11.3: Seeing the Complexity of Nature

Seeing the Commonality of All Life

Living organisms exhibit the highest degree of complexity that we know of, far higher than any systems we humans have designed, and it must be admitted that, as extensive as our scientific knowledge is to date, we are still far from understanding the nature of the phenomenon of life itself. As Meadows explains systems thinking, all systems have a purpose, and all of their ‘parts’ function together in order to fulfill that purpose. We humans construct nonliving systems that function to fulfill purposes of our choosing, from simple thermostat-controlled heat sources to computers. Natural living systems—organisms, and at another level of analysis, ecosystems—function to fulfill the purposes of staying alive, expressing their genomes, and evolving. They have been termed autopoietic systems in light of their properties of self-organization and self-maintenance. When an organism dies, its parts disintegrate into their nonliving components, but while it is alive it maintains its extremely complex, highly organized structure through constantly active biochemical processes, processes that are largely shared throughout the living world.

All life as we know it is based on a set of chemical compounds containing the elements carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur, a small, select subset of all the chemical compounds found in nonliving nature. These chemical compounds are joined together into proteins, lipids, complex carbohydrates, and nucleic acids, the building blocks of living matter, but the metaphor is misleading if it leads one to envision static structures; the biochemical constitution of living organisms is in constant motion, the vital processes of photosynthesis (in green plants) and respiration (in all living organisms) are ongoing—the engines of life—and continually feeding into more specialized pathways involved in life maintenance and continuation for specific types of organisms. Many of these metabolic processes are said to have been highly conserved, meaning that there has been very little change in them over evolutionary time—they are processes that we all have in common, all of us beings, as living parts of nature, the larger whole.

Seeing the Purposiveness of All Living Organisms

Even as the core biochemical processes of life have remained much the same, the bodily forms taken by living organisms have evolved over time. The discovery that a process of evolution has taken place on this planet, however, has often been said to have ‘taken teleology,’ or purposiveness, ‘out of nature,’ but that claim, in itself, is misleading. What can be said is that we have no evidence of natural processes seeking some externally imposed ‘final goal’ such as we might postulate a detached ‘designer’ dictating. But our planet, Earth, ‘is riddled with purpose,’ as the late Mary Midgley observed; it’s “full of organisms, beings that all steadily pursue their own characteristic ways of life, beings that can be understood only by grasping the distinctive thing that each of them is trying to be and do” (Anthony, 2014). Evolution, ‘descent with modification by natural selection,’ is conceived in terms of heritable changes occurring within a population of organisms over time as a result of factors within their environment selecting, for survival, those individuals exhibiting particular traits–bodily manifestations of genetic variability—that make them best suited to live in that particular place. But individual organisms are certainly ‘purposive’ in striving to do just that, to survive and, if so fortunate, to reproduce, and along the way to live their lives to the fullest according to what their own nature’s genetic toolkit enables them to do—as we humans, no more and no less products of evolution, also do.

We are finally coming out, thankfully, of an era dominated by reductionism, so it is no longer necessary to ‘flatten’ all living beings (excepting ourselves—and we do typically make exceptions of ourselves, inconsistently with an appreciation of evolution) into agency-less, subjectivity-less bits of matter being bumped about, at the mercy of the determinism of their DNA combined with the brutal mechanism of competition and conflict for ‘resources.’ This view of living organisms is wrong; it is the purposiveness of life, each individual organism pushing itself forward into the available affordances of its habitat, that provides the motive force behind the process of the evolution of life over time, a purposiveness that all of us living beings share. The fact that there is something known as ‘convergent evolution’—that certain abilities, such as the ability to see, the ability to fly, the tendency to socialize with conspecifics, even the capacity for engaging in ‘higher cognition,’have evolved in multiple, distantly related lines—may indicate that there are a certain limited number of ways of ‘living out ones genetic toolkit to the fullest’ on this planet, as a result of this ‘push’ from inside toward self-elaboration; it need not be taken as evidence for a predetermined pattern imposed from without, but the evolution of some of these abilities could legitimately give rise to speculations about mutual recognition among living organisms as a kind of strange attractor.

While we still, to reiterate, do not fully understand the nature of the phenomenon, nor its origins, we are bringing into focus an increasingly detailed picture of the development of life once it appeared, which some scientists are now claiming may date back as far as four billion years, almost to the origin of the Earth itself. Innumerable species have come into being and passed out of it again over this multibillion year span; the phenomenon of life has surged forward to elaborate a Biosphere of great complexity many times, suffering setbacks, and a few great die-offs, but so far always recovering, even if ecosystems have taken millions of years between cataclysms to attain the degree of diversity we enjoy today, or at least did until recently. A species, according to Holmes Rolston, “is a living historical form, propagated in individual organisms, that flows dynamically over generations” (1985, p. 721). Stepping back to view it from afar, we can thus see life flowing over time, its myriad specific forms adapting to changing circumstances, and simultaneously flowing over space, as forms differentiate and interact within environments. In its most recently generated wave of forms, moreover, life can be seen elaborating itself in multitudes of individual organisms with increasingly sensitive ways of becoming aware of what’s around them, and of responding to what’s out there, with many possible currents of interaction setting up between the different living forms as ‘mind’ has blossomed and subjectivity within them deepened.

