Rather, evolution is a general-purpose and highly powerful recipe for finding innovative solutions to complex problems. It is a learning algorithm that adapts to changing environments and accumulates knowledge over time. It is the formula responsible for all the order, complexity and diversity of the natural world.
—Eric Beinhocker1
We have just reviewed, however superficially, the rich diversity of Earth-bound life and its long history. But how did all this come about? Why should our lovely planet have so many species of plants and animals? Why are there lush rain forests and flower-filled prairies? As we've just seen, the fossil record makes evident how this glorious diversity has developed and enriched itself over time. Our precious planet has provided a comfortable and stable foundation for the advance of biodiversity over time. It was Copernicus who explained how the Earth, spinning at a tilted angle as it circles the Sun, provides us with the seasons. Each year, this same tilt has the Sun moving north and south, sweeping monsoonal rains back and forth across the tropics, enhancing tropical biodiversity. Today, we understand how plate tectonics has forced mountains high into the sky; another boost for the world's biodiversity. An extensive fossil record makes clear that larger marine life first appeared four billion years after the solar system's formation. Terrestrial vegetation and land animals were more recent innovations.
Life's grand proliferation over time has been propelled by two principal factors. Perhaps the most common is that of simple speciation, as lineages split to colonize ever more habitats. Competition, both within and between species, has driven this proliferation. Evolution, however, has been marked by yet another powerful factor: increasing complexity over time. Here, simpler forms of organization have given rise to more elaborate levels of organization. Beetles are a lot more complex than earthworms, both in their architecture and in their life trajectory. Jellyfish are even simpler than earthworms, but far more complex than single-celled life. Though instances of escalating structural complexity have been quite rare in the history of life, their significance has been grandly amplified with time.
Endosymbiosis helped build the larger and more versatile eukaryotic cell. Powered by their mitochondria, these cells were able to carry more information and—with time—fashion larger organisms. Once multicellular animals rolled their embryonic cells into a hollow ball and invaginated, they created two layers of cells, which then exploded into a riot of designs. Likewise, and together with a new photosynthetic partner, plant cells began building a world of more nutritious greenery. Slowly, escalations of design, coupled with the simple drive of diversifying speciation, raised the biosphere to new levels of interaction and ever more elaborate ways of living.
Evolution manifests a number of tendencies but the most visible of these trends is the long-term move toward complexity.
—Kevin Kelly2
Unfortunately, the concept of complexity eludes simple definition. Though dictionary synonyms include “consisting of parts” and “complicated,” science can't seem to agree on how to quantify complexity. Is it a measure of the energy needed to put a complex object together? Might it be algorithmic information content or internal levels of hierarchy? Must complexity have internal nodes for relaying information? Such efforts quickly descend into a philosophical muddle. (Using many examples from the natural world, Melanie Mitchell clarifies these challenges in her book, Complexity: A Guided Tour.)
Mathematicians have also examined complexity. But, as Steven Strogatz points out, “Complexity theory taught us that many simple units interacting according to simple rules could generate unexpected order. But where complexity theory has largely failed is in explaining where that order comes from, in a deeply mathematical sense, and in tying the theory to real phenomena in a convincing way. For these reasons it has had little impact on the thinking of most mathematicians and scientists.”3
Despite these problems, I think we can move right along using complexity as common usage defines it. Forests are complex because they are home to many living things; we are complex because we have many interdependent parts built by trillions of individual cells. But what is it that has propelled seemingly universal trends of increasing complexity in so many aspects of the living world? Over time, molecular, organismic, and environmental systems have all become more complex. When we discussed speciation, we mentioned a number of factors that result in the creation of increasing numbers of plant and animal species. This is a kind of multiplication; but why does evolution appear progressive as well? Daniel McShea and Robert Brandon answer this question in bold fashion by describing a “New Biological Law.” They claim that, in the absence of calamity or strong countervailing selection, biological systems naturally become more diverse and more complex. Their Zero Force Evolutionary Law (ZFEL) declares that, over time, diversity and complexity will increase, on average.4 Reproductive systems that cannot reproduce themselves with 100 percent accuracy will necessarily produce variation, and this variation will inevitably increase diversity. While Darwin's natural selection requires three components—reproduction, variation, and selection—the Zero Force Law requires only two—reproduction and variation. And since reproduction, at whatever level, cannot achieve 100 percent accuracy (thanks to Murphy's Law), increasing diversity and complexity are inevitable. Only strong stabilizing selection can keep living things from changing over time (stasis). Thus, today's world is far more complex than in times long past. Though a huge majority of mutations are deleterious, selection sweeps them away—even as a tendency to diversify (ZFEL) continues undiminished.
But what keeps the entire system moving forward? The answer to this question is simple: energy. In fact, all living things move energy around within the cell in the same way. Adenosine triphosphate (ATP) acts like a little battery; by losing its third phosphate group, ATP donates energy to wherever it is needed, becoming adenosine diphosphate (ADP). In turn, ADP uses energy-yielding reactions within the cell to re-acquire the third phosphate group, becoming ATP once more, ready to donate energy with the loss of its terminal phosphate group. But what are the sources of this living energy?
THE FIRST GREEN REVOLUTION
Energy is required to do work, whether moving a boulder or digesting your dinner. Because of this, energy is central to the survival of all living things. Here on planet Earth, we have only two primary sources of energy. Rocks deep beneath our feet still retain heat from Earth's early formation. In addition, unstable atoms within the Earth's core continue to break down, releasing nuclear energy. Together, these sources keep the Earth's center molten, magma extruding at deep-sea fissures, and volcanoes erupting. The strange biota of deep ocean vents are powered by the Earth's own deep energy. There, bacteria use chemical reactions to power their life activities and, in turn, support strange creatures in dark ecosystems.
Fortunately, planet Earth has a second and more accessible source of energy: sunlight! In the past, a few critics derided Darwin's evolutionary scenario because, they claimed, evolutionary progress contradicted the second law of thermodynamics. The second law tells us that everything in the universe runs down or moves to a state of lower energy and diminished organization. Evolution on planet Earth has done exactly the opposite! Not to worry: the second law applies to closed systems, and planet Earth is not a closed system—as anyone who has suffered a sunburn knows! By circling the Sun in the “Goldilocks orbit,” the Earth and its atmosphere maintain just those temperatures needed for keeping water wet. Clearly, the second law holds true: the Sun is “running down” on an enormous scale. Intercepting less than a billionth of that outward flowing radiation, our days are sunny, allowing photosynthesis to power much of the living enterprise. Though a number of bacteria can make a decent living at the edge of lightless sea-floor vents, or by using the energy of chemical degradation, all the rest of us are powered by our star.
