The Brain

Big Bangs, Behaviors, and Beliefs

Rob DeSalle, Ian Tattersall, Patricia J. Wynne

• Publisher: Yale University Press
• Year: 2012
• ISBN:
  9780300183566
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Evolution
• Index: Biological Sciences

After several million years of jostling for ecological space, only one survivor from a host of hominid species remains standing: us. Human beings are extraordinary creatures, and it is the unprecedented human brain that makes them so. In this delightfully accessible book, the authors present the first full, step-by-step account of the evolution of the brain and nervous system.

Tapping the very latest findings in evolutionary biology, neuroscience, and molecular biology, Rob DeSalle and Ian Tattersall explain how the cognitive gulf that separates us from all other living creatures could have occurred. They discuss the development and uniqueness of human consciousness, how human and nonhuman brains work, the roles of different nerve cells, the importance of memory and language in brain functions, and much more. Our brains, they conclude, are the product of a lengthy and supremely untidy history-an evolutionary process of many zigs and zags-that has accidentally resulted in a splendidly eccentric and creative product.

1 THE NATUREOF SCIENCE

Our Brains at Work

What does it mean to think about thinking? Only members of our species can ask this question. No other organism on this planet has the physical or neural makeup to come even close to posing it. And the answer involves a sometimes convoluted intellectual journey, albeit one with its destination steadily in the human brain. Understanding just how our unique modern human style of cognition was acquired necessitates looking at evidence of many kinds, much of it gathered from the study of a huge diversity of living organisms, but always in an evolutionary context. The evolutionary approach culminates in the viscerally most dramatic story of all—how, after several million years of jostling for ecological space with a host of other hominid species, a single lineage of our zoological family managed to eliminate all the competition, until there was only one actor left standing on the human evolutionary stage: us.

Our brains govern virtually every action we take. Behaviorally, it is our brains that make each of us what we are as unique individuals. And collectively, it is the extraordinary and totally unprecedented human brain that enables our species to be the psychologically complex, highly distinctive, and occasionally bizarre entity that it is. This book is about this delicate, elaborately configured, and yet highly adaptable mass that resides within our oddly large cranial vaults and that allows each of us to experience the world in the unique human fashion. It is about the simple origins and long evolutionary history of the brain: how it is structured, how it functions and changes through life, and how we have come to know what we do—so much, and yet so little—about this most mysterious of organs.

An Inelegantly Contrived but Rather Effective Evolutionary Mess

Much as we might wish to vaunt our modern human brains that have allowed our species to achieve so much, the New York University cognitive psychologist Gary Marcus has colorfully likened them to “kluges”: the inelegant but effective solutions to problems that are typically produced by garage tinkerers. This is because, as we’ll see, on examination they turn out to be a basically jury-rigged solution to a long succession of evolutionary problems. The human brain is hardly alone in this, for the “messy” organ that resides in our heads is in fact just one example of the kind of untidy history that most of the structures of living organisms accrue; but it may be the most wonderful example of all.

If one were to design a brain from scratch, it certainly wouldn’t look or work at all like our brains. It might look a lot like a modern-day computer, or something more elegant yet, but it most assuredly would not be the convoluted Rube Goldberg apparatus we have inside our heads. As a simple example of how inefficiently our brains are designed, we need only look at how we store memories. The easiest way to store something is to give it an address, place it where it belongs in storage according to this address, and keep track of all of the addresses. Librarians do this, museum scientists do it, and a lot of modern human daily life consists of giving things addresses so we can store them logically for easy retrieval, whether on notecards, sticky notes, or Excel spreadsheets or in computer desktop folders. Indeed, our computers are probably the epitome of how the human brain ought to store things. But even though human brains know enough to design computers as efficient information storage and retrieval systems—and indeed they have worked out very effective and nearly perfect methods in this domain—the memories that reside inside our brains themselves are not stored efficiently at all. This is why we forget so many things and why our memories are so frequently inaccurate. If recent estimates of the unreliability of eyewitness testimony at criminal trials are accurate, then our brains are pretty awful at storage and retrieval.

