I strongly recommend Darwin’s Blind Spot, a new book on the importance of cooperation in evolution. This morning, we feature a description from the publisher and an excerpt from the book.
Dr. Frank Ryan counters the belief that evolution developed through simple survival of the fittest. He argues instead that life on the planet is not only a bloody battle for supremacy, but is also the result of a labyrinth of cooperation. Darwin based his theory of evolution on competition between individuals, leading to the accumulation of gradual changes, dictated by natural selection. Evolution, he declared, could not make a sudden jump. He overlooked the vital importance of living interactions, whether as symbioses between different species and as cooperation within species, particularly humans. As Ryan explains, gigantic leaps in evolution have arisen from the blending of whole life-forms, giving rise to the great divisions of life, including the kingdoms of plants and animals, and ultimately to humanity itself.
Frank Ryan, MD
Coral reefs have been called the rain forests of the sea. In these precious domains, swimmers equipped with no more than masks and snorkels can watch creatures with the luminescent colors of precious gems behaviour rituals as enchanting as the mating of the most splendid birds.
The builders of these reefs are small marine animals that secrete their own external skeletons, whose slow accumulation over millions of years form the framework of the reefs. Members of the phylum Cnidaria, which also includes the jellyfishes and anemones, corals exist in a multitude of kinds. All consist of colonial animals, each of which is equipped with stinging tentacles (cnida is the Greek for the stinging nettle) that emerge like opening flowers to catch tiny prey and drag it into the saclike stomach for digestion. On the 2,000 square kilometers of Great Barrier Reef of Australia, there are about 350 coral species, including both hard and soft varieties. These reef-building corals grow in clusters, extending in a confluent surface layer over the graveyards of past generations. Each species has a different shape or color or blended shades of color; in association with other related cnidarians, they create all manner of sculptural forms. Some fashion huge rounded domes elaborated with sulci, like the human brain; others form twigs, bifurcating into staghorns or wide fan-shaped arborescent branches or the most exquisitely floral forms, all decorated with every subtlety of tone and hue.
Of course, coral reefs are much more than just a beautiful curiosity. They are the homes and nurseries for almost a million species of fish and other marine creatures and algae, many of which we rely on for food.
Nature films about the coral reef ecology used to focus on the predatory activities of its many residents, and certainly a reef can be a dangerous territory to inhabit. But lately the focus has become more balanced as filmmakers cater for a public delighted by the many symbiotic relationships among its lifeforms. Reef-building corals are exclusively symbiotic with unicellular yellow-brown photosynthetic organisms known as dinomastigotes, which live their entire lives inside the cells lining the coral’s gut. The coral’s stinging tentacles predate by night and the dinomastigotes capture sunlight by day, storing its energy in the form of carbohydrates. The ecosystem depends on this symbiosis for the zoothanthellae supply half or more of the coral’s energy needs, vastly increasing its ability to lay down calcification. This symbiotic union is believed to have evolved in the late Triassic period, some 210 million years ago.
Another group of tiny aquatic lifeforms is known collectively as the Foraminifera. With over 35,000 described species, “forams” are the most widespread of the organisms in the oceans. The individual shells are no bigger than grains of sand, yet in their billions they form an important component of plankton, the basic food source of all marine life. They also incorporate intracellular symbioses with a wide variety of photosynthetic microbes, which make the foram bigger and strengthen its shell, thereby improving its ability to survive. The symbiotic union is so self-sufficient it needs only a few additional vitamins to survive and reproduce. Mutualistic symbioses like coral and forams are important components of life in the sea.
On land one of the five kingdoms of life, the fungi, comprise some of the most ancient terrestrial lifeforms. Like bacteria, fungi are essential to the cycles of life. Underneath the soil certain fungi form immense labyrinths of filamentous mycelia, which can extend for miles and which play vital roles in plant nutrition. Above ground these fungi emerge as the fruiting bodies we recognize as mushrooms, which cast vast quantities of spores into the air. So enormous are the numbers of fungal spores and so efficient is their distribution they are found at every level and within every crevice of the biosphere. Fungi cannot live without carbon, which must be obtained from other creatures, whether bacteria, plants, or animals. This is why fungi are to be found growing in and around other forms of life, on their secretions, excretions, dead flesh, and – in saprophytic, parasitic, or mutualistic symbioses – in intimate relationships with a vast array of lifeforms.
