Evolutionary Biodiversity

Biodiversity is the term that embraces all the species represented in all the ecosystems of the world. Thus there are two complementary aspects to biodiversity—both necessary for understanding the earth's biodiversity in all its richness. One is ecological biodiversity, which is all of the world's local ecosystems. The other side of biodiversity is evolutionary biodiversity: the roster of species on the planet, mixtures of which supply the living components—the local populations of organisms—that compose the world's ecosystems. One can think of the evolutionary spectrum of the world's species (that is, everything ranging from bacteria and more complex microbes up through fungi, plants, and animals) as the roster of players in the game of life, while the ecological side of biodiversity (the world's ecosystems) is where

Figure 1

Evolutionary Relationships among the Major Divisions of Life

True bacteria

Archaebacteria

Protoctists

Green algae

Mosses and allies

Ferns and fern allies

Nonflowering seed plants

Flowering seed plants

Fungi

Sponges

Cnidarians

Flatworms

Rotifers

Roundworms

Lophophorates

Echinoderms

Cartilaginous fish

Bony fish

Amphibians

Turtles

Snakes and lizards

Crocodiles and birds

Mammals

Chelicerates

Crustaceans

Insects and millipedes

Mollusks

Annelid worms

Source : Based on Eldredge, Niles. 1999. The Pattern of Evolution. New York: W. H. Freeman.

the game of life is actually played. One important aspect of the general biology of each group of organisms is the characteristic roles that they play in ecosystems the world over.

Each of the major groups of organisms that, together, compose the evolutionary side of biodiversity is treated in a separate article in this encyclopedia; for more details, consult the appropriate entry. The purpose of this article is to develop an overall picture of evolutionary biodiversity, and in particular to show how all of life, from its very beginnings more than 3.5 billion years ago, is interrelated. For all of life has descended from a single common ancestor, and all of life on earth—at least 10,000,000 species, and possibly as many as 100,000,00—is interrelated in a way that can be shown as a great "tree of life." Figure 1 reveals the latest scientific thinking on how all of the major groups of life still living on the planet are related in an evolutionary (phylogenetic) sense. The diagram omits all those groups that have already become extinct and are known only through the fossil record. For more on how this form of evolutionary biological research is conducted, consult the articles on Sys-tematics and Phylogeny, as well as the general entry on Evolution.

Bacteria are the simplest forms of life—and, not surprisingly, the first to appear in the fossil record. Bacteria are prokaryotes—that is, they lack the complex organelles and a distinct nucleus housing DNA typical of the more complex forms of life (the so-called eukaryotes). It has recently been shown that there are two major, distinct forms of bacteria: the so-called eubacteria (or "true" bacteria) and the archaebacteria, which are considered by many biologists to be more closely related to eukaryotes than are the eubacteria. Cur rently there are nine major groups, or phyla, of bacteria on earth.

Ecologically speaking, bacteria still pretty much run the world: it is possible to see the eukaryotes (especially the complex forms of animal and plant life) as forms of life that not only came later in evolution but also in a sense very much depend on the microscopic bacteria for their very existence. Different kinds of bacteria are responsible for a variety of ecological roles; for example, some bacteria, living in conjunction with the roots of leguminous plants, can "fixate" nitrogen. Nitrogen is a vital component of the proteins of all animals and plants, and nitrogen makes up roughly 70 percent of the earth's atmosphere. Yet only a few forms of life—all of them bacteria—are able to take up free nitrogen and fix it so that it can be utilized by plants, and therefore by the animals that eat the plants. Still other bacteria are essential for the digestion of cellulose; together with some forms of fungi, they are the only organisms that can break down this material, which forms the cell walls of plants. Without the breakdown of cellulose, life would have ceased long ago— clogged as the earth would have been with undecayed vegetable matter. It is true that bacteria cause disease—anthrax being one of the more notorious examples. But the bacterium Escherichia coli, which lives by the billions in every human's gut, is absolutely essential for normal human digestion. The invisible bacteria truly are the key chemical engineers of the planet.

