2. Comparative Anatomy
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Success Criteria
Your learning has been successful if you can do the following:
I can explain the skull differences between a biped and a quadruped.
I can explain the difference in teeth and jaw of a human and an ape.
I can explain the difference in brow ridge, sagittal crest, foramen magnum and nuchal crest between a human and an ape.
Vocabulary
Learn these so you can communicate this concept well.
Larynx (voice box): Helps to create different sounds when we speak.
You may be required to compare trends in biological evolution of early bipedal hominins with living hominids. These trends involve:
Skeletal changes linked to bipedalism
Changes in skull and endocranial features
Changes in the manipulative ability of the hand.
Though closely related to apes such as gorillas and chimpanzees., there are a number of features that set hominins apart from apes. The main differences between apes and humans stem from human being bipedal.
In hominins, the body is concentrated into a narrow column above the feet. This means in humans, body weight is taken mostly by bone rather than by muscular energy as in apes.
Features of the Primate Skull
Being able to identify and compare different parts of a primate skull can help to identify the type of primate and various aspects of its biology (e.g. diet).
Knowing the names of major skull bones, as well as the features associated with a modern human skull will help you identify some of the evolutionary 'landmarks' in the development of humans.
For each of the male gorilla and modern human skulls below, you will use the internet to label each feature.
Comparing Skull Features of Apes and Humans
Foramen Magnum
In apes, the foramen magnum (hole through which spinal column passes) lies at the REAR of the skull whereas humans have it under the skull nearer the centre.
This means that apes need strong neck muscles, which are attached to nuchal crest on the skull, to hold up the heavy skull from an angle and prevent the head dropping forward.
In humans because foramen magnum lies more or less in centre of skull, the skull is balanced on vertebra, so little muscular energy is used to support the skull directly over the Centre of Gravity (COG).
Nuchal Area / Crest
This is the area where the neck muscles attach onto the back of the skull, to keep it balanced on the spinal column / looking forward. Apes have very large nuchal areas and associated neck muscles because greater muscle strength is required keep the skull looking forward when the spine is attached further to the rear of the skull.
Brain Organisation
The human brain has a much larger cranial capacity compared to other primates (Human 1330 cm3 vs Chimpanzee 500 cm3). This is because:
Humans have far more white matter in the brain, as there are more connections between the different parts of the brain.
However, there are other mammals with larger brains than our own. It is not simply a matter of how large the brain is (H. floresiensis), but how it is organized. An important factor is how large the brain is compared to body size.
Humans have a more folded brain. Increasing the surface area and therefore processing power.
Humans have a larger:
Frontal lobe (problem solving, judgement, impulse control, social behaviour)
Cerebellum (regulates motor movements - coordinates voluntary movements such as posture, balance, coordination, and speech, resulting in smooth and balanced muscular activity)
Wernicke's (speech comprehension)
Broca's areas (speech production)
Together, the Broca's and Wernicke's areas are areas of the brain specialized for production and comprehension, of language.
The modern human brain makes up 2% of our body weight but demands about 20% of our metabolic energy at rest - this makes the brain a very expensive (in terms of energy) organ to maintain. The selection pressures for increased brain size must have been considerable for additional energy to be made available - this could be provided during the course of human evolution by the acquisition of high energy food such as meat.
Brains are expensive organs to run - what allowed them to get bigger?
One theory is that: Bipedalism 🡪 free hands 🡪 increasing use of tools 🡪 increasing range of food sources (especially meat - high in calories) 🡪 increase energy intake and decrease energy output 🡪 more energy for brain to increase in size.
Selection pressure for brain expansion may have included:
Selection pressure to learn complicated skills such as tool making and language
Selection pressure to develop new brain structures to provide new functions such as language ability
Selection pressure to have an ability to imagine and connect ideas in new ways that enable new thought processes with survival advantage (e.g. understanding / predicting the rhythm of seasons / producing artifacts - tools from mental templates / understanding fracturing behaviour of stones)
Sagittal Crest
The sagittal crest is a ridge of bone running lengthwise along the midline of the top of the skull. The presence of this ridge of bone indicates there are exceptionally strong jaw muscles. The sagittal crest serves primarily for attachment of the temporalis muscle, which is one of the main chewing (jaw) muscles. It is absent or greatly reduced in most Hominins.
Humans do not have this because it is no longer needed for attachment for smaller jaws with less forces required, means less muscles needed to attach. This saves energy, which increases chance of survival resulting in energy that can be used elsewhere.
Temporal muscles
These are the muscles that pull up the jaw (bite). The temporal area is where these muscles attach onto the skull. In apes these are both much larger. This probably corresponded to a much more primitive diet that included a lot more fibrous plant material that needed to be ground down.
