Sacred-thorn-apple blossom (Datura wrightii) near Tucson
Sacred-thorn-apple blossom (Datura wrightii) near Tucson
Other than being outdoors, watching bees visiting flowers, my favorite haunts are natural history museums. Whether I’m strolling the dark, musty-smelling behind-the-scenes research and storage areas that most never get to see, or gaping at the dazzlingly well-lit high-tech exhibits in modern exhibition halls, natural history museums are spellbinding palaces to me.
Nose pressed against glass display cases, or body craning forward at the guardrails, I jockey with kids and fellow grown-ups for the best viewing position, not allowing age, size, or adult decorum to interfere. We peer at exquisite slabs of Paleozoic marine animal life, such as the stalked crinoids topped by their featherlike calyxes, looking more like plants than their near relatives the sea stars, and—my favorites—the segmented trilobites, those phenomenally diverse and successful marine arthropods.
We’re awestruck by the spectacles of life in the next era, the Mesozoic—exquisitely mounted skeletons of giant sauropods, and toothy carnosaurs standing on hind legs with mouths agape in reconstructed ferocious poses. But wait, something is missing from our image of Mesozoic life-forms. Where are the slabs of fossilized flowers dating from that time, perhaps some magnolias or water lilies? Did I overlook them in the neighboring botanical exhibition hall?
No, I didn’t miss the fossilized flowers—because they weren’t there. Although a few major US and European natural history museums have fossilized flowers, they rarely put them on public display. The Tyrannosaurus rex dinosaur fossils, or the mammals taken from Rancho La Brea—the dire wolves, the giant ground sloths, the saber-toothed cats—steal the show from them every time. On the “wow factor” scale, fossilized flowers don’t rate. Even the blackened fern fronds, horsetails, and the bark impressions of giant club mosses attract more interest from difficult-to-please museumgoers. So at the museums that do house them, fossil flowers are stored locked away inside cabinets in back rooms that only the occasional researcher might venture into.
Before we get to the earliest flowers, and the fossil evidence establishing their existence, we should look briefly at what came before the flowering plants. We now think that the first flowers appeared 125–130 million years ago. For hundreds of millions of years before that there were many forms of vegetation—but without flowers. About 472 million years ago (in the mid-Ordovician period of the Paleozoic era) the earth began greening up. The monotonous browns, blacks, and whites of terrestrial rocks and soils gradually gave way to the colors of plant life. Living in and near water, the earliest plants, including the one-celled algae known as diatoms and then multicellular, branched, filamentous algae, established their beachheads. Eventually a living, green biofabric, the plants developed, blanketing vast expanses of the earth’s surface. Matlike at first, those earliest colonizers of the land—algae, liverworts, and mosses—began growing taller in their competition for light and space, and evolved into plants similar to the ferns and horsetails we know today.
By the Devonian period, beginning about 419 million years ago (mya), vast forests of plants released airborne spores to reproduce. They depended upon the wind or, rarely, water currents to move their gametes. Spreading across equatorial continents (Eurasia, North and South America, Africa, Antarctica, India, and Australia) that made up the supercontinent Pangaea, these new plants took over the land so completely that they “terraformed” the planet. Their predominance brought the vast climatic changes and oxygen enrichment of the atmosphere necessary to sustain future animal life.
Animal life, during the Devonian period, also known as the Age of Fishes, consisted largely of the ray and lobe-finned fish that swam the oceans and lakes, and vast numbers of insects that flew through the air. Our distant ancestors, the first four-legged vertebrates, were just beginning to crawl ashore.
To conjure up a vision of this tangled, forested world, perhaps you can call to mind colorful dioramas in natural history museums that depict prehistoric scenes. The diorama created by French artist Édouard Riou portrays a scene from the Carboniferous period (359 to 299 mya), which I saw on a visit to the Chicago Field Museum when I was a teenager. It was so vivid an evocation of the plant and insect life in that ancient time that it made me feel I’d been transported back in time. It further inspired me to become a field biologist. Exotic as the scene was, some of the early plants depicted in similar paintings—brilliant green carpets of squishy mosses, horsetails and ferns of many types, flattened liverworts and hornworts growing on rocks, fallen logs, and earthen banks—would be familiar to anyone with a naturalist mind-set on a leisurely hike through a hardwood forest somewhere in Wisconsin or Arkansas.
