I can't even begin to draw a mental picture of me all alone in the world. What will it feel like? One thing is for sure, It won't be pleasant.
Drawing from human psychology and the behavioral pattern shown by different people who have experienced something similar to being all alone in the world, am talking about imprisoned criminals locked up in isolation or solitary confinement, records have show an intensive change in the structure of their brains resulting to psychological damage and distress. They suffered from first degree depression, allow me put it that way, delusions and other deteriorating mental states.
Take the above for cases instance, these are people who were left alone for only a couple of months. These people still had hope, knowing they weren't alone in the world. Now, imagine you all alone in the world, with no hope of coming across any human ever.
Hence, it is only right to say that prolonge isolation will likely damage the cerebral cortex - which is the part of the brain that makes us most human. I bet suicide will be the most constant thought and possible line of action if I was left all alone in the world.