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Since Aristotle’s day amber has been prized for the beauty of the insects, feathers and small reptiles imprisoned within it. The early philosophers already knew that it originated from the resin of trees; Greek mythology told the story of Phaethon’s three sisters who were turned into trees as divine punishment, and wept tears of amber. In later years, scientists admired the insects, frogs, scorpions and even mushrooms as intriguing novelties and, largely, left it at that. But they were turning their backs on a rich store of information about these fossils and the world in which they lived.

The creatures that met their sticky ends in this fossilised tree resin can provide valuable clues about ancient life. It was assumed that these entombed organisms, outwardly perfect, even down to the hairs on an insect’s legs, were merely empty husks, occasionally lined with the decomposed remains of tissue. But this is not the case. The soft internal organs of insects can be preserved faithfully in amber by a unique form of fossilisation. The variety and number of insects may also give an insight into the environment in which amber formed.

Amber is not especially rare, and has been found all over the world. It appears in rocks dating from the Carboniferous times, some 310 million years ago, and is still forming today. The greatest deposits known lie around the Baltic Sea and in the Dominican Republic, and scientists formerly assumed that they were the result of some sort of disaster in an ancient forest. They reasoned that only what they termed a ‘pathological event’ could have produced such enormous volumes of resin. Around the turn of the century, the researcher Hugo Wilhelm Conwentz, an eminent amber worker, caught the popular imagination with a vivid description of the Baltic landscape in Tertiary times, where ‘almost every tree in the amber forest was stricken with the disease succinosis causing them to weep prodigious quantities of liquid amber’.

But there is no need to invoke disease or unusual circumstances: some modern trees yield easily enough resin to account for the accumulations of amber in the fossil record. Sediments from south of the Caucasus Mountains, for example, yield around 200 grams of amber per cubic metre of soil. Pseudotsuga and Eucalyptus trees, for example, each year produce 45 litres and 60 litres of resin per tree respectively, although these trees are not sources of amber. The copal industry in the Belgian Congo (now Zaire), early this century provides further evidence of the volume of resin that trees can produce. Copal is derived from resin, but it is not fossilised like amber and is usually less than 100 years old. It was widely used in the manufacture of paints and varnishes and was once in great demand: in 1935 alone, 18 million kilograms were mined from the forests and reserves were described as inexhaustible.

Tropical origins

Today resinous trees are commonest in the tropics. As many as 45 per cent of the trees in some Amazonian rainforests are of species that produce resin or latex, and one study found resinous exudates on the bark of nearly 12 per cent of trees in a one-hectare plot. So it is not surprising that many amber deposits accumulated in what are, or were, regions of tropical or near tropical climate. Studies of resins from modern trees also highlight the environmental conditions that encourage the production of copious quantities of resin. Water appears to be of major importance: the more moisture is available to the tree, the more resin forms.

Unfortunately, the sedimentary rocks that contain amber are not especially informative about its origins. Most amber shows signs that it was moved by water before it settled into the sediment in which it now lies. The sediments that form from amber-bearing rocks probably did not form in the same place as the amber and so give little direct evidence of its origin. But comparing the fauna embedded in amber from a particular region with those that can be trapped in a variety of ecologies today should indicate where the amber formed. I tested this hypothesis with the Brodzinsky-Lopez Pena amber collection from the Dominican Republic which is held in the Smithsonian Institution in Washington DC. The amber in this collection formed some 40 million years ago, and contains more than 5000 insects. I compared these with insects collected from a range of modern environments – temperate forest, primary tropical rainforest, secondary tropical forest, tropical plantation, pasture, varying altitudes, and so on. The best match was with samples from moist areas in tropical rainforests.

Usually in fossilisation organic matter is replaced by minerals, which preserve the shape but not the substance of the organism. But insects appear to be preserved in amber in their original organic form. In this sense, they are not fossils, although the resin surrounding them has been altered by polymerisation. In effect amber delivers creatures from prehistoric rainforests to the laboratory bench. Whole insects preserved in amber from the Dominican Republic show that the body can remain full of soft tissue. The tissues can be preserved in three dimensions and so may contain information about their relative positions and interconnections.

