Primate Adaptations

Locomotory Adaption

Probably the primitive mammalian ancestor of the primates was a small, essentially quadrupedal and arboreal animal who scurried along the tops of branches and climbed by digging its claws into the bark.

Essential factors in upright posture of Primates:

• Reorientation of the body with respect to gravity, which necessitates many skeletal, muscular and visceral changes.
• With the face or muzzle being reduced and skull being set directly in the top of the vertebral column.
• Position of foramen magnum moves forward and the face is reduced so that skull nicely balances on the center of its base.
• Human require little muscle action to hold their heads erect because of this balance, whereas animals like gorilla with heavy muzzles require heavy neck muscles to hold them up.

Types of Locomotion among Primates:

1. Vertical clinging and leaping
2. Quadrupedalism (Sow climbing, Branch running an d walking, New world Semibrachiation, Old world semibrachiation, Ground running and walking, Knuckle walking)
3. Brachiation (True Brachiation, Modified Brachiation)
4. Bipedalism

Vertical Clinging and leaping

• Found only among the prosimians.
• Involves leaping from on vertical support to another, using the hind limbs for propulsion.
• Trunk is held upright position at rest, and forelimbs are used for clinging to tree trunks
• Prosimians can also hop on the hind limbs when placed on the ground.

Main Bodily featured associated with this type of locomotion:

• Elongation of hind limbs, which are used as springs in leaping.
• It may be a basic primate locomotor adaptation which was achieved very early in evolutionary history of the order and has been retained in a few prosimian groups today.

Quadrupedalism

Two distinct traits can be differentiated 1. Arboreal and 2. Terrestrial Quadrupedalism. In this category both fore and hind limbs are used in locomotion. There are also several sub-types in this category which differ mainly in which climbing, leaping and swinging are employed in addition to quadrupedal running or walking.

Arboreal Quadrupedalism:

1. Slow Climbing Quadrupedalism:
   
   • Portray slow climbing and deliberate movement with a powerful grip
   • Adaptation includes reduction in index fingers and the thumb.
   • Big toes are splayed out at a wide angle.
   • This allow them to use their hands and feet like forceps to enclose a branch in firm grip.
2. Branch running and walking quadrupedalism:
   • Their forelimbs and hind limbs are of equal length,though the hind limbsare slightly longer.
   • Gaps in the trees are crossed by leaping and the tails are used for balance.
3. Old world Semi Brachiation:
   • Found in the old world monkeys.
   • Anatomical change adapted to this type of locomotion are the presence of short, broad trunks, elongated arms, and long fingered hands.
   • These animals not only swing but also leaps.
4. New World Semi Brachiation:
   
   • The new world members have prehensile tails which they also use to suspend themselves.
   • In rest of the characteristics they resembles the Old world Semi Brachiation primates.

Terrestrial Quadrupedalism:

There are two distinct terrestrial quadrupedalism locomotive adaptation-The Ground Running & walking and Knuckle walking.

1. The Ground Running & walking:
   • Baboons and patas monkeys run and walk on the ground, on the soles of their feet.
   • Their limbs are of equal length.
   • Do not have to grasp the surface as their arboreal relatives do.
   • Relative lengths of the thumb and index finger, favour a more effective precision grip.
2. Knuckle walking:
   • This a secondary adaptation to ground living.
   • Chimpanzee and gorilla have become quadrupedal, probably because of their large body size, but at the same time have retained the long arms, long fingers, and short thumbs of their brachiating ancestors.
   • While they walk on all fours, their torso is actually slanted with the head end higher because of their disproportionally long arms.
   • The anatomical changes associated with this type of locomotion are the elongation of pelvis
   • The hand of femur not forming great angle with the shaft which restricts the movement of leg
   • Habitually bent knee and the whole foot making contact with the surface.

Brachiation

1. True Brachiation:
   
   • In Brachiation, the body weight is suspended from the arms and hands beneath the branches and progression is accomplished by swinging the body from one handhold to another.
   • The hind limbs are sometimes used for support and occasionally for suspension as well.
   • All the brachiators have some means of getting their thumbs out of the way during locomotion.
2. Modified Brachiation:
   
   • This type of locomotion is found in orangutans
   • It involves arm swinging with feet used for support, climbing and hanging by feet.
   • The orangutans are more cautious climbers than brachiators, and climb using any combination of their four grasping extremities.

Bipedalism

• Found in Human Beings
• Involves the hind limbs alone supporting the body weight and propel it along the ground
• The main bodily modification involves the elongation of legs and changes in the foot and pelvis.
• In the foot the big toe is no longer opposable and brought in line with the other toes.
• An arch has been developed and the heel has become longer to aid in balance.
• Thus grasping primate foot is lost in man.
• The pelvis has become short, broad and basin shaped.

The reason why man's ancestors first adopted bipedalism are unknown and speculative. Other primates have come to ground and abandoned the forests, but none have become bipedal except man. One of the advantage of this locomotion is that it has freed the hands from locomotion so that they can be used in carrying and manipulating objects such as tools.

SKELETAL CHANGES DUE TO BIPEDALISM AND THEIR IMPLICATIONS

1. Changes in Pelvis:
   
   • Has become short and braod
   • Formed basin shaped structure to support the abdominal viscera against gravity.
   • Pelvis has changed the position and has come to lie in the axis of hind limb bone, thus transferring the head and trunk's load directly to legs.
   • A bine in thigh called ILIUM has developed a number of spines that serve to provide attachment surface for muscles that help in the thigh movement and bipedal locomotion.
   • Another bone of Pelvis called ICHIUM has flattened, allowing humans to sit comfortably.
2. Changes in Hind Limbs:
   • Both the bones of legs id femur {Thigh}and tibio-fibula (Shin) are elongated.
   • The head of femur, after its attachment to pelvis has undergone such change that is capable of all round movement.
   • Linea aspera is particularly well developed, that keeps hind limbs straight during bipedal walking.
3. Changes in Muscles:
   • The leg and hip musculature has developed to facilitate upright standing, walking, running and climbing.
   • Three muscles gluteus Maximus, gluteus medius and gluteus minimus stabilize pelvis during walking.
4. Reshaping of foot:
   • The axis of the human foot lie between the big toe and the second toe.
   • The big toe has become no opposable to bear maximum load transmitted to the foot.
   • Foot develops two arches to support the weight of the body.
5. Associated changes due to bipedalism:
   • Changes in the spinal cord:
     The spinal cord developed four distinctive curves-cervical, thorasic, lumbar and scral. These curves aalow the erectness of the spine.
   • Changes in Sacrum:
     The vertebrae of sacral region unite with ilium of pelvis, the later in turn to femur. The small distance between these two joints allows the transfer of the load directly to femur.
   • Changes in Vertebral Column:
     From top to bottom, the vertebrae gradually increase in size. This gives the human vertebral column its characteristics cone-shape, which is more stable in load bearing function.
   • Changes in the skull:
     In a quadrupedal animal, the foramen magnum is situated more dorsally and posteriorly. In the human being it has shifted forward and downward.

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