman et al. 2016; Cisek 2019). A notion that is common to evolutionary biologists is that as the mass of an organism increases, the energy consumed per unit mass decreases according to a ¾ power (Figure 25). This relationship is known as Kleiber’s law (DeLong et al. 2010; Kleiber 1947; Wells 2007) and it is related to the ratio between the surface area of an object (i.e., the body) and its volume and to the central regulation of tissue metabolism by nervous and hormonal mechanisms (Kleiber 1947).[1] For a large organism to thrive (i.e., to reach its reproductive age, Melville’s Moby Dick, a Sperm whale, would have taken over 20 years of growth, Whitehead 2003)[2], the environment needs to be relatively stable for the largest organisms to mature; the dinosaurs of 64 million years ago went extinct when the environment underwent a radical change: an asteroid hit the earth which caused an ecological maelstrom (Alverez et al. 1979; Condie 2011; Kumar and Hedges 1998).[3] Hence, based on Kleiber’s law a multicellular organism is more energy efficient per unit mass than a single-cell organism and therefore evolution is biased in favor of promoting larger organisms with the limitation that this requires a period of ecological stability to support an extended maturation process. Indeed, the larger the organism the longer it takes to achieve full size, since adding new cells through mitosis takes time.

Invertebrate and vertebrate longevity is related to the total number of cells that need to be added during development via mitosis after insemination (Figure 26).[4] For example, the full development of a fruit fly takes two weeks, and it is composed of ~ 5 million cells at adulthood (Li et al. 2022); the full development of a human takes ~ 18 years (over 450 times longer), and it is composed of 30 trillion cells at adulthood (Hatton et al. 2023). Therefore, longevity scales with the total number of body cells required at sexual maturity (Tyshkovskiy et al. 2023). For the fruit fly, the rate of cell addition after insemination is 2.5 x 10^6 cells per week, whereas for the human, the rate of cell addition after insemination is 31 x 10^9 cells per week—note the ten-thousand-fold acceleration for humans, which is a property of ‘compound’ replication.[5] [6] Of course, the replication process in multicellular organisms is highly regulated (Barnea and Pravosudov 2011; Tyshkovskiy et al. 2023), otherwise an organism would be terminated by the willy-nilly reproduction of cells [e.g., Burkitt’s lymphoma is an example of unconstrained replication, such that the tumor doubles in size every 2 to 3 days displacing the organs of the gut if not treated (Tehovnik 2017)]. It is noteworthy that tissue-gene expression is a better predictor of longevity than is body weight (Tyshkovskiy et al. 2023). Thus, longevity is genetically programmed as are the other attributes that distinguish different species, such as the differential number of neurons between species; the more neurons the greater the presumed information capacity of a brain (Figure 27).

Vertebrates have existed since the Cambrian period and their basic brain structure—the telencephalon (which includes the hippocampus), the cerebellum, the brain stem, and the spinal cord has been preserved since this time. The lamprey, which is considered an ancient species with an evolutionary history of 450 million years (Grillner 2003), has all the component parts of a vertebrate brain (Figure 28). Birds and Mammals, as we understand them today, largely came about after the extinction of the dinosaurs 64 million years ago, and they are known to have a greater brain size to body size ratio than fishes, amphibians, and reptiles, including dinosaurs (Jerison 2009), and 500 million years ago the dominant species lived mainly in water since the land was not yet populated by plants (Darwin 1859). The augmentation of brain size in birds and mammals during the post-dinosaur period would have increased the overall energy consumption of an animal (Herculano-Houzel 2011), particularly as it relates to the telencephalon (see the enhanced relative size of the telencephalon as compared to other brain structures in birds and mammals, Figure 28), which may have been offset by a reduction in body size. Indeed, Kleiber’s law is operative in all animals, including those with an augmented telencephalon (Kleiber 1947; Kolokotrones et al. 2010; Moses et al. 2016).

As discussed, conscious operations are continuous during wakefulness and sustained by the mammalian telencephalon whose consumption of energy is 20 time higher per neuron than that of the neurons in the subcortex, and whose consumption is independent of whether an animal is mobile or immobile, a propensity that is not true of the neurons in the subcortex (Figure 7 of Chapter 4). It is unknown whether this fact about mammals generalizes to birds and other vertebrates (this needs immediate study—which could then be extended to the ganglia of invertebrates). Nevertheless, the telencephalon of birds, reptiles, amphibians and fishes, albeit relatively smaller than the other brain regions, is like the telencephalon of mammals in that it combines and stores sensorimotor information. For example, songbirds store learned melodies in the telencephalon and like humans they utilize the projections from the telencephalon that directly innervate the neurons in the brain stem to vocalize the melodies (Aboitiz 2018; Goldman and Nottebohm 1983; Rochefort et al. 2007; Simonyan and Horwitz 2011).[7] [8]The telencephalon of fishes contains neurons that are engaged in making sensorimotor associations and they store that information for long-term use (Gómez et al. 2016; Murray et al. 2017; Rodríquez-Expósito et al. 2017; Zacks and Jablonka 2023).

