[This is only a bit of an answer, so I just mentioned it in a comment] but Draconis suggested I post it as an answer]
From David W Anthony, The Horse, the Wheel, and Language, beginning of Chapter 8:
At the beginning of time there were two brothers, twins, one named Man
(*Manu, in Proto-Indo-European) and the other Twin (*Yemo). They
traveled through the cosmos accompanied by a great cow. Eventually Man
and Twin decided to create the world we now inhabit. To do this, Man
had to sacrifice Twin (or, in some versions, the cow). From the parts
of this sacrificed body, with the help of the sky gods (Sky Father,
Storm God of War, Divine Twins), Man made the wind, the sun, the moon, the sea, earth, fire, and finally all the various kinds of
people. Man became the first priest, the creator of the ritual of
sacrifice that was the root of world order.
After the world was made, the sky-gods gave cattle to "Third man"
(*Trito). But the cattle were treacherously stolen by a
three-headed, six-eyed serpent (*Ngʷhi, the Proto-Indo-European
root for negation). Third man entreated the storm god to help get
the cattle back. Together they went to the cave (or mountain) of the
monster, killed it (or the storm god killed it alone), and freed the
cattle. *Trito became the first warrior. He recovered the wealth of
the people, and his gift of cattle to the priests insured that the
sky gods received their share in the rising smoke of sacrificial
fires. This insured that the cycle of giving between gods and humans
continued.
(He does give some sources for this, which I haven't investigated. They include Calvert Watkins, Bruce Lincoln, Jaan Puhvel, and Mallory & Adams.)