Seeing Life Flowing over Time

The membrane-bound cell is the basic unit of all life as we know it, and a single cell is in itself an immensely complex system, the functioning of which we are just beginning to understand. All living organisms, single-celled or many celled, are given their bodily structure by proteins, assembled out of a set of twenty left-handed amino acids in a complex process under the direction of a DNA-based genetic code. On the basis of genomic similarities, LUCA (a Last Universal Common Ancestor), has been proposed, believed to have come into existence almost four billion years ago and to have given rise to all species that have emerged since, so that all presently existing species can be seen as deriving from one fundamentally interrelated Tree of Life (Hug et al., 2016). The first lifeforms on this Earth, our science tells us, were prokaryotes—bacteria and the more recently recognized archaea—single-celled organisms that lacked organelles and even a membrane-bound nucleus but that went about busily metabolizing anyway, picking up sensory cues, moving around in their environments, interacting in mutualistic, competitive and predatory relationships by themselves for a couple of billion years, give or take a few hundred million, until other forms appeared, other forms that are still going strong today. Prokaryotic cyanobacteria carried out photosynthesis, combining carbon dioxide and water to make sugars and ultimately many other organic compounds, thereby creating food for themselves and others out of the energy of the sun, and giving off oxygen, which gradually built up in the Earth’s atmosphere. Then somewhere around two billion years ago, the prokaryotes were joined by the eukaryotes, organisms whose DNA is packaged inside a nucleus and who come equipped with mitochondria to carry out oxidative phosphorylation, to keep energy flowing in their bodies, and, if plants, chloroplasts to house the processes of photosynthesis—these two essential organelles now recognized as quite possibly having arisen from prokaryotic forms becoming symbiotic with and later incorporated into larger cells, as first hypothesized by Lynn Margulis. “Whatever the exact series of events turns out to be,” explains Carl Zimmer (2009), “eukaryotes triggered a biological revolution,” since, while “prokaryotes can generate energy only by pumping charged atoms across their membranes,” eukaryotes “can pack hundreds of energy-generating mitochondria into a single cell,” and therefore could get much bigger, and develop multicellularity.

A little over 600 million years ago, multicellular forms known as Ediacarans appeared on the scene, with bodily forms unlike any organism alive today, some growing in strange, fractal patterns, some displaying three-part symmetry. These were replaced when the first ancestors of all the modern forms of animals—molluscs, echinoderms, arthropods, a group that includes insects and crustaceans, the chordate ancestors of the vertebrates, and so on—made their appearance in what has been called the Cambrian explosion, beginning around 540 million years ago. While its triggering factors are still scientifically controversial, this event has been summed up as follows: “after millions of years of quiet progress, animals had finally accrued the developmental recipes to build body parts and improve on basic themes,” an achievement requiring a genetic toolkit that was, in the words of paleontologist Nick Butterfield, “absolutely, astronomically, inconceivably complex” (Sokol, 2018, p. 884). The Cambrian marked the beginning of the Paleozoic era, which continued until roughly 250 million years ago, during which time vascular plants colonized land and vertebrates appeared—first the fishes, followed by amphibians, and then, with the evolution of the amniotic egg that permitted embryos to develop in a dry environment, the reptiles. The animal kingdom is generally thought of as divided between the vertebrates, animals with backbones, and the invertebrates, animals lacking backbones, but bodily development in both groups has been found to proceed along similar lines under the control of a small number of homeobox or Hox genes that serve to switch gene expression on and off in the growing embryos. The great majority of animal species, vertebrate and invertebrate, are classified as bilaterians, bilaterally structured with a right and left side that are mirror images of each other. The five classes of vertebrate animals–the fishes, amphibians, reptiles, birds, and mammals—all share the bilateral tetrapod body plan, with four appendages, be they fins, wings or limbs. ^[1]

The Paleozoic era ended with the most severe extinction event in Earth’s history, the Permian-Triassic (P-T) extinction event or the ‘great dying,’ occurring around 250 million years ago, during which reportedly about 70% of terrestrial vertebrates and up to 96% of species of marine life became extinct. ^[2] One likely contributory cause of this event is climate warming. Reconstructed seawater temperatures from the Triassic (the geologic period immediately following the end-Permian extinction), show an inverse relationship with biological diversity, and marine animals have been particularly vulnerable to warming because their need for oxygen increases with rising temperature while its concentration in seawater decreases, with water temperatures over 35 °C being generally lethal (Sun et al., 2012).

Ecosystems collapsed worldwide following the event, and while ‘disaster taxa’—weedy generalist species that can colonize many sorts of disturbed habitats rapidly—invaded relatively quickly, true ecological diversity was slow to recover, taking about 30 million years, well into the Late Triassic, for full recovery (Sahney & Benton, 2008). The Mesozoic Era, spanning roughly 250 to 66 million years before the present time and comprising the Triassic, Jurassic and Cretaceous periods, has been termed the ‘Age of Reptiles’; dinosaurs appeared in the Late Triassic and became the dominant terrestrial vertebrates over the Jurassic and Cretaceous periods, while the first birds and ancestral mammals emerged in the Jurassic, remaining relatively small and ecologically insignificant through the end of the Cretaceous. The Mesozoic Era came to an end with the Cretaceous-Tertiary (K-T) or Cretaceous-Paleogene (K-P) extinction, occurring around 66 million years ago, most often attributed to an asteroid impact spewing dust and sulfate aerosols into the atmosphere, blocking sunlight, inhibiting photosynthesis, abruptly cooling the Earth (Pope et al., 1998), and bringing about the extinction of an estimated three-quarters of terrestrial plant and animal species. Rebuilding again following the demise of the giant reptiles, the Cenozoic Era or the ‘Age of Mammals’ began, starting 66 million years ago and extending up through today.

