At the bottom of the ocean, where sunlight never reaches and pressures would crush a submarine like a tin can, volcanoes blaze and chemicals gush from cracks in the seafloor. It is here, amid utter darkness and crushing cold, that some of the most bizarre and unlikely life forms found on our planet thrive. These places—called hydrothermal vents—are more than just alien oases of life. They may also hold the answer to one of the greatest questions ever asked: Where and how did life begin?
Filamentous algae (Credit: Wünderbrot, CC BY-SA 4.0)
A world without sunlight
For much of human history, it was assumed that all life depended, directly or indirectly, on the sun. Plants absorbed sunlight through photosynthesis, herbivores ate the plants, and predators ate the herbivores. Each step in the food chain was based on solar energy.
But, in 1977, that notion was turned on its head.
Jack Corliss, of Oregon State University, and Tjeerd van Andel, from Stanford University, found a teaming ecosystem while exploring the Galápagos Rift submarine hydrothermal vents on 17 February 1977, in the DSV Alvin, a research submersible. Though, in some areas, the water temperature reached 400°C (752°F) and was laden with chemicals like hydrogen sulphide and methane, life was everywhere. Giant tubeworms, ghostly white crabs, mats of microbial life unlike anything ever seen before, and countless other unknown species filled their light beams.
Here, in total darkness, life was flourishing without a single ray of sunlight. It was an ecosystem based on the planet’s own heat.
Chemical fire: Life from the deep
The secret was chemosynthesis. Instead of relying on sunlight to make energy, microorganisms around the vents used chemical reactions to power themselves. Specialised bacteria were converting hydrogen sulphide, a toxic gas for most organisms, into usable energy. They formed the foundation of an entire food web.
Further research has discovered varying ecosystems around hydrothermal vents around the world, all looking like something from another planet: fields of red-plumed tubeworms, clumps of snails with iron shells, and shrimp with light-sensing patches on their backs instead of eyes. Some vents harbour yet undescribed species living in extreme heat, pressure, and chemical conditions that had been considered uninhabitable.
And these deep-sea ecosystems are ancient, possibly echoing the very first ecosystems that ever existed.
Life’s possible birthplace
The conditions around hydrothermal vents are strikingly similar to those theorised for the early Earth, around four billion years ago. Back then, our planet was a chaotic place. It was bombarded by asteroids, the sky was red, the air was unbreathable, and no plants had yet ventured onto the land.
So some scientists now theorise that life originated not in shallow, sunlit waters, as once assumed, but in the dark, chemical crucibles of the deep sea. The building blocks of life—simple organic molecules, like amino acids—could have formed in the warm, mineral-rich waters around hydrothermal vents. Natural structures called chimneys, made of porous rock and rich in metals, may have acted like primitive cells, concentrating chemicals and catalysing the first self-replicating reactions.
The deepest mystery
However, despite mounting evidence and promising theories, the true origin of life remains one of science’s greatest unanswered questions. How did simple molecules become complex? How did self-replicating systems arise from a lifeless stew of minerals and heat?
While hydrothermal vents offer a compelling setting—a rich brew of heat, minerals, and organic molecules—the precise steps by which non-living chemistry gave rise to living cells are still unknown.
The big problem is the mind-boggling complexity of the simplest form of life: the cell. From bacteria to human beings, the essential form of the cell is the same. Within the cell are many organs, each a complex machine, and each necessary for the whole to function.
It is thought that, in the warm seas of the young Earth, simple molecules, like amino acids and nucleotides, formed spontaneously. Under certain favourable conditions, they might have built up into polymers like RNA, which eventually became self-replicating molecules. So, possibly an “RNA world” was born, where molecules carried both information and catalytic function.
Eventually, simple, membrane-bound vesicles (proto-cells) could trap and concentrate these molecules. This might have brought selective advantages which, over an abyss of time and the favour of natural selection, evolved into more efficient and complex systems, resulting in the first true cell.
Scientists continue to debate whether the process was sparked by vent chemistry, lightning-struck tide pools, or even extraterrestrial material delivered by comets.
Black smokers and white smokers: The vents themselves
Not all hydrothermal vents are the same. Some are called black smokers, named for the thick, dark plumes they emit. These are filled with iron sulphides, and they resemble smoke. They are hot and acidic, yet often host dense communities of heat-tolerant life.
