Some animals evolved with forward-facing eyes and others evolved with eyes which face sideways. Evolution is very good at evolving features to suit each animal’s lifestyle and to ensure its survival. Eyes are placed where they give its owner the best possible vision for its own lifestyle.
Humans, apes, monkeys and most predators, including birds and mammals, have forward-facing eyes, in the front of their head. This gives them binocular or stereoscopic vision which enables the animal concerned to judge depth and distance. Judging depth and distance enables predators to track and chase prey animals. Monkeys need to be able to judge depth and distance in order to successfully swing and leap from tree to tree. When you live high in the treetops, an inability to judge distance and depth could mean a monkey falling to its death on the forest floor. Apes and humans have forward-facing eyes, and their large brains allow them to see in three dimensions and to judge accurately distance and depth.
Prey animals need to be able to see around, while they are grazing, which is why they generally have sideways facing eyes. Eyes each side of an animal’s head allow the animal a better peripheral vision, enabling it to see predators, who may be lying in wait near grazing animals. Sideways-facing eyes allow antelopes and other prey animals to see predators approaching from behind as well as from each side.
Occasionally animals have eyes on the tops of the heads for particular reasons. For example, crocodiles and alligators have eyes on the top of their heads, enabling them to remain submerged and hidden, whilst watching and stalking prey.
Both sideways- and forward-facing eyes have disadvantages. Animals which have eyes in the side of their heads have a blind spot right in front of them, and to make up for this defect nature gives many prey animals an excellent sense of smell.
Forward-facing eyes too have disadvantages. Animals with eyes in the front of their heads have large blind spots and very limited peripheral vision, meaning that it is very easy for other animals to come up behind them unawares. This is not a problem when the animal is a top predator, such as a tiger or lion, since healthy adult animals in the wild are not preyed upon by other animals, except for humans, but can be a problem for other animals with forward-facing eyes. Monkeys have forward-facing eyes and are often preyed upon by chimpanzees, snakes, leopards, crocodiles and other animals.
Many predators, such as wolves, cats, humans, hawks and owls evolved forward-facing eyes so that they could effectively track and hunt prey. Most prey animals, such as deer, rabbits and most small birds, evolved sideways-facing eyes to help them avoid the attentions of predators. Monkeys evolved forward-facing eyes to enable them to accurately swing between trees in their search for food and to evade predators safely in their arboreal world. Animals evolved forward- or sideways-facing eyes to give them the best possible vision for their survival. There is an adage which says “eyes on the side, animals hide, eyes on the front, animals hunt,” and to some extent this is a good guide.