Memory, what’s more, is only one example of our mental inefficiencies: many other aspects of our brains are even less logically constructed. Why? The reason is simple, and it is a historical one. We humans are only a single species in a great tree of life that has diverged and branched from a single common ancestor that lived more than three and a half billion years ago. As a result of this three-and-a-half-billion-year experiment, our genomes—and the genomes of every organism that has ever lived on this planet—carry data about our past, our present, and even our future potential. As we’ll see, evolution is not the process of optimization it is often thought to be but is instead often hostage to history and chance. And this is why our brains are such a mess.

Thinking about Thinking

Let’s start this book by posing the question of how we go about trying to comprehend the human brain and its properties, because it is crucial to understanding everything else that follows. All fields that contribute to our knowledge of the brain as an evolutionary and functional entity are scientific disciplines. And many of the products of the brain—for example, fiction, poetry, and spiritual ideas—are hardly susceptible to scientific analysis, because science deals solely with observations we can make about the material world. But to the extent that the human brain itself is a functioning material object, upon which repeatable observations can be made, it is indeed an appropriate subject—perhaps the ultimate subject—for the scientific way of comprehending the world that the unique properties of the human brain make possible.

Scientific understanding differs from every other kind of knowing, in being explicitly provisional. Perhaps one of the greatest myths of the modern age is that science is an authoritarian system, piling up mountains of immutable objective facts. The reality is quite the reverse. Because although it depends on human creativity and intuition as much as any other intellectual endeavor, science is founded on, and is most clearly distinguished by, the doubt and questioning it entails. Indeed, the oft-heard phrase “scientifically proven” is one of the most misleading clichés around. Science is not actually about proving anything. Instead, scientists gradually edge in on a more precise and accurate description of nature by proposing ideas about it (which is where the creativity comes in) and then discarding those that do not stand up to scrutiny (which is where doubt enters the picture). For scientists to do this, of course, those ideas have to be explicitly testable, so to be scientific an idea has to be formulated with its potential falsifiability in mind. If an idea is not somehow testable in the observable world, then it is not a strictly scientific one (though the larger fabric of science may also weave in notions that may not be directly testable in practical terms, at least with the equipment we have currently at hand).

Some scientific ideas have withstood examination by generations of scientists for decades or even centuries; others may be falsified tomorrow. If they are disproved, they will then have to be rejected, no matter how clever they are or how attached their authors may be to them. This process of proposing and testing hypotheses makes science the ultimate collective activity: it is a lot harder to imagine a world with just one scientist than to imagine one without any scientists at all. In the end, science is a self-correcting system that depends on its own internal system of vigilance, a system that is maintained by an essential plurality of ideas. This is actually one of the things that makes being a scientist so cool: unlike an engineer or a physician, whose decisions have to be right or the consequences may be disastrous, a scientist may be wrong and still feel that he or she is contributing valuably to an ongoing process—a process that is far bigger than any one individual. The corollaries of never knowing for sure that you are right and having to admit it immediately when you’re proven wrong are small prices to pay for this kind of satisfaction!

Scientific knowledge is thus a constantly moving target. It is a process, not a durable product. For the product itself is always changing—which is, after all, what the huge and exponentially growing scientific literature is all about. What’s more, contrary to common belief, even the process itself is not unified by the “scientific method” that we constantly hear about. The great biologist Peter Medawar, perhaps the finest scientific writer of all time, made our favorite comment on this subject in 1969: “Ask a scientist what he conceives the scientific method to be and he will adopt an expression that is at once solemn and shifty-eyed: solemn, because he feels he ought to declare an opinion; shifty-eyed, because he is wondering how to conceal the fact that he has no opinion to declare.” The reality is that there is not one scientific method but many, each specifically designed for its suitability to the phenomenon being investigated. Rather than a rigid formula that dictates how we should proceed, science is simply an approach to knowledge.

Still, despite the amorphousness of the larger scientific process, it is entirely fair to conclude that, as a result of its workings, the picture of the universe and its contents that we have today is more accurate than the one we had yesterday—even if it is almost certainly less accurate than the one we will have tomorrow. And the sum total of scientific knowledge at any one point in time definitely gives us a platform that is firm enough to allow us to build on it, constantly and confidently, toward tomorrow’s understanding. Indeed, some scientific ideas and observations of nature have proven so durable and resistant to falsification that we may regard them as the scientific equivalent of fact—although testing will always continue.