Biologist Lynn Margulis has proven that more complex cells—eukaryotes—are the products of the permanent evolutionary symbiosis of at least two distinct forms of bacteria. The telltale evidence for her conclusion is that both the mitochondria of animals and the chloroplasts of plants have their own separate complement of DNA, in addi tion to the DNA housed in the eukaryotic cell's nucleus. That is what might be expected if two different kinds of bacteria, each with its own supply of DNA, had fused to create a more complex form of cell. Mitochondria and chloroplasts are in effect the powerhouses of the cells of animals and plants, respectively—the places where stored energy is converted for use (mitochondria) or, in the case of chloroplasts, the site of photosynthesis, the conversion of water and carbon dioxide in the presence of chlorophyll and sunlight into sugars, a form of trapped solar energy. Without photosynthesis, no animal life would be possible. It is important to note that certain kinds of bacteria, such as the blue-green algae, developed photosynthesis billions of years ago.

The most primitive forms of eukaryotic life are the protoctists—of which there are some twenty-seven different phyla currently recognized. These include amoebae, the shelled diatoms and formaniniferans, as well as ciliates and the flagellated protoctists. Some, like the euglenids, photosynthesize and are rather like one-celled plants. Others, such as the flagellates, are very animal-like, and derive their nutrient and energy supplies by absorption of tiny particles. It has been clear to biologists for more than 100 years that the root of each of the "higher" multicellular forms of eukaryotic life extends down into this mixture of plantlike and animal-like single-celled eukaryotes. Further research, increasingly using the techniques of molecular biology, will no doubt continue to separate which of the protoctists would be classified with the fungi, which with the plants, and which with the animals. There is still a great deal of research and analysis to be done in the systematics (that is, the analysis of evolutionary relationships) among these primitive single-celled forms of life.

Although biologists have traditionally thought that fungi were the most primitive forms of multicellular, eukaryotic life, recent studies comparing DNA sequences of samples of fungi, plants, and animals have yielded the surprising result that we animals are more closely related to fungi than to plants.

Plants (Kingdom Plantae) of course are essential to the workings of all terrestrial ecosystems. There are some five major divisions of plant life recognized on the tree of life (Figure 1), including green algae (some biologists include these with the single-celled protoc-tists), mosses, ferns, gymnosperms (conifers), and angiosperms.

Photosynthesis, as we have already noted, is essential not only to the plants themselves but to all animal life as well. As producers of their own energy source from the simple ingredients of sunlight (the energy), carbon dioxide, and water, plants are autotrophic. But in addition to their ecological role as primary producers, plants play further crucial ecological roles: their roots fix soil, thus combating erosion. Trees in cities have been found to have a powerful effect in filtering chemicals from the air. And trees regulate the water cycle, especially in tropical rain forests. When vast tracts of tropical rain forest are cleared, not only does erosion set in but, in addition, rainfall cycles are disrupted. It takes, for example, some 52 million gallons of water to flush a single ship from the Pacific side to the Caribbean side of the Panama Canal; the water comes from the great artificial Lake Gatun, flowing downward by gravity to keep enough water in the canal for the ships to pass through. Without daily rainfall for much of the year—all supplied by the surrounding rain forest (which itself has been increasingly under threat to development in Panama), the Panama Canal would soon dry up.

Fungi—considered, like plants and animals, to be their own separate kingdom of life—are saprophytic, meaning that they absorb nutrients and energy from dead organic matter. Thus fungi are essential to the full cycle of life that makes it possible for new generations to replace the old. They include the familiar mushroom as well as yeasts (so useful in fermentation) and molds (such as penicillin). Only fungi and certain bacteria are capable of breaking down cellulose. Some species of termites in the tropics have huge fungal gardens deep within their mounds; the termites bring back pieces of wood fiber and leave it to the fungi to break down into a form that the termites can eat and digest, thereby obtaining the nutrients and energy they require.

Animals (Kingdom Animalia) are het-erotrophs—they need to eat something else organic simply to stay alive. Some animals (herbivores) eat fungi and plants exclusively, while others (carnivores) eat the plant-eaters; still others eat the animals that eat the plant-eaters, and so forth through a complex chain of matter-energy transfer that is the "food-chain" and lifeblood of every local ecosystem. Some animals (omnivores) eat a wide range of foods: fungi, plants, and many kinds of animals; our own species, Homo sapiens, is an omnivore par excellence.

