The true origin of the hazelnut is still unknown. But the most popular concept is that the hazelnut came from Asia, specifically China. Chinese folklore indicates its use dating back 5000 years, well into the ancient times. It was also found being used by the Germans, and the Scandinavians, the Swedish in particular. They believed it to be amongst the five sacred foods of the earth. Eventually, the French too started to grow different species of the hazel tree commercially, evolving different species of the hazel tree and growing it in varieties. Thus, its use spread through most of Europe. In fact, the Greek and Romans used it as medicine rather than a food item; they ate hazelnuts after their meals like we do pills. Perhaps, that is where the name hazelnut came from in the first place—haesel, the Roman warrior helmet, similar in shape to the hazelnut. So the hazelnut was popularly eaten by the UK, Spain, Italy, France and the ancient Greek and Romans. The nut used to be imported to USA, but after the 1940s, it began to be commercially grown in the Northwest country.
Another possible reason for why they are also called filberts is that they are usually harvested on St. Philibert’s Day (a time past mid-August when they are freshest and in peak state), although this is more speculation than fact.
There are more than a hundred varieties of hazelnuts, but the most popular ones are the ones grown in Portland, Oregon which produce the beaked variety, the “American” hazelnut, which is grown in the eastern part of North America, the Istanbul Hazelnut (the most common variety, used in most of Europe and Asia), and the Barcelona hazelnut, which by its name is native to Spain.