Human Classification

How humans are classified in the tree of life


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Humans and the Tree of Life

The most controversial part of the Theory of Evolution is the idea that humans are part of the story. As a species, humans must have evolved and must have a series of ancestral species.

Before we can answer questions about the evolution of humans we need to see what relationship humans have to the rest of life on Earth. In other words how are we classified as a species? How do we fit in to the Tree of Life?

Domain

There are four Domains of life. We belong to the Eukarya.

These are organisms that have a membrane separating the cell nucleus from the rest of the cell.

The presence of a cell nucleus keeps the genetic material separate from the chemical processing part of the cell. Cells with a nucleus can replicate in two ways giving rise to asexual and sexual reproduction. Eukaryotic cells also contain other membranes which separate other structures within the cell. The cells are compartmentalised.

All multi-cellular organisms are Eukaryotes but not all Eukaryotes are multi-cellular.

The other three domains of life are bacteria (which includes most pathogens), archaea (very ancient life forms living mainly in extreme environments and often having unique chemical processes) and viruses (crystalline life forms that can only reproduce within another cell).

The Eukarya have the fewest species but include the largest organisms. In a human body, for example, there are more about ten times more bacteria present than eukaryotic cells.

Kingdom

There are four Kingdoms of Eukarya. We belong to the kingdom of the Animals.

These are multi-cellular organisms whose cell walls are not strengthened.

Animals cannot generate their own energy. They must consume other living organisms to obtain energy plus the materials to build their own cells. Most animals can move and most have sense organs. Apart from sponges, all have differentiated cells forming different structures (e.g. muscle, skin, nerves).

The other three kingdoms are plants whose cell walls are strengthened with cellulose and which are capable of using sunlight to generate energy, fungi whose cell walls are strengthened with chitin, and protista, single celled organisms.

Phylum

The Animal Kingdom is divided into 25 or so Phyla (plural of Phylum). We belong to the Chordates.

Chordates have a dorsal notochord. This is a batch of nerves that runs along the back of the animal. Chordates are also symmetrical along an axis that includes the notochord. At some stage during development all chordates have a post-anal tail.

There are over 75,000 species of chordates.

Other phyla of animals include molluscs (snails, clams, squid and octopus), arthropods (the largest phylum which includes arachnids (like spiders), insects and crustaceans), segmented worms (like the earthworm) and echinoderms (starfish and other animals with five-fold symmetry).

Sub-Phylum

Within the chordates, we belong to the Sub-Phylum of Vertebrates where the notochord is surrounded by a series of bones called vertebrae. The structure is known as a backbone and the common name of the sub-phylum is the "backboned animals".

Vertebrates are the only chordates to possess a proper brain. The brain is located at the front end along with other sense organs. The nervous system runs from the brain along the back with branches occurring in pairs. This system is unique to vertebrates.

There are over 64,000 species of vertebrates.

There are two other sub-phyla of chordates; the tunicates (sea squirts) and lancelets (fish shaped animals that lack brains). These chordates have the notochord along the back but no bones around it.

Class

Vertebrates are divided into seven Classes. We belong to the class called Mammals.

Mammals are vertebrates with body hair. They can control their body temperature (i.e. they are "warm blooded"). They provide nourishment for their young in the form of milk secreted from the female body - these "mammalian glands" give the class its name. Apart from five species, all mammals have nipples from where the milk is secreted.

Mammals have one bone in their lower jaws. Other vertebrates have three bones in the jaw and one in the inner ear. In all mammals two of these jaw bones are now found in the inner ear making a total of three.

Another feature of mammals is a four chambered heart and the separation of oxygenated and non-oxygenated blood.

Mammals also have a region in the brain called the neocortex which controls higher functions like spatial reasoning and conscious thoughts.

All mammals are air breathing (using lungs), even if they live entirely in the ocean.

There are 5,338 recorded species of mammals.

The six other classes of vertebrates are jawless fish (like the lamprey - over 100 species), cartilaginous fish (sharks and rays - over 1,000 species), bony fish (perch, goldfish, carp, etc - this is the largest vertebrate class with over 30,000 species), amphibians (like frogs and salamanders - about 6,000 species), reptiles (snakes, lizards, crocodiles - over 9,000 species) and birds (over 10,000 species).

Sub-Class

Within the mammals we belong to the Sub-Class of Eutheria.

These are the majority of mammals. They give birth to live young that has been nourished by a placenta attached to the mother. All eutheria have a belly button left over from the attachment of the placenta. Eutheria are also called the "placental mammals".