Once living things learned how to capture and utilize the energy of sunlight, evolution began running uphill. One of the most crucial advances in the history of life, water-splitting photosynthesis provided hydrogen for building carbohydrates.5 No simple task, pulling apart the water molecule required a suite of complex molecules—acting in exquisite unison. At a minimum, ten photons of light are necessary to ‘fix’ one molecule of carbon dioxide in the business of building carbohydrates. Simply stated: photosynthesis transforms the physical energy of solar radiation into the chemical energy of food. Understanding photosynthesis required many years of research. Paraphrasing one of its early investigators: radiation physics was needed to understand the light; solid state physics detailed the light-capturing process; physical chemistry helped clarify initial oxidation; biophysics explained the electron-transport system; biochemistry helped us understand carbon dioxide fixation; plant physiology showed how the biochemistry was regulated; while botany situates these processes within the plant, and ecology makes clear how photosynthesis powers the environment.6
Fossil evidence, based largely on isotopic changes in ancient sediments, indicates that the Cyanobacteria had developed water-splitting photosynthesis at least 2.7 billion years ago.7 That was the good news: using the energy of sunlight and hydrogen from water to build energy-rich carbohydrates. The bad news was that highly reactive oxygen was being loosed upon the Earth. A new and threatening pollutant, free oxygen began to change the world. Because of its reactivity, oxygen unites with many minerals, or burns, and quickly leaves the atmosphere. However, with oxygenic photosynthesis now in play, bacteria were pumping oxygen continuously into the atmosphere, millennium after millennium. Though still a dangerous poison for some bacteria living today, in those early times a few bacteria devised a way to use oxygen! They reorganized their energy-acquiring metabolism to “burn” free oxygen. We call this oxygen-utilizing metabolism respiration. Oxygen-devouring respiration extracts ten times more energy from the breakdown of carbohydrates than does fermentation! Bacteria with this new metabolic system had a clear advantage. Simply stated, photosynthesis pulls apart the water molecule and respiration puts it back together again—a perfectly balanced cycle kept running by the power of our Sun.
INCREASING COMPLEXITY BY ADDITION
The emergence of oxygen-utilizing respiration laid the groundwork for another major advance in the history of life. Turns out, one of these respiring (oxygen-burning) bacteria became part of the eukaryotic cell. We call these organelles mitochondria. With oxygen more widely available for respiration, and powered by their mitochondrial fuel-cells, eukaryotes became the platform for further advances in biological complexity. Delivering hydrogen (from the breakdown of sugars) to oxygen, mitochondria gave complex eukaryotic cells the power to handle more elaborate genetic instructions and, eventually, produce more complex organisms. What respiration accomplished was to utilize a newly available resource—free oxygen—as a means of acquiring more energy.
Today, the division between the bacterial world and those organisms with larger nucleated (eukaryotic) cells is clearly the most significant division on the tree of life. Most bacteria lack sufficient volume to house a nucleus or other organelles. By sequestering DNA within the nucleus, fundamental genetic information was protected from the creative and destructive dynamics of metabolism. With more internal complexity and greater information-carrying capacity, the larger eukaryotic cell proved to be a singular advance in the history of life.
Just as the incorporation of the mitochondrion within the eukaryotic cell was a major step forward in the history of life, another bacterial endosymbiosis empowered yet another eukaryotic lineage. By making Cyanobacteria partners within their own cell walls, green algae became photosynthesizers themselves. As with mitochondria, these photosynthetic symbionts became essential organelles within plant cells; we call them chloroplasts. Using the same chemistry as their bacterial antecedents, chloroplasts absorb red and blue light, empowering themselves and those who eat them. By reflecting and transmitting the green light they're not absorbing, plants have turned our landscapes green.
These grand stages in the history of life, however, posed a very serious question for earlier biologists. How could minor and random mutations have fashioned so great an increase in living complexity? Clearly, addition is something more elaborate, and this is why the concept of endosymbiosis proved so satisfying. Discovering that both mitochondria and chloroplasts still carry a few of their own genes confirmed the hypothesis; these organelles had once been bacterial-grade organisms themselves. This was not a case of natural selection choosing among slightly differing base-pair sequences or new mutations. Instead, two highly structured entities came together to create a new and significantly more complex cell! Thanks to this very special kind of amalgamation, both respiring eukaryotic cells and photosynthesizing algae became major advancements in the history of life.
Mitochondria energize all larger, more complex organisms, from amoebas and fungi to plants and animals. And again, greater energy, supplied by oxygen-consuming respiration, allowed the eukaryotic cell to grow larger, house more chromosomes, store more information, and power the division and multiplication of a cell both larger and more elaborate than the bacterial cell. This innovation may have occurred as early as two billion years ago, but another billion years would pass before more elaborate multicellular plants and animals came upon the scene. And while free oxygen was being produced continually by Cyanobacteria in these earlier times, its presence in the atmosphere was minimal. Not until about 600 mya did oxygen pressure begin to rise, followed by the Cambrian Explosion of larger animals.
ANIMALS EVOLVE GREATER COMPLEXITY
Becoming a larger and more complex animal entails many costs. For starters, more energy is required when building a larger being. Subtle protocols are necessary to have hundreds of cells multiply and differentiate to join into a single coherent whole. A spirit of cooperation must be maintained as ever-greater numbers of cells unite to form the larger organism. Each cell must confine its own destiny to becoming a functional member of the larger individual. Differentiated cells within the larger organism perform specialized functions, providing the benefits of a division of labor. We humans have specialized cells to form our skin, our intestines, our muscles, and our brains. Though becoming bigger and more complex entailed many costs, the benefits have been astounding: from gelatinous jellyfish to many-legged millipedes, exquisite butterflies, and lumbering elephants. Larger animals garner more resources, cover more territory, and may reproduce more successfully. Unfortunately, greater organic complexity is fragile, ending in death and dissolution.
There's no avoiding the obvious: over time complex things break down. We are born, we grow, we reach maturity and reproduce, but then we decline and perish, to be superseded by the generations we helped bring forth. There comes a time when fixing an old machine becomes too costly, and it has to be abandoned. Larger, complex creatures eventually die. Not so bacteria or single-celled microbes! Their simplicity allows them to continue splitting in two for as long as their luck holds out. They are, in effect, immortal. While the human genome can be thought of as being immortal, changing only slowly over thousands of generations, we ourselves are mortal. Nevertheless, multicellular life has transformed the planet in ways the bacteria could not.
How did single cells paste themselves together, learn to communicate with each other, abandon their own agendas, and become organized into larger, more complex beings made up of thousands to trillions of cells? Second only to the enigma of life's origin, this question remains a major challenge for developmental biology. Free-living cells have a fundamental program: divide and multiply! Make more cells! But cells living within a complex organism cannot divide or multiply uncontrolled. To do so will destroy the organism itself. We know this only too well, and describe it with a single frightening diagnosis: cancer. When cells begin to multiply unconstrained within our bodies—no longer playing by the rules—we are in mortal danger. Think about this: each one of us is made up of trillions of cells working together to form a single integrated individual. Amazing! How do these many cells manage to work together, and, how were they assembled? These questions fall into two categories. The first is how did each one of us become the hugely complex individual we are? That's a developmental question, going back to our early embryonic origins. But here we are concerned with a more general question: How did the ability of complex animals to fashion themselves develop over time?
To build a larger organism, cells have to be able to both cohere together and communicate with each other. Once they were united, how did a system of constraints—a system that is basic to the functioning of all complex living things—develop? Only a delicately balanced system of intercellular communication can orchestrate coordinated development. Tissues need to develop where they are supposed to develop—and nowhere else! In developing animals, some cells must get out of the way to allow others to expand. This requires self-destruction (programmed cell death, or apoptosis). All the while, cells must duplicate in exquisite synchrony. We really do not know how our left and right arms end up so perfectly matched. Fortunately, most bilateral animals do have their right and left sides nicely symmetrical, so they can swim, run, or fly effectively. And finally, once the young animal has achieved a mature size, it must cease growing and begin reproducing.
