Part 2 (2/2)

PREDATORS.

I returned to the Jura Mountains and spent the following months reading and thinking about plants and animals. Martin Giurfa had made me look into the relations.h.i.+p between movement and intelligence. It is true that some observers claim that plants lack intelligence because they do not see them move. But this is an optical illusion caused by the different timescales we operate on. Plants, in fact, do move.

Most plants move slowly, but some plants move fast even in human terms. A Venus flytrap can snap its leafy lobes shut in a third of a second to catch insects lured by its nectar. The flytrap is a predatory plant that eats flesh by secreting digestive juices and dissolving its prey. Its reflexes are triggered by electrical signals similar to those that run along our own nerves.

Unlike the Venus flytrap, most plants do not eat animals. Instead they take nutrition from sun and soil. Plants are also eaten in large amounts, being the basic element in all food chains. They are clearly successful at surviving, as they make up 99 percent of the ma.s.s of Earth”s living organisms.

Movement can be a criterion of intelligence among animals, but it does not apply to plants. They eat freely available sunlight and soil nutrients, so they mainly do not need to move from one place to another. Those among us who lack this ability are obliged to move about in search of food. Animals are, by definition, organisms that move to feed themselves. Animals are animate. They move.

Over the course of evolution, animals with efficient nervous systems have had an edge over their compet.i.tors. A nervous system that conducts information down to the muscles in a matter of milliseconds, rather than seconds, helps avoid being eaten. We use our brains to escape from predators. And as predators, we use them to catch our prey. This neurological arms race between animal predators and animal prey has certainly contributed to the development of brains such as our own.

But plants have not remained inactive. They may appear to sit around merely absorbing sunlight and being eaten in large amounts, but these brainless organisms have developed thousands of chemicals to try to stop themselves from being eaten. Plants have contributed to the arms race of evolution in the domain of chemistry. Unlike animals, they never had to develop movement or nervous systems to avoid predation.

We humans operate on a very rapid timescale compared to most plants. To us, plants can look stupid just sitting there. In fact, the term vegetable is an insult when applied to humans. According to the Concise Oxford Dictionary, it means ”a person who is incapable of normal mental or physical activity, especially through brain damage; a person with a dull or inactive life.” We have animal prejudices against vegetables, and they come out in our vocabularies.

I wanted to reconsider things from the start and try to move away from my own prejudices. As an animal, I wanted to understand animals. For starters, I learned that not all animals have brains. The sponge, for example, does not even have nerve cells. It lives attached to the sea bottom, or to other objects. The natural sponge that can be purchased in a store is the skeleton that supports the sponge animal. Inside this skeleton, the body of the living sponge consists of a kind of perforated stomach lined by whiplike cells which create currents that draw in water and food particles. A four-inch sponge can filter one hundred liters per day in this way. Sitting stuck to a rock at the bottom of the ocean, a sponge just sucks in its food. Zoologists recently discovered that one type of sponge can respond to potential danger by generating electrical impulses similar to those that streak through the nerves of other animals, including humans. Electrical signals disseminate through the sponge body via a network of fine strands of cytoplasm, which are not divided into cells. The sponge uses these signals to shut down the intake mechanism when the water around it becomes murky with particles that would otherwise clog its pores. These electrical signals are part of a decision-making system that allows the sponge to gauge and exploit the world around it. Though sponges are brainless and nerveless animals, they appear to make correct decisions on a regular basis.

The hydra is another brainless, headless, and sedentary animal that lives in water. It looks like a thin, translucent tube about an inch long and has a nervous system called a nerve net, which crisscrosses its body without forming a particular concentration. The hydra lives attached to vegetation by the base of its tubular body. The bottom of the tube is closed, and an opening at the upper end both engulfs food and rejects residue. Around this opening is a circlet of retractable tentacles that sting and catch other small invertebrate animals such as crustaceans. When a hydra detects a prey, it extends its tentacles and reaches out to grasp it. How it carries out this precise action with no brain is not known. Studies reveal that the animal”s nerve net concentrates around its mouth area. This suggests that the earliest heads appeared about 700 million years ago in hydralike organisms that may have been the common ancestors of species from snails to humans. The early head was simply a net of nerve cells at the mouth of the organism. This concentration of neurons close to the mouth shows how important active feeding is for animals. We exist in our current shape, with heads containing brains close to our mouths, as a legacy of this.

I scratched my head thinking about the fact that my brain is close to my mouth. I used my predator brain to think about the long line of predators that had led to me. I could see an endless queue of mouthed ancestors stretching back hundreds of millions of years, snapping their teeth and laughing.

