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global exchange network of incredible power and efficiency. Here is how Lynn Margulis and Dorion Sagan describe it:

Over the past fifty years or so, scientists have observed that [bacterial routinely and rapidly transfer different bits of genetic material to other individuals. Each bacterium at any given time has the use of accessory genes, visiting from sometimes very different strains, which perform functions that its own DNA may not cover. Some of the genetic bits are recombined with the cell’s native genes; others are passed on again. ... As a result of this ability, all the world’s bacteria essentially have access to a single gene pool and hence to the adaptive mechanisms of the entire bacterial kingdom . 13

This global trading of genes, technically known as DNA recombination, must rank as one of the most astonishing discoveries of modern biology. “If the genetic properties of the microcosm were applied to larger creatures, we would have a science-fiction world,” write Margulis and Sagan, “in which green plants could share genes for photosynthesis with nearby mushrooms, or where people could exude perfumes or grow ivory by picking up genes from a rose or a walrus .” 14

The speed with which drug resistance spreads among bacterial communities is dramatic proof that the efficiency of their communications network is vastly superior to that of adaptation through mutations. Bacteria are able to adapt to environmental changes in a few years, where larger organisms would need thousands of years of evolutionary adaptation. Thus microbiology teaches us the sobering lesson that technologies like genetic engineering and a global communications network, which we consider to be advanced achievements of our modern civilization, have been used by the planetary web of bacteria for billions of years to regulate life on Earth.

The constant trading of genes among bacteria results in an amazing variety of genetic structures besides their main strand of DNA. These include the formation of viruses, which are not full autopoietic systems but consist merely of a stretch of DNA or RNA in a protein coating . 15 In fact, Canadian bacteriologist Sorin

Sonea has argued that bacteria, strictly speaking, should not be classified into species, since all of their strains can potentially share hereditary traits and, typically, change up to 15 percent of their genetic material on a daily basis. “A bacterium is not a unicellular organism,” writes Sonea; “it is an incomplete cell . . . belonging to different chimeras according to circumstances.” 16 In other words, all bacteria are part of a single microcosmic web of life.

Evolution through Symbiosis

Mutation and DNA recombination (the trading of genes) are the two principal avenues for bacterial evolution. But what about the multicellular organisms of all the larger forms of life? If random mutations are not an effective evolutionary mechanism for them, and if they do not trade genes like bacteria, how have the higher forms of life evolved? This question was answered by Lynn Mar- gulis with the discovery of a third, totally unexpected avenue of evolution that has profound implications for all branches of biology*

Microbiologists have known for some time that the most fundamental division among all forms of life is not that between plants and animals, as most people assume, but one between two kinds of cells—cells with and without a cell nucleus. Bacteria, the simplest life forms, do not have cell nuclei and are therefore also called procaryotes (“non-nucleated cells”), whereas all other cells have nuclei and are called eukaryotes (“nucleated cells”). All the cells of higher organisms are nucleated, and eukaryotes also appear as single-celled, nonbacterial microorganisms.

In her study of genetics Margulis became intrigued by the fact that not all the genes in a nucleated cell are found inside the cell nucleus:

We were all taught that the genes were in the nucleus and that the nucleus is the central control of the cell. Early in my study of genetics, I became aware that other genetic systems with different inheritance patterns exist. From the beginning I was curious about those unruly genes that weren’t in the nucleus. 17

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As she studied this phenomenon more closely, Margulis found out that nearly all the “unruly genes” are derived from bacteria, and gradually she came to realize that they belong to distinct living organisms, live small cells residing inside larger cells.

Symbiosis, the tendency of different organisms to live in close association with one another and often inside one another (like the bacteria in our intestines), is a widespread and well-known phenomenon. But Margulis went a step further and proposed the hypothesis that long-term symbioses, involving bacteria and other microorganisms living inside larger cells, have led and continue to lead to new forms of life. Margulis published her revolutionary hypothesis first in the mid-1960s and over the years developed it into a full-fledged theory, now known as “symbiogenesis,” which sees the creation of new forms of life through permanent symbiotic arrangements as the principal avenue of evolution for all higher organisms.

The most striking evidence for evolution through symbiosis is presented by the so-called mitochondria, the “powerhouses” inside most nucleated cells. 18 These vital parts of all animal and plant cells, which carry out cellular respiration, contain their own genetic material and reproduce independently and at different times from the rest of the cell. Margulis speculates that the mitochondria were originally free-floating bacteria that in ancient times invaded other microorganisms and took up permanent residence inside

them. “The merged organisms went on to evolve into more complex oxygen-breathing forms of life,” Margulis explains. “Here,

then, was an evolutionary mechanism more sudden than mutation: a symbiotic alliance that becomes permanent.” 19

The theory of symbiogenesis implies a radical shift of perception in evolutionary thought. Whereas the conventional theory sees the unfolding of life as a process in which species only diverge from one another, Lynn Margulis claims that the formation of new composite entities through the symbiosis of formerly independent organisms has been the more powerful and more important evolutionary force.