Certainly, biological development must be fine-tuned to produce and maintain symmetrical forms of proper size, whether beetle, lizard, or whale. A closely cadenced harmony of genetic instructions, developmental protocols, and self-regulating networks has fashioned the world's larger life forms. Organisms that failed the challenges of symmetry and cellular harmony are long gone. Here is an evolutionary story that began over 500 million years ago and has been transformed into a developmental program repeated in each and every one of us. Recall that every leafy plant, feathery bird, and furry mammal arises from a single fertilized egg cell. Indeed, our development does recapitulate—in a general way—the major stages of our long evolutionary history.
For animals, forming new layers early in the life of the embryo was a key innovation. After a number of cell divisions, the fertilized egg transforms itself into a hollow sphere only one cell thick. Then, one side bends inward and invaginates to form a two-layered sphere with a little hole outside. The future of these two layers, inside and outside, are very different. Each will contribute different tissues to the developing animal. The little hole that's left outside can become a mouth (in snails, squids, crabs, and insects) or an anus (in starfish and vertebrates). Each of these two very different groups of animals then proceeds to develop a second little hole, providing another opening, front or rear. Having an input-hole at one end and an exit-hole at the other provided an efficient one-way digestive system! (Flatworms, sea anemones, and jellyfish have only one opening and represent an earlier stage of animal evolution.)
Both the development of a human baby from simple embryonic beginnings and the elaboration of a mature butterfly within its pupal skin are awesome transformations. The fact that such developmental trajectories have produced millions and millions of humans and even more beetles, should not diminish our astonishment. As Neil Shubin points out, “Like a concerto composed of individual notes played by many instruments, our bodies are a composition of individual genes turning on and off inside each cell during our development.”8 The discovery that plants, animals, and humans use similar gene systems to guide their development is evidence for common origins very long ago.
What are the essential components in this story of increasing complexity? Genes are the starting template; they fashion a wide variety of proteins that do the work of the cell. Regulatory DNA activates genes when and where they are needed. Changes in developmental timing are a major source of evolutionary novelty. All the while, within-cell metabolic networks maintain stability (homeostasis), essential to keeping life processes running properly. In turn, cell-to-cell interconnectivity and communication build and maintain the larger organism. Cells generate ordered complexity by responding to subtle inputs from adjoining cells; they work together in a matrix of connectivity, producing larger tissues and organs. These, in turn, must be coordinated by a complex nervous system, keeping the entire organism on a proper course. Animals and plants use a variety of non-linear processes to form interactive networks that result in emergent self-organizing systems of great structural complexity. Each of us has developed and grown in exactly this way. But the more extraordinary claim is that natural processes, slowly fashioned over these last three billion years, have resulted in the biodiversity we see today.9
ELABORATING ANIMAL COMPLEXITY
One of the unusual revelations of the late twentieth century was that all animals share many of the same developmental genes. An eye-gene from a squid, placed on the embryo of a fruit fly, produced an eye at that location: a fruit fly eye, not a squid eye! Clearly, the “put-an-eye-here-gene” is the same for the squid and the fruit fly. Similar results were obtained transplanting mouse-eye genes. Again, the fly developed a fly eye at the site of the transferred mouse gene. These very different organisms, with very different kinds of eyes, have been using similar genes to build their very different eyes. No one had thought that the camera-like eye of squid or mouse had a common origin with the multi-faceted eye of an insect. And, in fact, they don't. But different lineages did use the same genetic tools in fashioning their different eyes over evolutionary time. Moreover, all animals use light-sensitive opsin molecules in the business of sensing light. We humans have three genes for making three slightly different opsins, allowing us to see the world in a great variety of colors. Apparently, where one gene would produce one opsin, a duplicated gene could produce a slightly different opsin, and the beginnings of color vision. Where one developmental gene might initiate a single lobe-like appendage on a velvet worm, additional—accidently duplicated—genes could create the several-jointed leg of an insect. Genetic analysis has revealed how each animal lineage came to have its particular Bauplan.10
Not that long ago we thought that humans had at least 100,000 genes. Why not? We are hugely complex beings and very, very smart! But after further study, it turns out that we've got about 24,000 protein-coding genes.11 Worse yet, the species closest to us in both form and behavior—the chimpanzee—shares between 94 and 98 percent of our genes (depending on how you score the genome). Unfortunately, these enumerations focused on protein-coding genes, and that's only a part of the hereditary story.
Let's change our focus for a minute, and look at plants. It turns out that our favorite laboratory plant, the thale cress (Arabidopsis thaliana), has about 24,000 genes! At the time of discovery, this was a huge surprise. Our other laboratory favorite, the fruit fly (Drosophila melanogaster) was estimated to have only about 13,600 protein-coding genes. Think about this! The weedy thale cress grows to little more than a foot in height. Then it just sits on the ground capturing sunlight to make flowers and produce seeds. The fruit fly, in contrast, has a full four-stage life cycle, can fly hither and yon, and has males that not only search for females but can dance little dances to entice the ladies. One would think something as complex as a fruit fly ought to have lots more genes than a static weed. This is significant. The few genes that differentiate Homo sapiens from Pan troglodytes, or the numbers of genes in the thale cress are telling us something.
Obviously, the number of protein-coding-genes is not the whole story. This is especially true in the plant-animal comparison. Cottonwood trees have recently been shown to have around 44,000 genes! Further genetic studies have indicated that many parts of an animal genome determine RNA sequences that are important in controlling the timing of cellular and developmental functions. In fact, RNA processing and modification are more versatile in animals and may explain why fruit flies have “fewer genes” than the little plant. This may also explain why we humans differ so profoundly from chimps, despite sharing so many genes. There is an intricate developmental trajectory that takes us from our genetic blueprint (our genotype) to what we end up looking like (our phenotype).
Problem was, geneticists began with a far too simplistic concept of “the gene.” Protein-coding genes make up less than 5 percent of our genome; the rest was called “junk DNA.” But if it was junk, why hadn't natural selection gotten rid of it? Turns out that much of our genome maintains the organization of the chromosome, regulates gene expression, codes for RNA regulation of cell activities, and does carry some “junk”—bits of viruses and “fossil genes”’ that are no longer functional. There's lots more to do than code for proteins. And here is a more significant point: larger amounts of “junk DNA” are correlated with the increasing complexity of the animals that bear them.12 Complex life depends on many different gene networks, precisely timed developmental cues, flexible physiology, dynamic morphogenetic fields, and lots more.13
Our early development (early ontogeny) indicates how developmental protocols have changed over evolutionary time. We humans build and replace two rather different kinds of kidneys during our early development. As you might imagine, a fish-like kidney forms first, but is resorbed. A kidney rather like that of reptiles forms next, and this too is abandoned. Finally, we end up with a mammalian kidney that serves us for the rest of our lives. Such embryological sequences prompted Ernst Haeckel, evolution's early proponent in Germany, to declare that “ontogeny recapitulates phylogeny.” Early embryogeny and development provide further evidence for our long evolutionary history. Blood vessels go down, around, and up again in the region of our throat. If you've got a Y-chromosome, testicles formed within your body, then exited through the abdominal wall to keep themselves cool (leaving a weak spot and making a hernia more likely). Such observations are testimony for our long evolutionary history (they do not support a “six-day creation 10,000 years ago”).