Shared, Derived. Derived, Shared. That Is All Ye Know on Earth and All Ye Need to Know

One of those robust hypotheses about the world is the notion of evolution. A century and a half after it was proposed, evolution is still the only scientific hypothesis we have that actually predicts the organization we observe in the living world—namely, that organisms visibly fall into a system of sets-within-sets that can be represented by a branching diagram (a “tree”) with repeating forks. This pattern has been corroborated time and again, as our knowledge of the biosphere has grown by leaps and bounds.

Today we trace the beginnings of modern evolutionary biology to the year 1858, when the evolutionary views of the British naturalists Charles Darwin and Alfred Russel Wallace were presented to a meeting of the Linnean Society in London. This historic event was hardly appreciated at the time for the turning point it was, but it was followed the next year by the publication of Darwin’s magisterial book On the Origin of Species by Natural Selection, which really did take the world by storm. Darwin’s own thumbnail characterization of evolution was “descent with modification,” a very neat way of expressing that all living organisms, no matter how disparate, are connected by ancestry. It is this which gives rise to the clear pattern of nature we’ve just noted.

To start at the base of the tree that describes this pattern, a single ancient common ancestor gave rise to the three major branches, or domains, of life: Bacteria, Archaea, and Eukarya. Each of these great groups then diversified repeatedly in the same way to produce the huge profusion of subgroups we see today. Eukarya (eukaryotes), for example, consists of numerous clusters of sometimes very different-looking organisms that are nonetheless united by possessing complex cells with membrane-bound nuclei. The nucleus was, of course, a feature that was present in the common ancestor of the group as a whole. Eukaryotes include, among certain other organisms, all of the animals and plants, ranging from simple unicellular creatures through mosses, green plants, fungi, sponges, flatworms, starfish, sharks, frogs, snakes, and crocodiles to platypuses and humans. Each of these and other large eukaryote subgroups is in turn subdivided, again and again, until we end up with literally millions of species.

We recognize the sequence of evolutionary splits that gave rise to this almost unimaginable diversity by looking for “derived” features (such as that membrane-bound nucleus in the case of the eukaryotes) whose possession binds the various groups and subgroups together. Any feature that is shared by all members of a particular group, to the exclusion of all others, will in principle be there by virtue of inheritance from the common ancestor of that group—and will thus testify to the group’s genealogical unity. Besides the cell nucleus, another classic derived feature is provided by the feathers of birds—the discovery of which in certain amazingly preserved dinosaur fossils has also recently allowed us to corroborate the early notion that these two groups are closely related. Characteristics like this permit us to recognize the boundaries of each group by excluding from it forms that do not possess them or that at least are not descended from ancestors that possessed it; they also serve to admit new potential members to the group as they are discovered.

In contrast, the many “primitive” features of the common ancestor that were not unique to it will undoubtedly be found more widely than just in the descendant group. They thus cannot be used to determine membership in that group—even if all group members possess it. So although a cursory inspection of their body form would almost certainly suggest to you that a salmon and a lungfish were more closely related to each other than either is to a cow, it turns out on examination that the lungfish and the cow belong to a single group that shares a much more recent common ancestor than either has with the salmon. The lungfish and salmon look superficially similar, because they still share the watery habitat that was occupied by the even more remote ancestor of all three. The cow has become so distinctive by virtue of adaptation to an entirely new environment, while the salmon and the lungfish, sitting on independent branches of the tree, remained relatively unchanged. Additional complicating factors in figuring out the geometry of the great Tree of Life are found where organisms have changed so much over long intervals of time that there are few recognizable features left to link them together or, in cases of “convergence,” where remotely related creatures have evolved remarkably similar adaptations.

There are traits, then, in organisms that can trick us into thinking they are the same thing. These are what Darwin called “analogies.” Today we call these traits convergences or, in the jargon of the tree-builder, “homoplasies.” Such traits arise independently in unrelated lineages. As an example, think of wings. There are many kinds of wings among animals. In fact, even some plant gametes can glide, float in the air, and move around by flying about. But let’s look at three kinds of animal wings: those of birds, bats, and flies.