I looked into the origin of central nervous systems. They first developed in small invertebrates like nematode worms. The present-day nematode Caernohabditis elegans looks like a mere speck to the naked eye. It has a body made up of fewer than one thousand cells, some three hundred of which are neurons that form a ring-shaped brain around the digestive tube not far from the mouth. The nematode brain, which is among the simplest known, is shaped like a saint”s halo. Centralized nervous systems have shorter and denser connections between neurons. This makes for quicker reactions to changes in the environment and for more complex behaviors.

The brown garden snail Helix aspersa has a central nervous system containing only a few thousand neurons. This is not much, considering it has a body the size of a walnut. Consequently, nervous signals take time to travel through the snail”s body, and its muscles can take several seconds to react to an outside stimulus. In fact snails perceive the world in slow motion. But this does not mean they make incorrect decisions. Snails are among the world”s most successful predators. There are about 65,000 species of snails, living in oceans, fresh water, and on land, in many different kinds of climates and environments. Snails are not stupid, but slow and efficient at what they do.

Octopuses have the largest brains among invertebrates, and scientists have noted their intelligence. Octopuses can run mazes, escape from locked tanks, break into other tanks and steal lobsters, open jars to get at crabs, disguise themselves, and even get angry and turn red. They have half a billion neurons” worth of brain power, which is about two hundred times less than ourselves, but a great deal more than snails. Octopuses are adept at finding food in concealed places”a skill usually a.s.sociated with big-brained vertebrates such as bears, pigs, and humans. Octopuses camouflage themselves by gauging their environment and, in a fraction of a second, transforming their body shape and the color, pattern, and texture of their skin. Octopuses are wily transformers.

Vertebrates include fish, amphibians, reptiles, birds, and mammals. We vertebrates have internal skeletons that allow us to achieve larger size than most other creatures. And we have backbones and skulls that partly enclose our central nervous systems, providing secure housing for eyes, ears, olfactory senses, and brains. This makes it easier to respond to the environment. But lacking a backbone does not make invertebrates stupid. Octopuses may be spineless, but they can run mazes as successfully as rats.

INTRIGUED BY THE CAPACITIES of invertebrates, I went to a zoology department at a Swiss university near where I live and asked if somebody could show me a nematode worm. I wanted to look at a living Caenorhabditis elegans through a microscope. The people at the zoology department were not used to dealing with such requests. After all, what business did an anthropologist have wanting to see a nematode? I explained I was writing a book about intelligence in nature and wished to see with my own eyes an invertebrate with a simple nervous system. My request was granted, and I was asked to wait.

On one of the walls in the corridor of the Zoology Department, there was a diagram of the complete body plan of a nematode. Each one of its 959 cells was mapped out in detail. A nematode is barely visible to the naked eye, but it is a complete animal, with skin, brain, mouth, digestive tract, reproductive tract including eggs and sperm, and a.n.u.s. Nematodes are among the animals that scientists have most studied. They are easy to keep in vast quant.i.ties and they reproduce very quickly. And they have transparent skin, which makes it possible to look into their living bodies with a microscope and see their organs function. They also have transparent eggsh.e.l.ls, so it is possible to watch their embryos develop.

Nematodes have brains that respond to taste, smell, temperature, and touch. And their neurons send one another an array of chemical signals including serotonin, which is a neurotransmitter that human brains also use. I may have several hundred million times more neurons than a nematode, but as a biological organism I share fundamental commonalities with it. Standing in the corridor looking at the worm”s body plan, I thought of myself as a kind of snaky organism with limbs. As a vertebrate, I differ from a worm in that I have a backbone and a brain encased in a skull. But like a nematode”and like most other animals”the bulk of my nerve cells is situated close to my mouth, and I have a long digestive tract. At the core of my being, there is a snakelike tube stretching from mouth to a.n.u.s.

Nematodes eat bacteria that they find in the soil. All animals feed on other organisms. Even vegetarians prey on plants. You cannot eat a carrot without killing it. Whether a vegetarian diet of plants is more ethical than an omnivorous one is a matter of opinion. I know I am a predator.

Putting an end to my reverie, a woman walked up to me and introduced herself as a geneticist working with nematodes. Her name was Monique Zetka. She came from the Czech Republic. We spoke in English. She was willing to interrupt her work to show me some nematodes.

I followed her into her office and asked about her work. She explained how she micro-injected DNA into nematode gonads in order to induce mutations in their eggs. She had several nematodes stuck on an oily slide and invited me to sit down at the microscope to take a look.

Once I got the swing of the apparatus, I focused on a single worm. The nematode was alive and moving. It looked like a transparent, Byzantine snake. Its internal organs had the intricacy of a racing-car engine, and it moved like a ballerina, ending each sideways weave with a flick at the tip of its body. I understood more clearly why the nematode”s scientific name includes the Latin word for elegant. I admired the nematode”s beauty for several minutes, feeling amazed that a creature with a brain of only three hundred neurons could move with such grace.

