It is more likely to be a historical question, isn't it?

Here is my academic guess, i wish I am right.
Originally, people exchanged their products with each.
Few of the products are shells.

Shell is different: light, easy to store, easy to take etc.

All the sudden, one of the consumers happened to use it to get whatever he/she wanted.
It is easier than sheep exchange, isn't it?

He/she will repeat this convenient exchange, day by day and year in and year out.
More and more people shift to this kind of practice.

Later on, the ruler found out that it was actually a very profitable business.
He executed his currency policy with authority.
His counterparts even ask people to admit that paper is valuable.
This is the origin of current currency.

I believe people are very creative, they keep writing history, don't they?

楼上说的有道理，可能最初的贝壳就像现在的黄金一样是很值钱的，人们乐于用自己的剩余产品去换取贝壳。

货币也是商品，只不过是特殊的商品而已。商品怎么来的就不用解释了吧。

我觉得并不是这样，一开始并没有货币，人们只是简单的物物交换，而那些比较容易交换，大家都需要的物品用的多，可是总是用物品交换不方便，如果是吃的可能坏了，但是当时人们都喜欢一种东西，贝壳，漂亮，稀少，应该就相当于今天的黄金，说白了就是一块石头但是大家都喜欢没有原因，所以大家都愿意用东西和贝壳交换，因此成为货币，但是并不是所有地方都用，有些地方离海远没有，所以只要是人们普遍愿意接受就会成为货币

芝麻姐的经济课：钱是怎么变多的
可以看看这篇文章
http://mp.weixin.qq.com/s?__biz=MzAxMjIwMDg1OA==&mid=202496557&idx=2&sn=ae450ebc0d34e1883a7b6c1ebc6d2e57#rd