Forever Amber

A tree once grew on the Caribbean island of Hispaniola that had the strange ability of freezing moments in time. A member of the honey locust family known as the algarrobo, it vanished millions of years ago. Fortunately for us it left behind tiny time capsules—fragments of amber in which we can read the story of a marvelous lost world. Entombed in some fragments are the remains of plants and various animals, the most common being the bodies of insects. For sheer beauty, mystery, and the capacity to astonish, these fossils are difficult to surpass.

The algarrobo tree produced a copious, clear, sticky resin. Whenever the tree bled from borer hole or fracture it extruded the substance in quantity. Sometimes the peculiar sap must have run down the trunk like honey, encasing whatever it encountered. At other times it sat atop the wound in a glistening pool, its reflection beckoning enticingly in the dark of the forest. Then a wasp, sweat bee, or beetle attracted to the substance might tumble in. Within seconds it would be engulfed. Life was extinguished almost instantaneously, but the corpse was rendered eternal.

It has long been known that resin has preservative and antibiotic properties, for it has been used in food preservatives, in medicines, and in winemaking since ancient times. Despite great recent advances in chemistry we still do not understand exactly how amber does its magic trick of preserving things so wonderfully. This is because amber is a cocktail of chemicals, including alcohols, sugars, esters, and terpenes whose complexity defies ready analysis. Somehow the brew withdraws water from whatever it engulfs, then fixes the dehydrated tissues and excludes bacteria. To top it all off it hardens when fossilized to a brilliant, transparent, gemlike material.

Although scientists have long appreciated the amber fossils for their ability to reveal something of a vanished world, their true potential as a key to unlocking the past has been recognized only recently. The senior author of The Amber Forest, George Poinar, was the first person to extract DNA from an amber fossil—that of a stingless bee from the Dominican Republic. It is now known that amber is the only substance that can preserve DNA in any form for more than a few tens of thousands of years.

The discovery of amber’s unique ability to preserve DNA over the long term has spawned a whole new industry, which began in 1990 when Michael Crichton published his book Jurassic Park. The story was to become famous and it went on to inspire one of the most popular recent movies. At the heart of the plot lay a plan to extract the DNA of dinosaurs from blood-sucking insects preserved in amber. The fictional researchers then reconstructed the beasts, with well-known consequences.

The authors of The Amber Forest inform us that Jurassic Park is still in the realms of fantasy, for even the DNA from animals preserved in amber is badly degraded, fragmented, and strongly cross-linked with other molecules in the cell. Yet we can still hope that one day the very improbability that the remarkable properties of amber exist at all will be matched by the creation of a real rather than virtual Jurassic Park.

The process by which algarrobo resin is transformed into amber is mysterious. Within a few hundred years of being exuded the resin becomes harder, though still remaining pliable and fragrant. At this stage it is known as copal. It takes another four or five million years to transform into the brittle, glasslike, and odorless substance we know as amber. Its gemlike qualities have long been celebrated. The Taino Indians who greeted Columbus presented him with treasured pieces, but the Spanish greed for gold soon extinguished interest in amber. It was only at the beginning of the twentieth century that Dominican amber once again regained its reputation as a precious stone.

The oldest fossils preserved in amber are plant fragments from southern Scotland that are thought to be around 300 million years old. The fossils that form the subject of The Amber Forest, however, are from the Caribbean and are between 15 and 45 million years in age. Today Dominican amber is mined predominantly in the high northern range of the island known as the Cordillera Septentrional. There, miners equipped only with a hammer, chisel, candle, and sack push deep into the steep hillsides, following the deposits for up to six hundred feet, where they work in tunnels so narrow that they have to crawl to the amber-bearing rockface.

The amber they seek was not formed in the rocks that they mine, for the Cordillera Septentrional is composed of sediments that accumulated under the sea. How the amber became incorporated in these sediments is not clear, but it seems likely that it was transformed from copal to amber in ancient swamp deposits. These may then have been eroded away and the amber washed out to sea by an ancient river. The amber was subsequently buried among the shells and other remains of crabs and other small marine creatures, some of which attached themselves to the glasslike shards. After the sediments were compressed and buried deep in the earth, great movements in the earth’s crust thrust them high into the air, forming the Cordillera Septentrional. The pressure generated by these earth movements crushed and fractured many of the amber nuggets, and indeed may have destroyed most of them. Consequently it is rare to find a piece of amber larger than a golf ball without internal fractures.

