Living Organisms

Biodiversity is the enormous variety of organisms is termed biodiversity (bios means life and diversity means variety). There is diversity in size among organisms and in complexity. For example, bacteria are simple single celled organisms and humans are made of a trillion cells and are highly complex.

All organisms have come to exist on earth because of evolution and are related through ancestry.

Biosphere and Ecosystem

Organisms live up to 8 km in air and up to 5 km below sea level. This part of the earth which supports life is the biosphere. Biosphere has diverse ecosystems such as the pond, river, oceans, and mountains, deserts, etc. Various kinds of organisms or different species live in these ecosystems. They interact with each other and also interact with the physical components of the ecosystem such as light, temperature, etc.


Biosphere is the physical part of the earth on which organisms can survive.


Ecosystem is definite geographical region in which various species of organisms live and interact with each other and the physical environment.


Group of very similar organisms which can interbreed to produce fertile offspring.

Levels of Biodiversity

All the varieties of living organisms on earth constitute biodiversity. Three levels of biodiversity have been recognized:

  1. Ecological / Ecosystem diversity
  2. Species diversity
  3. Genetic diversity

Ecological/Ecosystem diversity

Organisms evolved features which helped them adapt to their surroundings or the ecosystems in which they live. For example tortoises are terrestrial and turtles are aquatic. There is diversity of ecosystems - terrestrial ecosystems include forests, plains, deserts and mountains and aquatic ecosystems are sea, river, pond, etc. Organisms living in these have evolved suitable adaptations.

Species diversity

Variety of species living in a certain geographical area constitutes species diversity.

Genetic diversity

Organisms are made of cells and cells in their nuclei contain chromosomes which bear the genes. Genes control the features of a particular species. Every species has a gene pool. Gene pool means all the different kinds of genes found in a species. The gene pool of a species differs from that of another species.

Classification of Organisms

Grouping of organism according to similarities and differences is termed classification. When an organism is classified into various categories a hierarchy is maintained. Accordingly, an organism belongs to Kingdom, Phylum, Class, Order, Family, Genus and Species in hierarchical order.

Classification shows evolutionary relationships between organisms and is also termed Systematics. The science of classification or systematics is termed Taxonomy.

Five Kingdoms of Life

Earlier there were only 2 kingdoms of plants and animals. Whittaker in 1969 suggested that bacteria should not be in plant kingdom and protozoa not in animal kingdom. He gave the five kingdom classification. 

  1. Monera: Single celled, no well formed nucleus (Prokaryotes)
  2. Protoctista (Protista): Single celled with well formed nucleus (Eukaryotes)
  3. Fungi: Eukaryotes, multi-celled, saprotrophs
  4. Plantae: Eukaryotes, multi-celled, autotrophs
  5. Animalia: Eukaryotes, multi-celled, heterotrophs

Scientific Name of Organism

Every organism has a scientific name beside the name by which it is known in a particular language. For example, mango is its name in English, Aam in Hindi and Mangifera indica, its scientific name. In scientific naming, genus and species of the organism are mentioned. For example, Homo sapiens.

Scientific name consists of two words, name of the Genus to which it belongs begins with a capital letter and name of the species to which it belongs, begins with a small letter. For example, cat is Felis domestica where Felis is the genus name and domestica the name of the species.

Classification of Plants

Kingdom Plantae is divided into the following divisions:

  • Algae - No special roots, stem or leaves e.g. the algae
  • Bryophyta - Root like hyphae but no true roots e.g. Liverworts and mosses
  • Pteridophyta - True roots, underground stem, spore bearing leaves e.g. Ferns
  • Spermatophyta - Seed bearing plants

Kingdom Plantae has five divisions namely Algae, Bryophyta, Pteridophyta, and Spermatophyta. Spermatophyta further divided into Gymnospermia and Angiospermia or flowering plants.

Classification of Animalia

Kingdom Animalia is grouped into non chordates which are further divided into the phyla Porifera, Cnidaria, Platyheminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, and Echinodermata.

The chordates which have notochord at some stage of life form a single phylum Chordate. Chordate vertebrates are divided into the classes Chondrichthyes (Cartilaginous fish) Osteichthyes (bony fish) Amphibia (Frog, salamander), Reptilia (Lizards, snakes, etc.) Aves (Birds) and Mammalia (rats, tigers, horses, humans).

History of Life

The universe around us is so enormous that it is difficult even to imagine its dimensions. In a far corner of the Milky Way Galaxy, (one of the billions of galaxies comprising the universe), sits our solar system Within this system, Earth, the planet on which we live, is one of the planets revolving around the Sun.

The physical conditions on primitive earth were not congenial for life. The earth was extremely hot - a ball of hot gases.

