The original metaphor seems to come from the Taittiriya Upanishad. (It is also outlined in the Sarva-Sara Upanishad and the Paingala Upanishad.)
Here are some extracts from Swami Nikhilananda’s translation of the Taittiriya:
II.1.3. From the Atman was born AkAsha; from AkAsha, air; from air, fire; from fire, water; from water, earth; from earth, herbs; from herbs, food; from food, man. He, that man, verily consists of the essence of food. This indeed is his head, this right arm is the right wing, this left arm is the left wing, this trunk is his body, this support below the navel is his tail.
II.2.1. Verily, different from this, which consists of the essence of food, but within it, is another self, which consists of the vital breath. By this the former is filled. This too has the shape of a man. Like the human shape of the former is the human shape of the latter. prANa, indeed, is its head; vyAna is its right wing; apAna is its left wing; AkAsha is its trunk; the earth is its tail, its support. Continue reading
