Matsya avathar
God's first incarnation was Matsya meaning: Fish.
This goes well with the concepts of evolution when the earliest multi-cell living beings were aquatic. Once upon a time a demon called Hayagriva with the face of a horse stole all the Vedas and sasthras from Brahma - the creator when there was the deluge.He took all those and hid them deep in the sea. When Vishnu learn about this he took the Matsya avathar - incarnation as a fish which in time grew in size.

When the deluge ended Vishnu in the form of a huge fish like a whale killed Hayagriva and retrieved the scriptures and returned them to Brahma. The end of deluge also marked the end of night for Brahma and the beginning of the day. Millions of years of our reckoning form a day and night for brahma and the Gods. When the Vedas were returned and with the beginning of the day Brahma resumed his task of creation. Another reason given to explain why God incarnated..

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