“The evolution of complex creatures is no mere epiphenomenon; it is one of the marvels of the universe” claims Wallace Arthur. “From no head to rudimentary head to well-developed head to sophisticated head is definitely a series of steps up the ladder of complexity.”14 Just take a close look at a bug's head; check out the fancy mouthparts, big eyes, and slender feelers, all exquisite and purposeful parts. Though bacteria and jellyfish have remained constrained within their particular levels of organization over hundreds of millions of years, other lineages explored new possibilities. And because life had arisen within a world of water, living beyond the liquid realm necessitated new and extraordinary innovations.
THE SECOND GREEN REVOLUTION: A TERRESTRIAL FLORA
In the introduction we noted that today's biodiversity—in the sense of larger life forms—manifests itself most grandly on the land. However, moving from an aquatic lifestyle to survival on dry land was a major initiative, and especially difficult for plants. The key innovation may have been elaborating the plant's diploid state to become a more important part of the life cycle. The sperm of algae ancestral to land plants (Charophyta) must swim to the egg cell to form a zygote with two sets of chromosomes (the diploid condition). This produces a short-lived plantlet, which soon undergoes meiosis, building another generation of haploid algae that will produce new sperm and egg cells. Here, the diploid stage is short, and the entire lifespan is underwater. How might these aquatic algae have changed to live at least a part of their life span on dry land?
Land plant genomes possess two sets of developmental KNOX genes, while their ancestral algae have only one set! Apparently, the newly duplicated set of KNOX genes allowed the diploid zygote to become a longer-lived diploid plant!15 In effect, early land plants had two prominent life phases: a haploid generation and a diploid generation. The diploid plant—after meiosis—produces haploid spores and a new haploid generation. This haploid generation then produces gametes that will unite and form a new diploid plantlet. Botanists call this the alternation of generations and it is an important part of the lives of ferns, mosses, and liverworts. But what made this new two-stage life cycle so important?
Here we have a clear evolutionary advance based on a single duplication within a family of developmental genes: KNOX1 and KNOX2! Imagine the following scenario: our ancestral alga is living in a small pond; it is haploid and produces sex cells (gametes). Swimming sperm fertilize egg cells under water and begin the new diploid generation. Then the dry season begins, our little pond dries up, but the diploid plant now basks in the light of the Sun and lives long enough to undergo meiosis and produce haploid spores! Such spores can be released into thin dry air to be carried far and wide by the wind! In this scenario, water is still necessary for swimming sperm to reach the chemically attractive egg cell and produce a diploid zygote. But, with KNOX2, the diploid stage expanded to become a plant that could survive and disperse its haploid spores over long distances. Here we had a diploid plant able to live on land, and a haploid plant (formed by the spores) still living in water, where sperm could swim to egg cell. Later, after sperm cells “learned” to swim to the egg cell on thin films of water on plant surfaces, the life cycle became completely terrestrial.
Unlike the sudden explosion of larger animal life in the early Cambrian, one hundred million years would pass between the first evidence of land-plant spores and the appearance of tropical forests. Plants faced many challenges before they covered the land in greenery. A waxy surface constrained water loss; stomates regulated gas exchange; roots and plumbing brought water up within the plant, and, later, a tubular meristem would construct tall trees. A bit later, seed plants advanced into drier environments. The terrestrial world was now graced with tall forests and diverse vegetation.
A THIRD GREEN REVOLUTION
Beginning around 130 million years ago, flowering plants fostered yet another grand escalation in terrestrial biodiversity. Allocating less energy to defense and more to growth and reproduction, Angiosperms quickly overtook the other greenery.16 Colorful flowers and animal pollinators allowed many Angiosperms to reproduce effectively at lower population densities, packing a greater number of plant species into the same biome. More species, greater structural variety, and new flowering and fruiting modalities created environments that were both more elaborate and more nutritious. Nectar-feeders became flower-pollinators. Fruit-eaters became seed-dispersers. Between about 125 and 80 million years ago, the terrestrial fossil record displays a significant pulse of diversification. Given the acronym KTR (K for Cretaceous, T for terrestrial, and R for radiation or revolution), this was a time during which many lineages of both plants and animals expanded their numbers in grand synchrony. In fact, this may have been the moment in time when terrestrial species numbers surged beyond the numbers living within the sea.17
Today, plant variety is displayed most grandly within the lowland tropical rain forest; here flowering plants display their greatest structural variety. Trees with wide buttresses bracing tall trunks ascend into the upper canopy, even as thick woody lianas swing down from high above. Broad-leaved aroids adorn the forest floor or clamber up trunks toward brighter light. Palms reach into the lower canopy with broad feathery leaves at the tips of slender, unbranched trunks. Banana-like plants with leaves up to twenty feet tall cluster at stream margins and in clearings. Green foliage varies from simple elliptic leaves to those variously lobed or divided. Surprisingly, one sees few flowers within the rain forest; they've got all year in which to bloom, and most are found in the higher canopy. Greater structural diversity among flowering plants provided more niches for more species. Mosses, little ferns, and small orchids could grow on widespread branches, while primates, birds, and insects fill the canopy with noisy chattering. As we stated earlier: flowering plants have been the primary drivers of expanding terrestrial biodiversity over these last one hundred million years.
CHANCE OR NECESSITY?
Returning to the origin of land plants themselves, their apparent unity-of-origin brings up a troublesome evolutionary question. Stephen Jay Gould phrased it very nicely when he asked, “Would we [humans] appear at all, if we could rewind the tape to an appropriate beginning (say the origin of modern phyla in the Cambrian explosion more than 500 million years ago) and simply let it run again?” Gould thought that the role of accident and contingency were so fundamental that most plants and animals would, indeed, be very different “if the tape were run again.”18 Disputing Gould, Simon Conway Morris argued that environmental effects would tend to select creatures similar in their adaptations. To support his ideas, Conway Morris cites many cases of convergence, where different lineages of plants or animals had done much the same thing—independently—through convergent evolution.19 Obviously, if you are an animal planning to go somewhere it's good to have your head up front so you can see where you're going, no matter what kind of animal you are. That's a general convergence, but more specific trends are also common. The marsupial mammals of Australia have produced forms that resemble jumping mice, moles, and small dogs, quite similar to those found among placental mammals outside Australia. The tall columnar cacti of American deserts look rather like tall euphorbias in Africa, but they are unrelated, having evolved in widely separated hot dry climates. Surely, both Gould's and Conway Morris's arguments have validity. Both contingency and convergence have been part of life's history.
Unlike the situation among land animals, where a number of unrelated lineages pioneered the land, only a single lineage of plants succeeded in becoming terrestrial. Perhaps, without this “lucky break” Earth might be devoid of complex terrestrial ecosystems. Similarly, if early primates had not pursued insects into the tops of flowering trees, perhaps no similarly versatile—or clever—mammalian lineage could have become human. Both accidental contingencies and selective environmental constraints have determined the course of life's long history—initiating opportunities and then driving lineages down particular paths. But where did new genes and new developmental protocols come from?
MURPHY'S LAW: A WELLSPRING FOR DIVERSITY AND PROGRESS
Tradition has it that there really was a Murphy, an engineer in the United States Air Force during World War II. He proposed a rule for designing control mechanisms, but that rule was soon corrupted and transformed. Murphy's rule had suffered the fate decreed by Murphy's Law. Today, Murphy's Law is simple and universal: In complex systems, what can go wrong will go wrong! In the reproduction of living beings, things often go wrong. However, on rare occasions, “errors” can be the basis of adaptive change. Failures will be eliminated: that's what natural selection is all about. Our common intestinal bacterium, the much-studied Escherichia coli, suffers mutations with detectable effects all the time. Researchers estimate that each “good” mutation arising in E. coli is outnumbered by around 100,000 “bad” ones. Not a good ratio! But clearly, not everything gone wrong is deleterious. In fact, rare mistakes may open the door to new possibilities.
