The enormous quantity of vegetable debris necessary for the formation of even a single coal bed has led to the belief that the vegetation of Carboniferous times was ranker and more luxuriant than at any other time in the earth’s history, and that it grew in enormous swamps under torrid cloudy climate conditions.
—Edward Wilbur Berry, Paleobotany (1920)
“It’s going to be pretty much impossible to get you into a coal mine,” said Bill DiMichele, telling me precisely what I did not want to hear. “Coal companies have been stigmatized by the double whammy of safety regulations and getting the blame for global warming,” he explained. They didn’t welcome new faces on his team, particularly not nosey, book-writing biologists.
This dashed my hopes of strolling through a Carboniferous forest, but I couldn’t exactly second-guess Bill’s judgment. As the curator of fossil plants for the Smithsonian, he’d been leading coal-mine expeditions for years. Together with colleagues from various universities and government agencies, Bill had discovered an ancient river valley in Illinois, 100 miles long, where every detail of the forest was beautifully preserved in the rocky ceiling of the mine. “We simply look up and map the plants,” he told me. “See what was growing where.” He made it sound easy, but the forest emerging from those maps was anything but simple. In fact, it was redefining the whole context of seed evolution. The good news for me, he went on, was that there were plenty of places to see some of the same fossils on the surface. “Tell me what you have in mind,” he said, “and I’ll ask around.”
Six months later, I stood next to Bill at the bottom of a desert canyon, watching dozens of paleontologists from around the world scramble up the slope toward a dark seam in the rock. “This is only a coal bed to someone from New Mexico,” he said with a smile. But while it couldn’t match his Illinois mine in scale, the thin vein exposed on the wall above was otherwise remarkably similar: the carbonized remains of an ancient swamp forest, with beautiful examples of its plant life preserved in the surrounding rocks.
Soon the canyon echoed with hammers ringing on stone as people reached the coal and dug in. It was the first day of a conference devoted to what paleontologists call the Carboniferous/Permian Transition, a critical time in earth’s history when the climate abruptly changed from hot and humid to dry and variable. Traditionally, experts considered this a moment of triumph for seeds. The giant horsetails and other spore plants that dominated Carboniferous swamps relied on a warm, wet environment. They couldn’t adapt to the changing climate of the Permian, giving seed plants the opportunity to proliferate, overcome the spore plants, and dominate the global flora. It’s a nice story, but to Bill and a growing number of other specialists, there’s just one problem: it’s dead wrong. No one denies that spore plants declined in the Permian, but the real triumph of seeds probably came much, much earlier.
“I used to go to the field expecting certain things,” he told me, explaining how textbook knowledge can burden the mind with preconceived notions. “Now I go to the field looking. I’ve found it’s more productive to just dig a hole and see what I find.” In thirty years as a Smithsonian paleontologist, Bill DiMichele has dug a lot of holes. Compact and fit in his khaki vest and baseball cap, he moved around the dig site in New Mexico with the efficiency of experience, rarely swinging a hammer, but always there to comment on a new find. “You guys are in it, man,” I heard him shout at one point. “You’re in it!” Bill maintains the enthusiasm of a much younger scientist, but after a few hours of conversation I understood what really lay behind his long career: insatiable curiosity. For every question I asked, he seemed to have dozens of his own. They came out in a torrent, full of fresh ideas designed to wash away layers of old thinking. Just like a paleontologist in the field, he makes his intellectual discoveries by moving a lot of rock.
This approach opened Bill’s eyes to the glimmer of something new in that Illinois coal mine. Most of it looked like a typical Carboniferous forest, dominated by tree-sized spore plants related to modern horsetails and club mosses. But whenever the ancient terrain climbed upward, even a little bit, he and his colleagues saw more fossil seed plants. And when they encountered a side channel filled with debris from further up the slope, it was a jumble of conifers. No one doubts the dominance of spore plants in coal forests, but only a minor part of the Carboniferous landscape was swampy. What was growing in the uplands, on the hillsides, in the mountains?
