A Garden of Marvels Page 17
The oxygen that cyanobacteria produced immediately united with the vast quantities of iron dissolved in the oceans. Slowly, the oceans rusted, sending billions of tons of iron oxides to settle on the ocean floor in layers as thick as a half mile. (All the iron ore we mine today was formed in this era.) Finally, about 2.2 billion years ago, all the free iron, as well as other oxygen-hungry metals in the oceans like sulfur and manganese, had been oxidized. For the first time, oxygen bubbled into the oceans and floated off into the atmosphere, and the “Great Oxidation Event” began. The atmosphere gradually cleared, the sky and water became the blue we see today, and a protective ozone layer developed in the stratosphere. Many of those prokaryotes that had made their living by reacting iron and sulfur went extinct; other species were poisoned by free oxygen and either died out or were driven to the anoxic ocean depths. But those bacteria that reacted oxygen with carbon-based molecules thrived.
It was not, however, the single-celled bacteria that made the most of the new fuel. Some time between three and two billion years ago, another type of microscopic creature, a eukaryote (yoo-KAR-ee-oat), appeared in the oceans, likely descended from a one-time fusion between two single-celled individuals. Unlike a single-celled bacterium or archaeon, a eukaryote has a nucleus, which has its own membrane that encloses paired, threadlike chromosomes. Eukaryotes reproduce in a more complex way, by creating (through mitosis) two identical daughter cells that, by halving their genetic material (through meiosis), produce gametes with only one chromosome. The gametes from two different individuals then fuse to form a new individual with a mix of its parents’ genes. This sexual reproduction turned out be a boon for diversity, and that ur-eukaryote generated all multicellular creatures past and present, including, ultimately, us.*
Eukaryotes had another new feature. Unlike bacteria that have a rigid cell wall, eukaryotes’ cell walls are made of a flexible, dynamic network of fibers. Flexible walls meant eukaryotes could stretch and bend to envelop other creatures and devour them. One day, about two billion years ago, a eukaryote engulfed a particular bacterium (probably related to the one that causes typhus) that made its living by using oxygen to metabolize sugars. The eukaryote should have deconstructed the bacterium. But for some reason, and just this once, this bit of dinner proved indigestible. Not only did the prey survive inside the predator; it reproduced, and its descendants survived inside the host’s descendants. A symbiosis developed, with the bacterium reacting with oxygen and sugars provided by its host and the host snaring the resulting energy. The relationship, albeit born of an unsatisfactory dinner date, proved enduring and enormously productive. The engulfed bacteria became mitochondria, organelles that are the internal combustion engines of multicellular life.
Next, about 1.6 billion years ago, one of these oxygen-burning eukaryotes bumped into and engulfed a cyanobacterium. This time, and just this one time, it was the cyanobacterium that survived the eukaryote’s digestion. No one knows exactly how this happened, but there is recent evidence that a third party was involved. The research of Rutgers University biologist Debashish Bhattacharya indicates that another individual showed up for this particular dinner party, a Chlamydia-like bacterial parasite, and was simultaneously engulfed. This unique convergence allowed the cyanobacterium to survive inside the predator. The parasite did not survive, but certain of its genes became incorporated in the genome of its host. (This is not as unlikely as it sounds. Bacteria readily exchange genes with one another. This “lateral gene transfer” is part of the reason bacteria can evolve so quickly to thwart our antibiotics.) The parasite’s genes created a crucial conveyor belt that whisked the sugars produced by the cyanobacteria to the host cell. The cyanobacterium not only was spared, it was able to reproduce. Its descendants survived inside the eukaryote’s descendants, catching sunlight, building carbohydrates, handing them over to the host, and burping pinpricks of oxygen. Over time, some of the cyanobacteria’s genes transferred into the host’s nucleus, and eventually so many transferred that the guests could no longer survive independently. The cyanobacteria had become permanent residents, the chloroplasts.*
Some of the early seafaring photosynthetic eukaryotes, or algae as we can now call them, found a particularly salubrious niche in the freshwater shallows of river deltas and bays. Here, they found a good supply of mineral nutrients that weathered out of rocks on the nearby shore. Fast-forward to 500 million years ago. (When we talk in terms of millions instead of billions of years ago, doesn’t it feel almost like yesterday?) Some freshwater algae dropped a few fragile filaments down to the wet sand and prospered by anchoring in a harbor of plenty, instead of drifting in and out with the wind and tides. The algae reproduced and developed into colonies that floated on the water’s surface in sheets. They also formed a symbiotic partnership with fungi (see chapter 10) that had colonized land earlier. The anchored algae proliferated and diversified. About 450 million years ago, some of them moved fully onto land, becoming the progenitors of the two major divisions in the kingdom of Plantae.
