Paleo-environmental and tectonic events in the Miocene epoch created the Danakil Isthmus, a land bridge forming a broad connection between Yemen and Ethiopia. During the last 100,000 years eustatic sea level fluctuations have led to alternate opening and closing of the straits. According to the recent single origin hypothesis, the straits of Bab-el-Mandeb were probably witness to the earliest migrations of modern humans. It is presumed that the oceans were then much lower and the straits were much shallower or dry, which allowed a series of emigrations along the southern coast of Asia.