IELTS Reading: Note completion

Amber is fossilised tree resin, and the distinction between resin and sap matters more than it sounds. Sap is the fluid that carries water and sugars around a living tree, and it does not fossilise. Resin is something else entirely: a thick, sticky secretion produced in response to damage, loaded with compounds that kill bacteria and fungi and seal a wound. Over millions of years the volatile components of a lump of resin evaporate away and what remains polymerises into a hard, stable solid. The intermediate stage, in which the material has hardened but the process is incomplete, is called copal, and a great many pieces sold as amber are in fact copal that is a few thousand years old rather than a few million.

The reason amber preserves so extraordinarily well follows from what resin is for. A small animal caught in a flow is immobilised at once, and the resin then draws water out of its tissues while excluding oxygen and the microbes that would otherwise consume it. What remains after a hundred million years is a three-dimensional record of the animal's surface, accurate to a fraction of a millimetre: individual hairs, the scales on a moth's wing, the facets of an eye. Rock fossils give a flattened outline. Amber gives something closer to a photograph. It also catches behaviour, which almost nothing else in the fossil record does: specimens exist of insects mating, of a spider approaching a trapped wasp, of a parasite in the act of feeding, and in one celebrated case of a dinosaur-era bird's feathered tail.

It is necessary to be blunt about what amber does not preserve, because the popular understanding of it rests on a claim that turned out to be wrong. Reports published in the early 1990s described the recovery of genetic material from insects tens of millions of years old, and they were never successfully repeated. Careful subsequent attempts, using the contamination controls that the original work lacked, have recovered nothing at all. The reason is chemical rather than technical: even under ideal conditions the bonds in a strand of DNA break at a rate that leaves nothing legible after something in the order of a million years, and amber, contrary to what one might expect, does not slow this down.

Three deposits supply most of the material that scientists work with. Baltic amber, from around forty million years ago, is the most abundant and the source of the majority of specimens in European collections; the tree that produced it has never been identified with confidence. Dominican amber is younger, at fifteen to twenty million years, and came from a species of tropical legume that still exists. The most scientifically valuable is Burmese amber, which at about ninety-nine million years belongs to the Cretaceous, and is therefore old enough to record the world in which flowering plants were spreading and the modern insect orders were taking shape.

That third deposit has become an ethical problem of the first order. The mines lie in a region that has been the site of an armed conflict, the material is exported through channels that are difficult to trace, and the revenue does not obviously reach the people digging it. Several scientific journals and at least one major learned society now decline to publish descriptions of specimens acquired after a stated date, and others have imposed conditions on provenance rather than a ban. The argument within the field is genuinely difficult. A specimen that cannot be published is, for practical purposes, lost to knowledge; and a specimen that is published acquires a value that encourages the trade which produced it.

What amber has actually taught is considerable and comes with a caution attached. It has dated the origins of several insect orders far earlier than the rock record suggested, documented the relationship between flowers and their pollinators almost from the beginning, and preserved whole categories of animal that rocks lose entirely: mites, springtails, tiny parasitic wasps and other small soft-bodied creatures that leave no impression in sediment. The caution is that a resin flow samples a particular place. It catches what walks on bark and what falls from the canopy onto the forest floor, in forests of the kind that produce resin, and it tells you very little about anything that lived in open ground or in water.

Because the material is valuable, it is also faked, and the tests used are mostly simple. Genuine amber floats in a strong salt solution while most plastics sink. It fluoresces under ultraviolet light in a way that an imitation generally does not. A heated needle touched to a hidden surface produces a smell of pine from amber and of burning plastic from a substitute. The most troublesome forgeries are not plastic at all but young copal, or fresh resin with a modern insect deliberately embedded, and for these the only reliable test is an examination of the inclusion itself: an insect that belongs to a species alive today did not die a hundred million years ago.

It is worth pausing on what makes these objects unusual. A fossil is normally a mineral replica, a shape in stone that has to be interpreted. An inclusion in amber is the animal itself, or what is left of it, held in the position it was in on the afternoon it died, with the resin still preserving the direction it was trying to run. Very little else in the study of the past offers that, and it is not surprising that the material has attracted more speculation, more forgery and more bad science than any other kind of fossil.