Listen again to part of the lecture, then answer the question. Why does the professor say this?

Listen to part of a lecture in an archaeology class.
Professor: One of the most important changes in human history was a change from a nomadic lifestyle, wandering from place to place to find food, to a sedentary one, living in permanent year-round settlements. But what led to such a dramatic change? Well, our current thinking points to changing climatic conditions as the trigger, that climate change created conditions that enabled people to settle in one place. And the biggest such climatic change was the retreat of the glaciers after the last ice age.
    This lifestyle shift developed in several parts of the world at the same time, but I think the best-researched example, something we’ve spent a lot of time looking into, is in the coastal plains of the Eastern Mediterranean region. This area today is arid and barren, but about 18,000 years ago, after the glaciers retreated, this land along the Eastern Mediterranean was open grasslands and woods able to support numerous hunter-gatherer groups. In fact, food was so available, so accessible in this more open terrain that people were able to reduce the amount of moving around that they had to do. Then about 5,000 years later, there was another climate change that resulted in increased rainfall on the region, and this increased precipitation further enriched the ecosystems there. Finally, it became possible for people in that part of the world to remain permanently in one place because there were no more food shortages. What kind of evidence would you look for to support this theory of permanent settlement?
Student 1: Well, if you’re trying to prove that people were living in permanent settlements, then I guess you’d look for the evidence of houses, right?
Professor: That’s a good start! Architectural remains are one of the first things we look for, actually. The greatest quantity of evidence from this part of the world is from a group of people known as the Natufians. Natufians is the name we gave to a group of people whose settlements were first unearthed near an area in the Eastern Mediterranean known as Watee El Natuf (SP). In these settlements, structure we’re pretty sure they were houses, were arranged in a semicircle and they were partially dug into the ground. The part of the walls below the ground level were lined with stones, and the upper portion was made of brush or wood. There are small circular holes dug into the floors, suggesting that the houses had posts and beams supporting the roof. Any why do you think this could be considered as evidence of a sedentary lifestyle?
Student 1: Well, it sounds like they went through a lot of trouble. You wouldn’t go through all of that if you were just going to leave the house at the end of the summer or whatever.
Student 2: They probably weren’t built to be taken apart either with all that stonework underground also. You wouldn’t want to put all that effort into something you were just going to leave behind.
Professor: Right. Now, there’s another type of evidence we use to infer a sedentary lifestyle: the presence of tools, especially heavy ones. And quantities of stone mortars have been found at Natufian sites. The mortars would have been used to grind seeds or grains. The area the Natufians inhabited was rich in wild cereal grains like wheat and oats, and these mortars were huge, much too difficult to transport on a seasonal basis. And as you might guess, the Natufians also had tools for harvesting the grains.
Student 2: Didn’t some archaeologists claim that the Natufians planted grains also? I mean, you’d expect that from a sedentary people, right?
Professor: That claim would be hard to verify. The tools we’ve found indicate only that the Natufians harvested and processed grains, so, well, as for intentional planting... Now, I say "intentional" because it has also been suggested that regular harvesting of grains can lead to unintentional planting of seeds, like when seeds get gathered by wind and sprout the following season. That would have increased the amount of food available without the Natufians doing any actual planting. Now, there’s evidence of the animals that the Natufians hunted. We’ve found remains of various migratory birds that would have flown into the area to spend the winter, and we’ve also found bones of young gazelles, a type of deer that lives in open grasslands. Now, what about that combination of remains might indicate permanent residence?
Student 2: Well, the birds would have been there in the winter, and young gazelles would have been there, I guess, around spring or summer. So if both were found, the Natufians could have been eating birds in the winter and gazelles in the summer, living there year-round.
Professor: That’s right. The remains of both birds and young gazelles together indicate year-round hunting, which strongly supports the idea of permanent residence.