Deep at the bottom of the sea lies one of the most probable locations for the origin of life on Earth… hydrothermal vents. But what are they? How could they have created life? And what does it mean for life in our solar system?
It is a fact that at some point in our history simple inorganic molecules made the transition to organic replicators. Over billions of years those replicators slowly evolved into every living thing you see around you today. That transition point is known as abiogenesis, the origin of life. We know that it happened, we are living proof that it did, but how it happened is still somewhat of a mystery to the scientific community. However that doesn’t mean there aren’t any ideas. In fact there are many different competing theories about how abiogenesis occurred on Earth. Some are more plausible than others but none have been completely proved or disproved. One of the most likely theories is that life was kick-started deep at the bottom of the oceans in hydrothermal vents. And if these geological features are the origin of life on earth it could have interesting implications for extra- terrestrial life in our very own solar system.
What are hydrothermal vents?
Hydrothermal vents can be found across the ocean floor surrounding underwater ridges where tectonic plates meet. Simply put they are the result of hot magma from the Earth’s core meeting the cold waters of the deep ocean. The process of their formation begins when seawater drains down through fissures in the seafloor. The water then gets heated up by volcanic activity to around 400oC and is prevented from boiling due to the high pressure of the deep ocean. As it rises back towards the ocean the super-heated water mixes with multiple minerals in the earth’s crust. When the hot mixture is released into the cold seawater the minerals condense into the chimney-like structures that we call hydrothermal vents. There are different types of these vents based on the minerals that they deposit. ‘Black smokers’ are chimneys formed from deposits of dark iron sulphide and ‘white smokers’ are formed from deposits of lighter barium, calcium and silicon.
Hydrothermal vents are a relatively recent discovery only first observed in 1977 by scientists near the Galapagos Islands. What was perhaps even more surprising to the researchers was the abundance of life they discovered surrounding the extremely hot and toxic structures. They also continue to bewilder scientists today with new areas of vents being discovered all the time along with brand new species that accompany them. All of these discoveries have lead scientists to learn more about the geology of the structures and the ecology of the marine life that surrounds them. But some scientists also believe these structures may be where we and all other life on earth originated as well.
Simple replicators to complex life
The cells in our bodies are extremely complex and diverse but essentially they are just a rearrangement of chemicals and molecules found in the natural world around us. What has allowed for that transformation is the base code of all life neatly wrapped up inside every cell on the planet, DNA. It is the instruction manual for our bodies to create proteins, enzymes and organs to carry out the important functions that keep us alive. What makes DNA so special is its ability to self-replicate and pass itself on through reproduction. But DNA hasn’t just existed since the beginning of time, it was created. At some point inorganic molecules reacted together in such a way that they created the first compound capable of replicating itself. As those first simple replicators continued to copy themselves and make more changes they eventually became DNA, which eventually created living cells, which eventually produced living bodies.
The point at which those first simple replicators were created is the origin of life we call abiogenesis. Although it is not known exactly how it happened we do have an idea of the types of conditions and required steps needed for inorganic molecules to make the transition to self-replicators. We won’t get too technical or scientific but there are three main things that would have been required. Firstly is the building blocks, by that I mean the things that make organic molecules, most importantly carbon but also hydrogen and oxygen. The second requirement is a catalyst, a chemical that helps kick-start other reactions by reducing the energy needed for a reaction to occur or facilitate it through an extra step. Finally and most importantly is an energy gradient, a difference in energy across which chemical reactions can occur, this could be electrical, chemical or physical.
So do hydrothermal vents provide the basic requirements needed for the first simple replicators to form? Yes. Firstly the building blocks needed for organic molecules to be created. At the time abiogenesis occurred, over 4 billion years ago, the early ocean was extremely acidic meaning a high concentration of carbon dioxide. As well as this hydrothermal vents would have produced large amounts of methane and hydrogen so carbon, hydrogen and oxygen were all abundant around the vents. Secondly is the catalyst and as we’ve already seen hydrothermal vents contain a wide range of chemical compounds created when the superheated water reacts with the Earth’s crust. These compounds, such as iron sulphide from black smokers, would have made excellent catalysts not just for abiogenesis but also for the following reactions needed for life to increase in complexity.
Finally the most important thing hydrothermal vents contain is the vital energy gradient. In fact there are two, firstly there is the temperature gradient between the hot and cold waters mixing together. Secondly the water that comes out of most hydrothermal vents is very alkaline due to the chemicals it mixes with which would have been in contrast with the acidic early oceans, creating a chemical gradient. Either of these gradients, although most likely the chemical imbalance, could have provided the energy needed to kick-start the reactions to create the first replicators. On top of this the vents physical structures provide the perfect place for these reactions to occur. The way vents form creates a porous mesh across which the gradients between hot alkaline vent water and cold acidic ocean are at their greatest. This means chemical reactions could easily happen across their surfaces.
What makes hydrothermal vents such a compelling theory for abiogenesis is not just that it could theoretically provide the conditions for these vital reactions. But also that it has been scientifically tested. Researchers have actually managed to create nucleotides, the base components of DNA, in artificial vents by mimicking the conditions of the early oceans. However this doesn’t prove that’s how life was created, because nucleotides can be created simulating other abiogenesis scenarios as well. But it does make it one of the most reliable potential candidates.