The Amber Forest is profusely illustrated, and to look into an amber ellipse and see there the microscopic inhabitants of a tiny pool preserved for eternity, or a pair of beetles mating, is the best way to gain an appreciation of the miracle of preservation that amber has wrought. It’s a strange sort of fossilization, for it can preserve an intimate moment—the laying of an egg or the taking of a meal—and yet provide almost nothing by way of a historical setting for these fundamental acts. Thus, for example, we know that a mother millipede assiduously carried her single young about, cradled carefully in her forefeet, and yet we have no idea whether the perfectly preserved creatures last breathed 15 or 45 million years ago. We do not know whether they lived in a forest that resounded with the nocturnal cries of owls and the morning howling of monkeys, or whether rhinoceros, tapir, or sloth crossed their path. The forest may have been seasonally dry or eternally steamy, and we do not even know whether Hispaniola existed as an island at the time the algarrobo trees shed their deadly, embalming tears.

A reader trying to understand the nature of the amber-producing forest through the fossils illustrated in The Amber Forest is in very much the same position as a person trying to make sense of a greatly enlarged newspaper photograph from very close up. They can see the individual dots of ink making up the image very well, but the portrait as a whole is lost in the detail. That is because for all of their beauty the amber fossils can give us little direct feeling for what the ancient forest was like.

The sight of so many splendid amber fossils left me with a longing to know in a more intimate way what the world that they inhabited might have been like. The Poinars make an attempt to reconstruct the ecology of the ancient forest, but ultimately they fail to provide, for me at least, a sense of what it might have been like to wander in those ancient groves. For that we must turn to other sources, in particular the works of early travelers who ventured into the wilderness where the descendants of the algarrobo tree still grow.

One such was the eccentric wanderer Charles Waterton, who in 1812 left the town of Stabroek in Dutch Guiana in search of the mysterious Wourali poison, as curare was then known. His adventure—which he described in his Wanderings in South America—would take him deep into the rain forests of South America.^* There Waterton saw “the green-heart, famous for its hardness and durability; the hackea for its toughness, the ducalabali, surpassing the mahogany,… the locust tree, yielding copal; and the hayawa and olou trees, furnishing a sweet-smelling resin.”

Waterton says of the copal forest (in his rather florid style) that

the crowing of the hannaquoi will sound in thine ears like the day-break town clock, and the wren and thrush will join with thee in thy matin hymn to thy creator, to thank him for thy night’s rest. At noon thy genius will lead thee to the troely, one leaf of which will defend thee from both sun and rain. And if, in the cool of the evening, thou hast been tempted to stray too far from thy place of abode, and art deprived of light to write…the firefly…will be thy candle. Hold it over thy pocket book, in any position which thou knowest will not hurt it, and it will afford thee ample light. And when thou hast done with it, put it kindly back again on the next branch to thee. It will want no other reward for its services.

Evening in Waterton’s copal forest was an enchanted time: “The hayawa tree perfumes the woods around; pairs of scarlet aras [macaws] are continually crossing the river. The maam sends forth its plaintive note, the wren chants its evening song. The caprimulgus wheels in busy flight around the canoe, while whip-poor-will sits on the broken stump near the water’s edge, complaining as the shades of night set in.”

While The Amber Forest does not give the reader a larger, more fulfilling sense of the algarrobo forest, it more than makes up for it with a plethora of astonishing detail. Indeed the great strength of The Amber Forest lies in revealing and explaining the intimate moments in the lives of long-vanished, tiny creatures. For me the most extraordinary, if diminutive, of these intimate moments is fixed forever in a tiny fragment that has preserved the bodies of over five hundred once-living specks. They belong to an enigmatic, microscopic species that is represented in all stages of development. Their precise identification is still disputed, but they may be the tiny bear-like creatures known as tardigrades. Alongside them are preserved the strands of fungi on which they fed, while scattered throughout are the bodies of giant single-celled amoeba-like creatures, mites, and a single pseudoscorpion, all of which were carnivores and presumably preyed on the tiny hoard. Here is a miniature and fragile world preserved in toto. Just how amber could first engulf, then preserve, such a fragile spectacle defies comprehension.

Many of the larger creatures reveal the drama of life in that ancient forest. Peering out from one piece of amber is an ant, still apparently struggling mightily to free itself from a strand of spider silk. In another a butterfly is caught in a section of an orb-weaver’s web. It hangs helplessly, hoping perhaps by inaction to escape detection by the spider, just as I have seen countless moths and butterflies do today. In another tiny piece an entire flight of spiderlings has been overwhelmed just moments before they were ready to balloon off into the world. Had they not met their sticky end they would have been carried aloft on threads high above the rain forest, to come to rest perhaps far across the sea.

