The New Evolutionary Tree: Five Kingdoms of Life
Lynn Margulis

In the 19th and early 20th century the commonly accepted picture or model of the evolution of species was the form of a ladder-like tree, in which simple single-celled organisms, or protozoa, formed the base and successively higher segments of the tree were constituted by invertebrate animals (with branches of insects, sponges, etc.), vertebrates (with branches of fish, amphibians, reptiles), and mammals (with branches of ungulates, cetaceans, etc.). At the crown of the tree, or the top of the ladder, were the apes and man, the "crown of creation." This model is obviously anthropocentric: it erroneously equates evolution with progress, and implicitly promotes a vision of Homo sapiens as superior to all other life forms, by virtue of greater complexity of organization, and by virtue of coming later in time. This older view recognized two or at most three kingdoms of life (plants, animals, unicells), each of which was also pictured as a tree.

The new view of evolution, as now generally described in biology textbooks, involves a taxonomy of five kingdoms, first developed by the biologist Robert Whitaker: monera (bacteria), protoctista (water-dwelling microbes, such as amoeba), plants, fungi and animals. Instead of the image of a ladder or tree with many branches, with ever-increasing diversity and complexity, the evolution of species is now more accurately portrayed as a bush or a multi-trunk tree. Earlier periods saw many more diversified forms than those that have survived till now. Stephen Jay Gould, in his book Wonderful Life, which describes the mind-boggling fossils of the Burgess Shale, writes that "life is a copiously branching bush, continually pruned by the grim reaper of extinction, not a ladder of predictable progress, . . . not the conventional tale of steadily increasing excellence, complexity and diversity."

In an alternative metaphor, the biologist Lynn Margulis uses the image of a hand with five fingers, with monera forming the base of the hand and thumb. Such a model equalizes the evolutionary status of all five major life kingdoms, each of which still continues with its particular adaptation to this day. As Stephen Jay Gould notes in his foreword to the book Five Kingdoms, by Lynn Margulis and Karlene Schwartz, in this new taxonomy, "the greatest division is not even between plants and animals, but within the once- ignored microorganisms - the prokaryotic Monera and the eukaryotic Protoctista."

Margulis is one of the prime proponents of the theory that the bacteria evolved into eukaryotic organisms by endosymbiosis - by incorporating themselves into larger, more complex cells. From these eukaryotic cells, with true nucleus, evolved all other multi-cellular forms of life, including plants, fungi and animals. The following summary descriptions of the five kingdoms of life are based primarily on Margulis' writings.

The first kingdom, MONERA, is made up of 15,000 known species of single-celled bacteria, all of which are prokaryotic, i.e., do not have a membrane-bounded nucleus. The DNA of prokaryotes is a single, circular coiled molecule that floats freely within the cell. For bacteria, sex is separate from reproduction: they reproduce by simple cell division, but they also transfer strands of DNA to others - generating new forms by recombination of genetic material (a process now being simulated by genetic engineers). Monera are the hardiest and longest-surviving life-forms on Earth: the oldest bacterial fossils have been dated to 3.5 billion years BP. The first eukaryotic cells appeared less than a billion years ago, the fist animals 700 million years. "For at least two and a half billion years, more than half of the Earth's existence, the planet was the uncontested territory of Kingdom Monera." While for us, bacteria are associated with disease, there are many species of bacteria that are not harmful, or pathogenic, to animal or plant life, and others that are curative and life-supporting, e.g., the "friendly" bacteria in our intestines that aid the catabolic processes of digestion. Bacteria invented most of the basic biochemical, metabolic processes used in the "higher" life-forms, including photosynthesis, nitrogen fixing, fermentation and oxidation. The earliest bacteria were anaerobes, to whom oxygen was toxic; but about 2 billion years ago, photosynthesizing cyanobacteria caused a build up of atmospheric oxygen, triggering "a global catastrophe. Because oxygen was toxic to early life, it became an increasingly serious pollutant. . . The resolution of the oxygen crisis was a turning point in the history of the cell: microbes evolved the capacity to use in respiration the oxygen they pro-duced. . . They put the potentially poisonous oxygen to use in the elegant innovation of aerobic respiration." Subsequently, oxygen respiration became the basic metabolic process of those eukaryotic cells that evolved into animals, as photosynthesis became the basic process of plants.