I found the experience thrilling. I turned to Monique Zetka and thanked her sincerely. As the quality of nematodes is not a frequently discussed subject, and as some people get uneasy taking such tiny creatures seriously, I asked with some hesitancy whether she liked nematodes.

She seemed embarra.s.sed by the question and simply said, ”They are pretty nice.” Scientists sometimes view their business as keeping a cold gaze in the face of nature”s elegance and beauty. I thanked her again and let her get on with her work.

I tried discussing my newfound enthusiasm for invertebrates with people around me, but often they just laughed. Many Westerners place themselves above ”lowly” creatures such as nematodes. But humans are part of nature. Like so many other animals, we have eyes, noses, ears, mouths, teeth, brains, digestive tracts, skin, gonads, and so on. We are affiliated to even the simplest creatures.

The first animals were invertebrates. Animals with backbones and skulls only appeared about 500 million years ago. First came fish, then amphibians, reptiles, birds, and mammals. We humans are mammals. We belong to the order of primates, which includes marmosets, monkeys, apes, and chimpanzees.

Humans have several distinctive features, the most obvious of which is that we are the only living primates who walk full-time on two legs. According to the fossil record, some of the first bipedal primates belonged to a now-defunct genus called Australopithecus. These precursors of humanity lived about three and a half million years ago and had brains one-third the size of our own. Apart from their near-human posture, they were very much like chimpanzees, with similar diets and similar brain size. The first true hominids, commonly known as h.o.m.o habilis, appeared about two million years ago. They stood upright and had brains half the size of our own. Since then, hominid brains have continued to expand.

The fossil record is patchy and hard to interpret. Paleontologists do not agree on many details. When did the first h.o.m.o sapiens appear? Some believe that the roots of our species might extend back over four hundred thousand years. Others think that our immediate ancestors were a separate African species called h.o.m.o rhodesiensis, and that one should only apply the label h.o.m.o sapiens to fossils less than two hundred thousand years old. Some paleontologists believe that there have been different varieties of archaic h.o.m.o sapiens, including h.o.m.o rhodesiensis, h.o.m.o antecesor, and h.o.m.o heidelbergensis, from whom both modern humans and Neanderthals derived. Others view Neanderthals as an entirely separate offshoot of h.o.m.o rhodesiensis.

Our stocky Neanderthal cousins lived mainly in Europe and had a brain volume that was slightly superior to our own. Like us, they buried their dead, made musical instruments, and produced efficient hunting tools. Neanderthals were serious predators. a.n.a.lysis of their fossilized bones reveals that they had a heavy meat diet. Nevertheless, Neanderthals were also quite different from us. Their skulls were oval shaped, not round. Their foreheads were sunken rather than flat. Their chins were also sunken, whereas our own are pointed.

The fossil record suggests that anatomically modern humans, or h.o.m.o sapiens sapiens, emerged in Africa only about one hundred and fifty thousand years ago. This represents about seven thousand biological generations, and shows that we are a very young species. The word sapiens means ”wise” in Latin. Whether this label truly corresponds to humans remains to be determined.

I found it fruitful thinking of humans as a species. It seemed clear that our great strength is being able to adapt to a wide variety of environments and circ.u.mstances. The descendants of the small band of humans that left Africa spread out across the world and populated it. From the Arctic to the deserts of Australia and the rain forests of the Amazon, they learned to exploit the plants and animals in each new environment they entered. Humans have long perpetrated ecological depredation. Species that were easy to hunt tended to disappear shortly after humans arrived in a given area. The fossil record indicates this clearly in places such Madagascar, New Zealand, and Australia. Like lions and wolves, humans are social predators. And we are an invasive species. Our outstanding capacity of adaptation makes us the most dangerous of all macroscopic predators currently stalking the earth.

Archaeologists have compared human campsites to those of Neanderthals living at the same time in the same region. Our ancestors made sophisticated traps and carved fine tools, not only out of stone and wood but also out of bone and antler. They carved bones into needles, which enabled them to sew clothes. Neanderthals probably lacked the capacity to make warm clothes. Our species cohabited the earth with Neanderthals for more than one hundred thousand years, and even traded with them in some cases. But there were four major glaciations during this period, and the Neanderthals did not survive. Paleontologists now think that their mysterious disappearance twenty thousand years ago is best explained by their incapacity to adapt to a changing environment.

h.o.m.o sapiens sapiens has a vertiginous trajectory. The Cro-Magnon artists who painted Lascaux, the prehistoric cave in southwest France, lived less than a thousand generations ago. They were humans just like us”but they had neither electricity nor sophisticated technology. Now humans have developed indoor plumbing, was.h.i.+ng machines, s.p.a.cecraft, computers, and an understanding of the intricate workings of biology.

