Amber is actually a fossilized resin rather than tree sap. Whereas sap flows through the circulatory system of a a plant or tree, amber is a semi-solid organic substance that flows through the plant's epithelial cells. It is formed over millions of years when the resin fossilizes as the original organic compounds oxidize. Fossil resin can also become incorporated into surrounding soil to form sandstone and shale.
In the early 1800s, pioneering scientist Charles Darwin journeyed through much of the Southern Hemisphere, including the never-before-explored Galapagos Islands off the coast of Africa. In addition to animal bones and fossilized wood, Darwin found many creatures, mostly insects, preserved in amber. It was the well-preserved bodies of amber insects that helped Darwin determine that the creatures were direct ancestors of beings living today. The amber insects looked so similar to many of the creatures Darwin studied alive that he knew they were connected. He also learned that species can become extinct.
Though scientists once thought amber could not be found in or formed from a marine setting, researchers from the University of Barcelona in Spain presented a paper in 2008 to support the existence of marine amber. The Sharente-Maritime area of southwestern France produced the first known marine organisms preserved in amber. The samples featured several types of plankton found in shallow coastal waters. Through sample dating, the group found marine amber produced as early as the Early Cretaceous period. The amber forests of France likely were formed, according to the group, by remnants of shells that marine organisms introduced to the coast by wind and tide.
Though beliefs differ as to the purpose of tree resin, most scientists think it is a protective system. Resin may help the plant dispose of excess acetate; it may protect the plant from disease or injury inflicted by fungi; and it emits odors that can repel or attract different kinds of insects. Pine, cypress, cedar and juniper trees are among trees which secreted amber that became fossilized. Though the formation time of tree resin is unclear, most amber is found within sediment ranging from 30 million to 90 million years old. The oldest samples have been found in Kansas, Alaska and New Jersey, as well as Alberta and Manitoba, Canada.