This is the second time I've seen Jaynes come up on this sub. I was not impressed the first time with his ideas, and now having done more reading on him, I am even less so.
One contemporary critic of Jaynes described his ideas as "secular theology" in that his theory presents an all encompassing worldview that speaks to fundamental truths about humanity, but the purported scientific foundation of that vision is sorely lacking. Jaynes arguments "sound convincing and cover what is actually naked speculation with a cloak of scientific respectability" (Jones 1982). Ultimately, the ideas Jaynes put forth are, at best, unprovable, because doing so would require knowing the interior thoughts of our Pleistocene ancestors. This being impossible, the proxy evidence put forth to support the notion of a bicameral mind is also flawed.
A "weak" version of Jaynes' hypothesis, wherein increasing population density and linguistic evolution created a feedback loop of changes in human cognition is relatively jejune and probably why the concept has had staying power. It is adjacent to enough actual theories in anthropology and neuroscience that it can exist in their penumbra. The "strong" version of Jaynes' hypothesis though, requires us to believe that pre-modern humans existed a state of permanent schizophrenia, behaving like automatons in response to auditory command hallucinations. According to Jaynes, only with the advent of writing did this change, a proposition that is both shakily evidenced and profoundly ethnocentric in the worse possible way, given that the implication is that non-literate socities are technologically and mentally less "advanced" than the Western cultures on which Jaynes focuses. (See my previous comment about Jaynes and the Aztecs.)
On the specific topic of burial rituals proving the bicamerality of consciousness in the past, this is a good example of how Jaynes takes a common anthropological practice and leverages it into absolute surety about his particular thoughts about consciousness. There are any number of reasons that people did (and still do!) venerate the deceased as though they were still living people. This can be a function of grief, respect, or even denial, all expanded from personal emotion to institutional practice. Funerary rituals have a common theme of reassuring the living that the deceased had a "good" death which precipitates a "social death" wherein the dead individual transitions from their living role to their dead role (Engelke 2019). Jaynes might argue that the continued veneration of the deceased shows a lack of transition, but examination of cultures where the social role of the deceased is not completely extinguished still shows death acted as a transition point to a new role.
Preserved remains in the Andean area were given offerings of food and drink, and were periodically rewrapped with new cloths, but these actions were done as acts of veneration and symbolic sustainment of vital force (Lau 2008), not because the people literally thought the deceased were the same as living beings. Traditional funerary practices in Melanesia which maintained a sort of social relationship with the deceased were likewise not done with the literal belief that the corpse (or preserved parts of the body) was still alive (Lohmann 2005). Veneration of preserved body parts was a means of gaining the favor and spiritual assistance of the deceased, much in the way biological relics of Catholic saints are treated. Treating remains similarly as a living person is simply an extension of the respect shown while they were alive. In cultures where some spiritual aspect of an individual is thought to not only continue on after death, but also have the ability to effect the material world, this makes perfect sense and does not require Jaynes' hypothesis that these people where still hearing the voices of their dead relatives and where unable to make meaningful distinction between the living and the dead.
To return to the idea that death rituals are a mechanism for the living to process their own emotions -- including grief and denial -- the Toraja of Sulawesi famously have elaborate funerary practices, which include treating the dead as though they were still living and even exhumations to clean and redress corpses. If Jaynes were to be correct, such practices would be the norm for "bicameral" peoples. Yet among modern hunter-gatherer groups, which are often used as analogues for historical groups (a practice not without its own issues), elaborate funerary rituals are not the norm. Maintaining a prominent social role for the deceased is an outlier. Prevalent themes in death rituals are generally about aiding the bereaved in processing their emotions and returning to normalcy (Palgi & Abramovitch 1984), which is how the Toraja in fact conceptualize their practices (Wellenkamp 1988).
Death, being a universal fact of human life, tends to accrete a vast array of rituals and beliefs. There are numerous reasons as to why groups would chose to preserve some aspect of social existence for their loved ones or for culturally important persons. Their is grief to process, symbolic power to be preserved, the possibility of supernatural assistance, and various other beliefs that range from the personal to the institutional. According to Jaynes though, "this practice has no clear explanation except that their voices were still being heard by the living, and were perhaps demanding such accommodation" (p. 161). This is not supported by contemporary societies which practice such veneration occurs and is an extremely outlandish interpretation of historical and anthropological data. Jaynes idea of ancestor worship as a result of literally hearing the voices of the dead only makes sense if one presumes his hypothesis on consciousness to be correct. This is a case of the bicameral mind being a "solution" in search of a problem, brushing aside more plausible explanations.
Engelke 2019 The anthropology of death revisited. Annual Review of Anthropology 48.
Jaynes 1990 The Origin of Consciousness in the Breakdown of the Bicameral Mind (2000 Edition). Mariner Books.
Jones 1982 Julian Jaynes and the Bicameral Mind: A case study in the sociology of belief. Philosophy of the Social Sciences 12(2).
Lau 2008 "Ancestor image in the Andes" in The Handbook of South American Archaeology. Springer.
Lohmann 2005 The afterlife of Asabano corpses: Relationships with the deceased in Papua New Guinea. Ethnology 44(2).
Palgi & Abramovitch 1984 Death: A cross-cultural perspective. Annual Review of Anthropology 13.
Wellenkamp 1988 Notions of grief and catharsis among the Toraja. American Ethnologist 15(3).
You've misunderstood Jaynes, he didn't think that ancient people "didn't understand that their dead relatives were dead," he asserted that auditory hallucinations of leaders had an important function in ancient society and they helped tremendously with social cohesion. Auditory hallucinations of dead leaders would still occur many times and in some societies it seems that they consider the recently deceased to be in a sort of limbo between the world of the living and the dead. This was likely because they could still hear them at times.
The dead were then called huaca or godlike, which I take to indicate that they were sources of hallucinated voices. And when it was reported by the Conquistadors that these people declared that it is only a long time after death that the individual ‘dies,’ I suggest that the proper interpretation is that it takes this time for the hallucinated voice to finally fade away.