That period of time was in Japan a period of enormous change. From about 14,000 years ago until about 300 BC, there were humans living in Japan and they lived a hunter-gatherer lifestyle. They had pottery, but they did not farm rice, and didn't really do much agriculture at all, though there were some sedentary populations. They had no writing systems. Compared to China, which was an enormously complex land, the inhabitants of the Japanese islands were not very advanced or complex in their social organization or technology.

But around 300 BC, there was an influx of people, no one is sure where they came from, but most likely Korea or China--and they brought with them rice agriculture and more complex weapons. This was Japan's Iron Age. Agricultural communities can support a much larger population than hunter gatherers, so at this point, the hunter-gatherer population became smaller, either through joining the agriculturalists, or through competition for resources. Rice agriculture spread throughout the islands, and they also brought domesticated animals. One of the popular theories now is that the vast majority of the Japanese population of today are descended from these people who came to Japan around 300BC, while the Ainu people, who live in northern Japan, are the descendants of the population that had lived on the Japanese islands for 14,000 years.

the end of the Jomon period and the beginning of the Yayoi Period. The Jomon period lasted from many thousands of years ago until about 300BC.