Defining Human (VCE SSCE Biology): Revision Notes

Defining Human

Understanding biological classification

Biologists classify organisms using a hierarchical system called biological taxonomy. This system organises living things into increasingly specific categories, helping us understand what makes each species unique.

Modern humans belong to the species Homo sapiens, which is the only surviving member of the tribe Hominini. By examining which taxonomic groups humans belong to, we can identify exactly which characteristics define us as human.

How taxonomic classification works

The classification system arranges organisms into categories called taxa (singular: taxon). These range from the broadest category (domain) at the top, down to the most specific (species) at the bottom. As you move down through these levels, you exclude more and more organisms until only members of a single species remain.

The main taxonomic levels for humans are:

• Kingdom: Animalia (animals)
• Phylum: Chordata (chordates)
• Subphylum: Vertebrata (vertebrates)
• Class: Mammalia (mammals)
• Order: Primates
• Family: Hominidae
• Genus: Homo
• Species: sapiens

Mammals

Humans are mammals, placing us in the class Mammalia alongside approximately 5,400 other species. This includes familiar animals like cats, dogs, sheep, horses, whales, and bears.

Key characteristics of mammals

All mammals share five defining features:

1. Three bones in the middle ear - These small bones (the malleus, incus, and stapes) aid in hearing by transmitting sound vibrations from the eardrum to the inner ear.
2. Hair or fur - Mammals have varying amounts of hair or fur on their bodies at some point during their lifetime. This provides insulation and protection.
3. Single lower jawbone - Mammals possess one single lower jawbone that attaches directly to the skull, creating a powerful bite.
4. Mammary glands - These specialised glands produce milk to feed offspring, giving the class its name.
5. Variety of teeth - Mammals have different types of teeth for different purposes:
   • Incisors bite and pierce food
   • Canines grip and tear food
   • Premolars and molars grind food down

Primates

Within the class Mammalia, humans belong to the order Primates. This order includes around 400 different species such as orangutans, lemurs, gorillas, baboons, and chimpanzees.

Primates are often called "the highest order of mammals" because they have:

• Large brains relative to body size
• Increased reliance on visual information to find food and avoid predators
• Complex social behaviours

Key characteristics of primates

Primates share several distinctive features:

1. Prehensile hands and feet - Each hand and foot has five digits, with at least one opposable digit that can touch the other digits on the same appendage. This allows primates to grasp objects with power and precision.
2. Sensitive touch receptors - Primates have a large number of sensitive touch receptors in their fingertips, helping them use their hands to gather information about their environment.
3. Flexible spines and joints - Primates have flexible spines and a large degree of rotation around their hips and shoulders. This flexibility allows many primates to live successfully in trees.
4. Large cranium - Primates have a large skull relative to their body weight, housing their relatively large brains.
5. 3D colour vision - Forward-facing binocular eyes give primates three-dimensional colour vision, allowing them to judge distances accurately and better locate objects within their visual field.

Precision grip vs power grip

One defining feature of primates is the presence of an opposable digit, which enables two distinct types of gripping:

Humans have highly developed precision grips due to our hand and wrist structure. Our wrists are specialised for throwing and clubbing, whilst other primates have wrists adapted for suspensory behaviours like hanging from trees and swinging from branches.

Other apes, such as chimpanzees, have relatively shorter thumbs and longer fingers, resulting in a weaker precision grip. To compensate, they have much stronger power grips, which are particularly useful for tree climbing.

Hominoids

Within the order Primates, humans belong to the superfamily Hominoidea. Members of this group are called hominoids or apes.

The superfamily Hominoidea includes:

• Great apes: orangutans, chimpanzees, gorillas, and humans
• Lesser apes: many different species of gibbons

Key characteristics of hominoids

Hominoids possess several features that distinguish them from other primates. To illustrate these differences, we can compare a hominoid (gorilla) with an Old World monkey (baboon).

1. Y5-shaped molar teeth - Hominoids have distinctive molar teeth in the lower jaw with five cusps arranged in a characteristic Y5 pattern. This is visible when you look at the chewing surface of the tooth.
2. No tail - Unlike many other primates, hominoids lack a tail. This contributes to their ability to sit upright comfortably.
3. Shorter spine - Hominoids have a shorter spine between the rib cage and pelvis compared to other primates. This helps them maintain an upright sitting posture.
4. Broader rib cage and pelvis - The rib cage and pelvis are broader in hominoids, creating a wide, stable chest that supports upright sitting.
5. Longer arms than legs - Most hominoids (except Homo sapiens) have longer arms than legs. Their shoulder blades sit further back on the body, and their shoulder joints allow the arm to swing back behind the head. This permits movements like an overarm throwing motion and is useful for swinging through trees.
6. Increased cranium size - Hominoid brains tend to be larger and more complex than those of other primates.

