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 learnt 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 in forms other than human. All creatures were created by the same compassionate God. His descent into the earth should be the same for humans and sub-humans - other wise God becomes biased and parttial – an idea that is not accepted. This why God is shown as having incarnated as aquatic creatures and animals as also humans. Thus it was that Vishnu took the first avathar as 'Matsya' ie fish. This is the reason why god is shown as half-fish and half-god in the illustrations of the 'dhasa-avathara'.