During the Late Cretaceous, some 80-100 million years ago, the placental mammals split into four lines, one giving rise to the hoofed mammals, whales, carnivores,and bats, another leading to primates and rodents, a third to the elephants, among others and a fourth to the anteaters and armadillos (Marshall, 2009); early forms of most of the present mammalian orders emerged during the Eocene epoch, 56 to 34 million years ago. The first were small, but by the end of the Oligocene, 23 million years ago, there were large-bodied herbivores, specialized carnivores, and mammals inhabiting the air and water as well as land. Monkeys evolved during the Oligocene, 34 to 23 million years ago, with the ape lineage splitting from Old World monkeys about 25 million years ago; the apes differentiated over the Miocene, lasting from 23 to 5.3 million years ago, the human line diverging from its common ancestor with the chimpanzee and bonobo around four to six million years ago. By the Pleistocene epoch, the beginning of the Quaternary period, two million years ago, global temperatures having cooled throughout the preceding Pliocene, some very large land mammals and birds had come to inhabit the planet, all of which became extinct as the Pleistocene was winding down.

The factors contributing to the Late Quaternary Extinctions (LQE) have been reviewed and evaluated by Paul Koch and Anthony Barnosky (2006). As they discuss, 50,000 years ago the Earth was populated by many large mammals, including proboscideans—elephant-like mammals including mammoths and mastodons—giant ground sloths, camels, saber-tooth cats and a giant beaver in North America, woolly mammoths, rhinoceroses and giant deer with three-meter antlers in Eurasia, glyptodonts—giant armadillos the size of a car and weighing over 1,000 pounds—and litopterns—three-toed, camel-like mammals—in South America, and diprotodons—vegetarian, rhinoceros-sized wombats weighing up to 2.7 tonnes—in Australia, by 10,000 years ago, the start of the Holocene epoch, all of these had vanished. Reviewing evidence from archaeology, paleoecology, and climatology, Koch and Barnosky conclude that the worldwide disappearance of the Pleistocene megafauna—defined as animals weighing 44 kg or larger—can largely be attributed to human hunting, possibly aggravated by indirect anthropogenic effects like competition and habitat alteration, with changes in climate and other environmental factors also contributing to the patterns of disappearance. The impact was somewhat less severe in Eurasia, since ancestors of modern humans were present there from about 40,000 years ago, hunting with simpler tools, and this probably allowed the evolution of defensive behavior among prey. Africa, moreover, where humans originated, seems to have remained ‘a fortunate anomaly,’ losing only around half of its megafauna by the end of the Pleistocene, retaining the greatest number of large animals still alive today—although a modern extinction event, from uncontrolled hunting and habitat destruction, may be bringing about their demise right now, as will be discussed in the following chapter (Chapter 12).

The best-preserved paleontological record is from North America, where ‘extinctions were rapid and pronounced,’ and may even be compatible with the ‘blitzkrieg’ hypothesis—the notion that human hunters slaughtered the large mammals mercilessly over a short period of time—something that seems unlikely in most other regions of the globe where extinctions occurred over longer periods. The emergence of our own species, Homo sapiens, somewhere around 200,000 years ago, is thus considered to have been a major force leading to the extinction of many large mammals and significantly altering landscapes on all major continents, leaving us to inherit a planet with a post-Pleistocene fauna shorn of some of its more interesting and perhaps ecologically significant variants, and a planet that is now poised to lose many of its remaining specialized forms in the near future if we humans continue along on our current trajectory. Whether or not this trend will continue—whether we will go on waging so direct a war against nature—is currently under contestation; are we re-living out our early role as mega-killers, or will we become reflexive enough to activate our moral agency and change our behavior?

Seeing Life Flowing over Space

While the bodily forms of the Earth’s organisms can be seen as changing over time, the ecological relationships established among organisms can be visualized as large-scale patterns of interaction that show a kind of dynamic stability over space. Ecosystems are not simply collections of plants and animals, randomly or haphazardly thrown together; they are highly organized systems that are fundamentally structured by physics, the large-scale configuration of the system produced by the pathways through which energy flows. The basic conceptual framework for understanding ecosystem structure is often presented as a pyramid; the solar energy powering the whole system is first captured by photosynthesizing green plants, the ‘producers,’ at the base, and it flows upward through successive layers of ‘consumers,’ animals that can’t make their own food and so must eat other organisms power their own bodies . The collective biomass of these animals diminishes in a stepwise fashion, passing up the pyramid layer by layer, because the available energy diminishes at each step, since converting the body of one kind of organism into the body of another is energetically expensive. Aldo Leopold’s description of a terrestrial biotic pyramid is one of the best around:

Plants absorb energy from the sun. This energy flows through a circuit called the biota, which may be represented by a pyramid consisting of layers. The bottom layer is the soil. A plant layer rests on the soil, an insect layer on the plants, a bird and rodent layer on the insects, and so on up through various animal groups to the apex layer, which consists of the larger carnivores.

