Gottlieb gives the example of English physician William Harvey, who “figured out how [the heart] works two centuries before natural selection was discovered.” As precise, detailed and beautiful as Harvey’s descriptions of the heart and circulatory system were, they did not explain the origins of the heart as a functional organ. Why do different animals have different kinds of hearts? Why do some animals have blood but no heart?  When, how and why did hearts arise in the first place? Simply knowing how the heart works is not sufficient to answer these important questions. Rather, one needs to understand how the heart evolved. Such understanding contributes to more than basic biology—it also advances medicine. Tracing how gene expression in heart cells has changed over evolutionary time, for example, has simultaneously improved our understanding of congenital heart defects.

Just as evolution shaped the human heart’s structure and function, evolution sculpted the human brain—as well as the mind. This is an inescapable fact. The brain and mind are inextricable. In order to understand one, you must understand the other. Changing a brain’s structure changes how that brain behaves and what kind of mind emerges from its interaction with the environment. We have clear evidence of this from people who have endured swift and dramatic changes to their brains through traumatic injury, stroke and neurodegenerative diseases like Alzheimer’s. Likewise, the more gradual structural changes to the human brain during the course of its evolution mirror an evolution of the human mind. Consciousness, self-awareness, complex emotions, language, creativity—if you want to truly understand these aspects of mind, you must understand when and why they first evolved. To do that, you must understand how the brain has changed over time.

The evolutionary story of the human brain begins where life itself began: the ocean. The brain’s most basic building blocks have existed for billions of years: some of the simplest and oldest single-celled organisms use the same chemical messengers that our own brain and nervous system depend on. Sponges, one of the earliest groups of animals to have evolved, do not have nervous systems, but they do have some of the same genes and proteins that are essential for the construction of neural connections in our brain. The cells in a sponge’s body also communicate with waves of calcium ions not unlike the cascades of charged particles that surge down neurons in more complex animals. Jellyfish and their gelatinous relatives may have been the first group of animals to evolve genuine neurons—long, thin, branching cells adapted for the task of transmitting messages from one part of animal’s body to another. But these neurons were arranged in a diffuse net that enveloped the animals’ bodies. There was no central processor, no intricate organization, no brain. The next major chapter in the brain’s evolutionary history was a process known as cephalization, in which neurons cluster at one end of an animal, eventually becoming a brain linked to important sensory organs like eyes. Cephalization probably happened several times, and in different ways, in different groups of animals. Within the tiny, simple brains of worms and fish, particular brain regions began to specialize in different functions—one region largely devoted itself to vision, the precursor to our occipital lobe, while another focused on responding to threats, the progenitor of the amygdala. Even before life left the water, animals had evolved brains with much of the same basic neural architecture that we would eventually inherit.

Studying the brain and mind in ignorance of this vast evolutionary tale does not make sense. It would be equivalent to an archaeologist discovering the remains of an enormous tapestry, slicing out a particular figure from the cloth and claiming that he could learn everything he needs to know by examining that figure in isolation. Even if the archaeologist described the figure in exquisite detail, taking it apart thread by thread and sewing it back together, he would remain willfully oblivious of the whole story. In the same way, disregarding the human brain’s history limits psychology and neuroscience to a paltry understanding of our brains and minds.

With regard to our brain’s tumultuous past, evolutionary psychology is primarily concerned with what happened to the human brain during the Paleolithic, between about 2.6 million and ten thousand years ago. Gottlieb is right that evidence of Paleolithic psychology is scant, but it’s not nonexistent. Learning about the brains and behaviors of early humans is a difficult challenge, but not an impossible one[PDF]. By measuring fossil skulls—and creating models of the brains they once held—anthropologists have established that brain size tripled over the course of human evolution. The trend kicks off around 2 million years ago and the swiftest growth occurred between 800,000 and 200,000 years ago during a period of rapid shifts in climate. The National Museum of Natural History has a graph plotting changes in braincase volumes of early humans against changes in the climate. Anthropologists think that early humans with the largest brains adapted most effectively to such a mercurial climate.

Around 100,000 years ago, the human brain largely stopped expanding (and some evidence suggests it has actually shrunk a little since then). What scientists have not yet satisfactorily answered is exactly why the human brain began to swell in the first place and what benefits larger brains offered our ancestors. The most intuitive and tempting explanation is that the expansion of our brains during the Paleolithic paralleled the emergence of more sophisticated intelligence, as partially evidenced by the existing archaeological record of increasingly complex tools and cookware. Learning to cook with fire dramatically improved our ancestors’ diet—it’s much easier to digest and extract calories from soft, cooked foods than from raw, tough foods. In turn, a more nutritious diet likely fueled brain growth. As early human populations increased and spread across the globe, an increasingly diverse social environment would also have demanded a larger and more comlex brain.

One potentially distinct species of hominin called Homo floresiensis, also known as the Hobbit, bucked the trend of bigger brains. Although H. floresiensis went extinct relatively recently, only around 12,000 years ago, it stood just over three feet tall and boasted a brain only half the size of its predecessor, Homo erectus, and one third the size of our modern brains. Yet the remains of H. floresiensis have been discovered alongside evidence of butchery with stone tools and cooking with fire. How, then, do we reconcile the Hobbit’s small brain with evidence of such high intelligence? Is it structure, not size, that matters most? This is exactly the kind of evolutionary puzzle we need to solve to thoroughly understand the human brain and mind. The more we learn about the brains of early humans—and what those brains were capable of—the better we understand our modern minds.

Toward the end of his review, Gottlieb writes: “To confirm any story about how the mind has been shaped, you need (among other things) to determine how people today actually think and behave, and to test rival accounts of how these traits function. Once you have done that, you will, in effect, have finished the job of explaining how the mind works. What life was really like in the Stone Age no longer matters. It doesn’t make any practical difference exactly how our traits became established. All that matters is that they are there.”

Once again, Gottlieb proposes that understanding “how the mind works” is more important than understanding “how the mind has been shaped”—that once you have achieved the former, you need not bother with the latter. One could take a supremely utilitarian approach to the study of the brain and mind, confining oneself to research with explicit practical applications. All Why questions are off the table! We only care about how the mind works. Just explain what happens and move on. No need to think about what any of it means. To be perfectly honest, that sounds unbearably boring to me. More fundamentally, understanding how the mind works and why it works that way are indivisible goals. The human brain’s evolutionary past is not just some cute story we can leave on the shelf if we so please. Every cell in our brains—every moment of our mental lives—is intimately connected to entire history of life on this planet.