One division, the bryophytes, includes today’s liverworts, hornworts, and mosses. Bryophytes maintain close contact with the ground—nearly as close as algae does to water—because, like algae, they have no means of moving water up and around their corpus. They also need a damp environment because their sperm must swim to fuse with egg cells. The resulting organism grows to form a structure—the sporophyte—that then releases airborne spores. The other division, which encompasses all other modern land plants, is the tracheophytes. As the name suggests, tracheophytes have tubes that conduct fluids throughout the plant.
The earliest tracheophytes also kept a low profile. Although they could move water internally, their cells had no external coating to protect them from desiccation. (None had been needed in the watery environment from which they’d emigrated.) To grow tall, therefore, would be to risk death from drying winds. But if lying low was an excellent strategy for survival, it was not so good for reproduction. Like the bryophytes, the early tracheophytes all reproduced with spores, and the greater the height from which an individual could launch its spores, the greater likelihood of its spreading its genes the most widely. So, the little vascular plants began to evolve a clear, waxy cuticle. As it turned out, not only were taller individuals more likely to have more offspring, they were also more likely to reach maturity to reproduce: No neighbors put them in the shade. The race of the tracheophytes toward the sun was on.
A clear cuticle released the potential of tracheophytes. It kept internal water from evaporating while simultaneously allowing light to reach the chloroplasts. A completely impermeable coating, however, would have prevented the plant from absorbing atmospheric carbon dioxide. The solution was pores. And, wouldn’t it be nice if those pores could open and close? Opened, the pores would maximize carbon dioxide intake. Closed—say, in the desiccating heat of midday or at night, when the plant couldn’t photosynthesize—they would minimize water loss.
In fact, one of the earliest tracheophytes, the long-extinct Cooksonia, was a delicate, inch-high species equipped with just these features. Cooksonia was composed of a single, slender green stalk that bifurcated into two green upright and leafless stems. The stalk and stems were pocked with stomata. Each stomate was surrounded by two sausage-shaped “guard cells” that, expanding and contracting by filling and purging water, altered the size of the pore. This tiny plant emerged about 425 million years ago and proceeded to blanket great swaths of Euramerica, the ancient amalgamation of what is now Europe and North America. By 400 million years ago, Cooksonia had been joined by similar species, some as tall as two feet and more elaborately branched. The plants tended to grow in dense patches where, packed tightly together, their fragile stems supported each other. Tiny mites, centipedes, and the quarter-inch precursors of modern spiders scooted along in this miniature landscape.
The long-extinct Cooksonia.
None of these plants had leaves. It wasn’t that Cooksonia and its companions couldn’t
make leaves. Leaf-making didn’t require a once-in-history fusion or a fantastically rare undigested bacterium. The principal genes responsible for making leaves are the same ones involved in making branches, meaning that the ability to make leaves is as old as plants themselves. No, these plants were leafless because they had no reason to spend precious carbohydrates making and maintaining leaves. Earth’s atmosphere was brimming with carbon dioxide, at four thousand parts per million, ten times more plentiful than today. A few photosynthesizing stems were sufficient for satisfying a plant’s need for carbon. Leaves would have been a liability, capturing too much solar energy in this hothouse climate and causing the plant to overheat. Even if hypothetical leaves had been riddled with stomata, the roots of this era were still mere filaments, too attenuated to transport the volume of water needed for evaporative cooling from lots of leaves. So, Cooksonia and its cohort went unclothed.