This new view has forced biologists to recognize the vital importance of cooperation in the evolutionary process. While the

social Darwinists of the nineteenth century saw only competition in nature—“nature, red in tooth and claw,” as the poet Tennyson put it—we are now beginning to see continual cooperation and mutual dependence among all life forms as central aspects of evolution. In the words of Margulis and Sagan, “Life did not take over the globe by combat, but by networking.” 20

The evolutionary unfolding of life over billions of years is a breathtaking story. Driven by the creativity inherent in all living systems, expressed through three distinct avenues—mutations, the trading of genes, and symbioses—and honed by natural selection, the planet’s living patina expanded and intensified in forms of ever-increasing diversity. The story is told beautifully by Lynn Margulis and Dorion Sagan in their book Microcosmos, on which the following pages are largely based. 21

There is no evidence of any plan, goal, or purpose in the global evolutionary process and thus no evidence for progress; yet there are recognizable patterns of development. One of these, known as convergence, is the tendency of organisms to evolve similar forms for meeting similar challenges, in spite of differing ancestral histories. Thus eyes have evolved many times along different routes— in worms, snails, insects, and vertebrates. Similarly, wings evolved independently in insects, reptiles, bats, and birds. It seems that nature’s creativity is boundless.

Another striking pattern is the repeated occurrence of catastrophes—planetary bifurcation points, perhaps—followed by intense periods of growth and innovation. Thus the disastrous depletion of hydrogen in the Earth’s atmosphere over two billion years ago led to one of the greatest evolutionary innovations, the use of water in photosynthesis. Millions of years later this tremendously successful new biotechnology produced a catastrophic pollution crisis by accumulating large amounts of toxic oxygen. The oxygen crisis, in turn, prompted the evolution of oxygen-breathing bacteria, another of life’s spectacular innovations. More recently, 245 million years ago the most devastating mass extinctions the world has ever seen were followed rapidly by the evolution of mammals; and 66 million years ago the catastrophe that eliminated the dino-

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saurs from the face of the Earth cleared the way for the evolution of the first primates and, eventually, the human species.

The Ages of Life

To chart the unfolding of life on Earth, we have to use a geological time scale, on which periods are measured in billions of years. It begins with the formation of the planet Earth, a fireball of molten lava, around 4.5 billion years ago. Geologists and paleontologists have divided those 4.5 billion years into numerous periods and subperiods, labeled by names such as “proterozoic,” “paleozoic,” “cretaceous,” or “pleistocene.” Fortunately we do not need to remember any of those technical terms to have an idea of the major stages of life’s evolution.

We can distinguish three broad ages in the evolution of life on Earth, each extending for periods between 1 and 2 billion years and each containing several distinct stages of evolution (see table on page 234). The first is the prebiotic age, in which the conditions for the emergence of life were formed. It lasted 1 billion years, from the formation of the Earth to the creation of the first cells, the beginning of life, around 3.5 billion years ago. The second age, extending for a full 2 billion years, is the age of the microcosm, in which bacteria and other microorganisms invented all the basic processes of life and established the global feedback loops for the self-regulation of the Gaia system.

Around 1.5 billion years ago the Earth’s modern surface and atmosphere were largely established; microorganisms permeated the air, water, and soil, cycling gases and nutrients through their planetary network, as they do today; and the stage was set for the third age of life, the macrocosm, which saw the evolution of the visible forms of life, including ourselves.

The Origin of Life

During the first billion years after the formation of the Earth, the conditions for the emergence of life gradually fell into place. The primeval fireball was large enough to hold an atmosphere and

Ages of Life PREBIOTIC AGE formation of the conditions for life	Billion Years Ago 4.5 4.0 3.8 -	Stages of Evolution formation of Earth fireball of molten lava cooling oldest rocks condensation of steam shallow oceans carbon-based compounds catalytic loops, membranes
MICROCOSM	3.5	first bacterial cells
evolution of		fermentation
microorganisms		photosynthesis sensing devices, motion DNA repair trading of genes
	2.8	tectonic plates, continents oxygen photosynthesis
	2.5	bacteria fully extended
	2.2	first nucleated cells
	2.0	oxygen buildup in atmosphere
	1.8	oxygen breathing
	1.5	Earth surface and atmosphere established
MACROCOSM	1.2	locomotion
evolution of	1.0	sexual reproduction
visible life forms	0.8	mitochondria, chloroplasts
	0.7	early animals
	0.6	shells and skeletons
	0.5	early plants
	0.4	land animals
	0.3	dinosaurs
	0.2	mammals
	0.1	flowering plants first primates