Why does the professor talk about the layers of soil?

Listen to part of a lecture in an Anthropology class.
Professor: We are aware that early agriculture arose independently in several regions of the world, for example, the Middle East, China, Southeast Asia, and parts of the Americas, roughly 10,000 years ago. And then agriculture spread from those areas to the rest of the world, lt is now believed by some archaeologists that agriculture also developed independently in New Guinea. Up here, you can see a huge island on this map located in the Southwest Pacific, north of Australia.
    So for many years, it was considered that New Guinea domesticated crops and animals introduced from Southeast Asia about 3,500 years ago. Then in the 1960s and 70s, researchers explored sites in the island, hoping to find some evidence of independent agriculture development. Unfortunately, though, the research was unsuccessful to gather some conclusive evidence to support their speculation. For example, although evidence was found in deforestation, which is from at least 7,000 years ago, which is long before we’ve thought previously, it was unclear whether the forest had been cleared by farmers to plant crops or by hunter-gatherers to hunt more easily. And most plant remains like seeds and fruits don’t preserve well in swampy grounds. You know, New Guinea has a very humid environment, so really the proof was limited.
    But recently a group of archaeologists have come up with some pretty convincing support from a site that had been previously examined: Kuk swamp. As its name implies, it is located on a wetland margin, in the upper Wahgi Valley of the New Guinea highlands. According to their findings, they identified a succession of phases of agriculture development in the wetland, and it actually predated the earliest known agricultural influence from Southeast Asia. By using a modern archaeological method, they were able to analyze the sediment samples from each layer of the earth at the site in Kuk. From the oldest soil layer, dating back 9,000 years, they found some features such as pits, postholes and irrigation draining ditches, which provide evidence for very early phase of agriculture. That is, these all indicate that crops were being planted.
    From a higher layer of soil, the second phase, they identified regularly distributed mounds. Mounds were constructed in order to plant crops that don’t grow well in wet soil, such as bananas, because, remember, Kuk is a swampy wetland, and bananas can’t tolerate the conditions there. And in the layer from Kuk’s third phase, an extensive network of ditches and drainage channels have been found. They were excellent examples of transformation of agricultural practices. Since the archaeologists had more advanced techniques than were available from earlier researchers, the archaeologists also were able to identify microfossils in the soil from banana plants, and also grains of starch from taro date from about 10,000 years ago. lt was really significant to find taro remains because it meant that it must have been planted there, brought from the low areas, because taro doesn’t ordinarily grow in the high lands.
    When it comes to bananas, in sediment samples dating from about 7,000 years ago, researchers also found a high percentage of fossils from banana plants. This proved that bananas were deliberately planted, because where bananas grow naturally, the concentration of the plant fossil is lower. Bananas don’t naturally grow so densely. In fact, recent genetic comparison research suggests that the type of banana grown in New Guinea was domesticated there; it was then brought to Southeast Asia.
    Well, usually, we expect to see certain social changes brought about by the development of agriculture, structural changes in the society like rapid population surges and different social classes. But New Guinea? It’s largely unchanged. It remained an egalitarian society. So what does that tell us about the usual presumption?