“Hey Bill!” someone called, and motioned us over to a slab of rock at the base of the slope. There, etched in stone, was a good summary of the story I’d come to New Mexico to see. “Nice one, Scott,” Bill said, as he leaned in for a closer look. (Though people had come to the conference from as far afield as China, Russia, Brazil, Uruguay, and the Czech Republic, the world of Carboniferous/Permian specialists is a small one, and they all seemed to be on a first-name basis.) The rock had split neatly down the center, revealing mirror images of two plant stems lying side by side—a giant horsetail in the genus Calamites, and an early seed-bearer called a pteridosperm, or “seed fern.” The calamites stood out in sharp relief, its dark ridges and grooves like a scaled-up stalk of any modern horsetail. The seed fern’s trunk looked like lizard skin, scaly black and orange against the tan surface of the rock. Both species were long extinct, but for me, seeing them together embodied that ancient struggle between spores and seeds.
FIGURE 4.1. These fossils from a New Mexico coal bed sum up the ancient struggle between spores and seeds. They show the stem of a giant horsetail named Calamites right alongside that of an early seed fern. The plants grew side by side in the great wetland forests of the Carboniferous. PHOTO © 2013 BY THOR HANSON.
I snapped a picture and then scrabbled up the hillside to join in the hunt. The rock face above the coal broke apart easily, and soon I was finding my own fossils—a few ferns and horsetails, but mostly an unrecognizable jumble of leaves, stems, and spiky twigs. Around me, the paleontologists worked and talked excitedly. Where my eyes saw only dust and confusion, I knew theirs were reconstructing an ancient world. I tried to picture the calamites and seed ferns as living plants and my mind immediately turned to textbook images of the Carboniferous: a steamy swamp festooned with huge, mossy-topped trees like something from a Dr. Seuss book, and populated by newt-like amphibians as large as horses. It was an era long before dinosaurs, let alone more familiar creatures like mammals and birds. There would have been dragonflies and a few spiders, but no ants, beetles, bumblebees, or flies. While a swamp without mosquitoes sounds appealing, the forest would have seemed strange for all that it lacked. Then I reminded myself that, if Bill was right, the landscapes of the Carboniferous might actually have looked a lot more like home.
“It should be called the Coniferous!” he burst out during one of our conversations. “The evidence now really suggests that coal was a minor element.” Once Bill’s team began questioning conventional wisdom, they started seeing compelling signs of a hidden flora, a community of conifers and other seed plants that lived uphill from the swamps. Though it probably covered all but the wettest places, this forest left almost no trace of itself behind—just the occasional leaves and branches that washed down from above. “There’s a problem with terrestrial plants,” he explained. “They don’t preserve well in place.” Making good fossils requires fine-grained sediments and water, common ingredients in the swamps where spore plants ruled, but rare everywhere else. So while giant horsetails and club mosses may dominate the Carboniferous fossil record, that doesn’t mean they dominated the Carboniferous.
FIGURE 4.2. This classic view of a Carboniferous coal forest shows a swampy world dominated by ferns, horsetails, and other spore plants. Evidence now suggests that only a small part of the world was wet and hot, and that conifers and other seed plants dominated large swathes of upland habitats. Anonymous (Our Native Ferns and Their Allies, 1894). ILLUSTRATION BY ALICE PRICKETT, TECHNICAL ADVISER TOM PHILLIPS, UNIVERSITY OF ILLINOIS, URBANA-CHAMPAIGN.
New climate research makes the case even stronger, refuting the stereotypical image of the Carboniferous as a monotonous era of hot, humid weather. Instead, it repeatedly swung from sultry periods to ice ages and back again. Coal accumulated only at the wettest times, and the wet times were interrupted by long dry spells when seed plants would have covered even more of the landscape. In this view, spore plants fall from a position of prominence to that of a relative anomaly—minor players in both geography and duration. But because they grew in swamps, they left behind an overwhelming, disproportionate, and ultimately misleading number of fossils—what paleontologists call a preservation bias.
