Tea contains catechins, a type of antioxidant. In a freshly-picked tea leaf, catechins can compose up to 30% of the dry weight. Catechins are highest in concentration in white and green teas, while black tea has substantially fewer due to its oxidative preparation.[17][18] Research by the U.S. Department of Agriculture has suggested that levels of antioxidants in green and black tea do not differ greatly, with green tea having an Oxygen Radical Absorbance Capacity (ORAC) of 1253 and black tea an ORAC of 1128 (measured in ?molTE/100g).[19] Tea also contains theanine and the stimulant caffeine at about 3% of its dry weight, translating to between 30 mg and 90 mg per 8 oz (250 ml) cup depending on type, brand[20] and brewing method.[21] Tea also contains small amounts of theobromine and theophylline,[22] as well as fluoride[citation needed], with certain types of brick tea made from old leaves and stems having the highest levels.[23]

Dry tea has more caffeine by weight than coffee; nevertheless, more dried coffee is used than dry tea in preparing the beverage,[24] which means that a cup of brewed tea contains significantly less caffeine than a cup of coffee of the same size.

According to Mondal (2007, p. 519): "Camellia sinensis originated in southeast Asia, specifically around the intersection of latitude 29°N and longitude 98°E, the point of confluence of the lands of northeast India, north Burma, southwest China and Tibet. The plant was introduced to more than 52 countries, from this ‘centre of origin’."

Based on morphological differences between the Assamese and Chinese varieties, botanists have long asserted a dual botanical origin for tea; however, statistical cluster analysis, the same chromosome number (2n=30), easy hybridization, and various types of intermediate hybrids and spontaneous polyploids all appear to demonstrate a single place of origin for Camellia sinensis — the area including the northern part of Burma, and Yunnan and Sichuan provinces of China.[25] According to this theory, tea plants in southeast Asia may have been the products of the 19th Century and 20th Century hybridizing experiments.[citation needed]

Yunnan Province has also been identified as "the birthplace of tea…the first area where humans figured out that eating tea leaves or brewing a cup could be pleasant."[26] Fengqing County in the Lincang City Prefecture of Yunnan Province in China is said to be home to the world's oldest cultivated tea tree, some 3,200 years old.

In one popular Chinese legend, Shennong, the legendary Emperor of China and inventor of agriculture and Chinese medicine was drinking a bowl of just boiled water some time around 2737 BC when a few leaves were blown from a nearby tree into his water, changing the color. The emperor took a sip of the brew and was pleasantly surprised by its flavor and restorative properties. A variant of the legend tells that the emperor tested the medical properties of various herbs on himself, some of them poisonous, and found tea to work as an antidote.[28] Shennong is also mentioned in Lu Yu's famous early work on the subject, Cha Jing.[29] A similar Chinese legend goes that the god of agriculture would chew the leaves, stems, and roots of various plants to discover medicinal herbs. If he consumed a poisonous plant, he would chew tea leaves to counteract the poison.[30]

A rather gruesome legend dates back to the Tang Dynasty. In the legend, Bodhidharma, the founder of Chan Buddhism, accidentally fell asleep after meditating in front of a wall for nine years. He woke up in such disgust at his weakness that he cut off his own eyelids. They fell to the ground and took root, growing into tea bushes.[31] Sometimes, another version of the story is told with Gautama Buddha in place of Bodhidharma.[32]

Whether or not these legends have any basis in fact, tea has played a significant role in Asian culture for centuries as a staple beverage, a curative, and a status symbol. It is not surprising, therefore, that theories of its origin are often religious or royal in nature.