Nonetheless, a walk through a forest today would not prepare you for what it might be like to pay a time-traveling visit to one of those ancient forests. The club mosses and horsetails of today are small, only a few inches to several feet tall. But in the Carboniferous and Devonian periods many were enormous, and you would have walked around and under the towering giants, some with trunks and crowns towering a hundred feet overhead.
You would probably also have noticed the abundant pollen- and spore-feeding insects. Scavenging insects and other arthropods found a readily available food supply in the spores of ferns, seed ferns, and their allies. Later, they would have fed on the pollen grains and exudates from the gymnosperms, which you’ll read about in the next section. This was happening as long ago as the Permian period, 298 to 252 mya, and we have the smoking gun to prove it—pollen grains preserved inside the guts of fossilized early insects such as xyelid sawflies, a type of primitive plant-feeding wasp, rock crawlers, web spinners, and others. Palynologists (scientists who study fossilized and modern pollen grains) can often look at the pollen those early insects ate and identify which plants the pollen came from.
Before we meet the earliest true flowers, another vitally important group of plants must be mentioned, which also appeared on the earth sometime during the early Devonian period. Those ancient plants, including many still with us, are the gymnosperms, literally the plants with “naked seeds,” which is the hallmark of their evolutionary history—the characteristic that distinguishes them from their spore-bearing predecessors, and their flowering descendants. The seeds are naked in the sense that, unlike the seeds of most flowering plants, they are not embedded inside fleshy fruits.
You will certainly recognize at least one kind of still-extant gymnosperm—the evergreen conifers. Pines, spruces, firs, larches, and tamaracks are all conifers, and we find these trees not just in our forests, where they cover many millions of acres across the temperate regions of Canada, the United States, and Eurasia, but also in our backyards.
Conifers, like other gymnosperms, reproduce not via air- and waterborne spores, but by seeds. Each tree produces cones of both sexes—small, male, pollen-bearing cones that are so unobtrusive that most people don’t even notice them (unless they have a pine-pollen allergy), and the large, female pinecones that we all know. The reproductive act begins when the male cones shed their pollen into the wind. Most of the vast number of pollen grains wafted through the air die before reaching their mark. A lucky few, however, will be captured by a sticky pollination droplet on the surface of a large female cone, then pulled inside for sex. Conifer sex happens slowly; fertilization and development often taking several years. When this is finally complete, the female cone falls from the tree, its prickly scales opening wide to release the naked seeds—the pine nuts—to the ground, where some will germinate and take root.
Planted along streets and in gardens across America is another of the gymnosperms, the exotic-looking ginkgo (Ginkgo biloba), which was introduced two hundred years ago from its native Chinese homelands. With its elegant, fan-shaped leaves, which turn a brilliant yellow in the fall, the ginkgo has been admired and reproduced in Asian art and jewelry for centuries. Its history on the planet is traceable not in centuries, but in hundreds of millions of years, with evidence for its existence going back 270 mya, based on fossils found in two small regions in Zhejiang Province in eastern China. Unlike the conifers, which have male and female cones on the same tree, the sexes are separated on the ginkgoes. Male trees release their pollen into the wind, and fertilization occurs using the same pollination droplet mechanism by which conifers reproduce. It’s easy to tell the male from the female trees because after fertilization the females drop their nuts, which are so rancid smelling their odor has been compared to that of vomit. Although in this country people ardently select against the female ginkgo trees, in China the females are valued for their “stinky nuts,” which are popular both as a traditional food and for their purported medicinal uses.
Another extant order of gymnosperms is the cycads (Cycadales), many of which bear a resemblance to the palm tree, though they are unrelated. Zamia, the common Florida plant known as the Florida coontie, is the only genus of cycad native in the United States. Cycads are typically wind pollinated, but others are also pollinated by beetles, including weevils and leaf beetles, that are attracted to the edible pollen grains—another possible link to the flowering plants.
In my travels, one special place makes me believe I’ve been transported back to the late Jurassic period. A three-hour drive down the escarpment starting from Canberra in the Australian Capital Territory, on the way to the popular Pebbly Beach with its mobs of food-mooching eastern gray kangaroos, there is a towering forest of spotted gums (Corymbia maculata). This tree, from the eucalyptus family, is a comparatively late arrival and would not have made an appearance in any Jurassic scene. But the bizarre understory of the forest gives me the chills. In all directions, are millions of dark green cycads—more than you’ll ever see in any other single place. The scene is so eerily reminiscent of another era that I almost expect to see an ankylosaur raise its head, contentedly chewing away, with cycad fronds dangling from its mouth.
