The question makes no sense because anything can be used as a currency, such as beer or rice or gold or litecoin or even toilet paper issued at no cost to the issuers though of course that wouldn't be recommended  

Using a single currency will be a disaster if we talk only about Fiat because how could anyone supervise it, i mean a country could print an extra amount so...
But if you talk about cryptocurrencies including bitcoin then yea it is a good idea.
Aren't we using the same money (BTC) while we are from different countries? 

I think the overall effect on the world economy will be positive, because international trade is greatly facilitated - especially if all nations use a easily transferable and decentralized currency like Bitcoin. There will be significant differences regarding the extent to that specific countries will profit from a one world currency.

In general, competition will increase, because market access is facilitated for all countries. This most likely will result in lower prices for many products and services. The countries that will be able to produce cheaply will see the most positive impacts - regardless if cheap production is achieved via automation or low labor costs. In my view, developing countries might profit the most from Bitcoin as a one world currency, because labor costs tend to be low and many smaller businesses in these countries will have improved possibilities to offer their products on the international marketplace.

ya.ya.yo!

If that will happen a lot of people will be fucked.

Let's pretend for a second that this is happening.

The biggest and powerful country will want to have the lead.

By the lead I mean she will try to approximate the value of the coin we all use.

1.00000000 coin everyone use is 100 USD.

One person who is working in the country I mentioned get a paycheck of 50.00000000 coins

One person who is working in the weakest country on the same position will get 5.00000000 coins

And now comes the best part.

prices for bread in the fist country 0.00500000 coins

and guess what the price will be for bread in the weakest country? 0.00520000 coins

This scenario is inspired by real life salary and prices, the only difference is that everyone will know it for sure as a lot of people are not good at conversions and if they are they don't care about prices in other countries.

The World economy will collapse... We have seen what has happened with the Euro {Struggling countries} The world economy are being

manipulated by their governments... They print money to solve their problems, but instead of solving the real problems... they just delay

it further... At some stage, all of this has to collapse... One currency will not solve the problem, if it was based on the fiat model. 

If there is only one currency in the world, humankind on this planet will unite as one. There will be only one government, one country, one nation. And because of that, it will be easier for developing the Earth. And i think within a century, humankind can be able to explore and study universe which is far far away from Earth. We can find new species

Well if we have only one currency i think there is no trading currency online like this .. but it is impossible to happen you will never experience this kind of question. . better this thread is on off topic..

Do you mean as in One Fiat Currency? For example one "Earth" dollar.  In that case Purchasing Power Parity would be wiped from existence since the price level in each country would be based on the supply of that good.  It may work if the entire earth was aggregated into one entity with free trade since it would be based on the supply on the entire earth.

I think if that happen it will cause chaos and the economic will unstable, if the currency spread is not balance then it will cause inflation in the country that got least currency, so it's really important for each country to have it's own currency, so they have the power to control the inflation rate and their economic

I think so,and probably poor country will suffer more.Scarcity will prevail on said country and money will be hard to find.Though it is a good idea to have this centralized money systemI mean one currency to all, but those who has bigger money in their control can dictate the weak ones.