In Sumerian Mythology, Enki - or EA - is one of the supreme deities. He is the son of Anu, and the half-brother of Enlil. The name Enki means, 'Lord of Earth,' and was given because he was the first of the Gods to come to Earth. Enki was the one who built the first cities for the (extra-terrestrial) gods on Earth, in the Mesopotamian area.
Later on, when Anu and Enlil came to Earth too, the three of them divided their spheres of Influence. At that moment Enlil, instead of Enki, became the Lord of Earth and Space, while Enki's title changed into EA ('house in Water,' or 'dwells in water'). Enki thus became the Lord of the Seas (just like Poseidon), and of the Underworld or Netherworld. According to Sitchin, this netherworld is to be found in Southern Africa.
According to Sumerian Mythology, Enki also is the God who -together with his wife Ninhursag- created mankind. Sitchin claims this happened through a genetic manipulation wherein the genes of the gods were combined with the genes of Earth primates. This would have happened some 300,000 years ago (which coincides with the sudden appearance of the first type of Homo Sapiens). Unlike his brother Enlil, Enki always had a good relationship with mankind.
Enki is also considered to be "the one who imparts the knowledge of civilizations to mankind. He was also known to the Babylonians as 'Oannes' and to the Egyptians as 'Ptah.' He was the God of wisdom and knowledge, and throughout times his symbol has been the snake. According to the Sumerian texts, Enki was the one who instructed Noah to build the Ark so humanity could be saved. This was in direct defiance of the orders of Enlil, who desired the destruction of humanity." (PoL)

Enki's wife Ninhursag, also was his half- (or step-)sister. Because she was the goddess of Medicine, and because she helped to create mankind, she also had the title of Ninti. (Lady of Life). One of their children was Marduk, who, later on, would become the supreme deity.

Nice trivia: Sumerian astronomy knew of the existence of the planets Neptune, Uranus and Pluto. EA was the name the Sumerians gave to the planet Neptune. (And in Roman Mythology Neptune is the roman name for the Greek God Poseidon, mentioned above).

Personally, I believe Enki symbolizes the Sirian presence on Earth. His symbol is the snake or serpent. The snake, however, also refers to the inhabitants of Sirius. The title "Lord of the Seas," too, has a link with the watery worlds in the Sirian System. (cf. the Whales and Dolphins).
According to Earth's Galactic History, the Sirians were the ones who helped mankind to survive the great Flood.

[Apparently I am not the only one who came to this conclusion: a guy calling himself Matthew came to the same conclusion]