The sticklike plants of the era were unimpressive to look at, but they were gradually making an impact on the ground beneath them and the air above them. Their roots may have been thready, but in harness with mycorrhizae, they continually scratched away at rock, liberating minerals molecule by molecule. Communities of aerobic microorganisms flourished, feeding on the dead roots and fallen stems of the new ground cover. The carbon dioxide these microbes exhaled, trapped in a new layer of humus and mixed with rainwater, became carbonic acid. This mild acid further accelerated the weathering of rock. Simultaneously, calcium and magnesium silicates, newly exposed to air, reacted with carbon dioxide and slowly, slowly pulled billions of tons of the gas from the atmosphere, sequestering it in organic compounds. From the start of the Devonian era about 415 million years ago to its end 360 million years ago, the planet’s CO2 level plummeted by 60 percent to 1,600 ppm. As it fell, the greenhouse effect weakened, and the planet cooled.
As the atmospheric carbon dioxide level fell, plants could safely capture more solar energy without overheating. The tiny, forked plants that had spread far and wide developed more branches, thanks to mutations in a class of genes known as KNOX. When KNOX is expressed, a stem grows straight. When the gene is silenced, the stem produces a sideways growth. As the environment changed, mutations that turned off KNOX periodically were favored by natural selection. Plants with additional photosynthetic branches trapped more energy, seized more carbon dioxide, and converted it to more plant mass and more spores. More stems, then stems branching off of stems, developed. Other now-favorable mutations allowed a little photosynthetic epidermal tissue on the stems to extend beyond the stem, conferring a competitive advantage in the sunlight-harnessing business. Ultimately, the tissues between stems met and merged, an evolutionary process much like the one that would produce webbing between the toes of the ancestors of ducks. Typical of the leaves that emerged from this process were the pinnules (little leaflets) of a fern frond.
The pinnules of a fern frond. Cell divisions along the edges of small green branchlets may have originally produced the web of leaf tissue.
Although the individual pinnules were small, they collectively gathered far more solar energy than bare stems, and the plants bearing them could grow tall. By the late Devonian era, the world’s first forests—jungles, really—developed, covering the landscape in a dazzling diversity of green. Shrubby ferns grew in thickets on the forest floors. Other ferns grew into palm-tree shapes and sizes. The horsetail, or Equisetum, which grows in nodes like bamboo with a ring of needlelike leaves protruding at each node, rose ten stories tall. (The general morphology of their descendants remains unchanged, but extant species are only three to ten feet tall. These survivors, having withstood the test of many climate changes, are a gardener’s nightmare, nearly impossible to fully uproot and unaffected by most herbicides.) Lycophyta with trunks six feet wide and 140 feet tall—about twice the diameter and height of a mature oak—dominated the landscape. (The only survivors of these giants are tiny club mosses.) Some species had stiltlike roots, a brilliant adaptation to the often swampy conditions of the time. At the end of the Devonian, Archaeopteris, the first tree with a trunk made of concentric sheaths of lignin and cellulose—that is, wood—emerged and thrived. (Tree ferns had trunks made of stems woven together; Equisetum trunks were hollow, like bamboo; and the giant lycophytes had a hard casing, but a spongy interior.) We would have found Archaeopteris odd-looking: At the top of its trunk, lateral branches emerged supporting fernlike fronds. About twenty-five feet tall, it had the most extensive root system yet evolved, much more massive than that of the taller lycophytes and tree ferns.
By the end of the Carboniferous era (360 to 300 million years ago), the Earth had been home to 150 million years of madly photosynthesizing vegetation. The continents were blanketed with greenery filling every vertical niche, from the soil surface covered with mosses to the canopies of lofty trees. Wafting oxygen as waste, plants had transformed the atmosphere. As much as 35 percent of the atmosphere was then oxygen, compared to today’s measly 21 percent. That oxygen transformed the kingdom of animals that lived in this deep green world. Some of the descendants of the tiny fauna that had scuttled among the Cooksonia had evolved to gigantic proportions. Dragonflies with the wingspans of crows swooped through the air, three-foot-long millipedes and cockroaches the size of mice ran through the undergrowth, and two-foot-long water scorpions trolled the shallow waters. Amphibious newts were the size of crocodiles.