As we have seen, fungi are an integral part of lichens. Even more important, most land plants have evolved from a joint venture between fungi and green algae, a relationship that evolved over hundreds of millions of years to become the mycorrhizae that nourish the roots of most species of plants today. Only recently have scientists realized that a single pine tree may have several different fungi in a mutualistic symbiosis with its roots. Peter R. Atsatt, a professor in the department of ecology and evolutionary biology at the University of California, has proposed an even more radical concept: that land plants may have arisen from the very early incorporation of a fungal genome into that of a green alga and that this hybrid organism went on to play a central role in the evolution of embryos, then of pollen and seeds.
Many fungi enter into complex symbiotic relationships with insects, in which the fungus helps break down the tough cell walls of plants so that the insect can digest them, while in return the fungus has a seat at the insect’s dining table. Among the many species of insects that have mutualistic symbioses with fungi are scale insects, gall midges, wood wasps, and anobiid beetles.
One delightful example of such a relationship concerns the Atta ants of Central and South America, which harvest leaves and carry them back into their subterranean nests. After the ants masticate the leaves, they are further digested by a domesticated fungus growing in the nest’s inner garden. The fungus breaks down the leaves’ cellulose walls, liberating their hidden stores of nourishment for the ants, while the ants give the fungus protection and shelter and a constant supply of leaves to feed on.
Almost a third of all known species of fungi are involved in mutualistic symbiosis of one form or another, which clearly has evolutionary implications. In the words of Bryce Kendrick, ‘Several of these relationships have given rise to major evolutionary innovations, which have conferred on the interdependent organisms the ability to colonize habitats previously unavailable to them.’ In this way, symbioses of many different types made possible the expansion of life into hitherto barren and hostile ecologies in the water and on the land surfaces of the primal Earth.
‘Phyla’ are the major divisions, below kingdoms, in the hierarchical classification of life. All members of a phylum share a fundamental physical or physiological characteristic, whose evolution gave rise to that phylum. In 1987 Lynn Margulis and her colleague David Bermudes suggested that many, perhaps all, of the fundamental characteristics that define each phylum came about through the creativity of endosymbiosis.
They showed how 28 of the 75 phyla, excluding bacteria, depended on the incorporation of organelles that had once been free-living microbes. These include thorny-headed worms, the Cnidaria and the exotic creatures known as Ventimentifera that live around the deep sea vents; they also include all mycorrhizal plants, including conifers, angiosperms, cycads and ginkgos, as well as 15 protoctist and two fungal phyla. Lower down the taxonomic system, symbiotic mergers have given rise to legumes with their nitrogen-fixing rhizobial bacteria, many wood-eating cockroaches and termites as well all ruminant mammals and the luminous fish that inhabit the ocean deeps. In all these cases the enigma of heredity demanded an explanation that was far more varied and complex than Darwinian mutation alone. Each case of symbiosis had to be examined in its own light, a daunting prospect for evolutionary geneticists. In some cases it was clear that the symbiont was passed down through bacterial-style binary fission within the cytoplasm. Richard Law, a lecturer in the biology department at the University of York, described the hydroid Myrionemia amboinense, whose eggs carry algal cells within them. Giant clams have a more complex relationship with the dinoflagellate, Symbiodinium microadriaticum, for the symbiosis has to be reassembled in each new generation after the host reproduces. Mechanisms for transmission of exosymbioses rely on one partner’s ability to locate the other even if they may not live closely together; as in the symbioses between flowering plants and bees, butterflies, and hummingbirds. But the diversity of solutions to the enigma of heredity could be a strength of symbiotic evolution rather than a weakness. Simplicity in science has a compelling elegance, but as Ernan McMullin, the philosopher and historian of science, admits, simplicity appeals more to our human aesthetic instinct than to any real objective logic.
We need, moreover, to put these symbiotic ‘flashes of lightning’ into perspective. The evolutionary landscape they illuminate is not steeped in darkness but bathed in a pale, shimmering phosphorescent glow, the result of steady, unrelenting Darwinian change. The glow is cut through and altered by these lightning flashes and, quite occasionally, severely shaken and disturbed by the brilliant thunderbolt of a major endosymbiotic event.
The two different evolutionary mechanisms we see in this wonderful landscape are not separate from one another; they tightly interwoven not only with one another but, in the most astonishing fashion, with the landscape itself.
Get Darwin’s Blind Spot at Amazon.com.