How did life begin on planet Earth?

How did life begin on planet Earth?

Aug 09, 2021

No other question has perplexed humankind more than this one. From scientists to philosophers, everyone has expressed their amazement at this age-old riddle. How did life begin on planet Earth? The brief answer is – we are not sure but do not let that stop you from reading this article because we know enough to make an educated guess.

The oldest known fossil of bacteria goes back around 3.5 billion years ago! These are Stromatolites (layered deposits) formed by cyanobacteria/blue-green algae. We have many claims of even older shreds of evidence in the form of fossil and chemical residue. Some of them go back even 4.1 billion years ago. It is astonishing because the earth itself was created 4.5 billion years ago! That is a short period for life to spring into existence. However, all these claims have got no clear consensus among scholars.

In early times, the earth was a hostile place for life. There were frequent volcanic eruptions, a meteor shower and what not! Despite all this, somehow self-replicating molecule (or a set of the molecule) arose. From there on, natural selection took control and favoured any offspring that had the upper hand at survival. Soon, the first cell formed. Life begun! Isn't this amazing? But hold on, this raises more questions than it answers. How did those molecules come into existence? Why do we not see organisms popping into existence today? Would life develop the same way as it has done now if we ran the experiment again? Well, we will restrict ourselves to finding the answer to the first question.

Any hypothesis that explains the origin of life must answer three questions:

  • How were the building blocks – amino acids, nucleobases, and others assembled as a molecule?

  • How were these molecules bound in a cell?

  • Where did the energy come from to regulate life-supporting processes?

These are not simple questions. Even Charles Darwin did not take any stand about the Origins of life. Although, in a private letter to Mr Joseph Hooker, he talked of "warm ponds" as a viable cradle of life where the very first molecules could have formed. His exact words were,

“…we could conceive in some warm little pond with all sorts of ammonia & phosphoric salts,—light, heat, electricity &c present, that a protein compound was chemically formed….”.

Since then, people have proposed various theories, including hydrothermal vents, volcanic hot springs, volcanic ash, radioactive beach, and more.

The most widely accepted explanation is that of “Alkaline Hydrothermal Vents” because it answers those 3 big questions pretty well. Alkaline Hydrothermal Vents are a crack through which alkaline fluid keeps coming out. They are created by a process called “Serpentinization”. Water at the seabed reacts with minerals, olivine (magnesium iron silicate) to be specific, producing a ton of hydrogen and heat in the process (Yes, it is an exothermic reaction). Heat drives up the warm fluid to the ocean floor and comes in contact with cold seawater. Minerals precipitate out, forming a tall chimney-like structure (40-50 metres) composed of mostly calcium carbonate. This newly formed structure is central to the whole idea. Let us now examine what it has to offer.

How did amino acids, nucleobases, and etcetera were formed and assembled as a molecule?

We know DNA is central to the existence of life, so how was it formed in the first place? Protein does all the life-sustaining activity, including the creation of DNA, but information to create protein is embedded in DNA. We need protein for DNA and DNA for protein. We are running in a circle. So which came first? None! Scientists suggest that primary life-from started from RNA and moved on to DNA later on. RNA can hold genetic data (like DNA), function like protein, and also help in creating DNA and protein. Soon, DNA and protein took up the job of RNA because they can perform their respective task more efficiently. It is not very clear how RNA would have formed. Minerals like metal hydrides in vents could have easily helped create organic compounds like acid, sugar, and RNA from inorganic substances.

How were these molecules bound in a cell?

The obvious answer is that those structures acted as a containment space and concentrated the newly formed molecules. Further, the walls of these chimneys were rich in Iron and Sulphur which would have further helped in the creation/isolation of organic compounds.

Where did the energy come from to regulate life-supporting processes?

Again, the hydrothermal vent could have supplied the energy required to drive the life-sustaining processes. The seawater is acidic or proton rich whereas vent fluid is alkaline or proton-poor in content. When they come in contact with each other, there is some difference in the proton concentration. As you already know, protons are positively charged particles, and the gradient creates an electric potential that would have supplied energy. Even before we knew about these alkaline vents, Michael Russell suggested this very mechanism. What is even more interesting is that living cells nowadays are still powered by this gradient. Cell continuously maintains a gradient by pumping protons across a membrane. Today, we call it as Proto-Motive Force. 

Still, there are many questions to answer. How did life move out of vents? How were DNA and proteins formed? And many more...

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