In his 2021 book, “A Brief History of Earth,” Harvard geologist Andrew Knoll wrote, “What is life, anyway? What … differentiates us and dogs and oak trees and bacteria from mountains, volcanoes, and minerals?
“On the strength of our own lives, or those of our children, we might volunteer that organisms grow. True, but so do quartz crystals. But organisms not only grow, but …. reproduce, making more of themselves through time. Organisms harvest energy required for growth and reproduction from their environments – a set of processes that biologists call metabolism. And critically, life evolves.”
Crystals, on the other hand, grow only by adding more of the same, but none evolve into something else – only life evolves into something else.
Even the simplest of cells – bacteria and archaea – are very complex.
They make hundreds, if not thousands of different proteins, possess hundreds to thousands of genes, a complex system for generating high-energy phosphates and a fatty membrane studded with protein channels for ions and proteins. Clearly life began with something much, much simpler.
Life’s molecules – amino acids, phospholipids, cholesterol, sugars, RNA and DNA – are composed of differing combinations of five atoms: hydrogen, carbon, oxygen, nitrogen and phosphorus.
If in doubt, just look at the chemistry of those molecules – the same five atoms crop up, especially carbon, the matriarchal element that most readily forms bonds with the other four.
In 1953, Stanley Miller and Harold Urey combined water vapour, carbon dioxide, methane, ammonia and electrical discharges in an experiment meant to simulate early Earth’s atmosphere and managed to produce a few amino acids.
Their prediction about the makeup of early Earth’s atmosphere turned out to be wrong. Even so, they showed that combining a source of energy with some of the building blocks of life containing carbon, oxygen, hydrogen and nitrogen could produce amino acids.
Half a century later, John Sutherland and his colleagues created two nucleotides, the building blocks of RNA and DNA – under experimental conditions, which Knoll refers to as “plausible early Earth conditions.”
RNA is formed from nucleotides, which in turn are composed of three molecular building blocks – bases – which can be readily made from hydrogen cyanide (HCN), sugars from precursors such as formaldehyde (CH2O) and phosphate groups, which could have been produced by weathering of volcanic rocks.
Thus, all the building blocks for life were probably readily available on Earth and some of them have been found on asteroids, comets and in space itself.
The building blocks of life existed early on, but which came first, the phospholipid membrane, amino acids and perhaps combinations of amino acids to form small proteins? Or was RNA first?
The answer for some scientists is RNA. Why? Because only RNA carries genetic information for forming proteins, can reproduce itself and RNA carries the all-important property of evolving in response to environmental challenges or opportunities.
And RNA is capable of other tricks, such as facilitating and speeding up chemical reactions without itself changing (enzymatic properties).
RNA is less stable than its double-stranded derivative DNA, but that may have helped life gain a foothold by increasing the chance of mutations and variants on which evolution could act to favour the development of more complex biological molecules from which life might advance.
What probably happened many times and, in many places where life’s molecular building blocks were available in abundance in a universal solvent like water, was that nucleotides formed using bases, sugar and phosphate groups.
Some nucleotides would have linked together to form short strands of RNA. And through chance, some stretches of the RNA might have acquired the ability to direct amino acids to link together to form chains and thus proteins.
And some of those proteins might have been structurally and functionally useful to the evolving nascent cell and hence selected by evolution to survive.
Copying errors would have been common with early versions of RNA, some of which might have enhanced the structure and thus function of the proteins encoded by the RNA.
But long before the form and function of the simplest of single cells was established, there must have been countless biological experiments before coming together to form the simplest cells.
Fortunately, there was a lot of time for natural selection, acting on RNA to hone the structure, functions and interrelationships of life’s molecules within single cells before those protocells began to behave much like the bacteria and archaea we know today.
And last, but hardly least, RNA could attract free bases from the environment to match its own bases, to form copies of itself.
Without this property there would have been no molecular memory to carry forward favourable copying errors on which evolution could act to enhance the function of the RNA and its molecular products.
Those fundamental principles of life would continue with the development of double-stranded DNA.
That’s my imagined account of how the essentials of life came to be.
Dr. William Brown is a professor of neurology at McMaster University and co-founder of the InfoHealth series at the Niagara-on-the-Lake Public Library.