In Egyptian mythology, Aker (also spelt Akar) was one of the earliest gods worshipped, and was the deification of the horizon. There are strong indications that Aker was worshipped before other known Egyptian gods of the earth, such as Geb. In particular, the Pyramid texts make a sinister statement that the Akeru (plural of Aker) will not seize the pharaoh, as if this were something that might have happened, and was something of which to be afraid. Aker itself translates as (one who) bends, and thus Akeru translates as benders, though in what sense this is meant, is not fully understood.
As the horizon, Aker was also seen as symbolic of the borders between each day, and so was originally depicted as a narrow strip of land (i.e. a horizon), with heads on either side, facing away from one another, a symbol of borders. Since the sun reaches its peak (its solstice) in the zodiac of leo, these heads were usually those of lions. Over time, the heads became full figures of lions (still facing away from each other), one representing the concept of yesterday (Sef in Egyptian), and the other the concept of today (Duau in Egyptian).