It is very easy to see that the fly wing is not “the same” as the bat or bird wing. It has no bones to support the flap of tissue that is used as the flying device, and its musculature is not derived from the same precursor cells as those of vertebrates (the bird and the bat). So we can easily say that fly wings (and indeed all insect wings) are not the same as vertebrate wings. The bat and bird wings are a little harder to interpret, but it’s still pretty easy to see that they aren’t equivalent except in function because the arrangements of the bones of the wing and their origins from precursor cells are completely different. So, from looking closely at the makeup of traits, we can get an idea of what is really “the same” and what is not.

Analogy of animal wings. A bat wing, a bird wing, and an insect wing. Although these structures all have the same function (transportation of the owner through the air), they are not derived from the same ancestral structure. Hence, they are convergences, or homoplasies, with respect to one another.

The easiest way to go about determining the nature of the “sameness” involved is to look at the relationships of the species involved and to interpret the evolution of the trait we call “wings” in the context of those relationships. It is clear that, among the flies, birds, and bats, the birds and bats are more closely related, leaving the flies as “odd men out.” When we look at just these three kinds of organism, we see that having a wing is a good defining characteristic. But there are actually millions of other species out there, most of them insects, that we should add to this smaller tree. And if we do so, we notice that just having wings no longer defines our fly-bird-bat group. This is because there are a lot of creatures more closely related to vertebrates that do not have wings (such as sea urchins and starfish). Consequently, the best explanation for wings in insects is that they arose independently in the common ancestor of the winged insects. Next, we can look at the pattern for bird and bat wings. Birds are a subgroup of dinosaurs that happen to have wings. Bats, by contrast, are mammals, and the common ancestor of dinosaurs and mammals most certainly did not have wings. We need only to look at the marsupials and the primitive mammals to realize that these forms don’t have wings, and so the chain of common ancestry is broken, meaning that wings in birds arose independently of what we describe as “wings” in bats.

Could the Tree of Life Be a Periodic Table for Evolution?

In all of these cases, a powerful tool for resolving uncertainty is provided by the relatively recently developed science of molecular systematics (“molecular” refers to the hereditary molecule DNA that is the focus of this branch of science—we will hear a lot more about DNA later—while “systematics” is the name given to the study of the diversity of organisms and of the relationships among them). Serendipitously, and rather unexpectedly, it has turned out that certain parts of the long DNA strand are incredibly conservative and resistant to change over vast periods of time, so that distantly related organisms that have changed hugely in their physical aspect since they last had a common ancestor may actually share remarkably similar genes that can be directly compared. Where convergence has occurred, on the other hand, the structure of the genes responsible for the superficial similarities involved is likely to betray the underlying history involved.

The smallest individual unit into which nature is divided is the individual organism, which lives and dies independently of everything else that is out there. But from the point of view of someone trying to reconstruct evolutionary histories, the most important unit is the species. Scientists have never agreed on a definition of what species are (there are currently almost thirty definitions to choose from), but the most useful way to look at them is as the largest freely interbreeding populations of individuals. We members of the human species Homo sapiens, for example, don’t exchange our genes with any other organisms (although in the past there might have been some biologically insignificant interchange with close relatives that are now extinct). Today we are an in dependent entity, a distinct actor in the evolutionary play. We might go extinct in competition with other organisms, but we will never be engulfed by one of them. Genera, in turn, are groupings composed of closely related species that have descended from a single ancestral species by what is known as “speciation.” Over the eons, the splitting of lineages has been far from a rare event: there is, for example, a surprisingly large number of now-extinct species of our genus Homo known in the fossil record of the past two million years.

Fossils are the remains of ancient animals (in most cases only their tough teeth, carapaces, and bones) that have been preserved in sedimentary rocks laid down in the sea or, on land, mostly at the margins of rivers and lakes. These rocks accumulate as particles are eroded from preexisting rocks, to be transported by water or wind to new resting places on the landscape where they may cover and protect the weathering remains of dead organisms. In this way, a record of life is created as fossil-containing sediments are laid down, the younger on top of the older. Onshore fossils usually provide a highly sporadic record of past life, but marine sequences are often quite continuous over long periods of time. The end result is an admittedly incomplete record of earlier life; but nonetheless the fossil record is huge, and it is the only direct evidence we have of the history of life. We can work out the genealogical relationships of living organisms from their physical and molecular similarities, and from their molecular characteristics we can even mak...