The species of a layer are not alike in where they came from, or in what they look like, but rather in what they eat. Each successive layer depends on those below it for food and often for other services, and each in turn furnishes food and services to those above. Proceeding upward, each successive layer decreases in numerical abundance. Thus, for every carnivore there are hundreds of his prey, thousands of their prey, millions of insects, uncountable plants. (Leopold, 1949, p. 252)

That’s why ‘big, fierce animals are rare’ (Colinvaux, 1979): they need huge territories to support all the other animals that go into making up the lower layers of the pyramid, the prey animals and their prey animals and the plants that they eat, all together contributing enough transformed solar energy to maintain the large, fierce, active bodies of apex predators like lions and leopards.

The layers Leopold speaks of are called trophic levels, first the green plants (supported by microrganisms and nutrients in the soil) that form their own bodies out of air, water and sun, then on a level above them the animals that eat the plants’ bodies, the herbivores,a step above them the animals that eat some other animals as well as plants, the omnivores, and above them possibly several levels of animals that only eat other animals, the smaller, ‘meso’carnivores, below and at the top the apex predator, an animal able to feast on all the others and who usually doesn’t get eaten herself. A rule of thumb holds that the embodied energy goes down by about 90% in each step up a trophic level, such that the level above can contain only about 10% of the biomass of the one underneath—that’s why numbers of organisms generally get smaller, even as body size often gets larger (all the better to capture prey)—as they dine higher and higher up the pyramid. That’s also why humans draw an increasing amount of energy from the Earth the higher up the food chain they eat—much more energy, embodied in biomass, is required to grow the bodies of the animals on which they feast than would be required if people just met their needs primarily by eating plants directly—as our closest primate relatives still do today. Humans are not ecologically constituted to be apex predators. Aldo Leopold assigned humans to ‘an intermediate layer with the bears, raccoons and squirrels, which eat both meat and vegetables’ (1949), pointing out an ecological relationship that led environmental philosopher J. Baird Callicott to add, “as omnivores, the population of human beings should, perhaps, be roughly twice that of bears, allowing for differences of size” (Callicott, 1980, p. 326).

Real-world ecosystems are usually far more complex than this pyramid with its discrete trophic levels would indicate, of course, so the movement of energy and materials is better described as making up food webs, interconnected chains linking different kinds of organisms, and including the microbial and fungal organisms that break down the bodies of plants and animals, releasing nutrients for reuptake by plants or processing it into organic matter again consumable by other organisms. The fundamental role of plant life, whose photosynthetic trapping of the sun’s energy generates the ‘net primary productivity’—given the acronym of NPP–that powers the activities of virtually all of the Earth’s other living creatures, must be retained firmly in mind. Now we are aware, however, that quite a bit of ‘ecosystem engineering’ is a result of animal life. The prevailing view in ecological science once held that the large-scale structure of plant-dominated terrestrial ecosystems was primarily due to the climate and soil conditions facilitating plant growth, but more recent studies are showing the great extent to which top-down control of herbivores by their predators can affect the vegetative community.

One famous example of the way the presence or absence of a carnivore at the highest trophic level can ‘cascade’ down the system is the way aspen forests have been recovering following the reintroduction of grey wolves into Yellowstone National Park, their territories reducing elk grazing pressure on young aspen stands, ultimately changing the landscape.^[3] Another is the ongoing introduced instability of kelp forests in oceans around the world, as commercial exploitation led first to the extirpation of apex predators like sea otters and cod fishes, unleashing a rebound in their prey, populations of herbivorous sea urchins that subsequently overgrazed and diminished many kelp forests. Continued ‘fishing down’ of coastal marine food webs next led to extirpation of sea urchins in many places around the world, allowing kelp beds to regrow but this time ‘devoid of vertebrate apex predators,’ with large predatory crabs moving into the top spot in some places (Steneck et al., 2002); it remains to be seen where these systems will eventually restabilize, but one finding of this study is that the more biodiverse the system, the greater the likelihood it will be resilient to systemic kelp deforestation. Moreover, the diversity of species is proving to have important effects on ecosystem structure more generally, with the different kinds of diversity—genetic diversity, diversity in the functional roles played by different organisms in the ecosystem, and diversity of their interactions in biotic networks—having their own kinds of effects; so far, research is showing “compelling scientific support for the idea that maintaining a high proportion of biological diversity leads to efficient and stable levels of ecosystem functioning” (Naeem et al., 2012, p. 1405).

Larger-scale, landscape-level patterns of interaction among animals of different trophic levels are also discernable over time and space, such as ‘migratory coupling,’ where the migrations of prey induce the corresponding migrations of their predators (Furey et al., 2018), while at smaller scales the regular patterns of banding or clustering of organisms that can be seen in aerial surveys across many different types of terrain are being explained in terms of self-organization resulting from short-range positive feedback—more vegetation grows around pre-existing plants because they pull more moisture up through their roots–coupled with long-range negative feedback—roots from different plants compete with one another in the drier soil between vegetated patches—a principle that seems to hold across many different ecosystems (Rietkerk & van de Koppel, 2008; Pringle & Tarnita, 2017). Of course, as we humans increasingly take over space with growing urbanization and the installation of ever-larger agroindustrial systems for feeding our growing population, less and less room is available to support these patterns of interaction among lifeforms. Just how far this mega-scale alteration in the flowing of life over space will reach is going to be increasingly contested in the years ahead.

