Answer
Ecological conditions may constitute selective pressures that promote the evolutionary development of certain features and behaviors in different species. The following are cases in point:
Opposable thumbs. A thumb is called an opposable thumb when the animal is able to make pad to pad contact between the thumb and the little finger or other fingers (opposition or apposition). This makes possible the precision grip, and the power grip of primates and some other animals .
The need to catch and manipulate small insects as well as the need to care for immature young were pressures that drove the development of he precision grip. The need to hang from branches--sometime s with one hand-- drove the development of the opposable thumb; this made the forelimb prehensile. Some other animals besides primates have semi-opposable thumbs and prehensile grasping feet. but primates are the only animals with true opposable thumbs.
Mobile shoulder. The mobile shoulder confers skill and safety in tree travelling, and in holding on and swinging , sometimes with one hand. Arboreal travel, picking fruits and leaves, and the need to support immature infants while holding on, probably drove the development of mobility conferred by the possession of flexible shoulder joints.
Prehensile hand development is related to the evolution of opposable thumbs.
Opposable thumbs made the hands prehensile. that is, being able to grasp securely. This development facilitated swinging from branch to branch during arboreal locomotion with much reduced risks of falling to the ground from great height with disastrous consequences --especially while carrying a baby.
Primates also have prehensile feet--all except humans. Prehensility is driven by the evolutionary pressures of life in an arboreal habitat and the need to grasp and hold on.
Work Step by Step
Color vision. Primates are omnivorous, but most of their food is vegetable matter--shoots, leaves and fruits. Color vision enabled the former insectivores to distinguish shades of green, red, and orange. This made it possible for them to identify ripe fruits, and to distinguish between very dark-green, toxic mature leaves, and edible young leaves of lighter shades of green. The pressure for developing color vision was probably the need to find enough food.
Stereoscopic vision is vision in which each eye sees a different field; in the brain these are integrated into one image. This is the basis of 3-D vision that enables one to see in depth. Primates developed stereoscopic vision because it enabled more precise judgment of distances. Stereoscopic vision also facilitates better tracking of moving objects and gives a more accurate perception of relationships between viewer and objects being viewed. An animal with stereoscopic vision is less likely to miss when jumping/swinging from branch to branch, or from .tree to tree.
