Scientific achievement
With the continued interactions between the Human and Elven tribes, an eventual language is developed that is understandable by both parties.
Since their separation, Humans and Elves proceeded to work in conjunction with one another to cure their sick. The knowledge of herbalism was shared by the elves and a majority of human dialect was similarly adopted by the Elves from lessons taught by Humanity. After enough years, a very early but solid form of communication was possible that adopted words of both Human and Elvish origin, but turned them into a language individual from both. This language is modernly known as common, and will go on to adopt words of many more cultures.