The earliest plants didn’t need leaves because carbon dioxide was plentiful in the Devonian. During the Carboniferous, as soil levels increased, plants had access to more nutrients, and forests developed. During the Triassic, conifers—members of the gymnosperm division—thrived in the hot and dry climate. Although, with their narrow leaves, they were slow growing. Large-leafed, fast-growing angiosperms predominated during the Cretaceous. Chart by permission of Kenrick and Davis and Niklas.
Every minute of those 150 million years, trunks, branches, roots, and leaves had been growing, dying, and rotting. Soils became a meter deep and more in places, enough volume for ever-larger root systems. More plants grew and died than could be decomposed. (Whether this was because bacteria had not yet fully mastered the breakdown of lignin-rich wood or because plant material fell into anoxic swamps where decomposers didn’t live is unclear.) Over the eons, those plants were buried, compressed, and eventually became the coal—hence the designation of “Carboniferous”—we burn today. Practically overnight, considering that the continents had been essentially barren for 4.1 billion years before Cooksonia evolved, chloroplasts had transformed the planet. By steadily absorbing carbon dioxide, splitting water with solar power, and snapping together the carbon-oxygen-hydrogen molecules of sugars, they had created our familiar world.
nineteen
The Tenacity of Trees
Isn’t it always so? The beautiful hickory died an untimely death while our neighbor’s trees, a raggedy row of Leyland cypresses planted right on the property line, appear to be immortal.
In our close-in suburban neighborhood, most people’s lots are well less than a quarter-acre. We accept that we will live with a view, only marginally obscured by shrubbery and backyard fences, into each other’s yards and lives. I know, for example, when Doug across the street is sleeping in: His newspaper sleeps, too, wrapped in its plastic sleeve on his front steps. I knew before the Thompsons that sparrows were nesting in their eaves. Louise told me over the back fence that our dog had treed a raccoon one midnight. Bob across the street once called me to report that Alice, age seven at the time, was playing on our sloping porch roof. Our neighbors to the north, however, a physics professor with appropriately Einsteinian hair and his elegant French wife, have always been determined to have privacy, and more than twenty years ago, planted all four sides of their lot with Leyland cypress saplings.
If allowed to develop naturally, Cupressocyparis leylandii grow as much as fifty feet tall and in a dense, narrowly conical form, tapering from fifteen feet across at the base upward to a point. But because the professor
planted his saplings a mere three feet apart, the trees have grown up to look nothing like that. Most of their lower limbs, too deeply shaded, have died, so the trees that border our property now resemble thirty-foot-tall fence posts with branches only in their upper third. These branches, desperately seeking unobstructed light, extend mainly to the north and south (the row runs east-west). During winter, wet snows inevitably prove too much for one or two individuals, and we wake to find them listing drunkenly over our roof. In the spring, our neighbor’s yardman wires the fallen fellows to their sturdier comrades in order to reestablish the line. Nonetheless, although grotesquely pruned by snows and now laced together like a stockade fence, the Leylands survive year after year.
The trees pose some risk to our roof, but more to my spirit. Our house is close to the edge of our sliver of a lot, and the cypresses are a looming presence. One dismal winter day, aching for more light, I typed “how to kill Leyland cypress” into my browser’s search box, and discovered that I am far from the first to harbor sanguinary feelings about them. Leylands have become known, according to the BBC Newsmagazine, as the “scourge of suburbia” and “a by-word for neighbourly bust-ups.” In 2001, in the town of Talybont-on-Usk in Wales, one Llandis Burdon, age fifty-seven, was fatally shot in the course of a dispute over Leylands. The British government estimated in 2005 that there were as many as seventeen thousand unresolved disputes between neighbors over towering stands of these trees. In England, Part VIII of the Anti-Social Behavior Law, sometimes called the “Leylandii law,” gives authorities the power to force a homeowner to lower the height of his hedge should a neighbor complain.