Discover, October 1, 1995
The more you think about life on land, the less sense it makes. Life started in the ocean about 4 billion years ago, and for 3.5 billion years, it remained there. Evolution created organisms that had to stay wet- -they were essentially fluid-filled bags, and if they dried out, their circulatory systems would collapse, and most of their proteins and DNA would crumple up into uselessness. Without the ocean’s nutrient-filled currents, they would starve, and they and their fragile eggs and larvae would be immobile, unable to reach new or better habitats.
Seen from the sea, then, the land should equal death. Yet since animals, plants, and fungi first came ashore some 450 million years ago, life on land has been outrageously successful. True, land organisms have had to remain fluid-filled, DNA-based sacs, and they still rely on the old- fashioned, oceanic ways of getting food and energy, such as predation and photosynthesis. But according to the best estimates, there are now twice as many species on land as there are in the seas, and they produce some 50 times as much biomass. Furthermore, they manage this on only one-third the ocean’s breadth and in only a tiny fraction of its depth. And they achieved these luxuriant statistics in very little time. If ocean life were a 100- year-old man, life on land would be an 11-year-old child.
Researchers have tried to explain this land-sea paradox, in bits and pieces, without much success. But Mark and Dianna McMenamin, a husband- and-wife paleontological team, have an imaginative new hypothesis that they believe can explain it all, at one go. To understand the success of life on land, they say, you have to recognize that it is a unified whole. What makes it different from marine life is that unrelated terrestrial organisms–plants, fungi, and animals–form a vast number of direct, physical connections through which fluid can move. In effect, the McMenamins claim, life on land has not so much forsaken the sea as created a new sea within the sum of its tissue–something Dianna and Mark have dubbed Hypersea.
Hypersea is in many ways different from an ocean: for starters, it has no surface on which you can gaze, and it does not seek to be level. If you could look at life on land through a machine that registered only fluid, you would see great columns of nutrient-laced water rising–the columns would be where trees stand. You would see water flowing horizontally underground among plant roots and fungi, pouring into animals as they fed, moving as the creatures moved. According to the McMenamins, this liquid matrix has become Earth’s newest aquatic habitat, one that marine organisms have aggressively colonized. And in critical ways it behaves exactly like an ocean: the movement of fluid through Hypersea provides life with the same sustenance that ocean currents do. But there is one notable difference in the life it feeds: rather than being passive beneficiaries of an ocean that surrounds them, land organisms can control the currents within them. Thus, viewed as Hypersea, life on land couldn’t help being a smashing success.
The McMenamins propose Hypersea not as metaphor but as reality. If they are right, and Hypersea does actually ripple through all the plants on the surface of Earth, all the insects, the birds, the reptiles, the mammals, all the cells in all the bodies that crawl or walk upon the land, the implications are as vast as the sea itself. Hypersea not only offers, for example, an explanation for the largely mysterious emergence of life on land but it also suggests a number of bizarre life-forms that should once have existed and perhaps still do. It explains not only the land’s greater biomass and biodiversity but also such mysteries as why terrestrial food chains are so much shorter than marine ones. It could provide agricultural and medical researchers with new ways to understand pests and diseases. It could even reveal the future of evolution.
Of course, the Hypersea hypothesis can also be proved wrong. And the McMenamins have considerately offered their colleagues a number of ways to do that.
The idea of Hypersea began seeping quietly into the individual minds of the McMenamins in the late 1970s. It came to Dianna while she was an undergraduate at the University of California at Santa Barbara, coinciding with her introduction to the underappreciated kingdom of the fungi. There are several hundred thousand species of fungus, and they are more closely related to animals than are plants. Among us humans, they have an unfortunate reputation as freeloading or fatal parasites–we tend to notice them only when they appear on the soles of our feet or in our refrigerators, or blight a country’s worth of potatoes. But without a fungus, bread would be matzo and beer would be barley juice. Without penicillin and other antibiotics produced by fungi, infections would have claimed millions more lives this century. Without fungi in the soil, most plants would die, because they are joined with fungi in life-giving symbiosis.
