Listen to part of a lecture in an archaeology class.

One of the important aspects of the field of archaeology, one of the things that excites me about the field, is that seemingly insignificant things can suddenly change the way we think about a culture.We are always making new discoveries that have the potential to challenge widely held beliefs.

Take something like the banana, for example.It turns out that this ordinary fruit may be forcing scientists to rewrite major parts of African history.We know the bananas were introduced to Africa via Southeast Asia.And until recently, we thought we knew when they were introduced - about 2,000 years ago.

But discoveries in Uganda, that's In Eastern Africa, are throwing that into question.Scientists studying soil samples there discovered evidence of bananas in sediment that was 5000 years old.

Now, let me explain that it's not easy to find traces of ancient bananas.The fruit is soft and doesn't have any hard seeds that might survive over the ages.So after 5000 years, you might think there would be nothing left to study.Well, fortunately for archaeologists, all plants contain what are called phytoliths in their stems and leaves.

Phytoliths are microscopic structures made of silica, and they do not decay.When plants die and rot away, they leave these phytoliths behind.Because different plants produce differently shaped phytoliths, scientists can identify the type of plant from ancient remains.

So, those scientists in Uganda, dug down to sediments that were 5000 years old.And what do you think they found?Banana phytoliths.Obviously this meant that we had to rethink our previous notions about when bananas first arrived in Africa.But, well, this discovery had other implications for history.

As soon as bananas appear in the archaeological record, we know we have contact between Africa and Southeast Asia.It would appear now that this contact occurred much earlier than previously thought.

Although... now here's where the uncertainty comes in.We don't really have any solid evidence of trade between the peoples of these two regions that long ago.Presumably, if people were bringing bananas to Africa, they'd also be bringing other things too: pottery, tools... all sorts of objects made for trade or daily use.But any such evidence is missing from the archaeological record.

The early appearance of bananas also suggests that agriculture began in this part of Africa earlier than scientists imagined.You see, bananas, at least the edible kind, can't grow without human intervention.They have to be cultivated.People need to plant them and care for them.So if bananas were present in Uganda 5000 years ago, we would have to assume... that... that... that someone planted them.

But, there are questions about this too.We know that bananas can be a staple food that can support large populations, as they did in Uganda in the more recent past.If bananas were grown thousands of years ago, why don't we see evidence of large populations thriving in the area earlier?

So we are left with this mystery.We have what appears to be strong biological evidence that bananas were being cultivated in Uganda as early as 5000 years ago.But we are missing other kinds of evidence that would conclusively prove that this is so.

Clearly, more research needs to be done.Perhaps by some young scholars from this university?At least give it some thought.

What is the lecture mainly about?What excites the professor about the field of archaeology.According to the professor, what can scientists learn by examining ancient phytoliths?What are the implications of the theory that bananas arrived in Africa 5000 years ago?What does the professor consider a weakness of the study on bananas in Africa?Why does the professor say this?Clearly, more research needs to be done.Perhaps by some young scholars from this university?At least give it some thought.