Life: Definition, Characteristic, and Examples

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Life Definition

The ability of a living creature to distinguish itself from a dead organism or a non-living entity, as evidenced by its ability to grow, metabolise, respond (to stimuli), adapt, and reproduce. The flora and fauna of a certain area.

What is Life?

When it comes to the topic of when life begins, there is no universal agreement. The origin of life is likewise a subject of debate. Despite the ambiguous answers to concerns regarding life, the following are the essential qualities of a living thing:

Organization: Living things have a well-organized structure that allows them to carry out a certain purpose. A living item, in particular, is made up of a single or a group of cells (s). Any organism’s basic structural and functional unit is the cell. A living form would be able to maintain its existence by controlling its internal environment to maintain a constant or favourable condition, for example.

Metabolism: Through anabolic processes, a living creature would be able to transfer energy from molecules into cellular components. It would also be capable of catabolism, which would allow it to decompose organic materials.

Growth: is the process through which a living thing expands in size or quantity.

Response: An organism’s capacity to respond to stimuli or its environment, generally through a sequence of metabolic processes, is known as response.

Reproduction: The ability to reproduce, or the ability to create a new of its kind, is one of life’s characteristics.

Adaptation: An organism has the ability to change throughout time in order to adapt to its surroundings.

Evolutionary History of Life

Evolution is essential in biology because it promotes biodiversity. Over time, certain qualities will become more prevalent, while others will become rare. Life may not be as we know it if evolution does not occur. It will be less varied than it is now.

The Earth undergoes a sequence of transformations. At one time, the Earth was a livable world. The Earth’s primordial state was inhospitable to life. Life was thought to have begun around one billion years after the Earth initially came into existence. All living organisms are thought to have descended from RNA-based, self-replicating creatures. These living forms developed into single-celled creatures over a long period of time. Then emerged multicellular creatures. About 600 million years ago, they initially appeared.

Several major extinctions may be found in between the bursts of life throughout the history of life during distinct geologic epochs. For example, the Earth had a supercontinent named Pangaea surrounded by the Panthalassa ocean during the Permian epoch of the Paleozoic era. The interior became extremely dry and arid as a result of this. Reptiles thrived as a result of their ability to thrive in such environments. Dimetrodon is a reptile group that developed into therapsids. Therapsids developed into cynodonts, which were the first animal predecessors. Archosaurs, the first dinosaur ancestors, arose at this time.

According to legend, a catastrophic extinction known as “the Great Dying” happened, wiping out around 90% of life on Earth. The next epoch (the Mesozoic epoch) is known as “the Age of the Dinosaurs.” These creatures ruled the Earth’s land, oceans, and atmosphere. However, a catastrophic extinction occurred, resulting in the loss of dinosaurs and other big creatures. Mammals, on the other hand, seized the opportunity and expanded.

Evolution is necessary for life to survive on an ever-changing planet. Adaptability, both genetically and phenotypically, is required of organisms. It’s also possible that forming symbiotic partnerships with other creatures might help you survive and grow. Speciation happened in tandem with evolution. Throughout evolution, species have been divided into two or more descendent species. Unfortunately, most of the creatures that formerly thrived on Earth have already perished. Almost all of the Earth’s species have become extinct. These creatures died, and their species vanished completely. As a result, it appears that the extinction of species is unavoidable.

Last Universal Common Ancestor (LUCA)

The evolutionary connections between species are depicted in an evolutionary tree diagram. The classification is based on genetic and physical features that are similar and different. The branching pattern depicts how species or things descend from a common ancestor. Following the path of development of all living creatures on Earth would eventually lead to LUCA, the common ancestor (last universal common ancestor). The putative progenitor of all living creatures, LUCA, is thought to have appeared between 3.5 and 3.8 billion years ago.

There is still no consensus on how life began on Earth. Many others, on the other hand, thought that RNA-based, self-replicating organisms were the ancestor of all living beings. Single-celled creatures with cytoplasmic structures but no internal compartmentalization developed from these entities. Prokaryotes are single-celled creatures that lack membrane-bound organelles.