In addition to the patterns we can see in the world around us, moreover, our appreciation of the “little things that run the world” has been growing as well. The phrase is taken from the title of a talk by Edward O. Wilson, referring to invertebrate animals, but it could be extended now to include the single-celled organisms, which we are learning contribute a significant part of our own body mass and biochemistry. Wilson pointed out that invertebrate species outnumber species of vertebrates by a factor of more than twenty, and can make up over 90% of the animal biomass on a hectare of land; their importance in food webs and pollination and other ecosystemic interactions is so great that Wilson expressed doubt that we humans could last more than a few months without them. Should all the invertebrates disappear, he maintained:

Most of the fishes, amphibians, birds and mammals would crash to extinction about the same time. Next would go the bulk of the flowering plants and with them the physical structure of the majority of the forests and other terrestrial habitats of the world. The earth would rot. As dead vegetation piled up and dried out, narrowing and closing the channels of the nutrient cycles, other complex forms of vegetation would die off, and with them the last remnants of the vertebrates. The remaining fungi, after enjoying a population explosion of stupendous proportions, would also perish. Within a few decades the world would return to the state of a billion years ago, composed primarily of bacteria, algae, and a few other very simple multicellular plants. (Wilson, 1987, p. 345)

Wilson made these remarks at the opening of the invertebrate exhibit at the Zoological Park in Washington, DC, in 1987, and while the invertebrate-less world he presented seemed dismal, it also seemed far-fetched, since invertebrate populations appeared to be thriving in most places, and the occasion recognizing the importance of their conservation seemed to herald a new awareness of our need to treat them with care. More than 30 years afterwards, however, with populations of many kinds of invertebrates essential to ecosystem functioning on the decline now, his words sound a little more sinister. Meanwhile, a recent examination of the invertebrates ‘right under our noses’ has shown that, typically, more than a hundred species of insects and other arthropods live in and around people’s homes worldwide, and efforts to ‘go to war’ with chemicals against pests like cockroaches simply increase the evolution of their resistance. Moreover, the importance of even smaller ‘little things’ is coming to our attention as well, including the microbes that colonize our bodies, our houses, and other human-occupied spaces. A study of dust collected from forty homes in one American city documented an average of around eighty thousand species of bacteria and archaea, the vast majority of which are benign or beneficial to us humans, and despite people’s tendency to want to ‘kill them all,’ it’s being discovered that it is actually healthier to be surrounded by more microbial diversity rather than less (Dunn, 2018); the declining biodiversity in urban homes appears to be associated with an increase in the incidence of allergies and other chronic inflammatory diseases (Hanski et al., 2012). Trillions of microbes also inhabit the human gut, and enter into complex relationships with our diets, giving rise to metabolic products that have important effects on human physiology which are currently under investigation (Gentile & Weir, 2018).

Seeing Mind in Life

In the words of philosopher Evan Thompson, “a living being is not sheer exteriority . . . but instead embodies a kind of interiority, that of its own immanent purposiveness” (2007, p. 225), and it is recently being realized that this may apply to plants as well as animals and to the unicellular as well as the multicellular. The more we learn about life, its amazing complexity and its fundamental commonality as it extends over time and space, the more it becomes clear that there must be some kind of ‘mind,’ some purposive inwardness that pushes ahead, pursuing its own life in its own way, within each living organism, ‘all the way down.’

Microbial life, being life, by definition is of such organized complexity that we should not be surprised to find perception, motility, and evidence of subtle responsiveness to environmental conditions even in the single-celled. The green alga, Chlamydomonas reinhardtii, for example, has an eyespot composed of rhodopsin photoreceptors that, when stimulated, release a current of calcium ions that modify its flagellar motion, orienting it toward or away from light (Kateriya et al., 2004); the slime mold Physarum polycephalum, moreover, has been described as showing ‘primitive intelligence’ by solving a maze, finding the minimum length solution joining two nutrient locations at different ends of an agar labyrinth (Nakagaki et al., 2000). Plants, too, are exquisitely sensitive to factors such as light, moisture and nutrients, as well as predators and pollinators in their environment, and they respond to them in ways that further their growth and propagation; they also communicate with fellow plants, of the same and other species, within their ecological communities. Since plants are sessile (rooted to one place), their behavioral repertoire is necessarily more limited in terms of movement, but they exhibit many sophisticated responses that can rewardingly be studied along the lines of animal behavior, including anticipation of future events, memory, and communication with other organisms (Karban, 2008). They respond individually to the heterogeneity of light and moisture in their environment throughout their growth, not only by placing root and leaf development in the most favorable circumstances, but in ways that have been described as showing ‘choice’; the parasitic dodder plant, for example, actively rejects potential host plants of inferior nutrition by turning its shoot growth at right angles from such stems and elongating directly away from them (Kelly, 1992).

