Understanding how life began is one of the most profound areas of science.
The questions we ask and the answers we find don't just help us understand the origins of species on Earth, but also how — or whether — life has existed on the moon or Mars, or extra-terrestrial life exists today farther away in the universe.
As far as we know, the earliest evidence of life on Earth is a cyanobacteria, dating back 3.7 billion years — about 800 million years after the planet was formed. That evidence was found in fossils in Australia.
We also know that all known species on Earth are carbon-based life forms.
Carbon is the primary component of our cells, used by plants to build leaves and stems and by animals for cellular growth and replication.
But if you're wondering where these elementary building blocks of life originated — from space or a swamp? — the answer is manifold and disputed.
What is Darwin's 'primordial soup' theory?
Charles Darwin, the British naturalist, thought that life began in "some little warm pond" filled with inorganic chemicals that became complex organic chemicals or molecules. That led to the popular theory of a life-forming primordial soup.
These organic chemicals began to interact with one another, especially when stabilized and bound in a bag-like membrane.
Eventually, these cellular structures evolved to become life forms, such as the cyanobacteria found in Australia.
That's the theory, but possibly not the whole truth.
"The truth is that those events are almost completely shrouded in mystery. No one knows what sequence of events produced life from non-life, but there are many ideas, ranging from hydrothermal vents on the ocean floor to warm ponds on islands to subsurface environments," said Silvia Holler, a biochemist at University of Trento, Italy.
How did hydrothermal vents create early life forms?
Holler has been studying the earliest beginnings of life, replicating the conditions of hydrothermal vents — hot springs fed by underwater volcanoes — in what she calls chemical gardens.
Using these chemical gardens, Holler can tweak chemical compositions in a simulated environment — including the amount of energy and level of acid (pH value) — to understand what may have created perfect conditions for the earliest forms of life, or proto-life.
Holler's aim in a study published in the journal PNAS was to understand the stage just before life began, when inorganic chemical structures became organic molecules.
She found that the structure of hydrothermal vents is crucial for these "chemical transitions."
Her research found that when fatty organic molecules are incorporated into the inorganic walls of hydrothermal vents, their structure changes to form vesicles — another bag-like structure that can contain other molecules.
"Vesicles are similar to primitive cell membranes, or 'proto-cells,'" said Holler.
Nick Lane, a professor of evolutionary biology at University College London, UK, was not involved in the research, but said that the findings are important. It shows, said Lane, how cell-like vesicles may have emerged in specific conditions in hydrothermal vents.
Why is the biology of life still a mystery?
The vesicles that Holler created are too primitive to be defined as "life" — they didn't have any of the recognized processes of life: movement, reproduction, sensitivity, nutrition, excretion, respiration, and growth (simplified in the acronym MRS NERG).
Lane explained there is a long continuum from very simple prebiotic chemistry to genes and molecular machines that form the complex processes of life. The vesicles in Holler's study sit somewhere in the middle.
The evolution of simple cell-like vesicles, lacking genes, could have evolved relatively "quickly," said Lane, suggesting a timeframe less than millions of years.
But the evolution of cells containing DNA genetic code — the stuff that makes us who we are as individuals — probably took hundreds of millions of years, said Lane. And that process would have included life developing ways to create proteins from DNA. Protein is another vital element of life.
And it's this chain of events that remains a mystery.
Many experiments have tested whether life can arise spontaneously with the presence of organic molecules. But none of the experiments have succeeded in creating life from non-life, said Holler.
"[W]e can surmise that the gap between organic molecules and life is large, or the molecules themselves are not the key ingredient that causes life to arise," she said.
That chimes with Lane's view that organic molecules "technically have nothing to do with life — they are just molecules containing hydrogen bound directly to carbon." Organic molecules may only "appear to be necessary for life," he said.
Did life on Earth come from space?
Another way to test the primordial soup theory lies in our search for extra-terrestrial life.
Space is full of organic molecules. Molecules dubbed as "probiotic" have been detected in interstellar clouds in distant galaxies, as well as our own Milky Way.
These probiotic molecules are surprisingly complex, but researchers have yet to find vesicles or cell-like structures in space.
There is a hypothesis that life came to Earth from space via microscopic organisms that hitched a ride on meteorites or dust particles.
The idea, known as panspermia, is a fringe theory supported by few scientists, but nevertheless it is intriguing to think that the chemical building blocks required for life may not be unique to Earth.