Accidental gene duplication, resulting in two genes where one was quite sufficient, provided an opportunity to do new and different things. As we just mentioned, duplication of the KNOX gene may have allowed for the expansion of the diploid generation in early land plants: one of the most important advances in the history of life.20 Among land vertebrates, one developmental HOX gene gives rise to the upper arm, a second to the lower arm, and a third to the hand. Such development-altering duplications may be even rarer than the 100,000-to-1 mutation ratio reported in E. coli. However, over hundreds of millions of years, such rare events have propelled evolution along a trajectory of ever-greater complexity.
Recent work in developmental genetics has made clear the importance of duplicated genes. As already noted, we humans have three slightly different light-sensitive opsins, coded during their development by three similar but slightly altered genes. Together, they give us trichromatic color vision. Many other attributes appear to have been elaborated through gene duplication as well. The laboratory mouse has nearly a thousand genes devoted to the business of olfaction—sniffing and smelling. For animals that spend so much time close to the ground, scent provides important information. Marking their territory with scents, they communicate with their neighbors. Elaborating those many genes involved quite a bit of gene duplication, as well as modification of old genes for new tasks. Interestingly, the human genome carries the same olfactory genes as does the mouse. While this may seem like an academic detail, here's the big news: in humans, a large number of these olfactory genes are no longer functional! We no longer possess the olfactory sensitivity our distant ancestors once had, because primates decided to live in treetops. Here scent-marking was unimportant, but seeing in three dimensions and in color was important. Selection worked on our visual talents, even as our ability to smell declined. This loss of ability is now witnessed by the many genes that still sit on our chromosomes but no longer have a function. These “fossil genes” are further evidence of our deep history.
Speaking of adaptation by selection, we must remember that hereditary mechanisms are tightly constrained. Information carried by the DNA is translated into messenger RNA within the nucleus and then exported to other parts of the cell to build proteins or regulate reactions. This process cannot be reversed. Proteins cannot redirect or reconfigure DNA within the nucleus. No matter what experience the adult suffers, the consequences of that experience does not alter their DNA, safely sequestered in the nucleus. Jean-Baptiste Lamarck was wrong: experiences of a parent's life cannot be added to the information carried by the hereditary material. (Children of the stone mason are not born with stronger arms.) If scientists want to change genetic instructions, they must insert new DNA snippets into a chromosome residing within the nucleus. Today, genetic engineering can do exactly that—using plasmids and other vectors.
Over time, Murphy's Law has provided a continuing source of mistakes, duplications, and rearranged genetic instructions. Winnowed by selection, a very few of these changes have helped organisms modify their development or their abilities and live out their lives in new ways. (Actually, many mutations seem to be “neutral,” making no measurable difference.) It is the few good mutations, duplications, transpositions, and other things gone wrong that provided continuing variability—to be tested by an unforgiving environment. Sexual reproduction enhances this process by keeping a species’ genetic resources constantly in flux, allowing selective processes to continually screen the population. “Cruel and heartless!” you might think, but this process has allowed complex living things to adapt and transform themselves over hundreds of millions of years.
COMPATIBLE NOVELTY, CONSTRAINED DISORDER
Genes do not run our bodies or our lives. Living cells and tissues are incredibly complex, maintained by many interactions. Genes provide the basic instructions and controls, but cell machinery, mostly run by proteins, builds, and runs itself. Bad genes make bad proteins, which can compromise function; really bad genes can stop early development in its tracks, and the story ends there. It is the integrated organism that lives, reproduces, and, ultimately, dies. Each of us began as a plump egg cell inspired by a single sperm. That egg cell was where all our cell membranes began, where all our cytoplasm originated; it housed the mitochondria that would power all our life processes, reproducing themselves with every cell division. Thanks to their complexity, cells are the only source of new cells. All the while, DNA sits protected within the nucleus, duplicating itself with every cell division, and providing the basic instructions as they are called for. More versatile and dynamic, RNA carries instructions beyond the nucleus and participates in many life processes.
Genetic determinism is a concept that explains only a few exceptional traits. In reality, there are a long series of networked interactions that transform an organism's genotype into the characteristics of the mature individual, its phenotype. A multidimensional network of genetic instructions and interacting regulators provides flexibility and redundancy within each cell. Barbara McClintock's “jumping genes” (transposons) can alter the neighborhood of individual genes and change their functioning in that way. Neutral mutations may enhance the elaboration of a particular gene at a later time. A specific enzyme may be coded by different genes with a variety of different configurations, allowing the entire system to adapt and innovate. Within an individual cell, metabolisms are diverse and dynamic. However, the entire system must be robust and maintain effective homeostasis: the process of keeping everything in balance within the cell and the organism. We humans are composed of about a trillion eukaryotic cells, arrayed in some 220 different tissue-types. But when we take our temperature, the readings are usually around 98.6 F; this is an amazing feat of self-regulation! While some disorder allows for flexibility and innovation, the stringent control of disorder (homeostasis) has been a necessary requirement throughout the history of life.21
Key innovations, major breakthroughs, and new levels of complexity are all convenient ways to mark major steps in the history of life. Special attributes help us demark major lineages. Cladistic systematics defines lineages by their unique traits. Mammals, in contrast to other land vertebrates, have hair, nurse their young, and possess three little bones within their inner ears. Similarly, flowering plants have seeds enclosed within a protective ovary and use double fertilization to initiate seed development. These important and unique characteristics distinguish mammals and flowering plants from related lineages. However, it seems unlikely that such key innovations sprang into being suddenly. The idea of a single major mutational change (a macromutation, or a “Hopeful Monster”) suddenly ushering in new possibilities may satisfy simplistic views, but is utterly unlikely. Whether for a bacterium, a eukaryotic cell, or a multicellular organism, innovations must come in small steps that do not disturb dynamic balance within the organism. Whether at the level of the cell, the tissue, or the individual, dynamic integration between all the relevant processes must be maintained. A major new mutation in one aspect of the cell's functioning cannot disrupt other vital processes. The dynamic network can be tweaked and shifted, but if other elements in the network are adversely affected the new mutation cannot spread through further generations. Selection acts upon the entire organism; if the new mutation compromises the well-being of its host, it has little likelihood of being incorporated into that species’ future. To be maintained, “selfish genes” cannot impair the lives of those who carry them. The integrated functioning of the phenotype—the actual living being that the genotype produces—is paramount. And this means that genetic changes are most often minor, allowing all the players in the network to adapt and adjust throughout the developing organism.22
One of life's grandest progressive steps took place when one lineage of fish began to adapt to a new and challenging environment. The fish-to-amphibian transition took millions of years and required many subtle changes. As we mentioned in the last chapter, this advance began with lobe-finned fish having to negotiate obstructions in log-jammed streams and estuaries. Lobed-at-their-base, these fins had more flexibility than fins firmly planted on the body, allowing this particular lineage of fish to begin a new trajectory. Lobed fins soon developed something new—elbows—for greater flexibility. Pushing themselves through obstructions gave rise to further innovations: shoulders to attach muscles, and a neck to give the head movement. Snapping-up prey along stream sides fed these “evolving fish.” This scenario required more flexible forelimbs, a shoulder girdle to give the head and neck more independence, and digits for better traction. Quick-attack made greater demands on the central nervous system, vascular system, and musculature of the front limbs. Becoming better aerial hunters required better vision, olfaction, and hearing. Slowly a fish-like animal adapted to the land and transformed itself into a four-limbed tetrapod. A constellation of anatomical and physiological characteristics were changing together over evolutionary time. These hungry fish were not “evolving to live on land,” they were stalking tasty invertebrates along the shoreline.23
IT'S A WAR OUT THERE: THE RED QUEEN'S ADMONITION
Biologists often talk about fitness. “Survival of the fittest” became a catchphrase soon after Darwin published his bold new view of how life had elaborated itself over time. And, logically, fitness came to be equated with reproductive success; the more of your genes that become part of the succeeding generation, the more fit you have been. Critics complained that this made evolution-by-natural-selection circular! Fitness defined evolutionary success, and evolutionary success was defined by fitness. What these dimwits failed to comprehend was that fitness—measured as reproductive success—was a process over time.24 Operating over time and across generations, advancing fitness couldn't possibly be circular. Biologists have continued working with the idea of fitness, mostly from the point of view of the individual organism and its reproductive success. However, with a little bit of imagination, there's another way of looking at fitness.