Being in this place near the Australian coast makes me dream of the time before flowering plants achieved dominance over the land, when the gymnosperms were still in ascendance, and they and their ancient insect partners were taking the first trial steps along adaptive paths that would lead them to where they are today. We don’t know a great deal about those early insects, but recent discoveries are beginning to give us a tentative idea of how they lived.
In 2012 scientists published information about amber from Lower Cretaceous sediments (110–105 mya) in the Basque region of northern Spain that contained some interesting surprises—tiny female insects laden with hundreds of pollen grains they must have acquired while feeding. The insects are thrips, which today’s gardeners are familiar with as straw-colored, millimeter-long garden pests found in roses and other blooms. But long before they began to consort with angiosperms, thrips were likely pollinating the gymnosperm ancestors of flowering plants, as the thrips from Spain reveal. Analysis of the pollen grains covering their bodies suggests that the pollen was likely derived from either a cycad or a ginkgo tree. This is possibly the first direct evidence of insect pollination from the distant past.
But as we now believe, based on speculative but persuasive recent findings, insect pollination may go back further still. In 2009, Dr. Dong Ren of the Capital Normal University in Beijing, China, published an intriguing report about fossilized scorpion flies that were found in sediments dating from 167 mya. No one had suspected that insects from this ancient lineage had anything to do with pollination because the scorpion flies we know today (aka hanging flies) are either carnivorous or scavenge upon dead insects. However, on the basis of their appearance, Ren and his American colleagues speculate that the scorpion flies were indeed pollinators in ancient times. Their looks are certainly suggestive. Most notable are their long beaks, some of which were half as long as their bodies. What were they doing with such long mouthparts? The scientists who found them suggest that the scorpion fly used its siphonlike proboscis to probe and feed upon the nectarlike juices of seed ferns, conifers, and ginkgolike and other ancient plants. Regrettably, a close examination of the fossil slabs reveals no telltale pollen grains on the flattened scorpion-fly bodies. We have no definitive forensic clues as to their last meals. But their unusual feeding tool kit certainly supports the idea that they were pollinators. So, if this hunch is correct, and the dating of the sediments in which they were found is accurate, the discovery pushes animal pollination further back, almost 50 million years earlier than previously thought, into the late Jurassic, instead of the early Cretaceous period.
Recently my belief that various types of flies and beetles were the only pollinators to visit the early gymnosperms was forever altered. Butterfly lacewings have been preserved in 165-million-year-old fossils from Liaoning Province, China. I was lucky to be one of the first to see the actual fossils at the Smithsonian Institution. They are the size of the palm of your hand, with siphonlike mouthparts and vivid eyespots on their brown-and-black wings. They are relatives of the aphid-eating green lacewings familiar to gardeners.
What a thrill it would be to see them flying and foraging, I thought as I looked at these amazing fossils. Smithsonian Institution paleontologist Conrad Labandeira believes that the butterfly lacewings were pollinators of extinct gymnosperms. Candidates for their attentions may have included the male parts of Welltrichia (an extinct gymnosperm with flowerlike organs), and the female organs of Williamsonia (extinct gymnosperms resembling cycads).
In July 1879, just three years before his death at age seventy-three, Charles Darwin wrote a now-famous letter to Sir Joseph Hooker, then director of the Royal Botanical Gardens, Kew, in London, in which he referred to the origin and spread of the angiosperms as an “abominable mystery.” It was long believed that what was troubling Darwin when he wrote his letter was the lack of fossil evidence for the angiosperms during the early Cretaceous period (100–135 mya). During Darwin’s lifetime, few fossils of flowering plants had been discovered from that era.
Recently, however, a detailed historical analysis by Harvard botanist William E. Friedman has completely reframed our understanding of what was troubling Darwin. It wasn’t the scarcity of Cretaceous fossil evidence that concerned him; had there been many more such fossils, he would still have been confounded because, according to Friedman, what “deeply bothered” Darwin was the highly accelerated pace of evolution that culminated in the appearance of the first flowering plants. Darwin was a devout believer in the notion that natura non facit saltum—nature does not make a leap. But the seemingly abrupt appearance of flowering plants in the early Cretaceous period, followed by the highly accelerated diversification of flowering plants in the mid-Cretaceous, seemed to him to be far too rapid. This led him to years of speculation that there might have been a missing island or continent where the pre-Cretaceous evolution of flowering plants had occurred—a sunken Atlantis, as it were.
