Skip to main content

Verified by Psychology Today


The Evolutionary Origins of Consciousness

Part 2: The ancient beginnings of subjective experience.

Source: Dottedyeti/AdobeStock
Creatures of the Cambrian period, underwater scene.
Source: Dottedyeti/AdobeStock

This is Part 2 of a five-part series on the evolutionary origins of consciousness. I encourage you to read Part 1 first, for the overall context. Parts 2 to 5 look in a little more detail (but still necessarily in a summarized manner) at each of six books by scientists focusing on this intriguing, fundamental way of understanding consciousness. Here, in Part 2, we will discuss the theories of coauthors Todd Feinberg and Jon Mallatt.

In The Ancient Origins of Consciousness: How the Brain Created Experience,1 psychiatrist-neurologist Todd Feinberg and evolutionary biologist Jon Mallatt argue that an understanding of evolutionary biology is central to an understanding of consciousness. In this post, we will briefly summarize the evolution of nervous systems, as detailed in their book, and then focus on their discussion of how the increasingly complex and unique features of nervous systems made the emergence of consciousness and subjective experience possible.

Feinberg and Mallatt theorize that primary consciousness, also called sensory consciousness, originated in the first vertebrates, as well as, via independent evolutionary paths, in cephalopods (e.g., octopus, squids) and arthropods (insects, crabs, etc.). That’s a lot earlier than most people assume. Primary consciousness is the most basic form of consciousness—the consciousness of having any kind of experience at all—feeling “something that it is like to be” (as formulated by the philosopher Thomas Nagel in his famous essay “What Is It Like to Be a Bat?”2), being aware of stimuli, aware of the environment, but with no reflection at all—just the basic ability to have subjective experience.

Actually, as discussed in Part 1, the precursors of consciousness go back much further still: even single-celled organisms have molecular receptors on their cell membranes that can recognize the difference between "self" and "non-self," enabling those organisms to detect and approach sources of nutrition and withdraw from noxious stimuli. Single-celled organisms, of course, do not have a nervous system. In those organisms, the pathway from sensory input to motor response is simple, direct, and reflexive, with few intermediate steps.

Evolutionary pressure to develop a nervous system arose in multicellular organisms, where signals from sensory inputs had to be transmitted to more distant motor organs. In the most primitive multicellular organisms such as sponges, which first evolved about 600 million years ago (mya), those signals were probably mediated by signaling molecules similar to hormones or neurotransmitters. Somewhat later (still in the approximately 600 mya range), in primitive jellyfish-like animals, simple nervous systems evolved, enabling faster, more specific, and more direct transmission of signals from sense organs to motor organs. Those early nervous systems were still nothing like a brain. They were not centralized—they were just diffuse nerve nets.

The first brains, and their precursors

Precursors to brains evolved in primitive worm-like creatures in the Cambrian period, beginning around 540 mya. Neurons became clustered together, with more interconnections. With this clustering of neurons, the links between sensory inputs and motor outputs became separated by intermediate layers of nerve cells. These intermediate nerve cells with their rich interconnections allowed more extensive processing of the sensory inputs, even combining information from different senses (simple light detection, touch detection, and sensed chemicals) to help calculate and signal a motor output. These intermediate neurons also allowed the storage and recall of experience and thus learning and memory.3

Predatory behaviors first emerged in the Cambrian, leading to an evolutionary "arms race," in which natural selection drove the development of bigger and better brains, more acute senses and more rapid motor responses and coordination. In that more complicated competitive environment (still in the ocean), populated by predators and prey, more complicated senses were powerfully selected for—senses that could more effectively and efficiently detect those other animals. What had been simple light-detecting cells became organized into camera-like eyes that recorded full images for true vision; what had been simple chemical-detecting cells elaborated into sense organs for smelling and for tasting; and so on. And to process all that complex sensory information, what had previously been fairly simple reflexes and hierarchies of reflexes became more complicated, leading to the ability to form sensory images of the environment (internal representations), because that’s the most efficient way to organize all this sensory information about a food source, predator or mate. This led within several million years (still during the Cambrian) to the first brains and the first elemental forms of consciousness—in the limited sense defined earlier for primary or sensory consciousness.

Internal representations

Feinberg and Mallatt argue that the ability to create images (internal representations) is a key feature of primary consciousness.4 Vision may have been the first and most important of the distance senses to evolve, and may have laid the groundwork for the production of mapped/topographic images of the external environment, which may have formed the basis for sensory consciousness (primary consciousness). Eel-like lampreys might be an example of fairly early vertebrates with those kinds of early brains. All fish, amphibians, reptiles, birds, and mammals can build such maps or internal representations of the outside world in their brains.5

As brains became more complex and elaborately connected, sensory consciousness became further enriched, with internal representations not only of the external environment but of the self—a mapping in the brain of the body and its actions.

Types of awareness

Feinberg and Mallatt describe three domains of consciousness, defined by the ability to map the external and internal world, and to have emotional experience:

  1. Exteroceptive awareness: awareness of the external world; based on forming images or internal representations of the external world.
  2. Interoceptive awareness: awareness of the animal’s internal state, e.g., sensations from internal organs.
  3. Affective awareness. This precursor of emotion is basically the animal’s "likes" and "dislikes"—the attachment of valence or value to an experience. (This can be demonstrated by behavioral measures based on operant conditioning, which is trial-and-error learning through which the strength of a behavior is modified by reinforcement or punishment.) Affective awareness is the basis of emotion in higher mammals. Feinberg and Mallatt make the case that even early vertebrates such as lampreys that have image consciousness have "likes" and "dislikes" and therefore affective consciousness.

