Objection 1. It would seem that the body of man was not given an apt disposition. For since man is the noblest of animals, his body ought to be the best disposed in what is proper to an animal, that is, in sense and movement. But some animals have sharper senses and quicker movement than man; thus dogs have a keener smell, and birds a swifter flight. Therefore man's body was not aptly disposed.
Objection 2. Further, perfect is what lacks nothing. But the human body lacks more than the body of other animals, for these are provided with covering and natural arms of defense, in which man is lacking. Therefore the human body is very imperfectly disposed.
Objection 3. Further, man is more distant from plants than he is from the brutes. But plants are erect in stature, while brutes are prone in stature. Therefore man should not be of erect stature.
On the contrary, It is written (Ecclesiastes 7:30): "God made man right."
I answer that, All natural things were produced by the Divine art, and so may be called God's works of art. Now every artist intends to give to his work the best disposition; not absolutely the best, but the best as regards the proposed end; and even if this entails some defect, the artist cares not: thus, for instance, when man makes himself a saw for the purpose of cutting, he makes it of iron, which is suitable for the object in view; and he does not prefer to make it of glass, though this be a more beautiful material, because this very beauty would be an obstacle to the end he has in view. Therefore God gave to each natural being the best disposition; not absolutely so, but in the view of its proper end. This is what the Philosopher says (Phys. ii, 7): "And because it is better so, not absolutely, but for each one's substance."
Now the proximate end of the human body is the rational soul and its operations; since matter is for the sake of the form, and instruments are for the action of the agent. I say, therefore, that God fashioned the human body in that disposition which was best, as most suited to such a form and to such operations. If defect exists in the disposition of the human body, it is well to observe that such defect arises as a necessary result of the matter, from the conditions required in the body, in order to make it suitably proportioned to the soul and its operations.
Reply to Objection 1. The sense of touch, which is the foundation of the other senses, is more perfect in man than in any other animal; and for this reason man must have the most equable temperament of all animals. Moreover man excels all other animals in the interior sensitive powers, as is clear from what we have said above (Question 78, Article 4). But by a kind of necessity, man falls short of the other animals in some of the exterior senses; thus of all animals he has the least sense of smell. For man needs the largest brain as compared to the body; both for his greater freedom of action in the interior powers required for the intellectual operations, as we have seen above (Question 84, Article 7); and in order that the low temperature of the brain may modify the heat of the heart, which has to be considerable in man for him to be able to stand erect. So that size of the brain, by reason of its humidity, is an impediment to the smell, which requires dryness. In the same way, we may suggest a reason why some animals have a keener sight, and a more acute hearing than man; namely, on account of a hindrance to his senses arising necessarily from the perfect equability of his temperament. The same reason suffices to explain why some animals are more rapid in movement than man, since this excellence of speed is inconsistent with the equability of the human temperament.
Reply to Objection 2. Horns and claws, which are the weapons of some animals, and toughness of hide and quantity of hair or feathers, which are the clothing of animals, are signs of an abundance of the earthly element; which does not agree with the equability and softness of the human temperament. Therefore such things do not suit the nature of man. Instead of these, he has reason and hands whereby he can make himself arms and clothes, and other necessaries of life, of infinite variety. Wherefore the hand is called by Aristotle (De Anima iii, 8), "the organ of organs." Moreover this was more becoming to the rational nature, which is capable of conceiving an infinite number of things, so as to make for itself an infinite number of instruments.
Reply to Objection 3. An upright stature was becoming to man for four reasons.
First, because the senses are given to man, not only for the purpose of procuring the necessaries of life, which they are bestowed on other animals, but also for the purpose of knowledge. Hence, whereas the other animals take delight in the objects of the senses only as ordered to food and sex, man alone takes pleasure in the beauty of sensible objects for its own sake. Therefore, as the senses are situated chiefly in the face, other animals have the face turned to the ground, as it were for the purpose of seeking food and procuring a livelihood; whereas man has his face erect, in order that by the senses, and chiefly by sight, which is more subtle and penetrates further into the differences of things, he may freely survey the sensible objects around him, both heavenly and earthly, so as to gather intelligible truth from all things.
Secondly, for the greater freedom of the acts of the interior powers; the brain, wherein these actions are, in a way, performed, not being low down, but lifted up above other parts of the body.
Thirdly, because if man's stature were prone to the ground he would need to use his hands as fore-feet; and thus their utility for other purposes would cease.
Fourthly, because if man's stature were prone to the ground, and he used his hands as fore-feet, he would be obliged to take hold of his food with his mouth. Thus he would have a protruding mouth, with thick and hard lips, and also a hard tongue, so as to keep it from being hurt by exterior things; as we see in other animals. Moreover, such an attitude would quite hinder speech, which is reason's proper operation.
Nevertheless, though of erect stature, man is far above plants. For man's superior part, his head, is turned towards the superior part of the world, and his inferior part is turned towards the inferior world; and therefore he is perfectly disposed as to the general situation of his body. Plants have the superior part turned towards the lower world, since their roots correspond to the mouth; and their inferior part towards the upper world. But brute animals have a middle disposition, for the superior part of the animal is that by which it takes food, and the inferior part that by which it rids itself of the surplus.
Source: Summa Theologica (New Advent)