The origin of life, and how to get to that very point, have been as mysterious as they are fascinating. It is one of the greatest questions that humans have attempted to answer; how we and the rest of the natural world evolved?
One way of looking at it is through the primary necessity of our everyday life on our planet – oxygen. “However, many people believe that when life arose, there was very little oxygen in the atmosphere called reducing atmosphere. Others believe that there was some oxygen back then. However, the oxygen we have now is from living matter itself,” said Prof Vidyanand Nanjundiah, Faculty at Centre for Human Genetics at the Indian Institute of Science (IISc), during a lecture at the Jawaharlal Nehru Planetarium in Bengaluru, recently.
Prof Nanjundiah delved into various aspects about the origin of life, and where the keys to evolution can be found. According to him, the Earth’s crust plays out the most prominent part in figuring out the origin. The first sign of life was around 4 billion years ago. Interestingly, there are fossils today, called stromatolites – layered structures, with each successive layer being formed by bacteria over time. Whereas the eukaryote fossils (includes plants, animals, fungi, protists, and seaweeds) date back to 1.5-to 2 billion years ago. The single-celled creatures back then, had the same building blocks as modern humans.
Back then, the atmosphere was made up of largely reducing gases, like carbon dioxide, sulphur dioxide, nitrogen and methane, and most of the oxygen came from photosynthesis of blue green algae much later. Also, living matter has the same elements as in the Earth's crust and its oceans, only the proportion varies.
Essential molecule
For a common person, it is what came first, the chicken or the egg? But for scientists, there is a similar puzzle — did RNA or DNA come first? This question holds deep implications for understanding the origins of life on Earth.
If DNA is the blueprint for life and proteins, carry out the tasks that keep cells alive, how did life begin when neither DNA nor proteins existed? Prof Nanjundiah highlighted that the paradox has baffled scientists for years and led them to explore the idea that RNA, a molecule similar to DNA, might have played a crucial role in the early stages of life.
While research continues to reveal fascinating possibilities, Prof. Nanjundiah explained that RNA is a more versatile molecule compared to DNA, as it can both store genetic information and catalyse chemical reactions, unlike DNA, which primarily functions to store information. “This gives RNA a critical edge in the puzzle of life’s origins,” he pointed out, adding that RNA could have acted as both the genetic material and the catalyst for biochemical reactions in the early stages of life. The 'RNA World Hypothesis' suggests that life may have started with RNA molecules capable of replicating themselves and catalysing the necessary chemical reactions, a process that could have set the foundation for life, he said.
“But how did RNA form in the first place?” asked Prof Nanjundiah, highlighting that it is believed that the right conditions on early Earth, like warm, shallow pools and volcanic vents, might have helped create the building blocks of RNA from simple chemicals.
“Some researchers believe that these conditions, combined with the right amount of energy (from lightning, UV rays, or underwater volcanic activity), could have sparked the creation of the first RNA molecules,” he said, adding that these molecules would have been simple at first, but could have grown more complex over time. Additionally, scientists have observed how small RNA molecules can self-assemble under lab conditions, providing hints that life may have started in these simple, prebiotic environments.
Over time, as life evolved, RNA’s role was gradually taken over by DNA, which became the primary carrier of genetic information, while proteins became more specialised in carrying out the complex functions needed to support life. This shift allowed life to become more organised and diverse. But what if RNA did not completely disappear? Some scientists believe that remnants of the RNA world are still present in our cells today, such as the ribosomes that make proteins — a crucial link between ancient RNA and the DNA-based life we see now.
The search for answers continues today, as researchers study extreme environments, such as deep-sea hydrothermal vents, where conditions might have been just right for early RNA molecules to form. “By understanding how these early molecules could have come together, we are getting closer to solving one of science’s greatest mysteries — how life began,” Prof Nanjundiah mentioned.
So, could life have originated by DNA and protein coming together? Unfortunately, not! The reason being both DNA and proteins are sufficiently complicated in themselves. Both DNA and proteins require each other. DNA is used to make protein and protein is used to make DNA. This process is called an 'autocatalytic process'.
Tree of life
People now try to find the origin through evolution. As living creatures give birth to progeny, these are modified over the years. Therefore, we are confident that everything on this planet had ancestors. Working backwards gives rise to the evolutionary tree, which further gives rise to plants, animals and microbes. The tree represents the notion that as we go closer to the roots, most of the creatures today had common ancestors. The hope is that we can go back to one common ancestor of every living being, which is the last universal common ancestor.
Human beings belong to 'Eukaryota', a branch which contains a nucleus and other membrane-bound organelles. The other branches are bacteria and archaea. The tree confirms that we converge at a single root. However, as we study the evolutionary tree, there emerges side branches called horizontal gene transfer. This proves that there might be no single universal ancestor. Possibly, the single origin of life is misplaced. There are a bunch of candidates from the life which we know currently. The other theory is that the world of today came from only RNA and proteins, referred to as the RNA world. There is also the 'panspermia theory', which states that life did not originate on Planet Earth, but came from seeding from outer space, perhaps molecules and complex compounds. These complex compounds are found in meteorites. When meteorites strike the Earth, they are found to contain molecules pointing to the origin of life.
More theories
The other theory is Oparin-Haldane theory. Soviet biochemist Alexander Oparin had an idea that life could have originated in a primitive reducing atmosphere, with a source of energy, by spontaneous formation of coacervate, aggregates of organic molecules. These aggregations could show primitive metabolism, and could assimilate other compounds. Whereas in British geneticist JBS Haldane’s theory, water played a primordial role. The energy would be provided by the Sun through ultraviolet radiation. This radiation would be able to build compounds in the oceans, unless the density was high enough. The phrase he used to describe it is "Hot Dilute Soup".
Stanley Miller's 1953 experiment took a reducing atmosphere and passed electric discharges through it. After repeated discharges, there was formation of condensate. Some parts of the condensate consisted of organic molecules like amino acids and glycine. American biochemist Sidney Fox studied what would happen to these amino acids over time. These things could polymerise, giving rise to polymer ides or proteins. These polymer ides could encase in a shell, which he called microsphere.
Krishna Bahadur and Ranganayaki conducted studies on certain cell-like groupings, capable of metabolism and photosynthesis. They christened this, ‘Jeewanu’. By putting together a defined combination of elements, like carbon, hydrogen and oxygen, you can get photocells forming and growing.
Among the requirements of life forms, there are some means of utilising energy. One of the ways that anabolism might happen has come up recently. The energy is provided by high-temperature hydrothermal vents in which the temperature of the Earth can heat up water to such an extent that the geometrical configuration can work to ionise water molecules. This is called the Proton Motive Force. Hydrothermal vents are places where people can look for life.
The RNA-DNA Conundrum
RNA (Ribonucleic acid) and DNA (Deoxyribonucleic acid) are both nucleic acids that store and transmit genetic information, but they have several differences in structure and function:
DNA is double-stranded, while RNA is usually single-stranded. DNA is tightly coiled and stored in the nucleus, while RNA is smaller and moves out of the nucleus to the cell’s protein machinery
DNA contains deoxyribose, while RNA contains ribose. DNA also contains thymine, while RNA contains uracil
DNA stores genetic information and acts as a blueprint for life. RNA decodes the DNA blueprint to create proteins.
RNA is mainly found in the cytoplasm, but is synthesised in the nucleus
There are different types of RNA, including messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA)