E.V.O. is a side-scrolling platform game with role playing elements. The main character has three basic moves: a jump, tackle, and a bite attack, with an optional headbutt, and a kick attack available to some mammals, allowing them to attack behind themselves. By eating defeated enemies, players gain "Evolution points" that can be used to modify the creature they are playing as. These improvements include bigger jaws, various armours, horns, fins, longer neck, better jumping, swimming ability, flying ability and more. By evolving, your character gains more hit points, greater speed, stronger attacks, and even certain special abilities such as flight.
Evolution is not linear, as the player can choose at any point, to modify any part of their creature. Higher cost isn't necessarily better, as various changes can decrease some attributes.
There are also four types of crystals. Yellow crystals give tips and reveal points of the story. Red crystals allow the main character to morph into special powerful creatures that can be recorded in the evolution log so that they may be called upon later via Green crystals. Green crystals, of which the character may only possess one of at any given time, allow your creature to revert to a past form that was saved into the evolution log. Blue crystals grant the player an unusually large amount of evolution points.
The game uses a hub system similar to Super Mario World, with each level consisting of a single overhead map from which the player can choose a variety of stages to play. As the player completes stages within the map, new areas are unlocked for them to explore.
E.V.O. tells the saga of life's evolution on Earth, with a subtext of a creation myth and Theistic Evolution. The player takes the role of one of many billions of life-forms created by Gaia, the nurturing and benevolent daughter of Sol, the Sun. Among the creatures known as "life", there is a competition to evolve, and the greatest life-form will eventually be granted the privilege of entering the Garden of Eden and becoming the husband and partner of Gaia.
As the game progresses, it soon becomes apparent that some mysterious external force is interfering with evolution on Earth in a destructive manner. Strange crystals not native to Gaia appear across the planet, and creatures which eat the crystals are transformed into monstrously powerful beings that dominate all other life-forms, over-consume resources, and disrupt the flow of evolution. In each Age the player character is tasked with confronting the species transformed by the crystals and defeating them so that the evolution of life can continue on track.
In the final Age, the Age of Eden, the player learns that a mysterious entity is controlling the world's other life-forms and sending them against the player. This entity turns out to be Bolbox, a life-form that has evolved into a freakish and evil advanced being by consuming the crystals, believing itself to be the first human, but in reality it is a gigantic one-celled organism. In the game's final battle, the player and Bolbox fight to determine who will gain entry into Eden and become Gaia's partner. Bolbox is defeated, and the player joins Gaia in Eden and is granted the gift of intelligence.
It is also revealed that the crystals were introduced to Earth by an advanced civilization on Mars, who misguidedly wished to help Earth by speeding its evolution with the crystals. Upon realizing the dangers the crystals create, the Martians decide to leave Earth alone and simply watch it until it becomes advanced enough to interact with Mars.
In 1990, Enix published a game called 46 Okunen Monogatari - The Shinka Ron, a text/graphics based adventure game for the PC-98. E.V.O. is a loose adaptation of this game. 1
Upon its release and for many years after, reaction to the game was mild, but generally positive. Jonathan Dunder of The Free Information Society gave the game five stars, citing it as “a classic in gaming history” although admitting that “the gameplay can get somewhat repetitive”. Justin Holmes of AllRPG called it “an RPG that's a little more engaging than most" and "very original both in concept and gameplay”.