Humans (and other animals, and plants) can make their own salicylic acid (SA), the principal metabolite of aspirin. "It is, we suspect, increasingly likely that SA is a biopharmaceutical with a central, broadly defensive role in animals as well as plants," they maintain. "This simple organic chemical is, we propose, likely to become increasingly recognized as an animal bioregulator, perhaps in a class of its own."
Breakthroughs are like celebrity success; they seem sudden but they usually depend on hard work for a long time. As history tells -- Aspirin was synthesized for the first time in 1897 by a young chemist working for Bayer, Dr. Felix Hoffmann. However, people had been using SA for treatment of various diseases.
According to Bayer literature, "the preferred treatment for pain, fever, and inflammation in Hoffmann's day, the active substance SA, had one decisive disadvantage - it was very poorly tolerated. Legend has it that Dr. Hoffmann discovered acetylsalicylic acid in a fortuitous experiment while searching for a remedy that would be better tolerated by his father, who was plagued by rheumatoid arthritis."
Fast forward to 2008 -- when people still find the aspirin mechanisms baffling while praising its newly found benefits in treating a variety of diseases. Research has ranged from manufacturing "industry has long used carbon dioxide as a chemical building block--in the manufacture of the painkiller aspirin, for example" to the immune system of plants as in Reducing Pesticides In Plants With ... Aspirin?
The latter cited that after a plant is attacked by a pathogen, a disease-fighting hormone (SA) is produced at the infection site. "Some of this SA activates defenses locally, and some of this SA is converted to MeSA, [methyl salicylate, i.e. methyl ester of SA], which is biologically inactive" in immune response.
Previously, MeSA had been measured in laboratory plants. In 2007 researchers set up specialized instruments in a walnut grove near Davis, California. The levels of MeSA emissions increased when the plants, which were already stressed by a local drought, experienced low nighttime temperatures followed by higher daytime temperatures. Plants Have Their Own Form Of Aspirin And It Helps With Their Kind Of Stress Headache mentions two possibilities. "One of these is to stimulate plants to begin a process known as systemic acquired resistance, which is analogous to an immune response in an animal. This helps a plant to both resist and recover from disease." The other is "a mechanism whereby a stressed plant communicates to neighboring plants, warning them of the threat."
Reference 1 claims that SA is widely found in the animal kingdom. Indeed, they show a table where "those species regarded as primarily carnivorous" have "blood SA levels comparable to those measured in herbivores."
Earlier, these animal observations led Patterson et al to postulate that serum SA levels in aspirin-free individuals might be, at least partially, endogenous. Their new evidence maintains that SA exists in the blood of people who have not recently taken aspirin. Vegetarians had much higher levels, even overlapping those in patients taking low doses of aspirin. The scientists had previously concluded that this endogenous SA came from the diet because SA and its compounds occur in fruits and vegetables.
The new research of the UK team focused on studies of changes in SA levels in volunteers who took benzoic acid, also found in fruits and vegetables that the body could possibly use to make SA. They investigated
whether the SA source in humans consists solely of the fruits and vegetables they consume, or whether humans manufacture their own SA. Their results suggest that people do make SA.
"If we are right that SA is not just an important phytochemical but also a key biopharmaceutical," they write. "Its circulating level might well be subject to homeostatic control and not be solely influenced by dietary SA with or without benzoic acid intake."
Furthermore, Patterson and colleagues understand their monumental lead in reassessing the SA's role in human and animal, and plant, "pathopsysiology."
 John R. Paterson, Gwendoline Baxter, Jacob S. Dreyer, John M. Halket, Robert Flynn and James R. Lawrence. Salicylic Acid sans Aspirin in Animals and Man: Persistence in Fasting and Biosynthesis from Benzoic Acid. Journal of Agricultural and Food Chemistry, 2008; 56 (24): 11648 DOI:10.1021/jf800974z
 Ohishi, T., Nishiura, M.&Hou, Z. Carboxylation of organoboronic esters catalyzed by N-heterocyclic carbene copper(I) complexes. Angewandte Chemie International Edition 47, 5792–5795 (2008)