Prerequisites for consciousness

Feinberg and Mallatt identify three sets of defining features of consciousness (all these lists are of course fully explained and substantiated in their book):

  1. General biological features of all life (not listed here): these are features present in all living things, not just conscious animals. These general biological features are a necessary prerequisite for consciousness to evolve but are of course insufficient on their own. Additional neurobiological features are required:
  2. Basic features of nervous systems—neuronal reflexes and simple core brains.
  3. Special neurobiological features. For animals to be capable of primary (sensory) consciousness, they require the evolution of special neurobiological features that can create images and affects. These features include brains composed of large numbers of neurons with great neuronal complexity and inter-connectivity, elaborated sensory organs, neural hierarchies with lots of back-and-forth communication between the levels (and within the levels), neural pathways that create mapped mental images or affective states (valence), attentional mechanisms, and memory.

Explaining subjectivity

In their second book, Consciousness Demystified6, Feinberg and Mallatt elaborate on the model summarized above. They delve into the question of what gives consciousness its subjective quality. They identify four explanatory gaps in consciousness, which they refer to as "neuro-ontologically" subjective features. They provide explanations in their book bridging each of these presumed gaps:

  1. Referral: the brain always refers sensations to where they were sensed outside of the brain. We don't experience anything as coming from inside our brain, even though that’s where the sensations are generated.
  2. Unity: things are experienced in a unified, integrated manner, not as disjointed information processing.
  3. Causation: mental events have causal power, causing action that affects the outside world.
  4. Qualia: the very subjective quality of conscious experience, the feeling of, say, the redness of seeing red.

They argue that qualia cannot be separated from life itself, qualia owe their characteristics to the fact that they are neurobiologically unique, and to their being exclusively first-person, i.e. exclusively and entirely generated within the nervous system of that animal:

"Qualia are now seen as integrated living processes of certain complex brains. Whether we are talking about such neurobiologically diverse feelings as red, pain, hunger, or happiness, qualia cannot be dissociated from life processes in general. Qualia in this sense are alive in the same way that a cell is alive, or a heart is alive, or a person is alive. When viewed in this light, the widely varying special [neurobiological] features that are essential to the creation of images and affects […] are still living systems and processes." (p. 112)

"We hypothesize that experience and qualia are living processes that cannot be explained solely by a non-biological computation, and our view of the hard problem [explaining the very subjective quality of conscious experience] begins and rests on the essential role that biology plays in making animal experience and qualia possible." (p.119).

The subjective-objective divide

Feinberg and Mallatt’s goal in Consciousness Demystified is to try to explain subjective experience or the uniqueness of subjectivity in relation to what the brain does. They further tackle the subjective-objective barrier by explaining why we can't access our own brain processes, and why subjective experience is not accessible from outside. They refer to this as auto- and allo-ontological irreducibility:

  • Auto-irreducibility means that the subject has access to its own referred conscious experiences (see “Referral” above), but no access to its own neurons that create the experience.
  • Allo-irreducibility, conversely, means that an outside observer can objectively access the workings of the subject’s neurons but not the subject’s conscious experiences (as in the case of a brain scientist studying the subject’s brain).

It's worth noting that subjectivity is built into the very nature of life: as soon as the first cell evolved there was an inside and an outside, and therefore the beginnings of a subjective-objective divide between the body and the outside world. There is no physicalist/materialist principle that says you can’t have a divide between the interior and exterior world, with the interior being inaccessible to the exterior. There’s no need for mystery or mysticism in trying to understand this divide and why the interior of an animal’s experience is inaccessible to an objective observer.

Which speaks to Feinberg and Mallatt’s larger point: there’s no need to invoke magic (supernatural/paranormal phenomena) in explaining consciousness, once you have a solid grasp of biology and neuroscience.

Part 3 of this five-part series looks at the further insights provided by neuroscientist Joseph LeDoux into the evolution of consciousness.


1. Todd E. Feinberg and Jon Mallatt, The Ancient Origins of Consciousness: How the Brain Created Experience (Cambridge, MA: MIT Press, 2016).

2. Nagel, T. (1974). What Is It Like to Be a Bat? The Philosophical Review, 83(4), 435-450.

3. As mentioned above and discussed in Part 1, more elementary forms of learning and memory in the form of conditioning and molecular recognition occurred also in simpler organisms than these—this can be demonstrated in such organisms today. It’s important to relinquish our human preconceptions of what learning and memory are—there are very elementary, molecular mechanisms in very simple organisms that count as learning and memory. Indeed, the molecular mechanisms, the building blocks, of learning and memory are essentially the same in humans as in the humble sea snail [See Eric Kandel, “The Molecular Biology of Memory Storage: A Dialogue between Genes and Synapses,” Science 294, no. 5544 (2001)]. The vastly superior power of the human brain is partly just the result of the combinatorial complexity in which these simple molecular mechanisms have been arranged by evolution.


4. See Part 1, and the post “What Actually Is a Thought? And How Is Information Physical?” for further explanation of how internal representations are physical phenomena.

5. This takes place in the optic tectum of the midbrain in fish and amphibians, and in the cerebral cortex in reptiles, birds and mammals (reptiles are intermediate in the evolution of cerebral cortices; the tectum still plays a major role in their processing of sensory input) [See Part 5 of this blog series for discussion of the central importance of the tectum in attentional processes]. Most neuroscientists consider consciousness to be dependent on the cerebral cortex and therefore consider only birds and mammals to be capable of it. By arguing that primitive elemental forms of sensory consciousness are based on the ability to form internal representations from sensory perception, Feinberg and Mallatt push that capacity further back in evolution to apply to all vertebrates, and also to cephalopods and arthropods.

6. Todd E. Feinberg and Jon M. Mallatt, Consciousness Demystified (Cambridge, MA: MIT Press, 2018).

More from Ralph Lewis M.D.
More from Psychology Today