Synopsis

Short Summary (from the book jacket)

Deep History argues that the key to understanding important aspects of human behavior lies in viewing evolution through the prism of the first living organisms. By tracking the chain of the evolutionary timeline he shows how even the earliest single-cell organisms had to solve problems similar to those we and our cells do each day. Along the way, LeDoux explores our place in nature, how the evolution of nervous systems enhanced the ability of organisms to survive and thrive, and how the emergence of what we humans understand as consciousness made both our greatest and most horrendous achievements as a species possible.


Overview (from the Prologue)

When I mentioned to a friend that I was writing a book about the history of life, she asked, Why on earth are you doing that? She knew that I had spent most of my scientific career studying circuits in the rat brain that underlie behavioral reactions to danger, with an eye on how that informa- tion could help us understand at least some aspects of human emotions, especially fear and anxiety.

Part of the answer to my friend’s question is that if we really want to understand human nature, we have to understand its evolutionary history. As the biologist Theodosius Dobzhansky once said, “Nothing in biology makes sense except in the light of evolution,” and that includes behavior.

That behavior and evolution are interrelated is hardly a novel idea. Darwin emphasized it, as did pioneering ethologists such as Niko Tinbergen and Konrad Lorenz. The behaviorists who dominated psychology in the first half of the twentieth century paid little attention to evolution, but contemporary psychologists and neuroscientists accept it as a key factor.

Most efforts to understand the evolution of behavior, especially in neuroscience, typically focus on the relation between closely related groups, such as humans and other mammals. There are obvious reasons for doing so. For example, since the brain controls behavior, studies of how brains evolved in such groups help explain the evolution of their respective be- havioral repertoire, and also ours. But there are also good reasons to look deeper. Research comparing mammals (often rodents) and invertebrates (such as flies and worms) shows the connection between these, and is revealing how memory works in us.

For example, plasticity-related genes are shared between rodents and sea slugs, suggesting that they may each have inherited the ability to learn from a common ancestor that lived many hundreds of millions of years ago. But even more interesting, some of the same genes exist in single-cell protozoa. That’s relevant, since animals and current-day protozoa share a common protozoan ancestor that lived over a billion years ago. Some of the learning-related genes in our nervous system may therefore come to us via such microbial ancestors.

If you know anything about protozoa, you may be scratching your head regarding these findings. Most people, if they think about it at all, think of behavior, and especially learned behavior, as a product of a nervous system. But protozoa, being single-cell organisms, don’t have nervous systems, since that would require special cells—neurons—and they only possess one all-purpose cell. Yet they have a robust behavioral life—they swim away from harmful chemicals and toward useful ones—and they even use past experience to guide their present responses, suggesting that they have the ability to learn and remember. The logical conclusion is that behavior, learning, and memory don’t actually require a nervous system.

This was eye-opening to me, so I did a little research to see what was known about the behavioral capacities of these single-cell organisms. I found accounts not only of their swimming away from danger and toward nutrients, but also of moving toward or away from chemicals or sunlight to balance fluids or regulate temperature inside the cell relative to its environment. Protozoa even engage in mating behavior (sex) to reproduce their kind.

Protozoa are relatively recent single-cell organisms, having appeared about two billion years ago, when they evolved from another familiar single-cell creature—bacteria, which are the oldest living organisms, having emerged about 3.5 billion years ago. Bacteria exhibit many of the same kinds of behaviors that protozoa do, but did so first. They approach and avoid useful and harmful things in their world, and may even learn from experience what is useful and harmful in their world. They don’t, however, reproduce sexually; they simply divide in half. Sex is the behavioral claim to fame of eukaryotes, which evolved from bacteria, and which include protozoa and animals.

When animals engage in defensive, energy management, fluid bal- ance, and reproductive behaviors by freezing/fleeing, eating, drinking, and mating, scientists and laypeople alike often describe these activities as an expression of underlying psychological states—consciously felt experiences such as fear, hunger, thirst, and sexual pleasure. In doing so we effectively project our own experiences onto other organisms. Given how ancient these behaviors are, and how they arose long before nervous systems, we should probably be more judicious in making such attributions based on our mental states.

I will argue in this book that such survival behaviors have deep roots that date back to the beginnings of life. Animals later evolved neurons and circuits to make these behaviors more efficient and effective. But all organisms, whether composed of only one or of billions of cells, engage in these kinds of survival activities in the process of staying alive and well.

Because humans consciously experience feelings when we engage in our own survival behaviors, we intuit that these feelings and the behaviors must be intrinsically related—that the feelings are the causes of the behaviors. And because other animals close to us (other mammals, for example) behave like us in survival situations, and the circuits that control these behaviors are similar in us and them, their survival behaviors must be driven by their feelings.

But I will present evidence that turns this logic on its head. I will show you that there is indeed good evidence that the same brain systems control survival behaviors in humans and other mammals, but that these are not the systems that are responsible for conscious feelings we experience when we engage in such behaviors. The behaviors and feelings occur simultaneously, not because the feelings drive the behavior, but because their respective systems are responsive to the same stimuli.

Survival behaviors thus have very old roots that make them univer- sal. But the kinds of experiences humans call conscious feelings—that is, emotions—I propose are a much more recent development, possibly emerg- ing via evolutionary changes in the human brain a mere few million years ago that brought language, culture, and self-awareness to our species. This idea is likely to be controversial to some, perhaps many, since it seems to deprive animals of conscious experiences. But I hope that even those who are skeptical about this view of animal consciousness will hear me out.

I don’t, in fact, deny that animals have conscious experiences. My point, instead, is that any conscious experiences they have are likely to be very different from ours, as every species has somewhat different brains. And given that brain circuits with unique properties not found in other animals, not even other primates, are emerging as potentially important for the kinds of conscious experiences we have, we should be cautious in attributing humanlike conscious experiences to other creatures. But to say that they don’t have the kinds of experiences we have does not mean they have no experiences. For example, that they do not suffer the way we do does not mean that they do not suffer. While it is extremely difficult to scientifically assess consciousness in other animals, toward the end of the book I will speculate about what kinds of conscious experiences non- human primates and nonprimate mammals might possibly be capable of, given the kind of brain each possesses.

Recognition that the circuits that control our survival behaviors are different from those that assemble our emotions and other conscious experiences allows us to see our connection to the deep history of life in a new way. Like all species, we are similar to the species from which we evolved, but are also, by definition, different. And to fully appreciate our differences, we have to be as precise as possible about both our similarities and differences, using science, not intuition, to draw the conclusions.

Though I had been thinking about the ancient biology of survival since 2009, my first public foray into this topic began with a 2012 article entitled “Rethinking the Emotional Brain.” I have continued this line of thought in other writings and lectures ever since. Deep History consolidates and extends the nascent ideas I had been mulling over into an over- arching vision of how we came to be who we are, taking the argument all the way from the beginning of life in primordial microbes to the emergence of our ability to consciously know about the existence of our selves and our thoughts, memories, and emotions.