Could life have evolved on the longest mountain range on Earth, that is so far under the water it's invisible to most of us?
Many scientists believe life on Earth evolved in what's dubbed a 'primordial soup'. After the molten Earth cooled down, there were warm ponds of water, scattered across the rocky surface.
And somehow an external source of energy (perhaps lightning or ultraviolet light) triggered a series of chemical reactions in these ponds that led to life.
But this energy source would be intermittent, not continuous. You would have to wait for a lightning strike, or a break in the clouds so that ultraviolet could shine through.
A new (and still controversial) hypothesis is that life began on the sea floor, driven by the more reliable energy supply of molten magma just under the Earth's crust. Meet the ocean ridge — the longest mountain range on Earth.
Its mountain peaks rise from depths of about five kilometres up to a fairly uniform 2.6 kilometres beneath the surface. So you'll most likely never see it.
But despite being a mostly cold, dark wasteland, there are a few hundred locations along the top of the ccean ridge that squirt out hot water. And these hot spots could be where life on Earth began.
Warm, hot, hotter
In 1977, scientists discovered shimmering warm springs on the Galapagos seafloor. The water was about 20 degrees hotter its surroundings. And to their surprise, they found a strange biological ecosystem around the warm springs.
Two years later scientists in high-tech research submarines discovered the first seriously hot hydrothermal (hot water) vents on the peaks of the ocean ridge.
The only thing that stopped it from boiling was a few kilometres of water pressing down on it.
Like all hydrothermal vents, the water is heated by magma — the molten rock that continues to form the mountain range as it pushes upwards through the sea floor and spreads to the sides.
Since then, we have discovered water squirting out of hydrothermal vents deep in the ocean at temperatures up to 464 degrees Celsius!
But more importantly, the expeditions to these hydrothermal vents also discovered previously unknown biological communities.
These bizarre and very dense ecosystems live entirely off the energy in the hot water.
They are the only known lifeforms on Earth that don't get their energy from the Sun.
A single ecosystem can be up to a few hundred metres across. The distance between one hydrothermal vent ecosystem and the next can be tens, or hundreds, of kilometres. In between the vents, the ocean floor is in perpetual darkness, with very little life other than bacteria.
Bacteria and other single-celled organisms called archaea are the basis of the ecosystem, that surrounds the life-giving hot water vents. (Archaea are similar to bacteria, but run on different chemistry).
Both the bacteria and the archaea get their energy from strange chemical reactions involving sulphur, instead of oxygen.
These single-celled organisms coagulate into thick mats. Then small multi-cellular creatures such as copepods and amphipods feed directly on the mats.
Bigger creatures such as snails, crabs, clams, limpets, shrimp and fish then eat these smaller creatures. Since the early discoveries, hundreds of completely new species of snails and other creatures have been discovered at hydrothermal vents.
There are even giant tubeworms, two metres long and as thick as your arm, without a mouth or an anus!
So, the ocean ridge might seem an unlikely place, but as these bizarre lifeforms suggest, life on Earth could in fact have started around hydrothermal vents on the sea floor.
The hydrothermal vents are certainly able to provide the energy needed to sustain life. And their very structure shares a surprising similarity with the cells that form the basis of life.
All living cells use the energy from food to pump hydrogen ions to a 'holding tank' behind a membrane. There's a difference in the levels of hydrogen ions — high on one side of the membrane, low on the other side. When the hydrogen ions flow across the membrane, they create high-energy chemicals that in turn power everything we do.
In the hydrothermal vents, hot alkaline water flows upwards, and mixes with the more acidic ocean water. This creates a difference in the levels of hydrogen ions remarkably similar to the difference that powers our own cells.
If this new theory proves right, the elusive origin of life might indeed be as simple as hot water squirting through rock!