The oldest amber recovered dates to the Upper Carboniferous period ( 320 million years ago ).   Its chemical composition makes it difficult to match the amber to its producers – it is most similar to the resins produced by flowering plants ; however, there are no flowering plant fossils until the Cretaceous, and they were not common until the Upper Cretaceous . Amber becomes abundant long after the Carboniferous, in the Early Cretaceous , 150 million years ago ,  when it is found in association with insects . The oldest amber with arthropod inclusions comes from the Levant, from Lebanon and Jordan. This amber, roughly 125–135 million years old, is considered of high scientific value, providing evidence of some of the oldest sampled ecosystems . 
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In 1919 Phoebus Levene at the Rockefeller Institute identified the components (the four bases, the sugar and the phosphate chain) and he showed that the components of DNA were linked in the order phosphate-sugar-base. He called each of these units a nucleotide and suggested the DNA molecule consisted of a string of nucleotide units linked together through the phosphate groups, which are the 'backbone' of the molecule. However Levene thought the chain was short and that the bases repeated in the same fixed order. Torbjörn Caspersson and Einar Hammersten showed that DNA was a polymer.