Fungi exist as a web of slender threads known as hyphae, many of which are only one cell thick. They have no mouths with which they can eat like animals, and they can’t photosynthesize like plants. What they do have are enzymes that can break down living tissue, dead organic matter, or even rock; fungi get their nourishment by releasing these enzymes and then soaking up the resultant slush through their cell walls. The hyphae of many species, known collectively as mycorrhizal fungi, invade the roots of plants and sometimes even their stems and plunge into their cells. Though the fungi seem poised then to suck the plants dry, they are in fact gentle neighbors. Mycorrhizal fungi take some of the plants’ carbohydrates, but in return they give minerals and other compounds.
Fungi form underground networks that unite forests of different plant species. Sometimes a network acts like a nervous system. When a plant is attacked by insects, some species of fungus can pump pesticides into it. If one part of a stand of trees is poor in nitrogen and another is short of water, fungi can transport the substances needed. And if the plants are starving, fungi can give them lumps of oil to feed on.
When Dianna was an undergraduate, one of her interests was the bizarre sex life of fungi. In many species, when two hyphae meet, they probe each other. If they are of different sexes–a matter of genetic, rather than genital, compatibility–they fuse and exchange genes. (Chances were good for such exchange in the species that Dianna studied–it has 5,000 different sexes.) After mating, fungi often produce aboveground structures such as mushrooms and toadstools, loaded with up to a trillion spores.
By the time she was ready for graduate school, Dianna’s interests had focused on questions relating to the origin of life, and she decided to study paleontology at Santa Barbara. But she soon found that when she would ask other paleontologists about the origin of fungi, they’d simply shrug. With few known fossils, fungal origins were a blank. In 1980, Dianna met Mark, when he too came to study at Santa Barbara. They soon married, despite his being yet another animal-centric, fungus-oblivious paleontologist.
Mark’s own career had grown out of a boyhood fascination with early life-forms. When he was ten, he paged through a book of fossils and was struck by a picture of an inch-wide disk with three curved rays radiating from its center. The book explained that this creature, a tribrachidium, was 550 million years old, and that no one had any idea what it was. I thought, this is very strange, Mark recalls. The tribrachidium was among Earth’s first multicellular animals, known collectively as the Ediacaran fauna (named after Ediacara, Australia, where the first fossils were found). These animals were flat, boneless, eyeless, mouthless, and brainless. About 530 million years ago they vanished during a burst of evolution known as the Cambrian explosion, which populated the oceans with almost all the major forms of life that have existed ever since. The whole focus of my graduate work was trying to locate these things, to try to find a new field site, says Mark. He traveled the length of North America searching for one. Eventually he found a rich vein of Ediacaran fossils in Mexico. The site has yielded dozens of new species, and Mark has become an authority on the first chapter of the story of animals. This past March he discovered a jellyfish-like animal at the site that is 590 million years old–the oldest known fossil of a multicellular animal.
Like other Ediacaran fossils, it is a humble blob. In their book The Emergence of Animals, the McMenamins hypothesized that these creatures lived in a world very different from ours, one that had ample room for humble blobs. They created their own food, so even though they were large, they were not predators, nor were they preyed on, says Mark. Some animals harbored photosynthesizing microbes in their tissues, while others harvested the energy in chemical compounds in the ocean. Some were just passive nutrient absorbers, picking up amino acids kicking around in the water. Mark likes to call this tranquil world the Garden of Ediacara.
For reasons that are still unclear, the Cambrian explosion brought the world’s first predators, complete with mouths and brains, and they swiftly destroyed the Ediacaran garden. But as Mark points out, there was an evolutionary upside: When you get those first predators, they force their prey to do new things. Your lineage gets helped by the things that are trying to eat you. Help, in this instance, means you develop armors and poisons and escape maneuvers. And then the predators undergo a diversification and you get an upward spiral, and in a few million years, boom, you fill the ocean with a plethora of new species.