It has long been noted that plants respond to leaf-devouring insect attacks by releasing volatile chemicals, a response that not only leads other plants to beef up their own leaf level of insect-repellents but that sometimes draws in specific insect predators and parasitizing wasps (Pare & Tumlinson, 1999). The timing and intensity of release can vary in accordance with a multiplicity of environmental factors, and blends of different odor-producing volatiles can be produced in response to different leaf-eaters, possibly summoning particular carnivorous insects specialized to feast on each kind of herbivore, making it a highly sophisticated response that has been considered, according to a ‘behavioural ecological approach’ that speaks in terms of plant ‘decisions,’ and a ‘crying for help’ within the larger ecological community (Dicke, 2009). It has also been known for several decades now that many forest trees are linked together in underground networks by the mycorrhizal fungi associated with their roots, and they have been shown to send each other nutrients, communicate warning signals, and recognize kin through these networks. According to Suzanne Simard, another scientist who does not hesitate to draw a parallel with the behavior of animals, “the topology of mycorrhizal networks is similar to neural networks, with scale-free patterns and small-world properties that are correlated with local and global efficiencies important in intelligence” (Simard, 2018, p. 191). ^[4] The communicative properties of trees have also been conveyed to the public by Peter Wohlleben, a German forester, in The Hidden Life of Trees: What They Feel, How They Communicate (2016); he speaks of the ‘wood-wide-web’ that connects the trees in a forest, noting that the ‘mother trees,’ the big, old trees that serve as hubs, ‘suckle their young,’ pumping sugars through the network into the roots of young saplings too shaded to survive on their own (Grant, 2018).

The similarities between plant and animal behavior and, in some respects, their physiology prompted a group of scientists to announce in 2006 the founding of a new subspecialty, ‘plant neurobiology,’ maintaining that ‘the behavior plants exhibit is coordinated across the whole organism by some form of integrated signaling, communication, and response system,’ one that ‘includes long-distance electrical signals, vesicle-mediated transport of auxin in specialized vascular tissues, and production of chemicals known to be neuronal in animals’ (Brenner et al., 2006). The announcement was met with outrage from a certain quarter of the plant science community, more than thirty luminaries signing onto a letter noting that “there is no evidence for structures such as neurons, synapses or a brain in plants” (although the ‘plant neurobiologists’ had made no such claims) and challenging the proponents of the new field “to reevaluate critically the concept and to develop an intellectually rigorous foundation for it” (Alpi et al., 2007, p. 136). One of the signatories, Lincoln Taiz, interviewed by Michael Pollan, speaks dismissively of ‘a strain of teleology in plant biology’ and strenuously rejects the notion of ‘choice’ or ‘decision-making’ in plants, explaining that “the plant response is based entirely on the net flow of auxin and other chemical signals,” and maintaining that the verb ‘decide’ is a term that “implies free will.” He amends his stance, however, with the caveat “of course, one could argue that humans lack free will too, but that is a separate issue” (Pollan, 2013). This last statement is rather telling—when one is coming from a reductionist position that flattens down the purposiveness of all life into the bumping about of chemical compounds- one must be sure to keep that belief system ‘separate’ from our understanding of how we actually live our own lives. Whereas, accepting the evolutionary continuity that exists among lifeforms seen as whole organisms lets us recognize the purposiveness, intentional behavior and intelligence that exists throughout living nature—in us and in everything else that’s alive- with no need to make a special exception for ourselves. Pollan observes that “our big brains, and perhaps our experience of inwardness, allow us to feel that we must be fundamentally different—suspended above nature and other species as if by some metaphysical ‘skyhook,’ to borrow a phrase from philosopher Daniel Dennett.” But he notes that “plant neurobiologists are intent on taking away our skyhook, completing the revolution that Darwin started but which remains—psychologically at least—incomplete” (Pollan, 2013, n.p.). Monica Gagliano is another scientist who has already made the paradigm shift; unapologetic about speaking of learning, memory, and intelligence in plants (Gagliano et al., 2016). She is at the same time, critical of “those who make the big claims and write grand opinion pieces,” saying “we don’t need another opinion piece”—“we need to do the science.” Having started as an animal ecologist, she prefers to call her field ‘plant cognitive ethology,’ maintaining that, “for me, a plant isn’t an object, it’s always a subject that is interacting with other subjects in the environment” (Morris, 2018, n.p.). ^[5]

Unlike plants, however, animals typically move rapidly around in their environments and so must have a way of coordinating their movements rapidly—hence the emergence of the nervous system. Simple animals like sponges rely on cell-to-cell signaling, and radially symmetric animals like jellyfish make do with diffuse nerve nets, but the bilaterians generally coordinate their movements via well-developed nervous systems that are believed to have originated in a last common ancestor arising over 500 million years ago. The basic structure is a linear nerve cord with ‘ganglion’ enlargements supplying each body segment, and a larger ‘brain’ at the front end; in invertebrates, including many worms, crustaceans, and insects, the nerve cord is divided in two and placed ventrally, below the major organs of the body, while in vertebrates it is dorsally located and encased in a bony vertebral column. The insect brain is made up of three regions, the protocerebrum, deuterocerebrum, and tritocerebrum. The largest region is the protocerebrum that houses the mushroom bodies, paired neuron clusters making up the ‘higher’ brain centers, thought to be important in learning, memory, and behavioral complexity, especially in bees, wasps and ants; it is estimated that the mushroom bodies contain about 340,000 neurons in the honeybee. An example of complex cognitive behavior in insects is the ‘waggle dance’ of honeybees, which communicates information to hive mates about the direction and distance to sources of nectar and pollen. ^[6] Faced with the striking degree of organizational similarity among living animal forms, one scientist recently summarized, “as our knowledge of neural development increases, so does the list of conserved features, pointing to the existence of a highly sophisticated, single species as the origin of most extant nervous systems” (Ghysen, 2003, p. 555).