Who are we? We have skulls and backbones, just like other vertebrate animals. Everything indicates that we are animals. Yet we do many things that animals cannot, such as write books, debate the meaning of words, turn trees into paper, study invertebrates with microscopes, equip jaguars with radio collars and track them, ride bicycles, fly planes, pilot submarines, travel to the moon and back, make wine from grapes, smoke tobacco, manipulate DNA molecules, build nuclear reactors, and study the extinction of other species. We can also step back from the world and witness it as a spectacle separate from ourselves, which we call ”nature.”

We are rooted in biology, and we can also think about it. Words and concepts are our specialty. We are the symbolic species par excellence. We can treat words as symbols for things that are not in our immediate vicinity. Our linguistic and symbolic capacities enable us to devise new relations.h.i.+ps between unrelated concepts. Through language, we can exchange information, make plans, scheme, and strategize. Mastering language and symbols has led us to the top of the food chain. Lions and wolves have fangs and claws; we have cunning concepts that we can put to practice.

Language also allows us to pa.s.s on vast amounts of knowledge and experience to our children. The sophisticated technologies we have developed in recent decades grew out of the acc.u.mulated knowledge of our ancestors. Language has blasted us onto a steep learning curve.

These developments have been made possible by our brains. We have big brains. Relative to body size, the human brain is three times larger than might be expected in a primate”and primates already have enlarged brains compared to other mammals. The top part of our brain, known as the cortex, has mushroomed during the evolution of hominids. Rita Carter describes this in her book Mapping the Mind: ”One and a half million years ago the hominid brain underwent an explosive enlargement. So sudden was it that the bones of the skull were pushed outwards, creating the high, flat forehead and domed head that distinguish us from primates. The areas that expanded most are those concerned with thinking, planning, organizing and communicating. The development of language was almost certainly the springboard for the leap from hominid to human. It gave our ancestors lots to think about, and new brain tissue was needed. The frontal lobes of the brain duly expanded by some 40 per cent to create large areas of new gray matter: the neo-cortex. This spurt was most dramatic at the very front, in what are known as the pre-frontal lobes. These jut out from the front of the brain, and their development pushed the forehead and frontal dome of the head forward, reforming it to the shape of a modern skull.”

Our brains are organized into distinct areas. First, at the top of the spinal column, at the base of the skull, there are cells sensitive to smell and light. This corresponds to the fish brain. On top of this lies a clump of cells called the cerebellum, which coordinates movement. Together the two layers form the reptilian brain. Further areas lie on top of this, including the thalamus (involved in the primary sensory processing of vision, sound, and touch), the amygdala (involved in emotion), the hippocampus (involved in learning and memory), and the hypothalamus (involved in motivation and behavioral regulation). This corresponds to the mammalian brain, which also has an additional top layer of cells known as the cortex. Some mammals have more cortex than others. In humans, the cortex balloons out of all proportions.

The architecture of the human brain incarnates our hereditary connection to other vertebrates, in their order of evolutionary appearance: first fish, then reptile, then mammal. But the human brain differs from other animal brains in that it is equipped with specialized neuronal circuitry to deal with language. For decades, scientists believed that two specific parts on the left side of the human cortex, known as Broca”s area and Wernicke”s area, function as ”language centers.” But recent research based on brain imaging shows that language is handled by many different brain regions working in parallel. As Susan Greenfield writes in her book Brain Story: ”One of the most startling discoveries from such research is that saying just a single word causes a unique pattern of activity to ripple through the cortex. The experience of the word ”screwdriver,” for example, causes a part of the brain called the motor cortex to light up. The motor cortex is involved in controlling movement, so perhaps this word triggers memories of handling a screwdriver to become active. Obviously, language cannot be the preserve of just Broca”s and Wernicke”s areas”it involves an eruption of a.s.sociations and memories that are different for every word.”

Humans have remarkably big brains compared to the rest of their bodies. Our children come into the world so top-heavy that they take months just to sit up. Their heads are so large that our species has by far the highest maternal death rate during birth. And young humans require long years of nurturance, education, and compa.s.sion for their brains to reach full potential. Humans also have by far the longest childhoods and adolescences, and human parents sustain compa.s.sion longer than parents from any other species.

Having a large number of neurons relative to body size certainly seems to enhance intelligence, as octopuses and humans demonstrate. But if intelligence is defined as the capacity to gauge the world and make correct decisions, there is some doubt that humans are as smart as some people fancy. Our current tendency to deplete the natural world with little consideration of the future shows that we do not yet have a grip on our predatory behavior. True, our species is very young. In comparison, octopuses have been around for several hundred million years, which has given them time to hone their skills. By comparison, we are just getting started.

Chapter 7.

PLANTS AS BRAINS.

<script>