Later, Darwin came to believe that, if the rapid diversification of flowering plants in the mid-Cretaceous era was in fact a real phenomenon, it might be explained by coevolutionary interactions between pollinating insects and angiosperms that were so successful they hastened the pace of evolution beyond what was typical for plants and animals. Trying to account for this seeming exception to his belief in long, slow, gradual evolution was extremely important to Darwin, for he felt that if any species “had started into life at once, the fact would be fatal to the theory of descent with slow modification through natural selection”—and would provide support for a creationist’s view of life.
Although Darwin’s “abominable mystery” was not about the whereabouts of Cretaceous-period fossil evidence for flowering plants, the scarcity of such fossils during his lifetime probably exacerbated the mystery of what we might think of as the missing links. If we look at the “fossilization lottery,” we can see that flowers, with their soft expanses of petals, are bound to lose out to plants with tougher parts more likely to be fossilized. Woody stems and tree trunks, leathery leaves, pinecones, and the hard seeds of many fruits are usually what’s preserved in the fossil record.
Although flowers on a stem rarely fossilize, certain conditions did make that happen—flash floods or mud or ash flows that swept up everything in their path, burying the plants and preserving them for eons in sediments where we can still trace their carbonized outlines. Amber (fossilized resins) is an even better preservative. Some of the most complete examples of fossil flowers and early flower pollinators are found in amber. The golden gem of the ages, amber was formed from sticky plant resins that hardened over time and contains an amazing record of the insects and plants of hundreds of millions of years ago.
Fortunately, thanks to discoveries that have been made since Darwin’s time, today’s paleobotanists (students of ancient botany—not geriatric botanists) have a more complete fossil record of life on earth. Looking at the many flowering plant fossils that date back to different eras and have been found all over the globe, we can examine and trace the nuances of their evolutionary history—while acknowledging that we don’t yet know everything about them.
Some of the most amazing fossil flower discoveries have been made on American shores. In the 1990s thousands of tiny, exquisitely preserved flowers were discovered in clay pits in New Jersey—pits that had been mined to make bricks since the early nineteenth century. The flowers found in these Cretaceous-period clay and amber deposits are about 90 million years old. Known as charcoalized fossils, they are made entirely of carbon, and their preservation is astonishingly good, revealing every detail of the flowers’ anatomy, down to the cellular level, and much about their pollination requirements as well.
How the blossoms were preserved in such detail isn’t entirely clear. One possibility is that they fell into the dead leaves on the forest floor and were caught up in a flash fire that roared across the ancient landscape, quickly charring the leaf litter and turning the blossoms into some of the most perfectly complete yet discovered. Flowers preserved in sediment are typically mere carbon ghosts of themselves. Even when such a rock slab is brightly lit by a fiber-optic lamp and viewed under a stereo microscope, you are likely to see an incomplete, perhaps one-half, flower, folded in on itself so that its petals and shape are hard to make out. A painterly reconstruction of the original flower for a museum diorama or a line drawing for a scientific publication is quite a challenge, requiring use of your imagination to bring this flattened object to life.
No such act of the imagination is required to visualize the fossil flowers from the New Jersey clay pits as living organisms. Looking at these diminutive flowers, which average only one-tenth of an inch in diameter, we can see anthers, stamens, petals, stigma, style, and basal ovary (the latter three forming the carpels), nectaries, and what may be scent-producing glands, and enough details that we can also identify the family groups to which the flowers belonged. Descendants of these flowers that have survived to the present age include members of the blueberry or heath families, the magnolia family, and the laurel and witch-hazel families.
Nectar-producing glands present on the fossils of these miniature flowers tell us that they were attracting pollinators with their sugary-sweet secretions, just as flowers do today. We can also see, on some of the fossils from the ancestors of the blueberry family, detailed evidence of exactly how pollination occurred. Tetrads of pollen grains—each tetrad consisting of four pollen grains permanently joined to each other—were clumped into larger groups that were united by sticky filaments called viscin threads. Thus when a pollinator visited one of these plants and brushed against this gooey clump, the clump would stick to it, and the pollinator would end up transporting not just one grain but a tangled glob of grains to the next flower. This was, and is, an effective means of pollen transport, which made it more likely that at least a few of the grains would find their mark. Today, azaleas and rhododendrons have pollen grains held in bondage similar to those in the blueberry ancestors.
