What are life forms?

Life forms are the living entities or creatures present on Earth. They include all organisms ranging from tiny unicellular single-celled organisms such as bacteria, archaea, fungi, to giant organisms such as plants and animals.

The origin of life on Earth 

Earth evolved around 4.5 billion years ago and scientists believe that life formation started simultaneously.  According to scientists, liquid water and carbon are essential for life to start and thrive. They are the source of energy.  These sources of energy can be found in many places throughout the solar system, including planets, moons, and even comets, but only for a limited time. For life to emerge, it is a must-have hospitable environment. Some scientists also believe in extra-terrestrial life forms, or there might be a chance of impending discovery of alien life.

The atmosphere of Earth consisted of moisture and a few gases at first, and it is a common disposition that sunlight aided in the evolution of living forms. Earth, as a planet, lacked oxygen, an essential gas for the survival of life in its atmosphere. The production of a stable atmosphere containing gases such as oxygen arose from the breaking and forming of chemical bonds, and life began on Earth some 3.7 billion years ago. According to the hypothesis, the 'big bang' resulted in the formation of organic molecules from inorganic chemicals from the atmosphere. Those inorganic chemicals were not present in any other components of the solar system. The fossil record on Earth provides an opportunity to see what geochemical signatures are produced by microbial life, and how these signatures are preserved over geologic time. These possible bio-signatures include microfossils and stromatolites, molecular structures (biomarkers), and isotopic compositions.

All life forms are made up of some core elements that are required for the biochemical process to occur. These elements form the basics of a cell. These elements are carbon, nitrogen, hydrogen, oxygen, sulfur, and phosphorous. Together, they contribute to the formation of essential components such as nucleic acids, proteins, and lipids. All these elements are required for the development of DNA molecules. Among these, carbon is the most abundant element, as it can form infinite carbon-based organic molecules.

All the organisms initially started their life from a single cell, also known as a prokaryotic cell. There are two types of cells in general. Prokaryotic cells lack a distinct nucleus and specialized cell organelles. Eukaryotic cells are characterized by distinct nuclei and specialized cell organelles. All the mentioned elements combine and form macromolecules and organic compounds, which then form a cell structure. The earliest claimed microorganisms are fossilized microorganisms, believed to be found in iron and silica-rich rocks.

Different life forms during the different era(s)

The history of the Earth is divided into four periods, marked by the appearance and extinction of different species.

Precambrian era

The Precambrian era is not considered an actual era due to the lack of diversity in that period, but it may consist of some pieces of evidence required for the evolution of life. It started at the beginning of the Earth, around 4.6 billion years ago. Life did not come into existence until the end of Precambrian time. Toward the end of that era, there was life in the oceans, such as Jellyfish. However, no life forms occurred on the land due to insufficient oxygen, which would not fulfill the necessity of high-order animals. It is believed that bacteria were the ones that were present since the early Precambrian time. The bacteria were discovered in the rock dates of the Devonian period.

Paleozoic era

This era began with the Cambrian explosion. It was characterized by a relatively rapid period of speciation, a large number of species moved from water to land. Plants were the first to move on land, followed by invertebrates and vertebrates. Many new species appeared and thrived in the land environment. Different land plants and many vertebrates were acknowledged during this time.

The largest mass extinctions in the history of life on Earth came with the end of the Paleozoic era, which wiped out 95% of marine life and nearly 70% of life on land. The most likely cause of this mass extinction was climate change. The continents all drifted together to form Pangaea, and this mass extinction and devastation paved the way for new species to arise and a new era to begin.

Mesozoic era

This era is known as the age of dinosaurs. With the Permian extinction, many species went extinct and new species came into existence. During this era, many small mammals, herbivores thrived. Birds evolved from dinosaurs. With another mass extinction on its way, all the dinosaurs and many animals especially herbivores got wiped off and new species got their way onto the Earth.

Cenozoic era

With the extinction of Dinosaurs, the small mammals survived, grew, and became dominant on Earth. The climate changed drastically over a relatively short period, becoming much cooler and drier than during the Mesozoic Era.

All species of life including humans evolved into their present-day forms throughout this era, which has not ended till now; and most likely, it will not end until another mass extinction occurs.

From unicellular to multicellular

Both archaea and bacteria are unicellular organisms that fall under the category of prokaryotes. Archaea are believed to be the most primitive organisms. These are unicellular microorganisms and can adjust to extreme weather environments. These are morphologically similar to bacteria, but they metabolically resemble eukaryotes. Archaea have been found in a broad range of habitats such as hot springs and salt lakes.

Bacteria are the oldest forms of life on Earth and are one of the very first organisms to evolve on Earth. These are single-celled microorganisms without any cell structure and cell organelles. They consist of few necessary components such as a cell wall, genetic information in the form of a plasmid, a locomotory organ such as pili or flagella, and ribosomes for protein synthesis. The bacteria of different shapes typically live in soil, water, and organic matter. They obtain their nutrition through photosynthesis, decomposition of dead and organic debris, mutualism, and parasitism.

Eukaryotes emerged approximately 2.1-1.6 billion years ago during the Proterozoic eon. These are the most developed and evolved creatures with all membrane-bound organelles such as Golgi bodies and mitochondrion. Eukaryotes can be both unicellular or multicellular. Multicellular organisms may have first evolved from the colonies of identical cells. Scientists purported that around 600 million years ago, a minor genetic change may have allowed organisms to go from single-cell organisms to multicellular ones. There are some other organisms, which are not animals, plants, or fungus. These are known as protists. They may share close connections with eukaryotic animals due to their common ancestors.

Genetic material: RNA and DNA

In the year 1953, Oparin and Haldane suggested that life originated from non-living organic molecules such as proteins, DNA (deoxyribonucleic acid), and RNA (ribonucleic acid).