The reader is constantly drawn to the tiny, the enigmatic, and the unexplained in these rich pages. In one of Roberta Poinar’s plates we see a pair of fireflies frozen forever in the act of copulation. Copulating insects are a familiar enough sight in any garden, but to see them fixed timelessly in amber is strangely fascinating.

It is one of the miracles of the amber fossils that they encourage one to appreciate more fully the wonders of even commonplace insect life. Time and again when reading of a familiar, even pesky, insect species in the Poinars’ book, I found myself amazed at how they live their lives. Take, for example, the common and nearly universally distributed spider wasps. They construct the cells in which they lay their eggs in cavities such as old burrows or hollow plant stems. While making them, they feed from flowers and honeydew, but when the cells near completion these tiny predators seek out the spiders with which they will feed their young. If a spider remains too well hidden they may coyly pluck on a strand of its web, making the spider think that a meal awaits it. In fact the reverse is true. The tiny wasp rapidly stings the spider, immobilizing it, but as far as we know not affecting it in any other way. The wasp then uses its mandibles to gouge into the equivalent of the spider’s chest—its thorax—drinking deeply of its victim’s blood without killing it. After this vampire-like repast is complete, the spider is carried to the prepared cell where an egg is laid on its helpless body. Over the following weeks the egg hatches and the maggotlike larvae eat the spider alive. They commence with the nonvital parts such as legs and gonads, so that the meal is kept as fresh as possible.

Other parasitic wasps have evolved extraordinary adaptations to ensure that their parasitoid lifestyle works. Many, for example, have cultivated a relationship with a unique and sinister virus that seems to act rather like HIV does among humans. The wasps harbor the virus in their reproductive tract and transmit it to their immobilized prey as they lay their egg. Once inside the victim’s body the virus sets about demolishing its immune system, so that it is unable to recognize the egg or wasp larvae as foreign. The young wasp is thus free to grow and feed as it likes without fear of an immune response by its host.

Some wasps are very particular about where they deposit their eggs, and their genitals have the extraordinary ability to discern the nature of the tissue they are piercing as they drive into the host’s body. One kind of wasp, which evidently lacks a virus to help it, seeks out the nerve ganglion as the prime tissue in which to deposit its young. This is because once inside the nerve it can avoid a deadly immune response from the host.

The section of The Amber Forest dealing with the parasitoid wasps is extensive, fascinating, and ghoulish. By the end of it one feels like giving thanks that mammals are not as prone to parasitism as are invertebrates, for it seems that every tiny denizen of the ancient forest had at least one parasitoid searching for it, ever ready to subject it to the most awful demise. It is hardly reassuring that the authors inform us that “many more wasp parasitoids have been found entombed than can be discussed here. Their shear [sic] numbers emphasize the diversity of this successful group in the past as well as the high degree of parasitism that must have occurred in the amber forest.” Still, it seems that there is some justice in the insect world, for some wasps reserve their dastardly doings for their parasitoid kin, searching out the nest of a larger parasitoid wasp and laying their own egg on the egg or larvae of a wasp that is happily munching its way through its paralyzed larder.

For all the gruesome fascination of the wasps, the most extraordinary of the insects preserved in the deposits are surely the twisted-wing parasites or strepsipterans. No one knows exactly where these tiny, beautiful fly-like creatures with their expansive, translucent wings should be classified in the animal kingdom. One thing, however, is certain; their lifestyle is as bizarre as any on the planet. Only the males look anything like an insect, for the females are degenerate-looking blobs that lack wings, legs, and eyes. The female usually develops in the body of a “plant hopper,” or jumping insect, and unlike most insect parasites she does not remain hidden in the body but protrudes her head from between the plant hopper’s segments. From this bizarre perch she advertises her sexual readiness with a potent odor, and it is here that the winged male finds his lover.

But how to mate with this shapeless blob poking out from an alien species? Fortunately nature has provided a helping hand, for the female’s genitalia are located near her neck. To reach it the male must land on the plant hopper and insert his sperm into the partly obscured genital opening. Plant hoppers can move rapidly, springing away with a speed that evidently surprises even male strepsipterans, for hoppers have been found carrying about the severed male genitals of several unfortunate suitors, who apparently could not withdraw rapidly enough to avoid being dismembered by the sheer force that the hopper’s leap created.

The eggs of strepsipterans hatch inside the female, and the young wait inside a special brood pouch for the right moment to emerge. Then, as if at a signal, they pour out and begin searching for a plant hopper of their own to parasitize. They enter through the body wall, and doubtless do not use an anesthetic. One young strepsipteran was preserved in amber at the moment it emerged from its host.