The telencephalon (or the neocortex) supports consciousness evolutionarily by increasing the number of cortical neurons as a function of developmental duration, the number of years between insemination and sexual maturity (see Figure 29). To demonstrate the compounding nature of neocortical mitosis, the total number of neurons present in adulthood can be divided by developmental duration. From the smallest to the largest mammals (i.e., from the mouse to the killer whale) this value varies from 0.08 x 10^9 neurons per year to 2.2 x 10^9 neurons per year, a 28-fold difference while holding time constant. This difference is because the base (or principal) before a replication is small for the mouse and large for the killer whale. In the case of the killer whale, it is composed of a large number of cells requiring protracted environmental stability, while taking advantage of Kleiber’s law of energy efficiency. But the large number of neurons in the neocortex should facilitate an animal’s ability to solve problems related to survival (this will be discussed in a future chapter).

For consciousness to prevail, one needs to appreciate the embodied brain, which requires never separating the brain from the body (Clark 1998; Gibson 1979; Tehovnik and Chen 2015; Varela et al. 1991) and which runs contrary to Dualism, a biologically-bankrupt philosophy that severs the brain from the body as originally conceptualized by René Descartes. Embodiment can be traced back 600 million years to our ancestor, the amoeba (Lahr et al. 2019), which is totally embodied (the organelles and genetic material controlling motivational states are enclosed by one outer membrane), and which existed well before multicellular organisms.

It has been suspected for over 100 years that even single-celled animals have some degree of consciousness, given that they must survive the elements by learning to find food and avoid predators (Hebb 1968; Koch 2013; Morgan 1900; Nakagaki et al. 2000; Saigusa et al. 2008), and this is despite their short lifespan of two days or so in the case of the amoeba, which reproduces asexually and whose genetic variability is assured by mutations caused by interactions with the outside world (Noble and Noble 2024)[9]. All animals have a living constitution or ‘state of mind’ that is modifiable over the duration of their short or long lives (Hebb 1968), and which can be transmitted to the next generation if the changes are stored in the sex or a-sex cells at the nuclear and extra-nuclear level. Be aware that an egg carries with it all the extra-nuclear material of a parent, material that has been modified by parental experience, since no cell is completely isolated from the outside world including the reproductive cells (Noble and Noble 2024). An amoeba unlike a multicellular organism can engage in unconstrained reproduction, since each cell is an independent unit, whose numbers are controlled by the resources of the environment (Rogerson 1980).

Now let us consider the comparative cognitive capacity (or comparative intelligence) between species. This should be evaluated according to the duration of a species’ evolutionary presence. Crocodiles have survived at least two extinctions (Platt et al. 2013), which translates into an earthly presence of at least 200 million years. Humans, on the other hand, with their tremendous capacity for discovery and innovation (but only in recent times) have been present for a fraction of this time of under 0.5 million years (Kimura 2003); based on all the environmental damage produced by humans thus far (Rockström and Gaffney 2021) it remains unclear whether they will match the crocodile, evolutionarily. Some in the AI world believe that machines are being given the capacity for too much intelligence (e.g., Hinton 2024). Perhaps, evolution has endowed humans with too much intelligence—and without upgrading their emotionality. Just open any copy of The Economist magazine and it will become clear that economic growth—at all costs—is a human obsession, which does not make us that different from the common ant, who also operates according to a growth-bust model of existence (Wilson 2012).

Summary.

1. Kleiber’s law indicates that as multicellular organisms get larger, the amount of energy utilized per unit weight (or per unit cell) diminishes. In a stable environment this propensity should encourage the evolution of larger organisms. Larger organisms, however, may not have enough time to reach sexual maturity before being terminated by an unstable environment, which was the case for the dinosaurs. An advantage of being a smaller organism (and thus having a short longevity) is that genes of a species can be altered rapidly, as experiments on fruit flies have illustrated (a species of choice by geneticists).

2. Consciousness is a property of mammals such that the neurons in the telencephalon exhibit a high and steady metabolic rate per neuron during wakefulness. The same needs to be verified for the telencephalon of other vertebrates.

3. The vertebrate telencephalon, which also includes the hippocampus, acts as a storage facility of learned information by altering the synapses during development and adulthood. The expression of the stored information through the motor system represents consciousness.

4. Going from the smallest to the largest mammals, the number of neurons in the telencephalon (i.e., the neocortex of mammals) exhibits an explosive addition of neurons. This addition requires protracted environmental stability, even though the large number of neurons should enhance an animal’s propensity to solve problems related to survival.

5. The embodied brain for the mediation of consciousness can be traced back to the amoeba.

6. The comparative intelligence of an animal should be assessed according to a species’ evolutionary longevity. A species that has overcome many mass extinctions, such as the crocodile, deserves to be called intelligent.

