We will evolve an extra finger because we use our fingers so much. Once we recive our extra finger we will also unlock more of our brain powers.
We have more brain powers but that is another topic...
Creatures don't evolve extra body parts just because they over-use them, especially if the extra body parts serve no extra functionality. Human hands with 4 fingers can perform as well as 5, so 6 fingers is just a waste of evolutionary resources. However, organisms evolve improved usability and dexterity with excess usage of certain body parts, so instead of evolving 1 more finger, we would probably grow longer, stickier fingers for enhanced grip which all tool-using creatures need to better manipulate equipment.
Edited by Ashiene, 04 April 2013 - 06:04 AM.