All eutheria are missing epipubic bones that other mammals have. These bones help in locomotion but would hinder pregnancy as the abdomen needs to expand.

The other sub-classes of mammals are the marsupials (the pouched mammals like kangaroos, koalas and oppossums) and the monotremes (egg laying mammals which secrete milk not from nipples but from their skin - only two types are known: platypus and echidna).

Order

The Eutheria (Placental Mammals) are divided into 29 Orders. We belong to the order Primates.

Primates have forward facing eyes giving stereoscopic vision. Primates also see in colour. The sense of smell, important to most other mammals, is less important in primates.

They have nails instead of claws. The majority have an opposable thumb. The brain size of primates is large for the body weight. Many of these adaptations are due to living in a three dimensional tree environment.

There are 437 species of primates although several new ones have been discovered since the year 2000.

Other orders of mammals include rodenta (rats, squirrels, beavers, etc), lagomorphs (hares, rabbits, pikas), pilosa (sloths and ant eaters), soricomorpha (shrews and moles), artiodactyla (the even-toed hoofed animals like pigs, hippopotamuses, camelids, antelopes, cattle, sheep, goats), perissodactyla (odd toed hoofed animals: horses, zebras, tapirs, and rhinoceroses), cetacea (dolphins, whales and porpoises), sirenia (the so-called sea cows like dugongs and manatees), cingulata (armadillos), proboscidea (elephants), chiroptera (bats) and carnivora (cats, dogs, bears, seals, sea lions, etc).

Sub-Order

There are two Sub-Orders of primates. We belong to the Haplorhines (the "dry nosed primates").

This sub-order includes monkeys, apes and humans.

The Haplorhines all lack the ability to manufacture Vitamin C which most other mammals possess. They all have the same four genes that all mammals use for the manufacture of Vitamin C but the fourth gene has mutated and no longer works. All Haplorhines share the same mutation.

The upper lip is not connected to the nose allowing for a wide range of facial expressions. The primary sense is vision and the brain is larger even when compared to other primates.

The young have a long gestation period and are born relatively large but are dependent on the mother for longer than other animals.

There are 308 species of Haplorhines.

The other sub-order is the Strepsirrhins (the "wet nosed primates" which include the lemurs, lorises and the aye-aye).

Family

Haplorhines are divided into nine Families. We belong to the family Hominidae, also called the Great Apes.

The Hominidae consist of just seven species including Humans. The seven species are arranged into four Genera:

All Hominidae (Great Apes) share more than 95% of their DNA gene sequences among themselves.

Hominidae are large tail-less primates. The male is normally larger and stronger than the female (apologies to my female readers). They have hands which can be used for food gathering and the manipulation of tools. The Great Apes are omnivorous. They also show cultural differences between different groups.

Gestation lasts 8 to 9 months, resulting in the birth of a single offspring or occasionally twins. The young are born helpless and must be cared for. Compared with most other mammals, Great Apes are nor weaned for several years. They do not become fully mature for between 8 to 13 years. Females typically give birth only once every few years. There is no distinct breeding season.

Other families of Haplorhines include hylobatidae (the lesser apes, like gibbons), platyrrhines (New World monkeys) and catarrhines (Old World monkeys).

Genus

The Genus (singular of Genera) of Homo is about 2.4 million years old. It includes humans and their extinct relatives.

Homo is characterised by the following properties:

The only surviving species of this genus is our own species: Homo sapiens.

Humans are not in any way different from other life forms on Earth. They fall into place in the Tree of Life. We are a type of primate with a large brain that walks on two legs. Our closest relatives are the chimpanzee, gorilla and orangutan.

Physically, there is no structure in the human body that is not found in other hominidae. All differences are of degree rather than kind.

 Taxonomic Level   Human Classification   Common Name   Age
(Millions of Years) 
 Number of Species 
Domain Eukarya Cells with nucleus 2,100 8.7 million
Kingdom Animals Unstreangthened cells 590 2 million
Phylum Chordates Animals with dorsal notochord 530 75,000
Sub-Phylum Vertebrates Backboned animals 505 64,000
Class Mammals Milk producing animals 220 5,338
Sub-Class Eutheria Placental mammals 160 5,000
Order Primates Forward facing eyes and nails 75 437
Sub-Order Haplorhines Dry nosed primates 40 308
Family Hominidae Great apes 15 7
Genus Homo Humans 2.5 1
Species Homo sapiens Modern humans 0.5 1

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