Leigh Van Valen of the University of Chicago approached the notion of fitness from a new perspective by thinking of fitness in terms of the overall species, rather than any individual within that species. He suggested that fitness might be a measure of an entire species’ use of energy in the ecosystem in which it is living. And, he reasoned, if a species is increasing its overall fitness over time, it must be taking a greater bite out of the energy available in that ecosystem. After all, the amount of energy in any ecosystem is tightly constrained, ultimately dependent on solar energy. From this premise, Van Valen argued that as an individual species increases its fitness it must affect all other organisms in the system negatively! And, just as the Red Queen advised Alice in the story by Lewis Carroll, Van Valen suggested that each species finds itself in a world where “It takes all the running you can do, to keep in the same place!”25 Surviving in dynamic communities demands constant adaptation, not only to the physical aspects of the community but also to other ever-changing biological members of the community.
The Red Queen hypothesis highlights the competitive interactions between species living in the same environment. Other paleontologists studying extinctions over time had concluded that the random trials and tribulations of changing climates and other minor calamities were paramount. Focusing on unpredictable changes in the physical aspects of the environment, this has been called the Field of Bullets hypothesis.26 Here, a sudden physical event can terminate your existence just as effectively as a bullet. Actually, both the “Red Queen” and the “Field of Bullets” are reasonable scenarios; they are not mutually exclusive.
The pressure to keep running is not just in response to our parasites, pathogens, and things trying to make a meal out of us. We must compete with others of our own kind for nourishment, territory, and mates. Competition is an overall dynamic, sculpting many aspects of the ecosystem. Unless you find a special corner in which to hide, you better be able to keep up with the crowd. One way animals can “hide” is through camouflage, making it difficult for predators to find their prey.27 Mimicry allows a harmless insect to resemble a stinging wasp, or a moth to fold its wings and “become” a dead leaf. Both camouflage and mimicry are graphic evidence for a perilous world.
Over time and with new “inventions,” many lineages were able to improve their competitive standing. Vascular tissue in land plants allowed them to grow taller and shade smaller non-vascular plants. Pollen allowed seed plants to expand into drier climes. Recall the lichen symbiosis, which allowed these “composite organisms” to thrive in some of the world's most severe environments.
Pest pressure and incessant competition have populated the harshest environments, and the reason is easy to understand. In a dangerous world, peripheral environments may be physically more demanding, but less challenging in terms of disease and competition. John Bonner called this the pioneering effect and argued that this leads to the invasion of increasingly stressful habitats.28 Bonner's scenario claims that competition, together with disease and parasitism, have been the driving force populating some of the world's most uncomfortable habitats. Recall the Archaea; their ability to live in boiling water, high acidity, or high alkalinity may have taken hundreds of millions of years to develop, and they now have these nasty environments mostly to themselves.
Bonner's suggestion reveals a serious flaw in discussions regarding the possibilities of life elsewhere in our solar system. Such arguments have assumed that if bacteria can live in near-boiling water, in rock fissures deep underground, or within frigid Antarctic ice here on planet Earth, then it is reasonable to expect life forms in similar environments elsewhere in the universe.29 Nonsense! The flaw in these arguments is that they ignore the problem of life's origin and earliest elaboration. Bacteria living in Earth's extreme environments have had three billion years of competition to end up where they're at; it seems highly unlikely that they originated in these extreme habitats.
FOSSIL EVIDENCE FOR A MORE COMPETITIVE WORLD
Paleontologist Geerat Vermeij has examined what he calls Evolutionary Escalation by studying the rich fossil record of mollusks. Unlike most shell collectors, Vermeij has been especially interested in those specimens that were broken, damaged, or repaired during their lifetimes. Blind since childhood, Vermeij studies shells with his sensitive fingers and became fascinated by the kinds of damage he encountered. By studying damaged shells in the fossil record, he examined how some mollusks have changed over geologic time. In his book on escalation, Vermeij proposed several hypotheses to test the notion that competition and predation have played an important role in evolutionary trends.30 First, he hypothesized that competition and anti-predator capacities of individual lineages have increased over time. Second, he suggested that recently evolved individuals should be better adapted to a more hazardous environment than their earlier ancestors. Third, he predicted that, as time moves forward, hazards in the environment will have become more severe. Vermeij tested these conjectures with examples from many sources, but the damaged shells of fossil seashells were his primary data set. As the crushing pincers of some crabs have become larger and more powerful over millions of years, seashells—their prey—have developed thicker, stronger, and more elaborately ornamented shells. (The survival of many seashell lineages with thinner shells, or crabs whose claws have not become enlarged, does not negate the fact that a few lineages were part of an “arms-race.”) Consistent with both Vermeij's “escalation” and Van Valen's “Red Queen,” the fossil record supports the notion of increasing predation intensity over time.31
Competition really does appear to be a critical element in directing evolutionary trends. Our antibiotics to fight bacterial infections are a fine example. Penicillin, streptomycin, and similar compounds were first derived from Actinomycetes. These are bacterial-grade microbes that live in the soil, breaking down organic matter for their sustenance. The reason they produce powerful antibacterial substances is simple: bacteria are consumers of the very same resources. What better way to survive than to poison your competitors? These same “poisons” have become our antibiotics, saving millions of human lives.