The Amber Forest has a surprise in store for students of the strepsipterans, for one female has been found peeking out (albeit eyelessly) from the body of a stingless bee. No strepsipterans are known to use bees as hosts today, so here is a mystery. Another strepsipteran from the deposits is from a group whose males develop only in ants and whose females require crickets in which to grow. Just how the sexes find each other, and how they separate at birth, remains unexplained.

One remarkable fact about the amber fauna, and one that makes the lives of the insects relatively easy to interpret, is that almost all resemble living species. Thus we are not faced with the problem of reconstructing the lifestyles of entirely unfamiliar creatures, as is the case, for example, with scientists who study dinosaurs. Intriguingly, however, many species from the amber deposits no longer occur on Hispaniola, or indeed anywhere in the Northern Hemisphere. Prime examples include the giant termite Mastotermes and the honey-pot ant Leptomyrmex. If one wishes to see these creatures alive today one must travel to tropical Australia, for this is the only region where they have survived.

The New World is probably well rid of the giant termites preserved by the amber forest. At up to 1.2 inches long, they form colonies of over a million individuals and in Australia little is safe from them; they consume structural timbers, crops, living trees, and even some synthetic materials with equal facility. The honey-pot ants, however, could be considered a loss. They produce a cast of workers known as repletes that imbibe nectar until they are quite literally bloated. Australia’s Aborigines seek out these living food stores and deftly nip off the sweet marble-size abdomens.

One group that has disappeared from the Western Hemisphere is the Halovelvia bugs. They once strode across the mud flats and pools of seawater left by the tide around the ancient land. Today, the nearest these bugs approach the Americas is Western Samoa, some seven thousand miles from Hispaniola. What, one must ask, could have caused creatures such as these to become extinct, not only on Hispaniola but throughout half the world? At present, science holds no answer to this question.

Among the insects preserved in amber that are now absent from Hispaniola but may still be found in Central and South America are all seven of the bee species recorded from the deposits. This is quite remarkable, for few if any of the myriad wasp families represented fall into the same category. Why should bees be vulnerable to extinction on Hispaniola and not the closely related wasps? George and Roberta Poinar pose a number of possible solutions, such as the idea that climatic change affected the trees that the bees fed on, or that the bees were suffering from genetic problems because of their isolation on an island. Somehow these explanations ring hollow; a great and intractable evolutionary mystery still seems to me to be present in this tale of the wasps and the bees.

The fact that Hispaniola is an island, however, may have something to do with the extinction of some of the creatures whose remains are preserved in the amber deposits. And although Hispaniola is in one sense an island, it is also a high mountain range of a now submerged landmass known to geologists as Gaarlandia. The name is derived in part from the name of the isles to which Hispaniola belongs, the Greater Antilles. At the end of the age of dinosaurs, Gaarlandia stretched across the tropical ocean of the Western Hemisphere much as Central America does today, linking North and South America by an extensive land bridge. By 60 million years ago, however, it had moved east and begun to subside below the waves. Since then it has fragmented and subsided further, stranding its inhabitants on ever more distant and smaller isles.

We know from the amber deposits that when the algarrobo tree grew in Gaarlandia or its island fragments, carnivores (possibly primitive catlike creatures) stalked the land, for their hairs—replete with spikelets from a leafy, grasslike bamboo through which the creature must have wandered—have been preserved in the deposits. Zoogeographers (scientists who study the distributions of plants and animals) have long understood that carnivores stranded on islands are likely to become extinct. This is because they are large, and you can pack relatively few of them into a small landmass. What is surprising, however, is the very large number of tiny creatures—mostly insects—that have become extinct on Hispaniola since the time the amber formed.

The number of larger organisms recovered from the amber deposits is depressingly few. Lizards and frogs are the most frequent but even they are rare. One particularly large piece of amber has preserved an entire anole lizard. He was a young male, who was perhaps busily extending and retracting his dewlap in an attempt to win a mate or intimidate a rival when a predator arrived and knocked him off his perch and into the amber. Out of another piece stares the head of a gecko. The body is long gone, but the eyes still look out upon the world as if heedless of the millions of years that have passed. The mammals are very poorly represented. The partial skeleton of a shrewlike creature (not illustrated in the Poinars’ book) has recently been found, as have the hairs of primitive carnivores and rodents. The feather of a piculet (a unique, woodpeckerlike bird restricted to the islands of the Caribbean) has also been preserved.

One of the strengths of The Amber Forest is a handsome color section, depicting most of the fossils that are reproduced elsewhere in the book in black and white. Many are truly beautiful as well as informative, for nature imitated art magnificently when she captured a flower stamen releasing a cascade of pollen into the amber trap. Even the reader barely interested in the wonder of lost worlds can hardly fail to be moved by such images.