Europeans now rarely eat seaweed, but it was an important part of their diet until the late Middle Ages, archaeologists said Tuesday, calling for the eco-friendly aquatic plant to be put back on the menu. Seaweed, a healthy and sustainable source of protein, is a staple food in some Asian countries such as Japan. But it has long been shunned by traditional Western diets — with a few exceptions, such as the Welsh dish laverbread.