Following the Columbian Exchange, non-native crops such as tomato, papaya, corn, pineapple and chili peppers were introduced to Southeast Asia probably through the various sea ports of modern-day Thailand, Cambodia and Vietnam via the Philippines and Malacca. Through trades with the Portuguese and other Europeans cultivation of non-native crops and ingredients quickly spread throughout Southeast Asia. Thailand being one of the entry points where the Europeans have traded and where the papaya was introduced to mainland Southeast Asia have led historian Sujit Wongthes, to speculate wrongly that the variant of "tam som" using papaya as the main ingredient probably originated in the communities of ethnic Chinese–Lao settlers living in the Chao Phraya plains of what is now Central Thailand—where it became known as "som tam"—during the early Rattanakosin period (late 18th to early 19th centuries). The new dish, along with the papaya, then spread to today's Northeast Thailand (Isan) following the construction of the Northeastern railway line during the turn of the 19th–20th centuries, and became more popular after the opening of Mittraphap Road in 1957 helped bring new papaya cultivars into the region. The dish has since become widely adopted by the ethnic Lao people of both Isan and Laos. Likewise, the hot flavour also spread to Isan and Laos from Central Thailand, which had been introduced to chilli peppers first.