Amber oozes from the trunks of resin-producing trees, hardens, and becomes fossilized under the heat and pressure of layers of clay sediments. It holds an important part of our fossil record. Thousands of species of plants and animals that would otherwise have remained unknown to us are exquisitely preserved in amber and are easier to see than those preserved in clay—though flowers locked even in amber may have their details obscured by opaque areas within the golden nugget, or by bubbles, soil, or other debris.
Deposits of amber are found all over the globe, with particularly rich deposits in the Baltic area and the Dominican Republic, where enterprising merchants and artisans fashion it into jewelry—all the more unique (and much more rare and expensive) if it has a plant or animal fossil, an inclusion, entombed within. The most common of the flowers to be found in the Dominican and Mexican ambers are those of the resin-producing trees themselves—Hymenaea courbaril. Imagine the delicate blossoms dropping from the branches, tumbling and bouncing against the lower tree trunk, finally getting stuck in freshly oozing patches of Hymenaea resin, then being trapped as more gooey resin flows over them. Come back in 15 to 20 million years, excavate the sediments, and voilà!—exquisitely preserved flowers, with every detail clearly visible. Fragile petals, fat anthers atop slender filaments, they’re all there.
Much-older amber, dating to the early Cretaceous, is found in Burma, now Myanmar. Burmese amber has been traded for nearly two millennia, originally with the Chinese, and then, beginning in the nineteenth century, with Europeans. One of the most venerable of the Burmese mines, known since at least the first century AD, is located in the Hukawng Valley and dates to 105–100 mya. Some of the amber from this area contains early insect pollinators and exquisite fossil flowers, including one perfectly preserved specimen of a small flower. Perhaps the most extraordinary discovery from this amber mine, made in 2006, is of a beelike hymenopteran—Melittosphex burmensis—that dates back to the middle Cretaceous, about 100 mya. There have been other discoveries of stingless bees in amber—including Proplebeia dominicana specimens from mines in the Dominican Republic, which have been dated to roughly 20 mya, and several from the Baltic amber 45 million years old. Cretotrigona prisca, from New Jersey amber, has a minimum age around 65–70 mya—but the one from the Burmese mine is by far the oldest fossil superficially similar to a bee. It had branched hairs that are associated with pollen capture by modern bees, but there is no proof in the fossils that this protobee gathered pollen.
Although it would be extraordinary to find a fossilized bee or other insect pollinator inside a fossil flower, caught—literally—in the act, we have something close. Fossilized bees and other pollinators have been found with flower pollen clinging to their bodies or inside their guts. With a bit of forensic sleuthing, we can establish what flowers they were visiting just before their untimely demise, which can give us invaluable information about the flowers themselves. For example, the stingless bee Proplebeia dominicana in amber has played a significant role in our understanding of the origins of the orchid family, which had long been shrouded in mystery because there are no fossils of orchids. The mystery began to be unlocked in 2007, when orchid massulae were found clinging to the body of a stingless bee caught in 15–20-million-year-old Dominican amber. Here was a true example of pollen transport frozen in time, a glimpse into the history of orchid flowers and their bees. Using this evidence and extrapolating from what they assumed to be a relatively constant rate of orchid evolution, the Harvard researchers who discovered the pollen-bearing bee used a molecular clock-dating approach to arrive at a speculative late-Cretaceous date for the first appearance of orchids. If that’s true, then the earliest orchids coexisted with long-extinct dinosaurs.
For decades botany textbooks used flowers such as Magnolia grandiflora, with its gloriously large (as befits a plant named grandiflora), fragrant, white blossoms as the archetypes of primitive flowering plants. Botanists, paleontologists, and others with a serious interest in such matters long believed that the first flowers, at the start of the Cretaceous, were big, even by comparison with today’s flowers. But we were all wrong. What we now know from fossil evidence is that virtually all the oldest flowers (including the earliest magnolias, when they eventually came along in the mid-Cretaceous) were puny runts.
Over the years many contenders have appeared for first true flower in the fossil record. Some of these were eventually reclassified as nonflowers, while others were dated more accurately to a later geological time. Right now, the best and most unambiguous contender for the title of first true flower is Archaefructus sinensis, described in 1998 by Ge Sun at Jilin University and David Dilcher of the University of Florida. Archaefructus was found in Yixian lake-bed deposits in Liaoning Province of northeast China. Dating from the lower Cretaceous age, its scientific name means “ancient fruit from China.”