Zygomatic Arch
These are a bony arch just behind the cheeks. They provide a gap for the temporal muscles to pass through (see above). They are much larger in apes to accommodate much larger temporal muscles.
In humans, the majority of weight is now concentrated in the cranium. Smaller zygomatic arches because the temporal muscles have reduced in size due to diet changes (cooking of food). Smaller temporal muscles mean less muscle attachment sites required, so smaller mandible.
In apes, the majority of weight is concentrated in the jaw for attachment of large temporal muscles to chew through a tough diet. Larger zygomatic arches to allow for large temporal muscles to pass through. Large attachment sites required, so larger mandible.
Jaw / Mandible
In humans, the mandible is small in size because human teeth size is small (due to eating a softer diet). Less prognathism (shorter muzzle). Humans have a chin, to provide structural support to the gracile mandible.
In apes, the mandible is large in size because apes have large teeth (needed to eat the tough, fibrous diet). More prognathism.
Brow Ridges
This is a bony ridge located above the eye sockets. Its purpose is to reinforce the weaker bones of the face (reduces vertical stress).
In apes, they are much larger due to the tremendous strain put on the cranium by their temporal (jaw) muscles. Without this reinforcement, the eye sockets would collapse.
It is reduced in humans because they no longer have a large jaw or need muscles or much strength to move the jaw and so has led to a flattened forehead. This is important because it saves energy which increases chance of survival which can be invested elsewhere.
The brow ridge was one of the last traits to be lost in the path to modern humans.
Dental Arcade
The dental arcade is the shape of the jaw and arrangement of teeth. In humans, teeth are arranged in a rounded 'V' (or parabolic) shape arrangement. In apes, teeth are arranged in a 'U' (or rectangular) shape with parallel sides.
Teeth Shape
Humans have smaller teeth and are more uniform in size, suiting their omnivorous diet and softer, cooked food. They have no diastema. Canines become less sexually dimorphic (meaning less difference in canines between males and females).
Apes have larger specialised teeth for grinding and chewing. Canines are used for defence and displays of aggression. They have a diastema.
Teeth Structure
Humans have smaller premolars and molars with a thin enamel coating.
Apes have larger premolars and molars, and have a thick enamel coating.
Need help with Biology? Contact Mrs. Eleanor Adviento for some feedback or tutoring.
Success Criteria
Your learning has been successful if you can do the following:
I can discuss the spine, pelvis, femur, and foot differences between a biped and a quadruped.
I can discuss the changes/trends to the hominin hand.
I can explain the difference between the power and precision grip.
I can discuss how bipedalism led to changes in the manipulative ability of the hand.
Vocabulary
Learn these so you can communicate this concept well.
Larynx (voice box): Helps to create different sounds when we speak.
Do Now:
Why doesn't this work?
Do Now:
What doesn't work for a bipedal dog?
Features of the Primate Skeleton
Comparing Skeleton Features of Apes and Humans
The Centre of Gravity (COG)
Maintaining balance is critical when walking on two legs! When you consider the walking cycle of a biped, the biped must balance on one leg while lifting the other foot off the ground and swinging it forward.
The centre of gravity (COG) of any object is the point at which there is equal mass on every side around it. In bipeds, this is near the hips. In quadrupeds, the COG is higher in the abdomen.
As the legs alternate swinging forward during the walking cycle, the COG shifts from one side of the pelvis to the other, making a pattern similar to a figure 8.
Backbone / Spine
In apes, the back bone is a single gentle curve (C shape). All the vertebrae are of a similar size as the spine carries the load equally.
In humans, the backbone is S shaped, this enables the weight of the body to be carried directly above the hip joints directly over the COG. This forward curvature coupled with the backward curvature in the middle of the spinal column allows the backbone to function as a spring, facilitating movement and taking the shock of jumping down or striding out. The vertebrae get larger towards the bottom as the load is heavier at the bottom in a vertical position.
Chest
In apes the chest cavity protrudes forward and is deeper – this accommodates arm use for locomotion particularly brachiation.
In humans, the chest is flattened from front to back so that the body weight in human is concentrated as close to the spine as possible. The barrel-shaped rib cage of bipeds permits effective use of the arms for non-locomotory functions.
The scapula (shoulder blade) in humans is shorter and broader with little ridges. This further ensures the centre of gravity is over the pelvis. Less ridges in humans on scapula as less need for attachment of muscles. The scapula shape in apes supports knuckle walking and brachiation.
