I would recommend the following article:

Butzenberger, Klaus. 1996. Ancient Indian Conceptions on Man's Destiny After Death: The Beginnings and the Early Development of the Doctrine of Transmigration. Part I. Berliner Indologische Studien 9: 55-118.

Note that, in contrast with the UpaniSads, in the Gita (8.23-26) it is not one's actions/behavior that determines which path one will follow after death, but the moment of death, and this is consistent with BhISma's decision to postpone his death until the beginning of the uttarAyaNa.
The two paths seem to be combined in the JaiminIya BrAhmaNa (1.49), and this is probably an earlier version of the theory.
The devayAna and pitRyANa of the Rg Veda are probably not the same as the UpaniSadic paths.
The two paths are also based on the general Vedic division of time cycles into a light and a dark half (or, an ascending and descending half), where the light half of a cycle is auspicious and the dark one is not.  I have written on this (in Spanish) in:

Tiempo ciclico y eras del mundo en la India. Mexico City: El Colegio de Mexico, 1988:43-53.

Best,

Luis Gonzalez-Reimann
University of California, Berkeley


At 01:55 PM 03/07/2000 +0000, Eric Robert wrote:
>Answers CC to the author, please.
>
>---------- Forwarded message ----------
>Date: Tue, 7 Mar 2000 10:53:20 +0100
>From: "Robert, Eric" <Eric.Robert@um.be>
>
>I am searching for early references to karman and re-birth theory. I have
>several passages in the older upanishads: BU (eg 6.2), CU (5.3-5.10), KsU
>(1.1-1.2). These are relatively clear in their statements about the path of
>the gods, leading to brahman, and the path leading, through the world of
>fathers, to a new birth in a pleasant or foul womb, according to one's
>pleasant or foul behaviour. In KsU, it is said that the moon propels them to
>new birth in various conditions in accordance with his actions and his
>knowledge.
>Is there clear evidence or reference to such theory in prior material, ie
>samhitaas, braahmaNas or aaraNyakas?
>
>In advance than you
>
>Eric Robert
>