In an evolutionary rather than a poetic sense, perhaps we should consider Archaefructus as the mother, the Eve, of all living flowering plants. Because the fossil slabs that contain these flowers also contain small fossil fish, we speculate that the plants may have been aquatic or semiaquatic, growing in shallow lakes, extending their leaves, thin stems, and flowers above the water’s surface. The small size of the Archaefructus—it was only about eight inches tall—and its ability to flourish in both kinds of habitat may have given it an advantage over long-lived but slow-growing and less adaptive woody trees, allowing it to spread rapidly, colonizing new areas far from where it originated.
The Archaefructus had deeply lobed, finely branched leaves similar to those of modern buttercup or carrot, but flowers without petals or sepals to advertise their presence to passing lakeside insects—not exactly good candidates for winning any ribbons in a local flower show. We don’t yet know whether Archaefructus bore its small, unisexual flowers on long stems, or if the stem was itself part of the flower stalk. The fossils reveal that the plants had unfertilized ovules within the ovaries—which, following pollination and fertilization, would have become fertile seeds housed within a fruit similar to the woody pea pods found in today’s legumes. This characteristic—seeds enclosed within an ovary that becomes a dry or fleshy fruit—is one of the defining characteristics of the angiosperm. In fact angiosperm in Greek means “vessel” and “seed.” The entire female anatomy of the flower—stigma, style, and ovary—constitutes the carpel. Along with the double-fertilization maneuver described in the first chapter, the seed-bearing ovary is what makes an angiosperm an angiosperm.
Archaefructus seems to belong to a long-extinct plant family unlike anything currently known. The closest (albeit not very close) living relative to Archaefructus is a water lily. While it may not have been earth’s first true flower, Archaefructus is the oldest flowering plant we know about.
Flowers kept developing for the 125 to 135 million years of their existence on earth. Every few million years they varied in size, shape, color, scents, edible rewards, and reproductive morphology as the different species came and went. Their interactions with the ancestors of modern flower-visiting insects, many of them now extinct, also shifted—which resulted in further refinements in the flowers, and in changes to the insects themselves.
One major trend we see when we examine the fossil record is that flowers gradually increased in size. The modest-size Archaefructus and those carbonized pip-squeaks from the New Jersey clay pits were typical of flowers during the Cretaceous period, when they ranged from 0.04 to 0.23 inches in diameter, and their pollinators were similarly diminutive. Flowers didn’t just get larger, they became brighter, showier, and more fragrant. Bigger, more colorful petals helped make flowers more visible against the green background of leaves, better able to attract and hold the attention of pollinators. The scent-producing glands on their petals perfumed the air, to entice hungry insects to the blossoms. Bigger blossoms also contained more stuff—greater numbers of anthers and therefore greater quantities of pollen, as well as nectary tissues producing more of the nectar needed by flower-visiting insects.
All these coevolved features made flowers more attractive to their animal go-betweens, which helped flowers compete more effectively. Plants that are more frequently pollinated leave behind more seeds, reproducing more prolifically and spreading their genes more widely—which every plant and animal lineage does if it survives into future generations. Flowers just did it extraordinarily well.
Mutations among the early flowers resulted in ovules that were more enclosed and protected, giving the developing seeds a better chance at living long enough to fall to the ground and grow into new plants. In many early angiosperms, we see their evolutionary experimentation with reproductive organs, too. Some were unisexual plants with both male and female parts, while on other plants the male and female parts appeared on separate blossoms. Flowering plants also tried various genetically based breeding systems and population sex ratios during their long histories. Flowers weren’t prudish about their sexual contrivances.
In the earliest days of flower life on earth, small beetles and flies predominated as pollinators, and the small flowers they pollinated were relatively open, accessible to all. Bees arrived on the scene a bit later (witness those stingless bees in amber), and they, too, were small. Later, diverse and highly specialized morphologies began to evolve in flowers. They became bigger and more complex, with some of them hiding their nectar and pollen deep inside tubular-shaped blossoms so that they were no longer handed out freely. In response, flies and bees as well as moths and butterflies began evolving longer proboscides, and they used different foraging techniques to reach the hidden nectar and pollen, coming back again and again to the flowers to search for the precious substances, thus maximizing the flowers’ pollination strategy. From the fossil record we see that the pistils didn’t just get longer and more tubular, but also larger, which allowed pollen pickup and delivery by larger insects, and eventually by bats and, later still, hummingbirds. Flowers and their visitors were already engaged in a mutualistic dance—a dance that goes on to this very day.
