Can Humans feed Everyone on Earth?

Likely the answer to this is: No.
In theory we could but reality is a far state away from Theory.
Then let's ask another question: Can we feed more people than we do at present? No.

If we realistically answer both questions the answer has to be: No.

Then we might ask: "Why is this?"

A realistic answer might be: Because even feeding as many people as we do as humans is a fluke.
And the reason it is a fluke is because it was all based upon inexpensive energy since around 1900.

If we look at the population in recent history here it is: 700 million. 1804, 1 billion. 1850, 1.2 billion. 1900.
the above figures are quoted from:

World Population - The Current World Population

geography.about.com/od/obtainpopulationdata/a/worldpopulation.htm‎

1, 200 million. 1000, 275 million. 1500, 450 million. 1650, 500 million. 1750, 700 million. 1804, 1 billion. 1850, 1.2 billion. 1900, 1.6 billion. 1927, 2 billion ...

As of 2011 the world population was 7 Billion people. I believe that the primary cause for the world population going from 1.2 billion in 1900 to 7 billion in 2011 is cheap oil and cheap energy sources between 1900 and 2011. So, looking at the above chart of world population through time we see that between 1900 and 2011 the world population grew at least 5.83 times caused by cheap oil and technological innovations. But, now we see this just isn't sustainable.

Likely a population of 1.2 billion would have been sustainable but not 7 billion.

But now, cheap oil energy sources are gone. We still have some oil coming out of the ground on land and from under the sea but cheap energy from oil is gone. 

So, in the transition it is likely that earth's population will reduce.

Also, there are other factors that should be considered here. 

First, China can no longer feed itself. There are just too many people and global warming and in many cases bad sheparding of soil and water resources have contributed to this problem in China. I would say the biggest problem China has is that there are just too many people to properly educated them as to the best course forward which is to function more ecologically. The Soviet Union had this problem too regarding the ecology and so when it fell around 1990 or so there were many streams that were on fire from kerosene and other flammable liquids on the surface in Russia.

Also, in Russia in the last few years, Russia cannot survive without importing foods either. In some ways this makes China and Russia resource hungry.

It isn't as useful to consider unemployment in the cases of Russia and China without first looking at their food resource problems.

But, in the case of the U.S. we can still feed ourselves completely if we absolutely needed to in an emergency. However, this might change for us because of global warming. If tornadoes keep increasing east of the Mississippi River and from Texas north through Oklahoma and Eastern Colorado, many of these states are the breadbasket of the U.S. So, unless farmers live underground almost in bunkers you will see so many of their houses and barns devastated by tornadoes which are increasing in average size and the number of them almost every year now. Also, wind speed averages in tornadoes are increasing on average every year now.

So, within 10 to 20 years even the U.S. might reach a point where we can no longer feed ourselves completely without importing food from abroad.

Historically, resource problems often create wars worldwide. Everything from the French Revolution in the 1790s to World War I and World War II had their roots in not enough food in certain areas and countries. So, the lack of resources historically often creates big and small wars and conflicts all over the earth.

Even the actual causes of Arab Spring had more to do with an overabundance of young people in relation to the actual resources of the land available in the Middle East. When you have 50% of people under 30 unable to find useful employment because of this you then have something like Arab Spring. So, whether they should be or not, governments are overthrown but it is also inevitable that the people be disappointed in their new governments because still there will be no new jobs because food still needs to be imported from other nations which becomes more and more expensive and which means the poorest of the people are going to starve unless they are fed by organizations from richer countries or move to richer countries that might help them stay alive and still have food to eat. But, if too many people move to richer countries then the richer countries start to be destabilized by all the refugees coming into their countries. This is one of the problems Greece is having now because of the 4 million Syrian Refugees but also remember there are refugees also now from Libya, Tunisia, Egypt, Syria etc. In other words people are leaving the whole middle east in droves because it is so very unstable and overpopulated in relation to it's actual capacity to feed people without importing food from other nations.

Then on top of all this you have nations like Russia wanting to keep the status quo and deciding to keep Assad in place in Syria even though he has been raping and killing his Sunni people always and even though the majority of people in Syria are Sunnis not Shias. This is now destabilizing Syria, Iraq, Lebanon, and Israel both directly and indirectly. Also, though at present Jordan is not in danger of collapse, if things get much worse it could be threatened as a nation too.

So, once again in reality all the people of earth cannot be fed, only in theory. But, as we all know theory isn't the reality on the ground.