“Where’s Thor?” I heard someone call out. “The Czechs found some seeds!” I’d only been with the group for half a day, but everyone already seemed to know what I was working on, and just showing up had earned me a place in the first-name club. The trip leader walked over and handed me a small block of stone speckled with black marks. Through my hand lens they looked like watermelon pips ringed by thin membranes. I asked Bill what they were, but he only shrugged: “You’re best off just calling them winged seeds.” Fossil seeds rarely had names, he explained, because they were almost never found with the plants that produced them. Later that day I saw what he meant as I pored over trays of fossils at the New Mexico Museum of Natural History and Science in Albuquerque. There were scores of seeds, collected over decades, with labels like: “Seed?” “Ovule?” “Partial Cone?” or “Unknown Fruiting Body.” In one famous case, the “seeds” of a well-known ancient plant turned out to be fossilized pieces of a millipede.
“Man, I wish someone would work on paleo seeds,” a curator told me later at the conference social hour (wine, beer, and heavy hors d’oeuvres served up in a warehouse full of fossils). “We’ve got one that looks like a mango pit, but with a big keel like a sailboat, and it’s covered with hairs. What kind of plant made that?!?”
I agreed wholeheartedly. Studying ancient seeds would open a window on Bill’s hidden plant communities. After all, for every unknown seed lying in a museum somewhere, there must have been an unknown seed plant uphill from a swamp, raining its progeny down into the muck below. What’s more, those seeds dated to a time when all their critical traits were just evolving—nourishment, dispersal, dormancy, defense. For seed biologists, the most exciting aspect of Bill’s theory is what it does for the story of seed evolution.
The traditional view put seeds on the map at the dawn of the Carboniferous, or perhaps a bit earlier. Then, for more than 75 million years, nothing much happened. One had to accept that seed plants, with all their advantages, could only eke out a small living in the coal swamps until the climate changed in the Permian. This version of events left two glaring questions unresolved. First of all, if seeds represented such a substantial and successful evolutionary change, then why did they remain insignificant for so long? Second, if seed traits like nourishment, protection, and dormancy were so well suited to dry and seasonal climates, then how did they evolve in a swamp? Relocating seed evolution to the uplands makes these problems disappear. Suddenly, the seed strategy becomes a logical adaptation that allowed the early innovators to colonize huge swathes of unoccupied habitat. Bill and a growing number of his colleagues now think that seed plants dominated the Carboniferous, spreading and multiplying into diverse forms that the fossil record only hints at. The “rapid” rise of seed plants in the Permian finally makes sense. When the climate dried out for good, seed plants took over quickly for a very good reason: they were already there.
“I really put the pieces of this together over a long career,” Bill told me, making a point of crediting his many collaborators. But overturning long-held beliefs in science never comes without controversy. “There are colleagues of mine who violently disagree with me,” he admitted. “But I just try to be kind and keep smiling, keep saying it. My thesis adviser always told me, ‘Don’t argue; just keep working.’” Bill seems to have taken that advice to heart. After the field trip, the conference moved indoors, where people gave presentations on their research. Heated debates often erupted, but Bill always stayed out of them (and often did have a smile on his face). Later, however, I heard him restate his philosophy with a slightly different twist: “Never argue with a fool—an onlooker can’t tell the difference.”
If any of Bill’s colleagues really do “violently disagree” with him, I didn’t meet them in Albuquerque. Everyone I spoke with at the conference endorsed the notion of a dynamic Carboniferous climate, where coal forests were an interesting but by no means dominant part of the landscape. An affable Brit named Howard Falcon-Lang proposed moving back the origin of conifers by tens of millions of years, strengthening the notion of rapid seed plant evolution in the uplands. There was a Canadian graduate student who said his adviser had instructed him to “get close to Bill, and learn everything I can.” But it was Stanislav Opluštil from Prague who put it best. He had once believed strongly in the traditional view, he told me, but now considered the matter settled. “Bill changed my mind.”