There currently are some thirty-seven separate phyla of animals living on earth. Sponges are among the most primitive of the animal phyla (so much so that most people do not even realize that they are animals). Sponges have only a few different kinds of cells, some arranged into channels in which they can filter food particles from currents of water, others for constructing their often massive skeletons.

Animals of the Phylum Cnidaria (including corals and most jellyfish) are a bit more complex, their cells being arranged into a two-layer system of tissues, though they lack the true organs (such as gills, brain, and heart)

found in more complex animals. Flatworms have excretory organs and eyes, but round-worms and certain other animals lack the internal body cavity that ranks among the defining features of the higher animals.

The coelomate phyla—those with true organs systems and body cavities—are divided into two main branches, first recognized by biologists in the nineteenth century: the pro-tostomes and the deuterostomes. Protostome means "first mouth," and the word refers to the fact that the first opening (blastopore) in the round ball (balstula) that cells form early on in embryonic development becomes the mouth of the adult animal. Protostomes include such important phyla as the annelid worms, the mollusks, and the arthropods. Those phyla, plus some other, less prominent groups, also share many aspects of body organization and details of embryological development that mark them distinctively as a main branch of the animal evolutionary tree—and one that we now know has been separate from the other great branch, the deuterostomes ("secondary mouths"—that is, the blastopore becomes the anus and a new opening is created later in development to form the mouth), for at least the past 540 million years.

Echinoderms (meaning "spiny skins"), a phylum that includes starfish and sea urchins (as well as many extinct groups), are very strange animals indeed. None of them have anything even remotely resembling a head— and no eyes or brain, either. They have a complex hydraulic system (that is, "water vascular system") that operates their tube feet, which they use for locomotion and (in the case of starfish) for grasping prey.

Yet, odd as echinoderms are, their DNA and aspects of their embryological development clearly show that echinoderms are among the closest relatives to the Phylum Chordata—our own phylum! We belong to the Subphylum

Vertebrata—all chordates with a spinal column. The vertebrates include the cartilaginous fishes (sharks and rays), the bony fishes (salmon, tuna, and bass, plus some other more primitive forms), amphibians (frogs and salamanders), reptiles (turtles, crocodilians, lizards, snakes, and some others), birds (everything from ostriches to hummingbirds), and mammals: insectivores, probably the most primitive mammals (shrews, moles, and hedgehogs are insectivores), rodents (rats, mice, squirrels, and many others), rabbits, bats, whales, peris-sodactyls (horses, tapirs, and rhinos), artio-dactyls (hippos, pigs, sheep, deer, antelope, giraffes, and so forth), carnivores (for example, bears, dogs, cats, hyenas, weasels, and seals), elephants, and, of course, primates—our own group. Primates include lemurs and lorises, New World monkeys, Old World monkeys, the great apes, and ourselves, Homo sapiens.

Human beings are very much a part of the great "tree of life." And though our species is listed last in this brief kaleidoscopic overview of earth's evolutionary biodiversity, this is not to say that we sit alone atop some sort of pinnacle of evolution. Each and every kind of organism on earth, no matter how simple or complex, so humble as a bacterium or regal as an American bald eagle, is beautifully adapted to the life it leads.

—Niles Eldredge

See also: Entries on all major groups of life mentioned in this article, as well as: Classification, Biological; Ecosystems; Evolution; Food Webs and Food Pyramids; Linnaean Hierarchy; Species; Systematics

Bibliography

Brusca, Richard C., and Gary J. Brusca. 1990. The Invertebrates. Sunderland, MA: Sinauer; Eldredge, Niles. 1998. Life in the Balance. Princeton: Princeton University Press; Margulis, Lynn, and Karen V. Young. 1998. The Five Kingdoms, 3d ed. New York: W. H. Freeman; Raven, Peter. 1998. Biology of Plants, 5th ed. New York: Worth; Young, J. Z. 1991. The Life of Vertebrates, 3d ed. Oxford: Oxford University Press.

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