Others, called white smokers, vent cooler fluids rich in barium, calcium, and silicon. One of the most intriguing white-smoker systems discovered is Lost City, a towering landscape of venting chimneys made of carbonate minerals reaching up to 60 metres tall. Unlike black smokers, which rely on volcanic heat, Lost City is powered by serpentinization—a chemical reaction between seawater and the Earth’s mantle rock.
Lost City may more closely resemble the conditions of early Earth, making it a prime site for research into the origin of life. Its vents have been actively seeping fluids for tens of thousands of years—possibly much longer than black smokers, which tend to collapse and rebuild on shorter timescales.
Creatures of the abyss
The life that thrives around vents often looks as alien as the environment itself. Giant tubeworms (Riftia pachyptila) grow over two metres long and have neither mouths nor digestive systems. Instead, they rely entirely on symbiotic bacteria inside their bodies to convert chemicals into energy.
Scaly-foot snails are found only around Indian Ocean vents. They have iron-infused shells and scales, which work as a natural armour unmatched in the animal kingdom.
Around the Mid-Atlantic Ridge vents, vent shrimp are found. They are nearly blind and navigate using thermal sensors. Their only nourishment comes from the bacteria that cover their backs.
Pompeii worms live in tubes near black smokers. They are among the most heat-tolerant animals on Earth, living in temperatures exceeding 80°C (176°F). They host bacteria on their backs that may serve to insulate them to some degree.
Each of these creatures has adapted in ways that challenge our understanding of biology and evolution.
What does this mean for life beyond Earth?
If life can thrive in boiling, acidic, high-pressure environments with neither sunlight nor oxygen, it is more adaptable to extreme environments than science had formerly thought. The discovery of such a variety of life around deep-sea vents leads to the question of whether it might also have developed in extreme conditions on other planets.
Many scientists now look to Europa (a moon of Jupiter) and Enceladus (a moon of Saturn), both of which are believed to have subsurface oceans heated by tidal flexing and possibly containing hydrothermal vents. Enceladus has even been observed spraying plumes of water vapour into space, laced with organic molecules. It is not far-fetched to imagine deep-sea vents on these moons harbouring microbial life, or even more complex life forms—extraterrestrial “tubeworms”, silently swaying in alien currents, and even stranger forms of life, never seen on Earth.
Still, so many questions
Despite decades of study, hydrothermal vents remain mysterious. We still do not know how long many of these ecosystems last, how organisms colonise new vents, or how vent life changes over time. Much of the deep ocean remains unexplored—more than the surface of Mars—and new vent systems are still being discovered.
Science has barely begun to scratch the surface when it comes to understanding these remote places. While each dive into the abyss brings some intriguing new findings, each discovery deepens the mysteries.
Deep-sea exploration is continuing, with advanced submersibles and remotely operated vehicles (ROVs) that can withstand the pressure and temperature of vent environments. Recent missions to the Mid-Cayman Rise, the Indian Ocean, and the Antarctic Ridge have uncovered species never seen before and mapped vent fields the size of cities.
As hydrothermal vent fields age, move, and die, entire communities must migrate or perish. How they do so, and how fragile or resilient these ecosystems really are, remains unknown. With looming threats like deep-sea mining, it is increasingly urgent to understand these habitats before they are irreparably damaged.
Life forged in fire
The story of life on Earth may have started not in sunlit shallows but in the lightless depths—among steaming cracks in the seafloor and chemical plumes. Hydrothermal vents are not just scientific curiosities. They are living time machines, whispering clues from a time before plants, animals, or even DNA as we know it.
They remind us that life is tenacious and can begin in darkness. And perhaps, on some far-off moon or distant planet, it is beginning all over again.
Ethologist Ila France Porcher, author of The Shark Sessions, The True Nature of Sharks, and five other books on wildlife behaviour, conducted a seven-year study of reef sharks in Tahiti, resulting in several scientific papers. Her decades of first-hand observations of wildlife— from sharks to bears to birds— focus on the individuality and intelligence of wild individuals, challenging traditional views of animal minds. Her work has been featured on Shark Week, in scientific discussions, conservation debates, and international media for its unique blend of field observation, art, and science.