Hominins

Finally, we reach the tribe Hominini - the taxonomic group that includes humans and our most recent ancestors. Members of this tribe are called hominins.

The tribe Hominini includes:

• All members of the genus Homo (of which modern humans are the only living members)
• Some of our more distant upright-walking ancestors

Bipedalism: the defining feature

Bipedalism: using two legs for walking upright.

This distinguishes us from our closest primate relatives (chimpanzees and gorillas), who typically walk on four limbs with their knuckles on the ground.

Structural consequences of bipedalism

Becoming bipedal resulted in numerous structural changes that distinguish hominins from other primates. The following comparison between a gorilla and human skeleton highlights these differences:

Skull features:

1. Foramen magnum position - The foramen magnum is more centrally located in human skulls. This allows the head to sit forward whilst resting on top of the spinal column. In gorillas, the foramen magnum is positioned closer to the back of the skull.
2. Smaller canine teeth - Humans have much smaller canine teeth than gorillas. This change relates to differences in diet rather than bipedalism.
3. Reduced brow ridge - Humans have a much smaller brow ridge (the bony ridge above the eye sockets) compared to gorillas.
4. Absence of sagittal crest - Gorillas have a pronounced sagittal crest on top of their skull where jaw muscles attach. This crest is absent in humans. A large sagittal crest indicates powerful jaw muscles.

Spine and torso:

1. S-shaped spine curve - Gorillas have a C-shaped spine that curves forward. Human spines are S-shaped, with curves in both the lower and upper spine. These curves help support weight vertically and aid upright walking.
2. Barrel-shaped rib cage - Human rib cages are more barrel-shaped, whilst gorillas have funnel-shaped rib cages. This helps humans maintain an upright posture for extended periods.

Pelvis and limbs:

1. Bowl-shaped pelvis - Human pelvises are shallow and bowl-shaped, whilst other primates have vertically long, narrow pelvises. The bowl shape provides support for the upper body during standing and walking.

2. Greater femur angle - Humans have a relatively large femur angle (the angle between the top and bottom of the femur when standing). This increases stability during upright walking by ensuring the knee and foot are more centrally positioned below the body.

3. Modified hand structure - Human hands have shorter, straighter fingers and longer thumbs compared to gorillas, enabling a more refined precision grip.

4. Non-prehensile feet - The human foot no longer has grasping capabilities. The big toe is aligned with the other toes rather than being opposable. Human feet also have two arches and a wide heel, making bipedalism more energy-efficient and reducing impact on the foot.

   

Non-structural differences

Beyond physical structure, hominins possess other distinctive traits:

• Complex communication - Humans have the ability to speak and communicate using sophisticated language
• Large social groups - Hominins form large, complex societies with intricate social relationships
• Enhanced cognitive abilities - Humans can think about abstract concepts, create language, and develop tools
• Cultural transmission - Knowledge and innovations are passed from generation to generation through teaching and learning

Summary of characteristics

The following table summarises the defining characteristics of each taxonomic group humans belong to:

Classification	Characteristics
Mammals	• Three middle ear bones • Single lower jaw bone • Mammary glands • Variety of teeth • Hair/fur
Primates	• Flexible spines and hip rotation • Prehensile hands and/or feet • Binocular colour 3D vision • Opposable thumb/big toe • Sensitive touch receptors
Hominoids	• Y5-shaped molar teeth • Broad rib cage • Large cranium • Long arms • No tail
Hominins	• Bipedalism • Structural consequences of bipedalism (centralised foramen magnum, S-shaped spine, broader rib cage, bowl-shaped pelvis, increased femur angle) • Communication and formation of complex social groups

Complete taxonomic classification of modern humans

The complete classification pathway for modern humans is:

• Kingdom: Animalia
• Phylum: Chordata
• Class: Mammalia
• Order: Primates
• Superfamily: Hominoidea
• Family: Hominidae
• Tribe: Hominini
• Genus: Homo
• Species: sapiens

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