Listen to part of a lecture in an archaeology class.

One of the frustrating things about archaeology... especially for beginning students... is that theories are constantly evolving.

A theory that has been accepted for many years may suddenly be called into question.

But why would that happen?

Oh, there are probably a number of reasons.

Earlier finds are always being re-examined in the context of newer finds.

Or it may just be that someone looks at the evidence in a different way... has a different idea of what it suggests.

Take the theory about the earliest permanent settlements.

They were found in an area to the east of the Mediterranean Sea, called the Levant; and the people who lived there were the Natufians.

For quite some time, it's been widely accepted that about 15,000 years ago the Natufians developed a sedentary lifestyle.

Can someone remind us what that means?

It means that they stopped being nomadic... that they began staying in one area year-round instead of moving around all the time.

Right. And we think there was an abundance of edible plants and animals in the area at that time that made this shift away from a nomadic lifestyle possible.

Um, keep in mind that the Natufians were hunter-gatherers.

So in spite of other changes, they were always a preagricultural society.

Anyway, after being sedentary for around 2,000 years, something happened that forced the Natufians to change their lifestyle.

The general consensus is that there was a period of climatic cooling, which had a negative effect on the availability of food.

And this food shortage likely caused the Natufians to revert to a nomadic lifestyle.

Then around 11,500 years ago, the climate warmed again... food became more abundant... and the people in that area became sedentary again.

Now, no one is contesting that these people, probably descendents of the Natufians, were indeed sedentary by 11,500 years ago.

The evidence is quite strong.

Archaeologists have uncovered numerous circular structures... dating from that period... that appear to have been used to store grain.

We think this for a couple of reasons.

First, the remains of barley husks have been found inside them.

Barley was the main type of grain that grew in this area.

And secondly, the floors on these structures were elevated.

This design would have been consistent with the need to keep the barley dry and safe from rodents, so that makes sense.

And there are lots of these structures in the settlements; in one settlement that was only partially excavated, archeologists have already found four of these structures.

So it's the earlier part- that the Natufians were sedentary 15,000 years ago-that's changed?

Well, there's evidence, but some archaeologists have questioned the criteria used to identify permanent settlements.

See, circular structures have also been found in early Natufian settlements.

So archaeologists believed that these were also food-storage structures, based on their physical similarity to the structures in later settlements, and they would indicate that the Natufians were sedentary 15,000 years ago.

But now there are doubts about the use of these earlier structures.

Researchers point to the lack of grain remnants in these earlier structures.

In fact, things other than grains have been found in them.

So at the very least, they say, these structures probably had multiple purposes.

And another problem they point to is that most early settlements have only one of these so-called storage structures.

But do you think one structure would be enough to hold a surplus for an entire settlement?

Well, whatever these structures were used for, couldn't they just have built them at a place they came back to regularly?

Maybe to store things for their next visit?

Exactly... so-called base camps where the Natufians didn't stay all year-round.

But artifacts that were found at a number of Natufian sites seemed to present evidence of a sedendary way of life.

Large, heavy mortars... the sort of thing that would have been used for grinding grain.

Such heavy equipment could indicate that the Natufians would have stayed permanently in one place since the work involved in moving an item like this around constantly would have been substantial.

But this evidence of sedentism has also been called into question because the materials used to make the stone mortars did originally come from quite a distance.

And if the Natufians could move the materials over great distances...