7. And yes, humans live in an egocentric universe, cognitively.

Footnotes:

[1] The optimal shape to reduce heat loss is a sphere, which has the lowest surface area to volume ratio, but most animals are not shaped like a sphere. And the least optimal shape is a flat plane with little volume. The surface area of humans is related to body mass by a 2/3 power (Kleiber 1947), and a slim person would have a higher power value than a stout person. And to hold heat in a body, an animal can grow body hair (as do dogs) or behaviorally alter the surface area of the body to decrease or increase heat transfer, and in the case of humans clothing can be used for this purpose. Regulating body temperature is part of an animal’s goal-directed behavior and is regulated by the hypothalamus (Mogenson 1977).

[2] Large animals might have fewer predators due to a size advantage, although a whale can be overcome by a well-organized group of humans (Melville 1851).

[3] On record, evolving multicellular organisms have endured five mass extinctions due to environmental collapse, and after each extinction it is presumed that evolution underwent an acceleration to fill in the ecological void created by the collapse (Condie 2011; Kumar and Hedges 1998). Homo sapiens are the result of such a collapse 64 million years ago.

[4] Note that some animals reproduce using asexual reproduction, but sexual reproduction is most common in the animal world for it enhances survivability by mixing the genes (Noble and Noble 2023).

[5] By the age of 18 there are 972 weeks of development: 36 weeks prenatally and 936 weeks postnatally. The rate calculation is based on a total number of cells at adulthood of 30 x 10^12, and this value was divided by 972 weeks to yield 31 x 10^9 cells per week.

[6] Something that distinguishes fishes, amphibians and reptiles from birds and mammals is that the former classes typically continue to grow (defined as the growth of the vertebral column) for the duration of their lives (i.e., these animals have in-determinant growth, Hariharan et al. 2016), whereas birds and mammals grow to adulthood after which they live out their life at a fixed size/length (Williams 2017), but the vertebral column of Kangaroos continues to grow throughout life albeit at a lower rate following adulthood. By having continuous growth, the sensors of the body must be under constant adjustment so that the produced movements over an animal’s lifespan are re-aligned with the sensors (this is why animals need frequent efference-copy updates, especially animals that are always growing: not surprisingly all vertebrates have a cerebellum to recalibrate the relationship between the senses and the motor systems). The maximal growth of fishes, amphibians, and reptiles is restricted by predation, disease, or natural disaster, whereas the maximal longevity of birds and mammals is restricted by these factors.

[7] The direct innervation of the brain stem nuclei by the telencephalon assures that the telencephalon has maximal control over the muscles (Simonyan and Horwitz 2011; Vanderwolf 2007), which is a requirement for any motor system requiring precision.

[8] Frogs exhibited the earliest vocalization 250 million years ago and the larynx (which contains the vocal cords in humans) existed in the lung fish 400 million years ago, even though they may not have been used for vocalization (Simonyan and Horwitz 2011).

[9] An amoeba unlike a multicellular organism can engage in unconstrained reproduction, since each cell is an independent unit, whose numbers are controlled by the resources of the environment (Rogerson 1980).

Figure 25. Energy utilization versus mass of an organism (i.e., for mammals, Kolokotrones et al. 2010; Moses et al. 2016). The larger the organism the less energy consumed per unit mass. Is this what drives evolutionary bigness? Cost: the bigger you are, the harder you fall during periods of resource depletion, which can lead to extinction. Note that the slope of a log-log plot yields the exponent of a power function (slope = [log10 y2/y1] / [log10x2/x1]).

Figure 26. An animal’s full weight varies with longevity (cf. the red bars with the blue bars). Obtained from Tyshkovskiy et al. 2023.

Figure 27. Information is plotted as a function of the number of neurons. Two cases are considered, connected (exp > 1) and disconnected neurons (exp = 1). Brains are interconnected which enhances information capacity in terms of storage and transmission. Figure modified from Tehovnik and Chen (2015).

Figure 28. The vertebrate brain is made up of the telencephalon (cerebrum that includes the hippocampus), the cerebellum, the optic tectum, and the olfactory bulb. Not labelled is the brain stem, and not shown is the spinal cord. The cerebellum in the lamprey and amphibian is small and therefore not marked; it sits on top of the brain stem. The telencephalon co-evolved with the cerebellum, since the two structures work in tandem for regulating sensation and movement and they are connected anatomically in all vertebrates (Cheng et al. 2014; Murakami 2017; Murray et al. 2017; Nieuwenhuys 1977; Xu et al. 2016). The sizes of the brains are not to scale.

Figure 29. Number of neocortical neurons is plotted as a function of developmental duration in years. Data on the various mammals was obtained from the following papers: Herculano-Houzel 2011 (mouse, rat, baboon, monkey, human), Herculano-Houzel et al. 2014 (elephant), and Ridgway et al. 2019 (killer whale). The data are fitted to an exponential equation (dashed curve).