But access to this information is not straightforward. Scanning electron microscopy reveals details of the surface of a specimen, so cutting a fossil insect into many thin slices with a fine saw should, in principle, reveal much of the insect’s internal anatomy in three dimensions. In practice, the blade tends to rip material out as it cuts. One way round this is to slice the amber to the edge of the specimen, then break the insect’s body. Though this results in spectacular cross-sections, most of the soft parts of the insect stay hidden. Combining the two methods seems to be the most powerful approach so far, revealing locomotory, digestive, respiratory, nervous and sensory tissues in insects from Dominican Republic amber.

Spectacular detail of the digestive tract can survive. A nitulid beetle from early Tertiary amber – up to 40 million years old – still contained its proventriculus, the region of the foregut that functions as an accessory chewing apparatus and, as a valve controlling the movement of food. Its structure suggests that the beetle, like its living relatives, had a relatively soft diet, perhaps living on fungi or decaying plant matter.

Even subcellular features can be preserved. Transmission electron microscopy shows up the fine structure that generated the power for flight in muscle tissue some 40 million years ago. Muscle fibres are frequently left intact, and muscle used for flight is often exceptionally clear. Although the flight muscle of a fly embedded in amber has shrunk considerably, its structure can easily be related to that of modern flight muscle. In living insects, the structure of flight muscle is almost identical across the orders: it consists of fibres densely packed with bundles of contractile filaments, called myofibrils, and mitochondria, which generate energy. Characteristically, it contains very little else. Both myofibrils and mitochondria can be identifiable in the amber material, although the myofibrils are not as well preserved.

Under the microscope

One section through a beetle shows a complete view through the thorax, including two dense, symmetrical bands of flight muscle. Also visible are large numbers of tracheae, the tubes through which the beetle exchanged its respiratory gases. The large tracheae (each about 60 micrometres in diameter) split into a succession of smaller tracheoles less than 0.3 micrometres across. The tracheae are preserved in the position they took in life, and tracheoles can be seen running between individual adjacent fibrils of the flight muscle and across its broken surface.

Such detail is valuable for understanding the evolution of insects, and for tackling the origin of physiological innovations such as flight. Fibrillar flight muscle and, it is assumed, the physiological pathways associated with it, has been detected in insects from the Tertiary. This type of flight muscle, capable of contracting several times for each nerve impulse, has probably evolved independently several times within different insect groups. It may have allowed insects to evolve small size.

What is it about amber that leads to such extraordinary fossils? The resin from which it comes is part of a tree’s defence system, inhibiting the growth and entry of damaging fungi or bacteria and discouraging hungry herbivores. Sealant and antibiotic properties that make resin an effective protection are likely to contribute to its preservative qualities.

The idea that resin is a preservative is supported by evidence from ancient Egypt: the best-preserved mummies are associated with resin, both in the body cavities and impregnating the bandages that form the outer wrapping. When the mummy of Pum II, who died sometime between 240 and 100 BC, was unwrapped at Wayne State University in Detroit in 1973, it turned out to be exceptionally well preserved. Resin had been poured into the head, chest and abdomen and the body was sterile.

But some of the most intriguing clues to how amber preserves come from experiments on the decay of modern insects. Earlier this century, researchers suggested that hermetic sealing might be important. To test this idea I sealed blowflies (Calliphora vomitoria), freshly killed with chloroform, in airtight and watertight wax, and watched their decay. But after two weeks, all the internal tissue of the flies had disintegrated into a smelly mess, so there must be more to efficient preservation than an airtight tomb.

Cut and dried

Flies which were dehydrated were better preserved. The subcellular structure of flight muscle from a fly at least 40 million years old, preserved in Dominican Republic amber, turns out to be similar to that of equivalent tissue from a dehydrated modern blowfly. In both cases, structures that look like parallel stacks of membranes developed within their mitochondria after death. These specimens also showed more disintegration of the myofibrils than the mitochondria. None of the other methods of preservation investigated – such as immersing the insects in maple syrup – showed these distinctive features.

Egyptian mummies and amber probably both owe their extraordinary state of preservation to a similar combination of dehydration and the antibiotic properties of tree resin. In the most opulent Egyptian mummifications, a dry salt called natron was applied before the body was wrapped in resin-infiltrated bandages, and this helped to dehydrate it.

