When you start out in Maneater you will just be a little baby shark, but you will rapidly grow older as you play through the game. You can evolve and upgrade your shark, becoming a nightmarish monster of the deep, capable of all kinds of miraculous feats.
You grow older by earning experience, but evolving is a very different thing. The first thing you will need to do to evolve is eating, a lot. Eating other creatures gives you plenty of resources that you will need to upgrade your evolutions.
After that, you will need to get some evolve options. To get these, you need to finish certain missions or challenges in each of the games area. You can get different evolutions for your jaws, fins, tail, body, and head. You can also get different evolutions for your three organ slots.
Each area will have different rewards for different challenges, so be sure to check your mission in the Log in the main menu.
Just click on any of the slots to change your evolution, you are not stuck with it and can freely swap between them, and mix and max parts. Different evolutions provide different bonuses that will give you advantages in different aspects of the game. Some make you do more damage, while others will make you tougher. You can also become an electric shark, or a poison shark, doing tremendous damage to groups of enemies.
Playing around with the different evolutions is a huge amount of fun, and is something everyone who plays the game should do. To upgrade an evolution, just click on the part you want to upgrade, then hit the prompted Upgrade button. This is Y on the Xbox controller, for example.
You can then confirm the upgrade, paying the marked amounts of resources to upgrade it. There are five tiers in total, with progressively better stats each time.