Shoulder
Homo erectus has a different shoulder to us. Instead of placing the shoulder joint on the side of the rib cage, parallel to the neck, Homo erectus had a more forward-facing scapula with a forward-placed arm. This meant the transition from a high-shrug-like chimp shoulder to our modern shoulder was not a simple one-step process. See diagram below:
The forward-facing arm of Homo erectus was limited compared to our us. Homo erectus could not pull its arm back. Most detailed tool manufacturing and foraging does not require pulling the arm back very far, but there are a few tasks that can not be done without pulling your arm behind your body. Homo erectus could not throw well, and could not run well. Throwing and running both involve swinging your arm way behind your body.
In humans, the clavicle (collar bone) is longer, and the shoulder joint is on the back of the ribcage (instead of the side). Not only did this allow humans to pull their arms back to run, it also allowed humans to throw with power and accuracy, which turned humans into dangerous hunters.
Pelvis
In apes the pelvis is long and narrow, and has a box-like shape. In humans it is short and broad and is bowl shaped.
To accommodate the hip joints and muscles necessary for bipedalism, the pelvis of bipedal humans is lower and broader (bowl shape) than that of knuckle-walking apes.
Change in shape of pelvis in hominins means that spine and internal organs are directly above leg bones which puts the weight over the COG. Change in the shape of the pelvis means that muscle attachment point for main leg muscle - gluteus maximus is at the back in humans rather than on the side of the pelvis. This results in more efficient locomotion in that the stride is straight not side to side as in the ape.
The bowl shaped pelvis in humans results in the femur pointing inwards forming the valgus angle (humans are knock-kneed) and a change in the carrying angle for the femur. This results in more efficient locomotion as increases stability by making it easier to stand on one leg when walking, as movement is no longer “side to side” as in apes.
Lower Limbs
Bipedal humans not only have longer lower limbs than quadrupeds, the valgus angle (the angle that the femur makes with the midline of the body) is also different.
Longer lower limbs shift the center of mass towards the lower body. Angling the femurs inward moves the center of mass closer to the midline of the body. The altered center of mass allows stable bipedal locomotion and conservation of energy when standing.
Enlarged Joint Surfaces
Not only does the knee need to be restructured to accommodate the changed valgus angle, but joint surfaces must also be enlarged. This enlargement increases the contact area, helping the knee and other joints withstand the stress of standing or walking upright.
Foot (Heel, Arch, Toes)
In the apes, the big toe (hallux) diverges which facilitates tree climbing ability. Apes toes are longer and curved to facilitate better grip on the branch.
In humans, the big toe is elongated and faces forward in humans. This gives forward thrust necessary when walking on 2 legs and increases efficiency of bipedal walking allowing humans to walk considerable distances easily.
The human foot has two arches. The transverse arch of the foot also acts like a spring to take the shock of impact so walking long distances or endurance is possible, this helps to protect joints from damage when landing and striding out. Flat footed people find it painful to walk long distances.
The longitudinal arch stores forward momentum / energy and is used when pushing off to increase efficiency.
The calcaneus or heel bone is enlarged to take the impact of a “heel first” stride used in bipedalism.
Hand
The hand is the essential prerequisite for the manufacture of tools. The human hand differs from the apes in four ways:
The thumb is much more elongated and muscular.
The first metacarpal, which is the hand bone at the base of the thumb is connected to the wrist. This enables the thumb to touch the tip of every finger with ease.
The muscle that flexes the last joint of the thumb is separate to any other tendons allowing us to flex the last joint of the thumb independently.
The wrist is much more flexible, which improves our fine motor skills.
These adaptations allow a power grip, stone throwing, and precision grip, and writing.
Ape fingers are much longer and more curved which helps them with brachiating BUT reduce their fine motor skills.
The base of the thumb is joined to the wrist by a “saddle joint” – this enables the thumb to be brought acrosss the hand so that it can touch the tip of all four other fingers. The full opposability of the thumb with other fingers is much more developed in humans than in other primates
Humans have finer motor control and sensitive fingertips which in combination with precision grip enables them to manipulate small objects.
Human have much better manipulative ability in the hand than apes. Both apes and humans are capable of grasping objects (power grip) but humans can use the precision grip as well.
Power grip: Fingers and thumbs wrap around the object.
Precision grip: Fingers and thumb hold the object.
In apes – the fingers bones (phalanges) are more curved to allow for brachiation. The thumb is less mobile and is shorter. Apes do not have precision grip and are not able to accurately manipulate small objects. In humans the phalanges are straight
In humans - the thumb is considerably longer. Humans are capable of precision grip because of the fully opposable thumb. The thumb is able to touch the finger tips due to the saddle joint and this allows for the precision grip.