Beetles and flies, early pollinators of flowering plants, belong to extremely ancient lineages and were around long before there were true flowers. If we use the number of currently known species as our criterion for success and we exclude bacteria, beetles are by a wide margin the most successful living animals on the earth. Taxonomists have identified roughly four hundred thousand species of beetles worldwide. In what may have been an apocryphal exchange, responding to clerics who asked if anything could be learned about the Creator from studying the natural world, the late British geneticist J. B. S. Haldane was reputed to have said, “The Creator, if he exists, has a special preference for beetles.” The evidence would suggest that the Creator also liked flies, which played a key role in the pollination of early flowering plants and are very much with us today.
So how did primitive flowers entice their pollinators? A few examples will show nature’s ingenuity, which will be further explored in the next chapter.
Early angiosperms such as the water lily and the magnolia deployed multiple strategies for attracting flies and beetles. Not only did they have floral structures that mimicked those of the gymnosperms the insects had long been familiar with, but they had some special signals of their own. Using internal starch reserves as energy to heat up their flowers, they emitted pollinator-enticing scent compounds that made these flowers more alluring from farther away. Another possible advantage of the heat may have been to fool a female fly into thinking that she’d landed on a still-warm mammalian carcass on which to lay her eggs.
Although modern versions of the water lily and magnolia no longer deploy this heat-producing strategy to lure their pollinators, it still works for the common houseplant Philodendron, or “elephant ears.” In its homeland in the New World rain forests, scarabs and other beetles pile into the Philodendron’s flowers to feed and mate during the heating phase. Once mature, these mild-mannered plants burst forth with a dramatic phallic-looking, white spadix wrapped inside a green sheath. Over several days, the cream-colored spadices become noticeably hot to the touch, up to 57 degrees Fahrenheit above ambient temperatures. If you have a Philodendron in your collection of houseplants, you can see—and touch—this floral example in your home.
Another form of beetle pollination that has come down to us from ancient times is called mess and soil—a term that, for me, conjures up riotous images of wanton feeding, copulating, and defecating; orgies taking place inside each flower, which is exactly what is happening. The western spicebush, Calycanthus occidentalis, still makes use of this reproductive strategy. Even from a distance the spicebush is enticing, as I remember as a graduate student at UC Davis, roaming the Napa Valley foothills and their shiny green, serpentine rock outcrops where it flourishes. Its dark green foliage is set off by crimson flowers that emit a potent fragrance something like the aromas of the wines I often tasted at the Beringer, Nichelini, and Franciscan wineries.
Peering into a spicebush blossom often reveals a dozen or more slender, black-pointed-tail rove beetles, or black-and-brown, short-winged sap beetles. At the tips of the innermost spicebush petals are small, whitish food bodies, decoys that lure beetles to their feeding frenzies while keeping them at a discreet distance from the plants’ tender private parts. Sure, the munching beetles inflict some damage, but in frolicking about, they become dusted with pollen grains that get a free ride to the next beetle orgy a flower or two away—preferably on a neighboring spicebush. The same thing was happening in the Mesozoic, as we can see from chewing marks in fossils of the ancient gymnosperms and earliest flowering plants.
Bees have been on the planet for a long time. Exactly how long we still don’t know. The Burmese amber protobee fossil from about 100 mya is the oldest evidence we have of early bees or their ancestors. But today we view stingless bees as the culmination of eons of social evolution, and we feel sure that the oldest amber bees must have been preceded by ground-nesting bees that were solitary—the problem being that we have no fossil proof of their existence. All we know is that certain bees, similar to the modern stingless bees we know today, became highly social and also lost their stingers while adopting other defensive strategies—such as biting and chemical warfare—instead.
Stingless bees, like their ancient ancestors, live in tropical forests. Walk along any forested path in southern Mexico, Costa Rica, Panama, or Brazil and you find housefly-size black or orangish stingless bees such as Trigona, Tetragonisca, or the larger, striped Melipona. They seek out plant resins, saps, and gums—they hoard them, and you can find them scraping away at oozes and rot areas on tree trunks, and visiting resin-producing flowers such as Clusia. Combs, presses, and rakes on the legs of the females collect pollen from the flowers during their visits, and afterward, when the females groom themselves, they wet the pollen with saliva or regurgitated nectar, which forms a moist pellet of pollen that adheres firmly to the concave surface of their hind legs for the journey home. They do the same thing with resins, so that irregular blobs of the substance stick to their hind legs on their way back to the logs in which they live. Pollen is food for the adult bees and their young, and the resins are used to caulk up any fissures in their homes, to disinfect their nests against pathogenic bacteria and fungi, and to defend against ants and other enemies.
