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 (cf. Poseidon / Neptune), 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 half-sister 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 had a half-sister, called Ninhursag or Ninmah. He wanted her as his wife, but it was decided that she was to marry her other half-brother, Enlil, to whom she bore a son, Ninurta. Because Ninhursag was the goddess of Medicine, and because she helped to create mankind, she also had the title of Ninti (Lady of Life). In spite of being married to Enlil, Ninhursag and Enki had several daughters together, as well as two sons, Dumuzi and Nergal / Irrigal.
Enki's first-born son, however, was Marduk, whose mother was Damkina, more commonly known as Ninki.
Enki also had three other sons with concubines: Gibil, Ninagal and Nigishzidda.

Interesting piece of 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.

There are reasons to 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.