I left New Mexico with a completely revised mental image of the Carboniferous. The big newts and dragonflies remained, but now I pictured them against a backdrop that looked a lot more like home: a forest of conifers. Bill DiMichele’s work brings the story of seed evolution out of the swamp, putting it in a dry, upland context where a seed’s many adaptations for aridity make sense. But it’s still a long journey from spore to seed. To truly understand that leap, we must ask some indelicate questions about the private lives of plants.
When spore plants have sex, they usually do it in dark, wet places, and quite often with themselves. A fern, for example, casts off thousands or even millions of spores every year, microscopic blips that float like earthy smoke from the edges and undersides of its leaves. Each spore consists of a single thick-walled cell with no additional protection or stored energy. It will only sprout if it lands on just the right patch of damp soil, and even then it does not grow into another fern as we know it. Instead, fern spores produce an entirely separate and unrecognizable plant, a tiny, heart-shaped nub of green smaller than a fingernail. It is this plant, the gametophyte, that has the equipment necessary for fern sex.
When gametophytes make eggs, they also send forth swimming sperm that can paddle their way through muddy water in the soil for an inch or two (two to five centimeters). Only if that journey unites the sperm with a nearby egg will fertilization take place and a new, familiar-looking fern sprout up. The details of this system vary, but all spore plants relegate sex to a separate generation, and all of them require water for their sperm to find an egg. Those traits work fine in wet weather, but they were problematic whenever the great swamps of the Carboniferous started to dry up. Reproduction became a challenge, and having two stages to their life cycle made it doubly hard to adjust to the changing climate.
“If spore plants wanted to make major adaptations,” Bill explained, “then both phases of their life cycle had to adapt to it. And that’s very difficult.” In other words, the tiny gametophyte not only looks different, it might also have very different requirements for soil, moisture, light, or other conditions. “I used to tell my students, ‘Imagine that your sperm or eggs grew up into little one-third versions of yourself, and then those little yous had to have sex to produce another you. And what if they looked different? What if they were totally independent and had no knowledge of your existence? What if you decided you wanted to live somewhere different? If they wouldn’t or couldn’t go there, then neither could you!’”
FIGURE 4.3. Wallace’s spike moss (Selaginella wallacei). Like the common ancestor of all seed plants, this spike moss has taken the evolutionary leap of separating male and female spores. The males, precursors to pollen, are pictured on the upper right, emerging from their pouch like a smear of dust. The much larger female spores appear directly below. ILLUSTRATION © 2014 BY SUZANNE OLIVE.
In some ways, it’s as if seeds evolved in response to the limitations of spores. Instead of banishing sex to the soil, they united parental genes on the mother plant, equipped that progeny with food, and dispersed it in a durable, protective case that could withstand the elements and sprout when conditions were right. Eventually, they even replaced the swimming sperm with pollen, eliminating the need for water. With so few ancient seed fossils to look at, experts still argue about the details of this transition. But everyone agrees that it was well underway by the early Carboniferous. And while every step may not be preserved in stone, living examples survive in the modern descendants of spore plants that persist, and even thrive, all around us. I didn’t need to travel to a conference to see them—they grow right in my own backyard.
Every day, my short walk to the Raccoon Shack leads me past spore plants, from moss in the lawn to a patch of bracken fern that has survived years of mowing, weed-whacking, slash fires, and the depredations of our chickens. But the particular spore plant I wanted to see grew a few miles down the road, on a rocky bluff overlooking the sea where most visitors to our island gathered to watch for orca whales. On a clear January morning, I packed a sandwich and headed there to search for a somewhat smaller, but no less remarkable, species, Wallace’s spike moss.
