Other traces are much harder to find, and that group includes chemical traces of fossils, degraded biological compounds, and fossils of microorganisms themselves.

The epic journey lasts three or four life cycles, with the route marked by chemical traces left by earlier generations of the butterfly, scientists believe.

Chemical traces have revealed that the early hominid changed its feeding habits from month to month, indicating a nomadic lifestyle.

The agency took the line that it was perfectly safe to use milk with antibiotics for food production as long as the chemical traces were below maximum safety limits.

Chemical traces produced by climatic changes, such as quantities of particular isotopes, can be recovered from proxies.