Formation of Earth

The whole universe formed probably 12 to 14 billion years ago as a result of a Big Bang and subsequent expansion. Our solar system came into existence 5-7 GY ago (Giga Year). In its initial stages of formation (4.5 GY), the earth was impacted by another planet that caused the spin (that gave us day and night) and tilt (that gave us seasons) of our planet and also led to the formation of moon. For nearly 700 million years (up to 3.8 GY ago), earth experienced frequent and catastrophic bombardment by meteorites of different sizes.

Gradually earth’s crust solidified although volcanoes kept on spewing out harmful gases. These gases accumulated and combined to form methane, ammonia and hydrogen cyanide. These three gases along with minor gases like carbon dioxide and carbon monoxide, formed the atmosphere of the primitive earth. There was no oxygen then, a gas so essential for nearly all living organisms.

Origin of Life

There is a general belief that life on earth must have originated, not before 4.0 GY and no later than 3.5 GY ago. Some fossils (remains of living beings that once existed on earth) claimed to be cyanobacteria (blue green algae) were found in Australia from rocks dated 3.5 GY.

Regardless of where life had begun, how life emerged is still a mystery. Even if we assemble all the organic compounds essential for life we cannot produce from them a living organism capable of growing, reproducing and, storing and passing on a hereditary map to its offspring.

Diversification of Life

Life on earth started in the form of simplest unicellular (prokaryotic) microorganisms. In course of time, these organisms evolved to utilize solar energy through chemical process called photosynthesis. It is through the photosynthetic activity of the earliest autotrophs that oxygen built up gradually in the earth’s atmosphere making it possible for complex heterotrophic organisms to evolve.

For a very long time (nearly 3 GY) after the origin of life, earth had no life forms other than prokaryotes (cells lacking nucleus) comprising different groups of bacteria. There were neither plants nor animals. Eukaryotes (cells with nucleus) probably appeared about a billion years ago, but life was mostly in the form of unicellular (single celled) organisms.

Then suddenly, about 600 million years ago, in a geological period called Cambrian, there was a great, almost explosive, diversification of life into multicellular organisms with a variety of body plans and life styles. Biologists call this period the Cambrian explosion.

Fossils, the remains of plants, animals and lower living beings provide evidence for the sequence in which different kinds of living organisms came to exist on the earth.

Darwin’s Theory of Evolution

Darwin made important observations and drew inferences from them, which helped him in developing his theory of evolution.

The commonness of many features from anatomical to molecular is a clear indication that all organisms evolved from a common ancestor. Darwin concluded that living forms were not created but evolved by descent with modification from ancestral forms going back all the way to the earliest life more than 3.5 billion years ago.

Darwin suggested two very important points with regard to evolution.

  1. All living beings are related through ancestry.
  2. The mechanism which causes diversification of species from ancestors is Natural Selection.

Since most offspring do not survive, all organisms must be going through a struggle for survival, being eaten, suffering from disease and competition. The struggle for existence cause large number of individuals to die.

The ones who have characteristics that allow them to survive and reproduce better (i.e. possess most useful adaptations for surviving in the environment) will pass on these characteristics to their offspring. In other words, Nature selects the fittest individuals of the population. Natural Selection is the same as the famous phrase "survival of the fittest".

Therefore, the best adapted individuals were selected by nature to survive and leave offspring for the next generation. Darwin called this mechanism Natural Selection.


Incorporation of Mendelian genetics into Darwinian theory later by evolutionary biologists led to the emergence of Neo-Darwinism. Further refinements in the light of advances in population genetics and other areas of biology led to the modern synthetic theory of evolution.

The unit of evolution, in the modern synthetic theory of evolution, is the population. It is the population which evolves and not the individual. Variation occurs at the genetic level through mutation and sexual reproduction in the "gene pool" of the population (gene pool means all the different genes in a population of individuals). Natural Selection causes greater reproduction of the variant genes having adaptive advantage.

Evolution at the level of the hereditary material or genes that is the gene pool of a population is termed microevolution. Populations of a species differ due to microevolution. Macroevolution or adaptive radiation is the evolution and diversification at the level of species and genera.

Human Evolution

When human evolution began, forests had dwindled because of glaciation (ice age). Much of the land surface was however, still covered by forests. The common ancestors of apes and humans had to come down from trees where they lived and walk on the ground using all four limbs.

Recent molecular studies have shown that from common ancestors, evolution of apes (Chimpanzee, gorilla, gibbon and orangutan) and that of humans, diverged about 6 million years ago. 

The trends of human evolution are towards

  • bipedal gait or walking on two legs
  • acquiring a large brain

Fossil history reveals that human evolution began approximately 1.5 to 2 million years ago. Australopithecus is deemed to be the first human like ancestor. Fossils of an australopithecine named "Lucy" has been found in African rock deposits. Thereafter, fossils of Homo erectus which walked on two legs, were unearthed from many parts of the world.

Next to evolve was Neanderthal man and Cromagnon man. They were both Homo sapiens. Modern man, Homo sapiens sapiens meaning the wise one evolved about 50,000 years ago.