The vast majority of animal forms utilize the sensory information they take in from their environment in order to move in appropriate, survival-related ways. Hence they will have a great variety of perceptual abilities, forms of cognitive processing, and behavioral responses shaped by the different ecological niches they inhabit, something that we tend to take for granted but should recognize as a distinctive feature of animal life that extends far beyond the boundaries of our own species. Development of the human brain follows the same basic trajectory as that of all mammalian brains, the neural tube expanding into hindbrain, midbrain and forebrain regions, with the latter giving rise to an expanded cerebral cortex. Some other mammals also manifest a high degree of cortical development, including the other great apes, elephants, and cetaceans such as the bottle-nosed dolphin. To put our own brain power in perspective, we will look at what we now know about the brains of some other animals, bearing in mind that we are learning more all the time as careful investigations are carried out utilizing new technologies and with an open-minded attitude to what we may find.

The brain of the false killer whale, at almost 4,000 g, is more than twice the size of the human brain, at roughly 1,500 g, while the brain of the African elephant is almost three times larger, at four to 5,000 g, and the brain of the sperm whale, the largest of the mammals, is almost six times larger, at around 8,000 g. The cortical surfaces of the brains of the two cetaceans are also more highly convoluted, cetaceans showing the greatest degree of convolution among the mammals. Earlier comparisons have focused on the ratio of brain to body size, the ‘encephalization quotient,’ but this appears a rather crude measurement in light of a newly developed technology allowing for a quantitative assessment of the number of neurons and non-neuronal cells in different regions of the brain and in total, opening up insights into a greater degree of diversity in brain architecture than heretofore appreciated (Herculano-Houzel, 2009). Using this technology, it has been discovered that the different orders of mammals have different ‘cellular scaling rules’ determining the density of neurons present per gram of brain tissue. Larger brains in rodents, for example, will contain larger total numbers of neurons than will smaller rodent brains, but the brains of primates ‘scale in a much more space-saving, economical manner,’ such that neuron density is greater, and so increasing brain size in primates results in an even greater number of neurons, gram for gram, than would be found in rodents. By this measure, humans, with the largest brains among the primates, do have the greatest number of brain cells—in a 1.5 kg brain, 86 billion neurons and 85 billion non-neuronal cells have been found—but only when compared with the other, smaller-brained primates. ^[7] According to the author of these studies, “we need to rethink our notions about the place that the human brain holds in nature and evolution, and rewrite some of the basic concepts that are taught in textbooks” (Herculano-Houzel, 2009, pp. 9-10). Ours is not qualitatively different from other primate brains, but simply has the number of neurons expected for its size; it is basically just ‘a linearly scaled-up primate brain.’ Moreover, our cerebral cortex, which makes up 82% of our brain mass at an average of 1,233 g (out of an average 1,500 g brain), holds only 16 billion neurons (19% of the total in the brain), a fraction similar to that seen in other primates and some other mammals. While the cerebellum—a part of the brain until recently considered solely devoted to movement coordination, but now becoming the focus of increasing interest as its complex interconnections with the cerebral cortex are explored—weighs only 154 g but contains 69 billion neurons (Herculano-Houzel, 2009).

The new research not only gives us a new perspective on our own brains, and thereby our ‘cognitive’ place in nature, it is beginning to change our views of other animals, what they are really like and what they might be capable of, cognitively. The brain of the African elephant is not only roughly three times larger than our own, it contains roughly three times as many neurons—257 billion of them as calculated in the pioneering study (Herculano-Houzel, 2014). The vast majority of them, however—251 billion, or 97.5%—are found in the cerebellum, with only 5.6 billion in the cerebral cortex—and the neurons that are found there are thought to be an average of 10 to 40 times larger than those found in other mammals, with what this might mean for cognition being currently unknown. The size of the elephant cerebellum, which makes up more than 25% of the total brain mass, the largest proportionally of all mammals, has been speculated to be related to infrasound communication or possibly to processing the complex sensory and motor requirements involved in the sensitive, manipulatory use of the trunk—but much remains to be discovered about this fascinating animal.

The numbers and distributions of neurons in the brains of cetaceans are yet to be determined—one estimate was 11 billion neurons in the cerebral cortex of the false killer whale, but this could be off by a factor of ten, giving an estimate of between 21 billion and 212 billion for the whole brain, depending on the scaling rules for the order, as yet undetermined (Herculano-Houzel, 2009). One thing that is known is that the architecture of cetacean brains is even more divergent from the typical mammalian plan than that of elephants. While their brains are the most highly convoluted among the mammals, their cerebral cortex is comparatively thin and appears to lack one of the usual six layers of cells. Moreover, instead of an expansion of the frontal lobes, as observed in primates, there has been an expansion toward the sides, in the temporal and parietal regions, and there is a completely new lobe, the paralimbic lobe, not found in any other mammal, the function of which is so far unknown (Marino, 2002) but possibly may be related to echolocation or coordination of synchronous movements in groups of animals. The pattern of projection of visual and auditory information onto the cerebral cortex is also highly unusual among mammals, as is the marked degree of independence between the two cerebral hemispheres, which reportedly sleep independently of one another, and seem to be altogether lacking in REM sleep.