Amber looks like a promising source of ancient DNA. If this material can be sequenced, the information will complement research into the developmental genetics of insects such as Drosophila fruit flies (see New ÐÓ°ÉÔ­´´, Science, 17 October 1992).

Amber fossils should also help to plug many of the gaps in the fossil record, by supplementing the fossils on which many studies of evolution are based. Such fossils are a highly biased source, as they do not provide a representative sample of creatures from any particular age: organisms with shells, especially marine creatures, are far more likely to become fossilised than land-dwellers or soft-bodied animals such as jellyfish. The result is a relatively poor understanding of prehistoric life on land. Insects are probably the most diverse and abundant creatures in terrestrial ecosystems, yet they are comparatively scarce in the fossil record. Even when insects or other organisms are fossilised in rock, only the most robust parts of the skeleton remain: soft parts, such as nervous tissue, muscles and gonads, are rarely preserved.

Amber preserves organisms and parts of organisms that are otherwise relatively rare or absent in the fossil record. But even the amber record is subject to bias as some organisms are more likely to be trapped in tree resin than others – large insects could well have a greater chance of escaping from sticky resin than smaller ones, for example. But for insects, at any rate, this type of bias turns out to be limited, as the closeness of the match between the range of insects embedded in amber and those trapped from places thought to be similar modern environments indicates. In fact, the differences between faunas sampled from the same environment using different trapping techniques, or those from different environments sampled using the same technique may be greater than those between the amber-derived fauna and its supposed modern equivalent.

Further interesting results should come from Cretaceous rocks found worldwide that, at up to 140 million years old, were laid down at the time that the first flowering plants were evolving. These plants in turn provided new habitats and a greatly enlarged source of energy-rich food for the insects. This led to a dramatic increase in the number of species in several of today’s important groups such as butterflies and moths (Lepidoptera), bees (Hymenoptera, Apoidea) and beetles (Coleoptera). These insects developed an extraordinary range of morphological and behavioural adaptations for feeding on the leaves, sap and pollen of these plants. Insects preserved in Cretaceous amber will provide a unique window into the origins of their complex evolutionary relationship with plants.

Alison Henwood last year completed her PhD on insects in amber at the University of Cambridge and now works for Shell.

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Resin down the ages

Amber is the fossilised resin of trees. The earliest evidence for the synthesis of resin comes from tropical forests in the late Carboniferous period, about 310 million years ago. Cordaitalean conifers and possibly the seed ferns (Medullosans) that together constituted the bulk of these forests, produced resin that survives as amber in the rocks of the Northumberland coalfields. Resin canals, but little fossilised resin, can be seen in younger conifers from the cool and drier Permian and Triassic periods.

During the Cretaceous period (between 145 and 65 million years ago), resin production became more common, and consequently amber is more abundant in rocks of this age. At this time many new species of insects were evolving; resin production may have become more common as a result of an increased need for the trees to protect themselves from new types of insect herbivores.

Cretaceous amber is found among deposits of various conifer families along the Atlantic coastal plains of the US and from several sites in central Canada. Samples of this amber vary in their chemical composition, suggesting that it comes from several different source trees. Most Cretaceous amber is probably derived from a tree similar to the present day Araucarian species appropriately named Liquidamber.

Eocene amber from the Baltic region is perhaps the best known. Pinus succinifera of the family Pinaeceae was formerly thought to be the source of most of the Baltic deposits, because droplets of amber were found in the resin canals of wood preserved from this era. But recent examination of the preserved wood shows it to be different from modern pine wood in both its structure and chemistry. Chemical evidence points to a tree related to the modern Araucarian Agathis, not a pine, as the source of Baltic amber.

But amber is not only produced by conifers. All pre-Cretaceous finds must be coniferous, because the angiosperms did not evolve until the Cretaceous period but the Tertiary Dominican Republic deposits, 40 million years old, were produced by an ancient angiosperm related to the extant leguminous tree Hymenaea and these have recently yielded a wealth of inclusions, from frogs to fruit flies. Ambers of the same age from Mexico and Colombia were also produced by a species closely related to modern Hymenaea.