Aker was an ancient earth-god in Egypt. He was believed to guard the gates of the dawn from which the sun rose each morning. He was portrayed as a double-headed lion, or a two lions sitting back-to-back. In between them is shown the sun with the sky overhead. In this way they form the akhet symbol, which was a symbol of the horizon. Occasionally they were portrayed bearing the akhet on their backs (as in the image to the right).
The two lions were called Sef and Duau, which means "Yesterday" and "Today" respectively.
As Egyptians believed that the gates of the morning and evening were guarded by Aker, they often placed statues of lions at the doors of their palaces and tombs. This was to guard the households and tombs from evil spirits and other malevolent beings. Sometimes they gave these statues the heads of men and women. The Greeks called this class of statuary, "Sphinxes."