How Does Tree Sap Become a Time Capsule? Amber and What It Keeps

Resin is a sticky substance produced by certain trees in response to damage, sealing wounds and deterring insects, and it is chemically distinct from sap. Most resin decays, and becoming amber requires a specific sequence. The resin must be buried before it degrades, usually in waterlogged sediment where oxygen is excluded. Volatile components must evaporate or be driven off, which hardens it. The remaining molecules must then link into larger networks over long periods, a process called polymerisation, which converts a brittle soluble substance into a stable insoluble one. Material partway through that process is copal, which is younger, softer and dissolves in solvents, and distinguishing the two is the standard test against modern fakes.

Inclusions are preserved with a completeness nothing else matches:

• •Insects and other arthropods, complete with hairs, wing membranes and eyes
• •Spiders, including some caught with their prey
• •Plant fragments, flowers, pollen and fungi
• •Feathers, and in rare cases parts of small vertebrates
• •Behaviour frozen mid-event, including mating pairs and parasites attached to hosts
• •Fine structural detail at a scale ordinary fossilisation destroys entirely

What amber preserves is a biased selection and the biases are severe enough to matter. Only organisms that encountered resin are represented, which means creatures living on or near the trunks of resin-producing trees and excludes everything living elsewhere. Only small organisms are trapped, since anything large escapes or pulls free, so the record is overwhelmingly of insects and other small arthropods. Only forests with resin-producing trees are sampled, and those occurred in particular places and periods, so the record is a series of snapshots of specific forest types rather than a continuous history. And deposits are concentrated in a few regions, principally the Baltic, the Dominican Republic and Myanmar, which is where nearly all studied material comes from.

A large share of the amber studied in the past decade comes from northern Myanmar, and the material is extraordinarily productive scientifically and extremely problematic. It dates to the mid Cretaceous, which fills a gap other deposits do not cover, and it has yielded a stream of remarkable finds including feathers, a dinosaur tail and numerous new species. The mines are in a conflict zone, and revenue from the trade has been linked to armed groups, with the material reaching researchers through commercial dealers without documented provenance or export authorisation. Several journals introduced restrictions on publishing material collected after a stated date, professional societies issued statements, and the debate about whether to work on it at all has divided the field.

The most famous claim about amber does not hold. Recovering usable ancient genetic material from inclusions was reported in the early 1990s and became widely known, and those results have not been replicated, with subsequent work concluding that the original findings were contamination and that genetic material degrades far too quickly to survive tens of millions of years even in amber, where it is not in fact protected from water or chemical breakdown. The oldest genetic material recovered reliably comes from permafrost and is measured in hundreds of thousands to about two million years. What amber does preserve is physical structure, which has turned out to be enough for a great deal, including reconstructing anatomy in three dimensions by scanning without opening the specimen.

Resin must be buried before it decays, lose its volatile components and then polymerise over long periods, and material partway through is copal rather than amber. Inclusions preserve hairs, wing membranes and behaviour frozen mid-event. The sample covers only small organisms that met resin-producing trees in a few regions, and usable genetic material does not survive in it.