When Did Consciousness Begin?

Consciousness may have originated with humans, mammals, fish, or bacteria.

Posted Jan 11, 2019

A provocative new book by Arthur Reber argues that bacteria are conscious and that the origins of mind are found in the simplest, single-celled organisms that arose billions of years ago. Here are some alternative answers to the question of when consciousness began. 

Hypotheses About the Origins of Consciousness

1. Consciousness has always existed, because God is conscious and eternal.

2. Consciousness began when the universe formed, around 13.7 billion years ago (panpsychism). 

3. Consciousness began with single-celled life, around 3.7 billion years ago (Reber). 

4. Consciousness began with multicellular plants, around 850 million years ago. 

5. Consciousness began when animals such as jellyfish got thousands of neurons, around 580 million years ago. 

6. Consciousness began when insects and fish developed larger brains with about a million neurons (honeybees) or 10 million neurons (zebrafish) around 560 million years ago. 

7. Consciousness began when animals such as birds and mammals developed much larger brains with hundreds of millions neurons, around 200 million years ago. 

8. Consciousness began with humans, homo sapiens, around 200,000 years ago.

9. Consciousness began when human culture became advanced, around 3000 years ago (Julian Jaynes).  

10. Consciousness does not exist, as it is just a scientific mistake (behaviorism} or a “user illusion” (Daniel Dennett). 

I think that #7 (consciousness began with mammals and birds) is currently the most plausible hypothesis, but the issue deserves examination. 

How Can You Tell Something Is Conscious? 

Except for your own introspection, consciousness is not directly observable, so it can only be inferred by an inference to the best explanation of the available evidence. This form of inference is common in science and everyday life, for example when scientists accept the existence of non-observable entities such as the Big Bang, electrons, forces, and genes because they provide a better explanation than alternative hypotheses of the full range of available evidence.

How do you know that you are conscious? One piece of evidence is that you feel you are conscious, but this might be a mistake, as behaviorists and some philosophers have argued. Fortunately, there is additional evidence that you are conscious, including your verbal reports of conscious experiences, and your complex behaviors such as ones related to pain, emotions, and imagery that can be explained by your having these conscious experiences. Moreover, there is beginning to be deeper neurological explanations of how consciousness comes about through interactions of numerous brain areas. So inference to the best explanation supports the hypothesis that you are conscious as superior to the alternative hypothesis that you only act as if you are conscious. 

The same form of reasoning supports the conclusion that other human beings are also conscious. You do not have direct access to the experiences of others, but you can observe their behaviors related to pain, emotion, and imagination, and you can hear their reports of conscious experience. Moreover, the brain structures and processes in other people are very similar to yours. Alternative explanations such as that other people are zombies without consciousness have no evidence to support them. Therefore, it is plausible that other people are conscious just like you. This is not just a weak argument from analogy, but an inference to the best explanation that relies on the fact that the evidence and explanations for the consciousness of other people are almost as convincing as the arguments for yourself.

The evidence for consciousness in nonhuman animals is weaker, because they cannot report their conscious experiences. In a previous blog post, I conducted a debate on whether animals have emotions, which is difficult because there are alternative explanations for why animals such as cats and dogs seem to have emotions. Maybe their apparent happiness is just reward-related behavior, and maybe their apparent fear is just threat-related behavior.

However, as I will report in a future blog post, I have become convinced that grief is widespread in mammals such as elephants, chimpanzees, and dogs, where their actions are too complicated to be explained by simple behavioral accounts. Therefore, I now think that the best explanation of mammal behaviors related to pain, pleasure, and complex emotions is that they have conscious experiences. The same arguments apply to big-brained birds such as ravens and parrots that are capable of complex problem solving and learning.  

The evidence becomes much sparser if you move down to smaller-brained animals such as bees and fish. Honeybees do exhibit reward-related behaviors, and fish exhibit pain -related behaviors, but it is not at all clear that these require an explanation based on conscious experience. At best we can put a question mark beside hypothesis #6. Similarly, simpler animals such as jellyfish and even plants can show behaviors such as sensing, reacting to sensory inputs, and signaling in response to environmental influences, but there are simple, stimulus-response explanations of what they were doing that do not require the attribution of consciousness.


