The Evolution of Life on Earth

Photo by Irina Iriser from Pexels

We talked a little about the basic unit of life called cell earlier, and I also hinted on how  the whole life was actually created out of Bacteria. So, let’s sit back and enjoy this story of a single-celled Bacteria called Cyanobacteria (also known as the Blue-Green Algae), which in a way, made the Earth we know today. 

First, let us try to address the conundrum that we had in the previous story – about how life could survive – let alone evolve – in a hostile atmosphere on earth filled with all toxic gases like methane, ammonia, and carbon dioxide and with no atmospheric oxygen at all. 

I’m sure we all don’t want to imagine even for a second our life without our beloved oxygen. But to simply recall it, let us rewind back to the simple science. 

For a cell to survive, it needs energy; and a cell derives this energy from the breakdown of the sugar in the presence of oxygen, a process known as respiration. And how do we get sugar? Animals basically get it from plants whereas the plants produce sugar by combining carbon dioxide and water in the presence of sunlight, a process known as photosynthesis. 

Now that we’re up to date with this basic science, let’s travel some thousand years back. Oxygen was not present in the earth’s atmosphere, and minor amounts of oxygen was  present in small areas of Earth’s ancient shallow oceans. And in the deep ocean lied our savior – the Cyanobacteria. 

The Cyanobacteria present in the deep ocean were able to perform photosynthesis – that is combining carbon dioxide and water in the presence of sunlight – to release oxygen. And bit by bit, or rather Cyanobacteria by Cyanobacteria, oxygen got accumulated in our  atmosphere. 

This whole oxidation of earth’s atmosphere, thanks to Cyanobacteria, is known as the Great Oxidation Event (GOE).

Moreover, these cyanobacteria just didn’t stop there. They went on to evolve into more  complex cells and organisms leading to the world that we have today. And this is where the evolution of cells comes in. 

The single-celled Cyanobacteria are the primitive single-celled organisms known as the Prokaryotes – meaning without a nucleus and any membrane-bound cell organelles. And these single cells could perform very basic stuff necessary for their sustenance only – like being capable of aerobic (in the presence of oxygen) respiration, being able to perform photosynthesis, or just ingestion of foreign particles for survival (known as Phagocytosis). 

These prokaryotes were able to further evolve into the complex cells known as Eukaryotes through a process known as Endosymbiosis. 

The endosymbiotic theory suggests that the complex eukaryotes were formed by the  ingestion of one Prokaryotes by the other. Contrary to the obvious expected outcome of  death or disintegration, some of these prokaryotes were able to survive inside another cell and collectively evolve as a new cell (hence the name symbiosis). 

Some of these amoeba-like organisms ingested prokaryotic cells that then survived within the organism and developed a symbiotic relationship. Mitochondria (one that produces  energy inside the cell) formed when bacteria capable of aerobic respiration were ingested and chloroplasts (one that undergoes photosynthesis) formed when photosynthetic bacteria were ingested. 

The fact that both Mitochondria and Chloroplasts have their own membrane, their own DNA, and cannot be produced by the parent cell further corroborates the theory. In fact, there are plenty of examples around today that support this theory. Like the presence of  single-celled Paramecium with numerous Algae inside its cytoplasm, or even multicellular hydras with algae inside its cells. 

And after this, kicks in the more widely known theory known as the theory of Evolution. 

While there are many theories associated with the theory of Evolution, the widely  accepted one is the one by Charles Darwin which stresses the idea of Natural Selection.

Natural Selection basically says that the members of a species better suited or adapted  to their environment are able to produce more offspring than others, and also to pass their  more useful traits to their offspring. And this, over number of generations, would lead to  major changes in the characteristics of the species and even origin of the new ones. 

Darwin published this theory in 1859, and there wasn’t much knowledge about genes and DNA back then. But now, with our understanding of genes and DNA and technology  around, we can compare how alike or how different organisms are genetically, and see if they were closely related long before or not. For example, Humans and the Chimpanzees share about 98 percent of DNA (yep, that large a number), leading us to believe that we  evolved from them through the years. 

So, to sum up, the miraculously generous Cyanobacteria not only led to the creation of  oxygen filled atmosphere, but also led to the creation of advanced forms of cells known  as Eukaryotic cells, and later to complex living beings and different species, aided by the Natural Selection, to the creation of advanced forms of life on earth and eventually us humans. 

But hold on a minute. 

This is one mistake us humans always do. We always try to think of human beings as the epitome of evolution and everything that has happened beforehand as some recipe for the final masterpiece that is us. 

However, that is not quite the case! We are just another species on earth amongst millions  of other species, with plenty of species extinct on the way and plenty still evolving, and  plenty on the verge of extinction. From Cyanobacteria to humans and all other species  present on earth today, it has been one adventurous journey of evolution – the one filled  with all amazing characters ranging from Terror birds to the mighty dinosaurs and scenes  replete with massive explosions and extinctions. And we humans have been part of the  journey for the tiny of the frame, maybe tiniest of the tiny. 

Just to give you the context, I will paraphrase Billy Bryson here – writer of this super amazing book called “A Short History of Nearly Everything”:

“If you imagine the 4,500-billion-odd years of Earth’s history compressed into a normal  earthly day, then life begins very early, about 4 A.M., with the rise of the first simple,  single-celled organisms, but then advances no further for the next sixteen hours. Not until  almost 8:30 in the evening, with the day five-sixths over, has Earth anything to show the  universe but a restless skin of microbes. Then, finally, the first sea plants appear, followed  twenty minutes later by the first jellyfish and the enigmatic Ediacaran fauna first seen by  Reginald Sprigg in Australia. At 9:04 P.M. trilobites swim onto the scene, followed more  or less immediately by the shapely creatures of the Burgess Shale. Just before 10 P.M.,  plants begin to pop up on the land. Soon after, with less than two hours left in the day,  the first land creatures follow. 

Thanks to ten minutes or so of balmy weather, by 10:24 the Earth is covered in the great  carboniferous forests whose residues give us all our coal, and the first winged insects are  evident. Dinosaurs plod onto the scene just before 11 P.M. and hold sway for about three quarters of an hour. At twenty-one minutes to midnight, they vanish and the age of  mammals begins. Humans emerge one minute and seventeen seconds before midnight.  The whole of our recorded history, on this scale, would be no more than a few seconds, a single human lifetime barely an instant.” 

No more than a few seconds! 

But we humans are narcissistic by nature, and we cannot help but put ourselves first, so let’s just chuck it up as human nature, we are what we are. Well, I, for one can’t wait to  get started on talking about our brief albeit adventurous stay on earth. So, next up, I will write about the origin of humans on Earth. (Sorry Dinosaurs, I am skipping about you altogether!)

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