Calm water stretched away below in glints and ripples of tide as I walked down the short path. I couldn’t help stopping for an early lunch and found an open place where I could soak up as much warmth as possible from the winter sunlight, a rarity in our neck of the woods. Before I’d even unwrapped my sandwich, however, I spotted the object of my quest, peeking out from a crack in the rock beside me. To be honest, I knew I’d find it without much effort. I often led field trips to this site, and had once looked on proudly as the majority of my botany students ignored a passing group of orcas to focus on these tiny plants. (Having grown up on the island, they were familiar with whales, but this was their first spike moss!)
I knelt down for a closer look. Spike mosses trace their ancestry straight back to the giants of the coal forests. While this plant grew only a few inches tall, the leaves pressed to its little stem would have looked right at home on the fossils I’d seen in New Mexico. But what the spike moss knows sets it apart from almost every other spore plant that has ever lived. I pinched off the tip of a branch and held it up in the sunlight, squinting hard. Then I rubbed my eyes and sighed. It was time to admit that I’d reached a point in life where I could no longer enjoy the pleasures of spore viewing if I forgot to bring my reading glasses.
What I was looking for came into clear focus back at the Raccoon Shack, with the help of a dissecting microscope. There the spores practically glowed, tucked into speckled golden pouches at the base of each leaf. But it’s not unusual for tiny things to look beautiful under magnification. What made these spores remarkable was their size, or rather, their sizes. Low down on the stem each spore looked bulky and smooth-edged, like a big river stone, but near the branch tips they were minuscule, spilling out of their golden sacs like smears of reddish dust. The spike mosses know something that ancestral seed plants also had to learn: how to separate the sexes. The large spores are female, the precursors to eggs, and the small ones are male, the beginnings of sperm. This system not only increases genetic mixing, it allows the plants to start “packing a lunch,” investing their energy in the female spores destined to produce a new plant. While both the males and females must still travel off and grow into gametophytes, and they still require water for their swimming sperm, this clever adaptation evolved at least four times in the spore plants. And on one of those occasions, it led to seeds.
Like unearthing a perfect fossil, looking at spike mosses is a glimpse into the past. As modern ambassadors of an ancient line, their mismatched spores mirror a critical step in the evolution of seeds. With the sexes separated, it becomes much easier to imagine the rest of the story. Over time, early seed plants learned not to cast off their female spores but to cling to them, letting the eggs develop right there on the tops of their leaves. Male spores continued dispersing and with a few tweaks became windborne grains of pollen. When that pollen landed on an egg, the plant suddenly found itself in possession of all the basic elements of a seed: a fertilized baby that could be protected, provisioned, and sent off to grow directly into the next generation. This system gave seed plants immediate advantages whenever the weather turned dry. Where spores required water for their swimming sperm and moisture-loving gametophytes, seed plants could reproduce on a gust of wind. And their durable, well-stocked offspring landed in the soil prepared to wait for just the right conditions in which to germinate and grow.
The fossil record for seed evolution remains hazy, but as with spike mosses, other modern plants help fill in the gaps. Most people recognize the ginkgo tree as a popular ornamental, or the source of herbal elixirs sold to boost memory and improve blood flow. But it’s also the sole survivor of an early seed-plant family whose pollen still produce swimming sperm, a holdover from the spore era. A group of palm-like trees called cycads also retain this trait, and one of them boasts sperm so huge they can be seen by the naked eye. (Festooned with thousands of waving tails, the sperm of the chigua from coastal Colombia exceed those of any other plant or animal.) Together with the conifers and a handful of lesser-known species, these plants make up the gymnosperms, or “naked seeds,” so named because their seeds mature unadorned on the surface of leaves or cone scales.
Gymnosperms dominated the world’s flora from the dry periods of the Carboniferous all the way through the time of the dinosaurs, and they remain extremely common today. Anyone who has enjoyed pine nuts on a plate of pesto is familiar with naked seeds. So are the billion or so people who live in or around temperate forests, where pines, firs, hemlocks, spruce, cedar, cypress, kauri, and other conifers still cover more land area than any other plants. But while they may be widespread, these venerable trees and shrubs long ago passed the crown of plant diversity down to a younger group of seed innovators.