The sudden expansion of mammals after the end-Cretaceous extinction has been explained as a release from intense predation after the dinosaurs had been eliminated. Once those fearsome beasts were no longer part of the landscape, mammals diversified explosively. Here, a change in the competitive landscape allowed an underclass of smaller furry animals to proliferate. A similar scenario occurred after South America became linked to North America via the new Panamanian land connection around three million years ago. Surrounded by oceans for more than 50 million years, “Island South America” had developed a very peculiar mammalian fauna. Sloths, anteaters, armadillos, neotropical monkeys, and a rich variety of smaller marsupial mammals (such as the opossum) are living representatives of South America's original fauna. Extinct giant ground sloths, large ungulate herbivores, and huge armadillo-like creatures were also part of South America's distinctive fauna. But with the “Panamanian bridge” suddenly available, land animals were able to move between the two continents. Two distinct faunas came into direct contact, giving rise to what has been called “the great American interchange.”32 Giant ground sloths, opossums, armadillos, and monkeys moved from the south into North America. These animals added to the rich fauna of the north, but the story on the other side of the exchange was different. From the north came a more lethal crowd. Wolves, bears, large and small cats, raccoons, deer, camellids, and various rats and mice made South America their new home. For many indigenous South American animals these new neighbors proved deadly. A distinctive fauna was no longer protected by its isolation, and many of its unique animals became extinct. Initially both North and South America had about twenty-six families of mammals. However, after the intercontinental exchange, 50 percent of South America's original mammalian genera were gone, while North America lost around 28 percent. (Ice age fluctuations account for the northern losses. But then, beginning around 12,000 years ago, humans with a new hunting technology eliminated many large mammals throughout the Americas.33)
Among neotropical plants, there is no evidence for extinctions after South America became connected to North America. Clearly, among animals, competition plays a more critical role in fashioning the biota. This is especially apparent on isolated islands, where less competitive animal lineages still survive, whether marsupials and monotremes in Australia, or lemurs on Madagascar. Isolation provided protection. All of South America's endemic ungulates became extinct after they were challenged by a more aggressive northern fauna.
COMPETITIVE ARENAS: LARGE AND SMALL
What made the North American mammals more effective competitors than their southern neighbors? Why have island animals been weaker competitors than continental invaders? The answer appears to be: more intensive competition over a larger land area for a long period of time. Contiguous Africa, Eurasia, and nearby North America provided an immense “playing field” on which competitive scenarios could play themselves out. We've seen much the same thing in more recent times, as the introduction of Eurasian plant diseases wiped out the American chestnut and decimated American elms. Worst of all was the loss of Native American people after Columbus had opened the New World to European immigration. With the introduction of human diseases from Eurasia and Africa, indigenous American populations suffered huge declines. Native Americans had not been part of the “arms-races” between humans and their pathogens ongoing in Africa and Eurasia over the previous 15,000 years. Isolated from these nasty interactions, Native Americans lacked the immunities that people in the Old World had acquired over this time span. During the two hundred years following the discovery of the “New World,” population decline among Native Americans may have been as high as 80 percent. Whether for North American trees or Native America peoples, the scenario was the same. The larger contiguous Old World land masses, with continual pathogenic interactions, provided a more harrowing environment than a smaller isolated land area. When the survivors of the larger war encounter those who have not undergone this same trial by fire, the former prevail.
The take-home message from these various tales is clear: competition, whether in the form of disease, predation, or crowding, has been a driving force of evolutionary advance. A majority of species formation appears to be driven by niche division and ecological differentiation (chapter 2). This too is a direct consequence of competitive interactions, where peripheral populations avoid competing with others of their own kind, by entering territories new to members of their own species. Similarly, the pioneering effect drives populations into ever-more challenging environments. Competition results in a world of increasing crowding and nastier neighbors. Nevertheless, some species can “facilitate” the survival of other species, whether weeds beginning plant succession or insects pollinating flowers. Together, these many factors give us a world that is both dynamic and has become progressively more complex. (This view is not an artifact of the “hindsight fallacy” where, looking backward, we seem to see directional trends.) Let's consider two grand triumphs in the history of increasing biological complexity: social insects and mammals.
A NEW LEVEL OF COMPLEXITY: THE SOCIAL INSECTS
As we discussed in chapter 1, those insects with a four-stage life cycle have amassed the highest species numbers within the animal kingdom. By dividing their lives into a start-up stage (the egg), an eating/growing stage (the larva), a transformative stage (the pupa), and a dispersing/reproducing stage (the adult), these animals have become the most numerically abundant on the planet. But now, let's switch from the number of species to the number of individuals.
In chapter 6 we saw how between-species cooperation could lead to greater overall biodiversity. Whether fig trees and their wasps or soil fungi and plant roots, cooperative symbioses have helped many species live together successfully and multiply. But there is another form of cooperation we didn't mention: within-species cooperation. No single wolf can hunt with the effectiveness of a pack of wolves; no single ant can do what a colony of ants is capable of. Social species are yet another advance in biological complexity. The most advanced of these are eusocial animals, living in multi-generational communities with a stratified division of labor. Here, individuals sacrifice their own interests—even their lives—for the good of the community. Leaving our own super-social species for the final chapters, let's examine the social insects.34
Eusocial insects have non-reproductive castes within their communities to perform specific tasks. The best studied example of this lifestyle is in honey bees (Apis melifera), where sterile worker bees maintain the hive, nurture the young, and gather food, but do not participate in reproduction themselves. The young worker bee begins by nursing the larvae, becoming an active forager only later in her life. Here is a clear division of labor within the life span of the individual worker—all in the service of the colony's welfare. The beehive and its queen make up a single entity, often called a superorganism. Other eusocial insects include termites, some wasps, some bees, and many ant species. Though eusocial insects comprise only 2 percent of described insect species, surveys suggest that they make up a majority of the world's actual insect biomass! In one Amazonian rain forest survey, 75 percent of total insect biomass was estimated to be made up by colonies of eusocial insects.
Found only in neotropical forests, leaf-cutter ants carry leaf fragments down from high trees into their underground nests, nourishing the fungi they feed upon. Large underground colonies can number as many as five to ten million ants. Their leaf-gathering activities are often clearly evident on the forest floor, where narrow trails, free of debris, lead to favorite trees. (They remove litter from their pathways!) Leaf-cutter fungus culture is estimated to have evolved from debris-gardening ants around ten million years ago.35 These little ants, thanks to their numbers and their industry, play a significant role in the life of lowland neotropical rain forests.
Astounding: only two percent of insect species, but accounting for more than half of insect biomass! Stratified social hierarchies have paid off for these lineages. Just as Adam Smith declared: the division of labor provides economic efficiencies. Among some ants, aggressive “soldiers” are ready to sacrifice their own lives for the protection of the colony—a very effective way of maintaining the fitness of the group. But why is eusociality so rare? The hurdle appears to be that a large percentage of individuals must become genetically reconfigured to abandon their own reproductive potential for the “greater good” of their community—and this doesn't come easy.36 Eusociality seems to be a bold contradiction to ordinary natural selection, where the reproductive success of each individual is the object of selection. Special preconditions in the behavior, genetics, and lifestyles of these animals were necessary before eusociality became possible.37 Here, the fitness of the individual is sacrificed for the greater success of the community, in what has been called group selection or multilevel selection. The cooperative community is the unit-of-selection and not the individual!
The hive or the colony stands or falls on the effectiveness of all its castes working together for the common good. Despite their little brains and limited behaviors, natural selection has given rise to a “swarm logic,” making these insect societies remarkably resilient. While rare, this strategy of increasing social complexity and stratification has proven successful over a wide range of habitats. Not a big jump in species numbers perhaps, but surely a large increase in numbers of individuals.
Termite mounds are often a conspicuous feature in tropical grasslands, where they play a role in soil dynamics and where termites make up a significant portion of the living biomass. Eusocial ants are numerous and important around the globe. From a broader perspective, the elaboration of insect societies is another fine example of how our living world has become increasingly elaborate over recent evolutionary time. But while insects are the most numerous of all land animals, they are not the most dominant.