Remarkably, the bees have somehow developed ways to prevent themselves from getting stuck in the resin—perhaps with a resin solvent?—but once in a while they become hopelessly fouled in the sticky goo, and some of them end up fossilized in the amber treasured by collectors and scientists alike.
Small bees, beetles, and flies were not the only pollen eaters in ancient times. The 50-million-year-old mid-Eocene oil shales of Messel, south of Frankfurt, Germany, contain exceptionally well preserved fossils of larger animals such as bats, which have huge amounts of pollen in their stomachs. Just like certain living bats, the Messel bats must have visited flowers to feed on their pollen and nectar, moving pollen from flower to flower, pollinating them.
No matter where we go—wandering through alpine meadows, strolling along the paths in a botanical garden, or picking our way through our own home gardens—we come across displays of brilliant flowers. Yet never do we pause to wonder why the monotonous greens and browns over the long expanse of time before flowers arrived on earth gave way so suddenly—abruptly enough on the evolutionary scale to cause Darwin great concern—to an earthscape where flowers became nearly ubiquitous.
The answer to “Why flowers?” is that their evolutionary strategies were highly adaptive. From their simple and miniaturized beginnings the angiosperms developed an extraordinarily diverse set of biological innovations and “learned” to rely on a seemingly infinite variety of means of reproduction. Flowering plants come in just about every life-form imaginable, from tiny floating duckweed to the tallest rain-forest emergent trees, from columnar cacti to prostrate shrubs, and they can flourish in most climates. Some tree species live for thousands of years, capable of enduring withering droughts and freezing winters. Flowering plants also excel at chemical warfare. They ooze, drip, and volatilize chemicals that inhibit the germination of unrelated plants below them.
One of the most valuable contributions to this 125–135-million-year-old success story is double fertilization, which ensures that each flowering plant seed comes with its very own “survival space suit,” or onboard starchy or oily food reserves. These stored nutrients get the young seedling up out of the soil, where it can start making its own food from sunlight, carbon dioxide, and water.
Also critical to the success and spread of the flowering plants is the diversity of their reproductive strategies. Some use wind or water to move their pollen around, while the vast majority enlist myriad hungry insects, some birds, a few bats, other mammals, and a lizard or two as their pollinators.
The versatility of the flowering plants, their double fertilization, and mutually rewarding, coevolutionary dance with pollinators are all factors in having made them one of the most successful forms of life on the planet—organisms that have the ability to colonize almost every terrestrial environment, from arid deserts to shallow marine waters along the coasts of the North Pacific, from lowland rain forests to mountaintops where they can be found above the tree line. Flowering plants are survivors.
In the next chapter we will return to the flower-visiting animals, this time to take a look at the living descendants of those we’ve glimpsed here in the fossil record. We will explore the animal visitors to flowers, focusing on intriguing and often bizarre examples of the sexual relationships between them, and providing case histories of colorful blooms and their guests. We’ll tour the diversity of volant insects that feed on and pollinate flowers, from tiny thrips to beetles, flies, wasps, ants, bees, butterflies, and moths. We’ll examine generalist blooms that welcome all comers, and superspecialized mutualisms such as those between yuccas and their moths, figs and their fig wasps, and bucket orchids that force their pollinators, the metallic green-and-blue orchid bees, to swim out of the pools that collect inside the bucket-shaped petals of its flower. We’ll watch the workings of various trap flowers, such as Dutchman’s-pipe, which lure flies down the throat of its curving tubes, where they are held in captivity for a day before being released, dusted with pollen, to repeat the experience at the next flower. There are wonders such as the solitary oil-baron bees and bees that turn themselves into living tuning forks to harvest pollen. Among the animals with backbones, we follow the foraging lives of specialist hummingbirds, sunbirds, honeycreepers, and nectar-feeding Arizona bats, charismatic honey possums and elephant shrews in South Africa, along with a remarkable lizard, a pollinating, nearly fluorescent green-and-red day gecko on the remote island of Mauritius in the Indian Ocean. All these animal visitors play vital roles in the life cycles and remarkable stories of flowers.