The Red Sea was formed when in the Eocene period, the Arabian Peninsula began to move away from the continent of Africa. This prevented further exchange of genes between African and Arabian species. Furthermore, the late Tertiary and the early Quaternary eras saw a period of climatic cooling that drove vegetation bands southwards, and the Arabian Peninsula received an influx of species from Eurasia. With increasing aridity, conditions became inimical for many of these and they retreated to the damper, southwestern mountainous regions, becoming relict populations.