ANOTHER PARAGON OF LIVING COMPLEXITY: THE MAMMALS
Warm and furry, mammals are a marvelous and unusual crowd. Numbering around 5,500 living species, these animals are found in even the most severe terrestrial ecosystems. Their unusual success is due to a variety of factors. Most notable is a warm covering of fur, especially important for little mammals that lose heat rapidly because of their larger relative body surface. Consistently warm body temperatures allow mammals to be active over a wide range of external temperatures, metabolize more rapidly, and respond immediately to challenges. Maintaining higher temperatures, however, demands a steady supply of nutritious food. This, in turn, required the differentiation of teeth able to cut, tear, and grind. In addition, all mammals share a uniquely complex inner ear. Better hearing may have been essential for living a nocturnal lifestyle during those times when quick carnivorous dinosaurs ruled the daylight hours. Mammals are further distinguished by mammary glands, providing high-quality food for their newborn young.
The earliest mammals diverged from mammal-like reptiles about 200 mya, and produced a variety of lineages over the ensuing 150 million years.38 Like many other lineages, mammals show some increase in diversification as the flowering plants expanded, around 80 million years ago.39 Today's most primitive mammals are the monotremes, still laying eggs, and with their arms and legs splayed out sideways in a reptile-like configuration. More advanced living mammals fall into two groups: the marsupials and the placentals. Marsupials give birth to small, poorly developed young that must crawl to the mother's teats, where they attach themselves and begin nursing. The reason the young are born so small is that they must escape the uterus before the mother's body produces antibodies against them. Since the little ones contain genes from their father, they are, in effect, foreign bodies within the mother, and the mother will produce antibodies that can kill them. Developing largely while nursing has constrained the variety of marsupials. Nevertheless, some marsupials resemble mice, a few look like dogs or little bears, and some become quite large, hopping around on their hind legs. Overall, the marsupial lineage has been limited in its morphological diversity—all because the little ones must escape the womb early in their development. Not so the placentals!
One of the most sophisticated inventions in the history of animal life has been the placenta. This intricate organ allows the blood vessels of the mother to transfer food and oxygen to the blood vessels of the fetus, while accepting carbon dioxide and waste products from the fetus. Thanks to the complex placenta, fetal growth transpires without triggering antibodies in the mother's own body. What this means is that a mother elephant can carry her calf for twenty-two months before giving birth. It means that the fetus can develop, fully protected, within its mother's body for however long it takes. Newborn whales and porpoises come to the surface for air and swim alongside their mothers immediately after they are born. Newly born antelopes can run quickly only an hour after birth. Once the dinosaurs departed, the complex placenta has allowed placental mammals to diversify into a huge variety of forms.40 The smallest mammals, little shrews and the tiniest bats (weighing around two grams, or 1/16 oz.), and the blue whale (reaching up to one hundred tons) are all “Placentalia”! No other animal lineage comes close to being as diverse in size and form as do the placental mammals. All thanks to a placenta that effectively prevents the mother's defense system from destroying the fetus she is carrying.
Another advance in complexity is found in the ruminant mammals with a four-part stomach and the ability to regurgitate their food so that it can be chewed again for further digestion. Because of the many microbes living in their extended alimentary canal, these are among the few mammals that can derive energy from the digestion of cellulose. While all mammals have microbial symbionts helping them digest their food, ruminants are the most numerous large herbivores in the modern world.
Returning to the placenta, one of its most important benefits has been the elaboration of larger brains. Like so many other “experts,” behavioral scientists disparaged animal intelligence over many decades. Any attempt to describe mammal behavior in human terms was derided as anthropomorphism. In contrast, Darwin was deliberately anthropomorphic in his descriptions of animal intelligence and their expressions. Fortunately, recent work, ranging from parrots to chimps, has brought us back to Darwin's perspective. Here's a recent report I think revealing: It seems that “Rico,” a German family's pet border collie, had been given many toys over time, each toy with a specific name. When scientists heard that Rico could retrieve two hundred different toys by their two hundred names, they decided to investigate. Rico and the scientists were situated in one room, as Rico was instructed to fetch a specifically named toy from another room holding about ten different toys. Rico proved to be correct 95 percent of the time. However, the scientists varied their procedure, and they did this several times. In these instances, they told Rico to retrieve a toy whose name he had never been taught, and Rico returned with a toy he did not know.41 This clever canine knew what he did not know! (Really smart, seems to me!) But why should mammals have been enlarging their brains over time?
THE NATURE OF INCREASING BIOLOGICAL COMPLEXITY
While the history of life has been one of increasing numbers, the occasional input of greater complexity has provided new platforms for further diversification. Though the fossil record is best preserved in marine sediments, it is on the land where both species numbers and biological complexity have reached their peak.42 Sculpted by natural selection imposed by an unforgiving and dynamic environment, further propelled by incessant competition, we now find ourselves in a gloriously diverse biosphere.43 But what might be the underlying mechanisms that foster increasing complexity?
In an essay titled “On the Evolution of Complexity,” W. Brian Arthur proposed three mechanisms that drive increasing complexity, whether in biological, economic, or cultural systems.44 He calls his first mechanism Growth in Coevolutionary Diversity. Here entities coexist in interacting populations, and these interactions can drive the system into greater numerical richness. Within-species competition may drive some individuals into peripheral isolation, where new species are likely to form. New species, in turn, create new niches for others and enlarge environmental complexity.
Arthur calls his second mechanism Structural Deepening. Here entities can break out of their earlier limitations by adding functions or subsystems that improve their competitive or adaptive abilities. One can imagine how the invention of flight opened myriad possibilities for insects. For humans, learning how to walk on two feet allowed them to do new and amazing things. Arthur argues that competition drives these mechanisms ever forward.
Finally, Arthur proposed Capturing Software as a third mechanism, driving rare but sudden escalations in complexity. Endosymbiosis, whether with mitochondria or chloroplasts, embodied the “capturing” of new abilities. Recently discovered, a unicellular red algal species (a Eukaryote) has acquired genes from both Bacteria and Archaea in order to survive as an extremophile.45 In our own species, the “software of language” accelerated cultural innovation. Today, scientific methodology drives technological advance ever forward.46
Increasing complexity appears to be universal. In a short review of super conductivity, physicist Dirk K. Morr states that: “The emergence of complexity is often tied to novel forms of collective behavior driven by strong interactions.”47 Whether in condensed-matter physics or exploding stars, our universe has become richer in complexity over time. Within a living cell, complexity allows redundancy and dynamic networks to maintain equilibrium in the face of both internal and external challenges. Over the history of life, competition has made survival increasingly difficult. As a consequence, and only in recent times, one lineage has responded by becoming very, very smart! Thanks to our agricultural symbionts, we humans became increasingly eusocial, relaxed the selective pressures that constrained us, and made possible increasing social complexity.48
In the final chapters, we will examine two of the world's most extraordinary examples of advancing complexity. The first was biological: the ascendance of human intelligence. The second has been a direct consequence of this same advance, but something entirely new and dramatic: the elaboration of human cultures. Moving from small pagan chiefdoms to strong unifying theistic religions allowed large societies to maintain close internal cooperation. Such societies drove the purposeful advance of human technologies.49
Figure 2. Hominid cranial evolution. “Lucy” (Australopithecus afarensis) had a cranial volume of about 400 cc, similar to modern chimpanzees. Average cranial volume expanded from about 800 cc in early Homo erectus to as much as 1,500 cc in Neanderthals, and 1,300 to 1,400 cc in modern humans.