The second kingdom, PROTOCTISTA ("first builders"), includes all the eukaryotic single-celled microrganisms previously labelled protista (protozoa and protophyta), but also certain multi-celled organisms, such as kelp, that don't belong to the plant, animal or fungi kingdoms. There are known to be thousands of species of these organisms, all of them aquatic: they include the amoebas, algae, seaweeds, slime molds, ciliates, diatoms, paramecia, forams and many others. Protoctists invented two-parent sexual reproduction, and the capacity to form multi-cellular colonies, both of which evolved into the forms of plants, fungi and animals. Eukaryotic cells typically are much larger (up to a thousand times) and more complex than prokaryotic monera, and contain membrane-bounded organelles, which perform specialized metabolic functions, including mitochondria (for oxygenation), chloroplasts (for photosynthesis), and a nucleus (containing DNA). "According to the symbiotic theory of the origin of the eukaryotes, once-independent microbes came together, first casually as separate guest and host cells, then by necessity. Eventually, the guest cells became the organelles of a new kind of cell." Thus, the bacterial ancestors of mitochondria and chloroplasts incorporated themselves into the much larger eukaryotic protists to form bac-terial cooperatives, or as Margulis and Sagan say, ". . . communities of interacting mcrobes. Partnerships between cells once foreign and even enemies to each other are at the very roots of our being." They provide one of the first and most powerful examples of evolutionary adaptation by symbiosis.

In the older classification, the FUNGI, the third kingdom, which include yeasts, molds and mushrooms, were placed within the Plants, or the Protista. In the new taxonomy, the fungi, estimated at 100,000 species, are given their own kingdom. While their cells are eukaryotic, they have a fundamentally different life-cycle and ecological adaptation than plants or animals. Plants produce, animals consume, and fungi absorb. They absorb minerals and other nutrients directly through their membrane, and also transport complex chemicals out to plants with which they are in symbiotic association. An example is the mycorrhizal association between many species of fungi and the roots of trees and shrubs, which are fundamental to the ecological viability of forests. Many kinds of fungi obtain nutrition as parasites or as decomposers. Molds and yeasts are used in the production of cheese and beer, and antibiotics such as penicillin are fungal in origin. Margulis writes that "various fungal strategies for survival include the production of complex organic compounds, such as the ergot and amanita alkaloids, which can induce hallucinations or even death in animals." In addition, there is the possibility, proposed by Terence and Dennis McKenna, that tryptamine-producing psilocybe mushrooms, which are vision-inducing, stand in a potentially symbiotic relationship with the human species, and may have contributed to the growth of language in proto-hominids.

The fourth kingdom, PLANTS (half a million species), consists of multicellular, sexually reproducing eukaryotes, whose cells contain chlorophyll. This gives them the capacity to photosynthesize: capturing solar energy, using it to convert carbon dioxide and water in-to complex organic compounds, and releasing oxygen. This is the basic autotrophic food-producing process of the biosphere. The oxygen excreted by plants is used by all aerobic organisms, including animals, in respiration. Green land plants are descended from green algae, and it is generally agreed that it is the plants (with fungi in their roots) that made the move from sea to land possible. In response to the new conditions on land, plants with vascular structures evolved to enable the transport of water and chemicals through the plant. The life-cycle of plants is distinct from the other kingdoms: "Unlike animals, most of whose cells are diploid, and fungi, which are mostly haploid or dikary-otic, plants alternate haploid and diploid generations in an orderly fashion." The fruiting and flowering plants (angiosperms) appeared relatively recently, about 100 million years ago, and "that evolutionary innovation changed the living world by producing an environment in which man and other mammals could survive."

The fifth kingdom, ANIMALS, consists of multicellular, heterotrophic (feeding on others), diploid (dual chromosomes) organisms, that develop from the fusion/fertilization of an egg and a sperm cell. Mitotic cell divisions lead to the development of first a morula, then a blastula and finally a gastrula - an invaginated, hollow sac, that forms the embryonic precursor to the digestive tract, by means of which animals ingest nutrients and excrete waste. Animals are distinguished from all other kingdoms of life by the complexity of their morphology - tissues made up of specialized cells are organized into complex organ systems. The other chief distinguishing feature of animals is the adaptive capacity for varied forms of movement. Animals are Nature's great experiment in movement. The mutual interaction between microbes which cycle sulfur gases, methane and amonia, plants which release oxygen, absorbing carbon dioxide, and animals which excrete CO2 and breathe oxygen, is the fundamental, metabolic energy exchange of the biosphere.