Here's a theory I came up with that I haven't heard anyone talk about, but it seems to fit quite nicely, as far as I can tell. Anyway, the theory is that the Fall was the beginning of agriculture, and the subsequent turn-over from matriarchy to patriarchy.
After placing Adam and Eve in the Garden of Eden, God commands them to go forth and multiply and to subdue the Earth. According to the Biblical description of the location of the Garden of Eden, it was located in a valley which was extremely fertile, providing ample food in terms of plant and animal life. God tells them that the Earth will bring forth fruit spontaneously, that they may eat freely. Of course, if small numbers of humans existed in a rich and fertile valley as nomads, the Earth would have spontaneously brought forth all the food they needed without them tilling the earth.
First, Eve partook of the fruit, then convinced Adam to partake as well. Women were the first to dabble in agriculture, being the more stationary of the two sexes. Men only took over later. Of course, in the Biblical account, God cursed both Adam and Eve. Eve's curse was that her conception and sorrow would be multiplied and that her husband would rule over her. Before agriculture, women would breastfeed their babies for roughly four years, or until they were able to walk and keep up with the rest of the people since they traveled a lot. Breastfeeding kept the women from ovulating, and therefore, from becoming pregnant. Now, any doctor will tell you that breastfeeding isn't a surefire birth control method, but this is because people now eat grains. The carbohydrates disrupt female hormones making it so that a breastfeeding woman can ovulate. Before agriculture, grains and carbohydrates were not a major part of human diet. This explains the phenomenon of the “Irish twin”, as the Irish were famous for their potato-eating tendencies. Also, women started weaning their babies earlier because they had grain cereals to feed the babies, and waiting between children became less essential since they weren't nomadic.
God then cursed Adam that he would till the earth and eat bread for the rest of his days. This fits right in, since no one tilled the earth or ate bread before agriculture. Also, this is the first mention of any grain-based food. Before, it only says they ate fruit and meat. Also, as the population increased and people stopped traveling, the amount of food needed to support a large, sedentary population would need to be cultivated through hard work and labour, rather than simply going out and finding it.
As for the promises of death and suffering, it is believed that the infant mortality rate was much higher among agriculturists. They were also subject to far more diseases, as a sedentary lifestyle and close proximity to livestock and their own waste would promote. Small pox, black plague, measles, and several other diseases were all due to agriculture. A comparison of the skeletons of hunter gatherers and agriculturists living at the same time and in similar areas reveals that the hunter gatherers were much healthier and lived much longer. The farmers, in addition to all the unique diseases, had evidence of severe arthritis, stunted growth, and tooth decay. The repetitive, labour-intensive tasks of farm life led to the arthritis and growth problems, and carbohydrates, which begin to break down in the mouth, led to tooth decay.
But, of course, they also were able to "go forth and multiply" as the women were giving birth to far more children. It also increased the knowledge of both good and evil, since populations could then support an elite - scholars, spiritual leaders, and artists. The potential for evil was also increased. Since grain can be stored, agriculture was the beginning of commodities, and therefore, of money and property. Money, the infamous root of all evil. Now that there was ownership, there could be stealing, conquest, and inequality. There began to be an a small percentage of people who were vastly richer than the general population. Now there were kings, slaves, and wars. An increasing population would also mean that people encroached on each other's space more, leading to more conflict. There was also literacy and an increasing number of new technologies, which could be used for good or evil.
This general idea fits in with myths from various cultures that involve a trickster, or fallen god that gives fire, agriculture, or other technologies to mankind as a way to make them worship him, or simply to cause trouble. This certainly fits in with the idea of Satan beguiling Eve, giving her the knowledge of good and evil, and launching mankind into an era of great growth and development, as well as evil and suffering. Agriculture is quite the boon, but obviously had unforeseen consequences, which no primitive person could possibly have understood.
There are other aspects of the Fall, as well. For instance, they realised they were naked and began wearing clothes to hide their shame. I haven't quite figured out the significance of this, other than to note that nudity does not seem to be nearly as taboo in ancient and matriarchal societies. The onset of Semitic patriarchy seems to bring with it a new standard of modesty. Before then, figurines of the gods/goddesses had, not only visible, but prominent and exaggerated sex characteristics.
As for what happened to the hunter gatherers, I've read conflicting things. Some say they simply adopted agriculture slowly and some say they were taken over and killed by the agriculturists, who were able to out-breed them and build permanent settlements. They also would have caught diseases created by agriculture, which people in agricultural communities began to have immunity to. I would say that, according to the Bible, it wasn't a peaceful takeover. Cain, the badboy earth-tiller met Abel, the herder (hunter gatherers did sometimes keep flocks of domesticated animals as they could be moved) in a field and killed him. They also would have killed the nomads inadvertently with their unique diseases, which they incubated, grew immunity to, then transferred to their nomadic brothers. This has happened throughout history – including when Europeans met with aboriginals on their various conquests. They decimate the population through disease before they even meet face-to-face, in many instances.
In conclusion, agriculture allowed both the curses and blessings laid out in Genesis from partaking of the fruit. People were more knowledgeable, more fruitful, and had greater potential for both good and evil. They had "subdued" (domesticated) many species of plants and animals. There was also more disease, death, hardship, and violence. Thus, I submit that perhaps the forbidden fruit was wheat, the first plant to be domesticated. Also, it seems likely that the Adam and Eve story as told in Genesis is more likely to describe the beginning of Semitic religion, rather than the beginning of mankind. Most subscribe to the idea that Adam and Eve were the first humans, although many LDS believe that there were “pre-Adamites”, including some general authorities.
Anyway... that's my theory... I came up with it after reading Against the Grain by Richard Manning and Adam's Curse by Bryan Sykes. I later found out that this is the theme of the book Ishmael: An Adventure of the Mind and Spirit by David Quinn, in which a telepathic gorilla tells a Jewish man about the Fall. I haven't read it yet, though.