So why does Reber think that bacteria are conscious? He correctly notes that single-celled organisms have powerful ways of sensing their environments to detect sources of food and toxicity. Moreover bacteria live in biofilms of large numbers of individuals that communicate with each by secreting chemicals that spread important environmental information about food and toxins. Bacteria are capable of moving individually and collectively to get closer to food and farther from toxic substances. Perhaps sensing, reacting, communicating, and moving are best explained by the hypothesis that bacteria have some degree of consciousness. 

But machines are also capable of sensing, reacting, communicating, and moving—for example, the self-driving cars that are being developed by Google, Uber, General Motors, and other companies. Reber thinks not only that such machines are not currently conscious, but that they never could be, because he accepts the discredited thought experiment  of John Searle that artificial intelligence is impossible because the symbols used by machines are inherently meaningless. Christopher Parisien and I argued a decade ago that self-driving cars are capable of semantics in the same way as human brains, through interacting with the world and learning about it. So machines that interact with the world can have meaningful representations even though they do not yet have consciousness. 

Engineers know exactly how self-driving cars work because they built them, and can explain their operations without invoking consciousness. Self-driving cars do not display behaviors such as pain, emotions, and imagery that consciousness helps to explain in birds and mammals. Self-driving cars and even thermostats refute Reber’s claim that when an event is sensed it is felt.

Another oddity of Reber’s view is that he thinks that plants, which evolved from single-celled organisms, lack consciousness, even though they are capable of sensing, reacting, signaling other plants, and reorienting themselves toward the sun. 

Reber's main reasons for attributing consciousness to single-celled organisms are not that it provides the best explanation of the available evidence, but rather that this attribution solves philosophical problems. He thinks that his theory of the cellular basis of consciousness provides the most plausible answer to the problem of emergence. Consciousness is a property of objects very different from simple properties such as consisting of atoms and molecules, or even firings of neurons, so all of hypotheses 2-9 face the problem of figuring out how consciousness became a property of wholes when it is not a property of their parts or a simple aggregate of the properties of their parts. 

Fortunately, there are new theories of how consciousness could emerge as a property of large numbers of individual neurons even though it is not a property of individual neurons. Stanislas Dehaene thinks that emergence comes from the broadcast of information across brain areas, whereas I argue in my new book Brain-Mind that the key properties are patterns of firing of neurons, binding of these patterns into more complex patterns, and competition among the resulting patterns. 

Both of these hypotheses about the emergence of consciousness in large brains have the advantage that they attribute consciousness to just those organisms for which there is evidence concerning pain,  emotions, and imagery. We have no reason to attribute pain, emotions, or imagery to bacteria, so the attribution of consciousness is superfluous.

Another philosophical reason that Reber gives for his cellular basis of consciousness is that it provides a solution to the philosophical "hard problem" of consciousness: there is something that it is like to be conscious. But Reber's view does no better than others in accounting for the feeling aspects of consciousness, which can better be handled by breaking the problem down into specific aspects of pain and specific aspects of emotion. Without wallowing in the vagueness of "what it is like", specific aspects of conscious experience of emotion and imagery can be given neural explanations, as I show in Brain-Mind.

Therefore, Reber’s theory of the cellular basis of consciousness helps little with the philosophical problems of emergence and experience. Given his appreciation of scientific evidence, he should be able to recognize that the evidence for consciousness in single-celled organisms is much worse than the evidence for consciousness in self-driving cars, which already exhibit much more complex sensing, reacting, moving, and communicating than bacteria. Moreover, there are progressing alternative hypotheses of how consciousness emerges through the complex operations of large brains capable of representing the world, learning about it, representing representations, and communicating with other brains. 

Although I think that Reber is wrong about bacterial consciousness, I recommend reading his book. It is full of interesting scientific information, trenchant discussions of important issues, and entertaining stories.  Sometimes mistaken ideas can contribute to intellectual progress.


Reber, A. S. (2019). The first minds:  Caterpillars, ‘karyotes, and consciousness. New York: Oxford University Press.

Thagard, P. (2019). Brain-mind: From neurons to consciousness and creativity. New York: Oxford University Press.