The brains of birds, too, have recently been found to be more remarkable than once believed. Birds have a pallium instead of the neocortex found in mammals; the surface of their brains is smooth rather than convoluted, and the cells in their cerebrum are arranged in nuclear clusters instead of layers. It has recently been discovered, however, that their neurons are even more tightly packed than in the brains of primates, with parrots and songbirds having about twice as many neurons as primate brains of the same mass, and their brains are truly ‘miniaturized,’ since the short distance between neurons necessitated by their high densities likely results in a higher speed of information processing (Olkowicz et al., 2016). Parrots, like primates, show an increased connectivity between the telencephalon and the cerebellum, possibly indicative of an interplay between fine motor skills and complex cognition in birds (Gutierrez-Ibanez et al., 2018), along the lines of what is being investigated in mammals. What is being learned about the brains of birds, moreover, is spurring a new look at the brains of reptiles and even fish. The mobulid rays, a group of cartilaginous fishes comprising the manta and devil rays, have high encephalization quotients, a relatively large telencephalon making up over 60% of the brain mass, and a high degree of cerebellar foliation thought to be due to their active, maneuverable lifestyles and highly developed social and migratory behavior (Ari, 2011). A study of selected genes from mammalian neocortex and homologous genes from avian and turtle brains found, once again, a ‘highly conserved’ pattern of gene expression, supporting the conclusion that many of the cell types, neurotransmitters, and circuitry are widely shared among the vertebrates, preserving the major connections and performing very similar functions despite major differences in brain structure and tissue architecture, attesting to fundamental continuity since the last common ancestor, over 500 million years ago.

Among the ‘brainier’ members of the mammalian and avian classes—particularly the primates, elephants, whales and dolphins, parrots, corvids and some other songbirds, and even the mobulid rays (Ari & D’Agostino, 2016)—we are finding many, many examples of ‘higher cognition.’ Over the last five to 10 years or so, there has been a veritable explosion of research reports, popular articles and books detailing what’s being discovered about their abilities, and it is now widely accepted that some of these animals engage in tool use, mirror self-recognition, imitation, vocal learning, and complex social cognition likely including ‘theory of mind,’ to name a few indicators. Frans deWaal discusses the cognitive abilities of some of these other animals, from apes and monkeys to crows and parrots, elephants and octopuses, and even ants, wasps and bees, raising deep questions about our common assumption: that humans are the only living beings capable of intelligent thought (and that only the human kind of thought should be considered ‘intelligent’), an attitude that, because it is exclusively ‘centered upon the human,’ is termed anthropocentrism. ^[8]

One way to see how our thinking has changed can be illustrated by consideration of what we have been learning about birds, both in terms of behavior and in brain structure. As discussed by Ackerman (2016), birds have now been extensively documented to have complex cognitive abilities, including memory and spatial mapping (Clark’s nutcrackers can bury and retrieve pine seeds from up to 5,000 caches spread over hundreds of square miles), tool use (New Caledonian crows fashion elaborate tools from branches and bend wires into hooks for obtaining food), vocal learning (mockingbirds can imitate, with near perfection, as many as two hundred different songs of other birds), social learning (a few great tits learned to open milk bottles in a single town in the 1920s and the behavior spread widely over Britain over subsequent decades; crows can recognize individual humans and spread information about the ‘dangerous’ scientists who capture them across large social networks), mirror self-recognition (Eurasian magpies will scratch away a mark put on their throat when seen in a mirror), and complex social interaction, manipulation, and possibly ‘theory of mind’ (western scrub jays keep track of other birds that might be watching them when they cache their food, and will recache it later if necessary; male Eurasian jays seem to understand their mates’ specific desires for certain foods). But until recently, little effort was put into making such observations, since until very recently we had little respect for ‘bird brains.’

The lines giving rise to the primates, elephants, and cetaceans probably diverged over 95 million years ago, with independent evolution occurring in these lines ever since, so it is not surprising that differences are to be found in the overall structure of their brains. The split between what became mammals and birds came even earlier, sometime around 300 million years ago. Nevertheless, parrots and primates “show impressive convergence of complex cognitive abilities, and this is accompanied by convergent changes in the brain,” including relatively large brain size, telencephalon size, size of associative areas of the telencephalon, and increased connectivity between the telencephalon and cerebellum- though this increased connectivity has evolved over different neural pathways (Gutierrez-Ibanez et al., 2018, p. 5). “It has been suggested that intelligence in these taxa can only have arisen by convergent evolution,” observes cognitive biologist Nathan Emery:

driven by the need to solve comparable social and ecological problems; simple examination of six ecological variables across corvids, parrots, other birds, monkeys, apes, elephants and cetaceans reveals that certain preconditions correlate with the development of complex cognition: omnivorous generalist diet, highly social, large relative brain size, innovative, long developmental period, extended longevity, and variable habitat, [and] this exercise suggests that the evolution of intelligence was highly correlated with the ability to think and act flexibly within an ever-changing environment. (Emery, 2005, p. 37)

The same can be said about the conditions under which our own species evolved, of course, placing us within the spectrum of cognitively complex animals, one with a very high degree of behavioral flexibility indeed.