Evolution of Living Organisms

The planet Earth first appeared between 4 and 5 billion years ago. Around 3.5 billion years ago, life began on Earth. Approximately 15 million distinct species of creatures have developed since then. However, only roughly two million have been detected thus far. In this lesson, we will discover how life began on Earth and how such a diverse range of organisms, colloquially known as biodiversity, arose via variation and natural selection.

Origin of Self-Replicating Genetic Material

As a result of condensation and polymerization, the original earth, which was a dilute boiling soup, developed complex organic molecules. At least three conditions were required for the genesis of life as −

  • Formation of polymers that can replicate themselves continuously;

  • Copying of these replicators must have been subject to error through mutation; and

  • Continuous supply of free energy for self-replicators and partial isolation from the general environment.

The early Earth's high temperature may have met the mutation criteria. Self-replications accidentally produced variations. Nucleic acids appear to have been the first replicators. The evidence supporting the laboratory creation of Coacervates, Protenoid microspheres, and Protobioints like liposomes supports the formation of primitive living cells. The notion was bolstered further by the origin of RNA, which served as an enzyme and genetic material, and DNA. According to the data, the RNA molecule came first. RNA can replicate itself and act as an enzyme (Ribozyme).

A copying mistake may result in a self-replicating molecule that is stable. In extant creatures, RNA has a multifunctional character and a wide range of tasks, as indicated by RNA primer translation in DNA replication, RNA interference, splicing, and RNA processing, among other things. These functions played essential roles in the early evolutionary process. Polypeptides might be coded using primary RNA nucleotide sequences (stored information).

Protobionts carry the ancient RNA. Protobionts can divide and convey information to daughter cells. Natural selection may result in a favourable generation. The following are some of the probable causes for the substitution of RNA for DNA −

  • Because of its low reactivity, DNA is a stable transporter of genetic information.

  • Rapid replication of RNA, as required in complex creatures, was not achievable. Chemicals may cause RNA damage.

Origin of Prokaryotes

In the history of life, evidence suggests the transition of the earth's atmosphere from primary (which lacked oxygen) to secondary (with a small quantity of oxygen). The first primitive Anaerobic living cell (Prokaryotes) may have evolved in the following manner.

  • Development of primitive membrane-bound vesicles with short-chain molecules.

  • RNA serves as both a hereditary material and a ribozyme.

  • In the protocell or protobiont, primitive metabolic machinery resulted in basic biochemical processes.

  • Natural selection has a role in protocell development.

Prokaryotes, which lacked a nuclear membrane and organelles, are thought to have evolved 3.5 to 3.8 billion years ago. Because of the lack of free oxygen in the atmosphere, the environment was anaerobic. These prokaryotes donate their nuclear membranes, cytoskeletons, and sophisticated organelles. They split apart via binary fission. The first prokaryotes used energy created by the fermentation of organic compounds in marine broth in an oxygen-free (reducing) environment. They were Heterotrophs because they required preformed organic material as sustenance.

The nutrient in seawater began to deplete and exhaust progressively due to the fast growth in the number of heterotrophs. This resulted in the emergence of autotrophs. Autotrophs might produce their organic compounds through chemosynthesis or photosynthesis. The production of organic compounds in seawater ceased due to a drop in temperature. Some early prokaryotes developed into chemoautotrophs, which could manufacture organic food using the energy released during inorganic chemical processes. These anaerobic chemoautotrophs looked like modern anaerobic bacteria.

As oxygen in the atmosphere increased, UV radiation turned part of it into ozone. This ozone produced a layer in the atmosphere that blocked UV radiation, leaving visible light as the primary energy source. The shielding of the earth's surface from UV radiation and the increase in oxygen levels aided in the evolution of new forms. The oxidizing environment also breaks down organic molecules, which were not ideal for the first rudimentary living cell.

Alternatively, mutation, selection, and evolution all had a part in developing aerobic creatures and aerobic respiration. Aerobic species are advantageous over anaerobic organisms because they produce many ATP (Adenosine triphosphate) molecules from the ingested (engulfed) food material.

Origin of Eukaryotes

Eukaryotes evolved from essential prokaryotic cells around 1.5 billion years ago. There are two ideas on how eukaryotes evolved.

Symbiotic Origin

Margulis (1970-1981) of Boston University claimed that specific anaerobic predator host cells absorbed but did not digest primitive aerobic bacteria. As a symbiont, these aerobic bacteria established themselves inside the host cells. The first eukaryotic cells evolved from anaerobic predator host cells. Animal cells developed from anaerobic predator host cells that consumed aerobic bacteria, whereas those that engulfed aerobic bacteria and blue-green algae evolved into eukaryotic plant cells. Aerobic bacteria formed mitochondria, whereas blue-green algae established themselves as chloroplasts.

Origin by Invagination

According to this theory, eukaryotic cell organelles may have evolved through the invagination of primordial prokaryotic cell surface membranes. Endosymbiosis of Archaebacteria and Eubacteria provided the earliest eukaryotic common ancestor with the genes for its nucleus and mitochondria. These primitive eukaryotes diverged further due to cyanobacterial endosymbiosis. This resulted in the evolution of chloroplasts and, eventually, separated lower and higher plants. The evidence is founded on −

  • Fossil Records

  • Computational Phylogenetic Techniques

  • Biophysical Approaches

  • Radioactive Zircon Fragments

The Origin of Multicellularity

It is still unknown how multicellular life came to be. These multicellular animals are thought to have evolved 600 to 700 million years ago (Pre-Cambrian epoch) in the following ways.

  • In single-cell organisms, cell division occurred, but daughter cells did not split.

  • Single-cell organisms developed numerous nuclei and established borders for two/many cells.

  • Cells' permanent associations resulted in specialized functions such as nutrition absorption, reproduction, and so on. Cellular specialization might have been the key to size expansion.

The origin of complexity aided the emergence of organized and functional variety in primordial eukaryotic cells. Comparisons of fossil records and DNA sequences indicate the presence of early eukaryotes, such as algae, which began around 1.2 billion years ago. Soft-bodied creatures arose as complexity increased. The cold period (about 750 million years ago) hindered development.

Many live animal phyla (Porifera, Coelenterata, Mollusca) arose during the Cambrian epoch. (About 530 million years ago). This is based on paleontological evidence. Other outliers based on a Chinese fossil imply that sophisticated creatures may have evolved earlier. Another watershed moment in life history was the advent of microorganisms, plants, and animals on land. Tetrapods first appeared 360 million years ago. Around 6-7 million years ago, the process of divergence of tetrapods, namely hominoids (apes), from the evolutionary tree laid the groundwork for early humans. Humans evolved around 200,000 years ago.


Previously, molecules were linear and formed membrane-based vesicles called Protobionts. This resulted in the formation of the protocell. Variation and selection were vital in forming several vesicles that gave rise to the earliest cells. The primordial cell was most likely related to extant prokaryotes. As a result of mutation and environmental changes, the cells developed various properties.

The phanerozoic aeon, split into three periods, was dominated by Eukaryotic, multicellular creatures. (i.e. Paleozoic, Mesozoic and Cenozoic). The fossil record, computational phylogenetic techniques, biophysical methodologies, and radioactive record of zircon fragments provide evidence for life's history.

Updated on: 20-Apr-2023


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