The evolutionary history of life suggests that RNA molecules were the self-replicating molecules that existed before the evolution of DNA and proteins. RNA contains ribosomes, an organelle that carries out protein synthesis, but RNA as a genetic material had many drawbacks. It is less stable due to the presence of unstable nucleotides and is responsible for the incorporation of mutations in the genome.

Most DNAs are biopolymers and double-stranded. They consist of nitrogen-based nucleotides, and because of the presence of more stable nucleotides in DNA than RNA, it is more stable as genetic material. With evolution, the lesser stable genetic material RNA got replaced by a more stable DNA. It is now considered the molecule carrying all the genetic, hereditary directives needed to grow, develop, function, and reproduce all known life forms.

Within cells, DNA is packed into long thread-like structures called chromosomes. Eukaryotic organisms such as animals, plants, fungi, and protists store most of their DNA inside the cell nucleus and some of their DNA in organelles such as mitochondria or chloroplasts. In contrast, prokaryotes such as bacteria and archaea, store their DNA only in the cytoplasm. Chromatins, which are made up of DNA-binding proteins called histones and DNA, tightly packed inside the nucleus of a eukaryotic cell, is found within the chromosomes, whereas in prokaryotes chromatins are found inside the cytoplasm.

Earth’s biosphere

The biosphere is the total of all the ecosystems. It is the zone of life on Earth. It integrates all living beings and their relationships, which include their interaction with the lithosphere, geosphere, hydrosphere, and other atmospheric elements such as water, gases, and so on. Life forms are present everywhere in the Earth’s biosphere; be it the hot springs or cold glaciers, inside the rocks or deep in oceans, or high up in the sky.

The dynamic diversity of life on Earth is the result of the interplay among genes, metabolism, environmental factors, and symbiotic relationship. Earth's habitable zone is dominated by microorganisms and as a consequence of these microbial activities, the physical-chemical environment on Earth is changing through subsequent years. For instance, the release of an oxygen molecule by cyanobacteria as a by-product of photosynthesis induced global changes in the Earth's environment. As oxygen was toxic to most life on Earth at the time, these evolutionary changes ultimately resulted in the formation of Earth's major animal and plant species. This interplay between organisms and their environment is implicit for the living systems on Earth.

Covid-19: A threat to life form

Coronaviruses are a group of viruses that can cause disease in both animals and humans. The severe acute respiratory syndrome (SARS) virus strain referred to as SARS-CoV is an example of a coronavirus. SARS spread rapidly in 2002–2003.

The new strain of coronavirus is named severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The virus causes coronavirus disease 19 (COVID-19). Coronavirus is common in many animal species. It affects animals such as camels and cattle. However, the transmission of coronavirus from animals to humans was rare. The first case of coronavirus is believed to be originated from a seafood market in China selling bats. The infected human spreads this in the form of droplets which when inhaled by another person the virus starts dividing inside the host and attacks the immune system. It spreads easily due to its high infectivity rate. On March 11, 2020, it was declared a pandemic. The pandemic led to a dramatic loss of human life around the globe. It continues to affect human health, the economy, and the food chain.

Context and Applications

The concept holds relevance in various undergraduates and postgraduate courses. It is mainly inculcated in degrees courses such as:

• Masters in Biology
• Masters in Palaeontology
• Masters in Microbiology
• Masters in Zoology

Practical Problems

Q1 Which one of the following is the characteristic feature of DNA?

a. Biopolymer and single-stranded

b. Biopolymer and double-stranded

c. Synthetic polymer and single-stranded

d. Synthetic polymer and double-stranded.

Answer-Option b. Biopolymer and double-stranded.

Explanation-The DNA molecule is double-stranded as it provides interlocking of nitrogenous base pairs and thus less exposure to the environment and hence, less mutation in the organism.

Q2. Who among these are believed to survive in harsh conditions?

a. Bacteria

b. Eukaryotes

c. Archaea

d. Protists

Answer- Option c. Archaea

Explanation-Archaea are believed to survive in extreme conditions such as boiling water and a highly acidic environment, hence they are called ‘extremophiles’. They can survive in such conditions because their plasma membrane is different in composition from other bacteria. It is a phospholipid made up of glycerol-ether lipids, which makes archaea unique.

Q3. In which era did Dinosaurs dominate?

a. Mesozoic era

b. Precambrian era

c. Cenozoic era

d. Paleozoic era

Answer-Option b. Mesozoic era.

Explanation-This ear was considered as the age of dinosaurs, as they were the most evolved and strong animals present at that time on the Earth.

Q4. Which of the following elements is the most abundant on Earth?

a. Sulfur

b. Phosphorous

c. Oxygen

d. Carbon

Answer-Option d. Carbon

Explanation-Carbon-based molecules form the structure of life forms, carbon is present in abundance in the cell.

Q5. Who dominates the Earth’s habitable zone?

a. Human beings

b. Carnivores

c. Microorganisms

d. Dinosaurs

Answer-Option c. Microorganisms

Explanation- They are the most abundant and diverse group of organisms on the planet. With the changing conditions, they have adapted to almost all the environment and continue to be dominant.

Common Mistakes

The students might get confused with the different theories that came into existence about the origin of life. Originally, people used to believe that life is a creation of god and that’s how life came into existence. However, as humans started wondering more and more about life’s existence and origin several theories were put forward such as the theory of spontaneous generation, chemical theory of evolution, Lamarck’s theory of inheritance of acquired characters, Darwin’s theory of natural selection. Out of all these, the most accepted theories were the chemical theory of evolution of life and Darwin’s theory of natural selection.

Related Concepts

• Cell Biology
• Molecular Biology Life Sciences
• Anatomy of a cell