The final major step in seed evolution occurred when a few gymnosperms learned to cover up. They did it in much the same way people do after a bath, and for similar reasons. At three years old, my son Noah still uses the blue plastic tub we bought when he was an infant. He can climb out on his own now, but when he does I wrap him up immediately in a big fluffy towel. I do this not out of some prudish aversion to nudity, but because his little naked body seems so vulnerable. For me, it triggers an instinctive parental response to protect and nurture. While plants don’t run around making conscious decisions about towels, the same evolutionary drive led one line of gymnosperms to wrap their naked seeds, folding up the underlying leaf to enclose the developing egg. Botanists call this chamber the carpel and the plants that have one are known as angiosperms, Latin for “seeds in a vessel.”
I didn’t see any fossil angiosperms in New Mexico. “Wrong conference,” one attendee told me gruffly. The rocks were wrong, too, off by several major geologic time periods. While it sounds like a simple, even obvious step to wrap a protective leaf around a seed, angiosperms didn’t work it out until the early Cretaceous, after naked seeds had been commonplace for more than 160 million years. To put that into perspective, the entire diversity of placental mammals, from rodents and bats to whales, aardvarks, and monkeys, has evolved in a time period less than a third as long. Botanists still puzzle over this delay, but no one disputes that putting seeds in a vessel turned out to be a good idea. Once established, angiosperms spread so fast that Darwin considered their rise an “abominable mystery” that threatened his concept of measured, incremental change. They now make up the vast majority of all plant life, and their seeds dominate the discussions in this book.
From an evolutionary standpoint, the leap from spores to gymnosperms was the paramount step for seeds. Bill DiMichele laments our tendency to focus on angiosperms. “It misses the story,” he told me. “There just happen to be a lot of them.” But there’s no doubt that wrapping those naked seeds refined the system and opened a range of new opportunities. After all, a towel is just the beginning. Noah’s wardrobe trends toward striped pajamas, but people can cover up their nakedness with whatever they want: shorts and a Hawaiian shirt, a cocktail dress, or even a suit of armor. Seed coverings soon evolved from simple leaf tissue into the dizzying array of structures we know collectively as fruit. Like clothing, fruit can be protective but it can also attract, giving angiosperms a powerful way to hoodwink animals into dispersing their babies. (We will explore the ties that bind fruits, seeds, and animals, including people, in Chapter 12.)
Even more important than the evolution of fruit, however, was the way that covering seeds affected pollination. With the egg hidden away inside its vessel, wind became a less reliable tool for pollen delivery. Instead, angiosperms turned increasingly to animals, and particularly insects, to move pollen from flower to flower. Colorful petals, nectar, fragrance—all the allure we associate with flowers—developed in response to this need, transforming pollination from a random wind splatter into one of nature’s most precise (and beautiful) methods for mixing genes. This system helped propel the rapid diversification that so mystified Darwin, and it also gave rise to another name for angiosperms: “the flowering plants.”
In nature, the flowering plants put sex, seeds, and dispersal on full display, spurring not only their own evolution but also that of the animals and insects with which they became so entwined. In most cases, the diversity of dispersers, consumers, parasites—and, most especially, pollinators—rose right alongside that of the plants they depended upon. But the evolution of floral sex has also proved vital to people. Without the ability to manipulate pollination and save the result as a durable seed, it’s hard to imagine our ancestors ever succeeding in agriculture. Author and food activist Michael Pollan takes the case a step further, calling the practice of plant breeding “a series of experiments in coevolution” that has changed both plants and people forever. Pollan has argued that human desires for sweetness, nourishment, beauty, or even intoxication have become encoded in the genetics of our crops. Selecting for these traits both pleases us and benefits the plants as we dutifully disperse them from their original habitats to gardens and farm fields across the globe. But our intimacy with seed plants fills more than our bellies—it also feeds the human imagination. The knowledge we’ve gained from this long relationship may be our deepest reservoir of insight into the workings of nature. Without it, the most famous experiment in history might never have taken place.
