WHAT WAS THE SPARK FOR LIFE ON EARTH?

How life on Earth began is the ultimate cold case. Most of the evidence has been lost along the way. We do know a few things, however. One is that for the last 3.6 billion years, Earth has been home to one bizarre life form after another. And that at the cellular and molecular level, all living things are related. That's right. The simple DNA code that holds the recipe for all living things links all life on Earth to a single common ancestor.

So what was it, and where did it come from?

HOW DID LIFE ON EARTH BEGIN?

One of the few remaining clues to our origins lies in Western Australia. These ancient rock formations, stromatolites, are the earliest evidence of life on Earth. They're the product of fossilized cyanobacteria or blue-green algae. Old as they were, these cyanobacteria were biologically complex. They had cell walls protecting their protein-producing DNA. They were so advanced that scientists think life must have begun much earlier. But evidence of earlier life has been hard to find, and there's a good reason.

Scientists think the cyanobacteria in these rock formations killed almost every form of life that came before it. So what happened?

Our planet was a much harsher place 3.6 billion years ago than today. The atmosphere had no oxygen; no plants, animals, or insects existed. The sea was loaded with bacteria, but most of it was anaerobic, the only kind capable of surviving in such an oxygen-starved primordial soup. Then a stranger appeared on the horizon, and the planet changed forever little by little. This upstart arrived in the form of cyanobacteria.

Cyanobacteria are photosynthetic; they convert sunlight into energy and produce oxygen as a waste product. As cyanobacteria flourished, their oxygen wastes started building up in the water and atmosphere. And as oxygen levels increased, the existing anaerobic bacteria began dying off. To them, oxygen was toxic. As the thriving blue-green algae slowly poisoned the atmosphere, the surviving anaerobic bacteria fled to the bottom of the sea. The oxygen users may have won the day. However, the die-off destroyed all of the evolutionary links to our first ancestors, the anaerobes.

How they made that first giant leap from non-living to living remains one of life's greatest mysteries.

WHAT WAS THE SPARK FOR LIFE ON EARTH?

Around 4 billion years ago, molecular reactions switched on and became life somewhere on Earth, creating little molecules. These little molecules began interacting with each other. The reactions produced compounds that fed back into the cycle, creating more reactions and molecules.

The process culminated in one of these molecules being able to copy itself, and by doing that, genetic information could be passed down.

But what was the switch that was flipped? What breathed life into those first Franken molecules?

Several theories have been put forward. One early experiment proved that lightning might have supplied the energy needed to trigger life on Earth in its early days. Researchers created an early Earth atmosphere by sealing water, methane, ammonia and hydrogen in flasks connected with a loop. The liquid water was heated to induce evaporation. When the researchers simulated lightning by adding a spark, they found that the solution in the flask contained something it hadn't before; amino acids.

Proteins play a crucial role in almost all biological processes, and amino acids are their building blocks. It seemed the lightning experiment worked. But there are other sources of energy that could have triggered the same chemical reactions needed for life.

Another popular theory is that simple metabolic reactions occurred near ancient sea floor hot springs, enabling the leap from a non-living to a living world. These events still fuel strange underwater ecosystems inhabited by giant tube worms, blind shrimp, and sulfur-eating bacteria.

Many researchers believe these complex, high-pressure, mineral-rich environments may be the source of all life on our planet. However, a radical new idea has recently been put on the table.

Scientists are close to demonstrating that the building blocks of DNA can form spontaneously from chemicals thought to be present on early Earth. If they succeed, that would mean DNA could have formed before the birth of life.

Then the question would become, how did DNA originate?

We're still far from finding out how life appeared on Earth, but clues may be closer than we think. Our DNA may hold the answer to one of life's most significant unknowns, the mystery of life itself.