In 1984 the McMenamins went to Mount Holyoke College in Massachusetts, where Mark had taken a teaching position. As they worked on their book on animals, Mark started overhauling his class on the history of life. When I looked at the textbook I was using, there seemed to be something missing, he remembers. For one thing, fungi were barely discussed. For another, he says, plants were mentioned, but peripherally, in little boxes. This seemed like a serious oversight. The one thing I kept coming back to was coal. In their study of early life, the McMenamins had noticed an odd coincidence. During the Cambrian explosion, dry land was occupied by only a varnish of bacteria. But within 60 million years, plants and animals arrived, and less than 100 million years after that, life on land was already more diverse than in the oceans. Land plants were forming forests so vast that they created a kind of rock never seen before: coal. For billions of years there’s no coal, says Mark, and then suddenly there’s a new rock. No one had been able to solve the mystery of how a biological process so powerful that it became a geologic force could burst on the scene so quickly.
Mark became consumed with studying this second explosion. We spent a lot of time talking back and forth after he started doing his research, says Dianna. Mark would toss out explanations for this second explosion of life, and Dianna would shoot them down. (He has more ideas, I have better ones, Dianna claims.) In particular, she reminded him that any theory for the rise of life on land had to explain the mysterious origin of fungi. I kept hammering on him, saying, ‘Look at the fungi!’
Mark turned to a weighty fungus textbook for inspiration. I’m reading, and I think, I’m not going to read this whole thing. In all the articles, the interesting things are either in the throwaway comments at the end or in footnotes. So I open up the book to one of the later chapters, and there’s a discussion of weird cases. I’m reading along, and one weird case is this fungus called Septobasidium. And suddenly it just hits me: this is the essence of why life works on land.
Septobasidium is a fungus with a taste for animals–specifically, for waxy, mothlike creatures known as scale insects that live as seemingly immobile bumps on the branches of trees. The fungus forms a blanket that traps the insect against a trunk. Then it inserts its hyphae into the insect and absorbs its fluids. But the paralyzed insect doesn’t die–it actually lives longer and has more young than its uninfected counterparts. It survives by sticking a long feeding tube into the tree and sucking out the tree’s fluids. The tree, meanwhile, is drinking from a mycorrhizal fungus entwined with its roots underground. In other words, fluid is flowing from fungus into plant, from plant into animal, and from animal into fungus. Mark began to think of the other organisms connected to this association: the parasitic fungi that infected the mycorrhizal fungus, and the wormlike nematodes that latched onto the roots of the tree.
Septobasidium, the tree, and all the other organisms, he realized, fed on currents of fluid, just as many marine organisms do, but this fluid flowed through their own tissues. The various organisms had, in effect, created an ocean inside themselves. The more Mark and Dianna talked about this concept, the more it seemed to them that the whole effort of evolution on land had been to re-create the ocean internally and exploit it. Perhaps it started as an act of evolutionary desperation, but it was eventually able to create the staggering bulk and diversity of life that has lived on land ever since.
Comfortable as life might be in the ocean, it has its intrinsic limits. The organisms that live in the marine surface waters are in a perpetual state of famine, says Mark. They strip the surface water of its nutrients, and it takes a long time to replenish them. They scrub everything clean. With few exceptions, plankton can do nothing but wait for an upwelling of deep water. In the ocean, productivity is just little twinkles where you get upwelling, and at the fringes along continents. The rest is just desert.
The short pause between the Cambrian explosion and the advent of plants, animals, and fungi on land was probably no coincidence, according to the McMenamins. You had these brain-driven predators ruining the neighborhood, so the only safe place left was a marginal place where the animals hadn’t yet become serious predators, says Mark. Perhaps some protoplants and protofungi tried to scrape by along the shores, where they had to endure dry periods. The protofungi would attack the protoplants and usually kill them. But suppose a defective protofungus failed to kill its victim–it could stumble into a symbiotic relationship with the plant that allowed them both to thrive. As the protofungus broke up rock and consumed it, it supplied some to the protoplant. The protoplant, in turn, could stop depending on ocean currents for its nutrients and start evolving the structure and shielding to grow upright into the air. Once a plant could escape the water, it could immediately gather far more solar energy. Drying out from evaporation was a risk, but evaporation was also beneficial because it acted like a pump, pulling more water from the plant’s roots, laced with the fungal goodies, to its photosynthetic tissues.
Copyright 1995 Discover Magazine. Reprinted with permission.