Endosymbiotic Theory

The prokaryotes appeared first, followed by the eukaryotes. They were able to withstand the Earth’s initial harsh environment. Around 1.6 to 2.7 billion years ago, single-celled eukaryotes emerged. The endosymbiotic theory suggests that bigger organisms, such as bacteria and cyanobacteria, take in smaller cells for a cooperative relationship (endosymbiosis). They went through coevolution as a group. Smaller prokaryotes eventually developed into semi-autonomous organelles. Bacteria developed into mitochondria, whereas cyanobacteria developed into chloroplasts. Eukaryotes emerged as a result of the existence of membrane-bound organelles inside the cell.


The earliest multicellular organisms appeared during the Neoproterozoic era, namely during the Ediacaran epoch (about 600 million years ago). Until now, the origins of multicellularity have remained a mystery. In this sense, Haeckel’s hypothesis is the most popular. Multicellularity, according to his Gastraea Theory, arose when cells of the same species gathered in a blastula-like colony, and certain cells in the colony underwent cell differentiation over time. Based on Ediacaran biota fossils, sponge-like creatures emerged during this time period. They were thought to be the earliest creatures.

Cambrian Explosion

The Paleozoic era follows, with geologic eras ranging from the Cambrian to the Permian, each marked by important evolutionary events. A rapid explosion of life happened during the Cambrian epoch (about 541 million years ago). The Cambrian explosion was the name given to this geologic event. Plants and animals of all kinds arose. Plants and fungus began to colonise the land. Arthropods and other creatures came ashore soon after, most likely to mate and lay eggs.

Rise of Invertebrates

Invertebrates were the main creatures throughout the Ordovician epoch (485 to 440 million years ago). Primitive fish continued to evolve, and a mass evolution of fish occurred in the Silurian geologic epoch. Arachnids and arthropods began to occupy the land, not simply venture it, during the Silurian (440 to 415 million years ago). Internal gas exchange systems, waterproof exterior layers, skeletal structures (endo-or exoskeletons), and a method of reproduction that did not require water were developed, making life on land a possibility.

Age of Fish

The Age of the Fish refers to the Devonian period (415 to 360 million years ago). The fish has become the most common marine vertebrate. Plants developed on land, creating new habitats in the form of primitive plants, trees, and shrub-like forests. Animals developed and diversified in tandem with the emergence of terrestrial plants. The earliest tetrapods to arise were amphibians. They first appeared around 364 million years ago.

Emergence of Amniotes

A significant evolutionary event happened during the Carboniferous epoch (360 to 300 million years ago). Amniotic eggs-laying tetrapods appeared. Tetrapod amniotes were able to migrate further away from the waterside and therefore dominate further inland by depositing amniotic eggs in a drier environment. As a result, towards the end of the era, these early amniotes had become very diverse.

Permian Reptiles

Reptiles and synapsids flourished throughout the Permian epoch (300 to 250 million years ago). Soon after, a significant evolutionary event took place, resulting in the appearance of beast-faced therapsids. The cynodonts evolved from these therapsids (the early ancestors of mammals). In the Permian epoch, the first archosaurs (early dinosaur ancestors) emerged.

Age of the Dinosaurs

The Mesozoic period (252 to 66 million years ago) follows the Paleozoic era and is known as “the Age of the Dinosaurs.” Dinosaurs inhabited the Earth and ruled it. However, there was a huge extinction catastrophe. By the end of the period, they had died along with the other huge animals (weighing more than 25 kg).

New Life

The next epoch, the Cenozoic (66 million years ago to the present), is known as the “New Life.” Mammals have grown in number and diversity. The evolution of giant apes led to the evolution of hominids, which led to the emergence of the Homo species. Homo sapiens is the sole living member of the genus Homo (anatomically modern humans).

Life Citations


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