Base of the web
Further evidence for hydrothermal vents as the source of abiogenesis is the abundance of life that lives around them today. The longstanding view of marine food webs is that they are based underpinned by phytoplankton and sunlight. But recent studies suggest hydrothermal vents may play an equally important role in marine food webs. That is because of animals like giant tube worms, Riftia pachyptila, that grow on the outside of some hydrothermal vents. What makes these organisms so remarkable is that they don’t eat anything. Instead they have a specialised organ called a trophosome which contains bacteria capable of creating energy from the sulphur in the vents. They provide the chemically generated energy to the worms as part of a symbiotic relationship which is not only important to the worms but also underpins an entire deep sea ecosystem. So even if life did not originate at hydrothermal vents they are still vital to ensuring that life today continues to flourish in the oceans.
So life may very well have originated in hydrothermal vents but it is not the only plausible candidate. There are a number of other theories which we will now briefly look over. Firstly is the primordial soup theory, the oldest in the list first put forward in 1924, which is based on the idea of life emerging from chemically rich shallow pools on the earth’s surface. In this scenario the energy gradient that created the first replicators was either intense UV due to a reduced atmosphere or a lightning strike. This is one of the only other theories to be successfully tested for in laboratory conditions. The main problem with this theory is that life would have had to start and develop into complex life in a small isolated system that likely would not have enough resources to sustain it. On the other hand if life developed in the oceans it would have near unlimited resources, time and space to evolve.
Conflicting theories also suggest that extreme energy gradients were not the origin of life but instead slower constant reactions in a more stable environment were the cause. There are two such stable environments that have been put forward. Firstly is thick ice sheets, like those during an ice age. During these periods the ice surrounding the earth’s oceans would have been miles thick creating a constant and isolated environment in the oceans below. The other theory is that life developed slowly over time within clay rich in minerals. The main problem with both of these theories is that the emergence of life would be due more down to chance than any particular driver which is much less likely.
Finally for sci-fi fans out there we come to the theory of Panspermia. In this theory life did not originate on Earth at all but travelled here from somewhere else in the cosmos. And no I’m not talking about ET and his buddies flying here on their space ships, instead single celled organisms or simple replicators may have crash landed here. Panspermia is the idea a basic form of life from a different life covered world destroyed in a catastrophic collision or event survived within meteorites or other debris. You may think this is a ridiculous idea but it actually holds merit and is taken very seriously in astrobiological studies. Its main issue is life surviving the extreme temperatures of entry through our atmosphere and thousands of years in cold space. But there are already some organisms on Earth who are capable of surviving such extremes. Maybe we really are all just aliens living on a strange planet far from home?
Water in the solar system
So there are many plausible explanations behind abiogenesis, but for a little while let’s assume that life was definitely created in hydrothermal vents. What would that mean for extra-terrestrial life in our universe or even our own solar system? Because Earth is not the only place in the solar system you can find oceans. As recently as last year astronomers discovered a sub-glacial lake on the pole of Mars an indication the planet may once have been covered in oceans much like ours. If we venture further out to Saturn its moon Enceladus is now believed to have a subsurface ocean. It was discovered in 2005 by NASA’s Cassini space craft and since has been shown to have geysers and even small amounts of salt, nitrogen, carbon dioxide.
As we continue outwards to Jupiter two of its moons contain water, Europa and Ganymede. It is believed an ocean resides beneath Europa’s surface, kept from freezing by Jupiter’s gravitational effects. Some estimates even suggest the ocean could be up to 100km deep making it twice the volume of our own oceans. Whereas Ganymede may have a subsurface ocean that is rich in salts but resides beneath 150km of ice. But less is known about these two than other examples closer to us. Other potential candidates include an asteroid called Ceres and the Ice giants Neptune and Uranus who lie on the very edge of our sun’s reach. The question is could any of these watery worlds harbour any form of life?
Are we alone?
So could foreign oceans in our solar system harbour life born in hydrothermal vents? The answer maybe, but probably not. The reason is that to have hydrothermal vents you need high levels of tectonic activity within the planet or moon that the ocean belongs to. Unfortunately none of the potential candidates are believed to be active enough for this to occur. Saturn’s moon Enceladus does have geysers and has even shown some precursors to life making it possibly our best shot. But it’s not actually tectonically active and the geysers are instead a result of Saturn’s gravitational effects. But that doesn’t mean these oceans are necessarily lifeless.
As you may have noticed most alternative explanations for abiogenesis involve water and in particular oceans. Take for instance the ice sheet theory where life slowly sparks in to life through chance in an isolated system. Both Europa and Ganymede have thick ice sheets and potentially large isolated oceans so if this theory has any merit so does the idea they could have life. Maybe a primordial soup is cooking in the geysers of Enceladus or even some form of life trapped inside the asteroid Ceres waiting for a panspermic collision with another world. Then there are countless exoplanets surrounding alien stars across the cosmos. Surely one of those must contain an ocean whilst also being tectonically active.
The beautiful thing about abiogenesis is that it doesn’t really matter how it happened just that it did happen and it only has to happen once for life to evolve into something as complex as we see around us today. Maybe life on Earth was created by hydrothermal vents but that doesn’t mean there isn’t an extra-terrestrial race out there who were created in a primordial soup or in some mineral rich clay. When the dinosaurs were killed who’s to say a piece of life filled Earth wasn’t hurled through the universe just to set up shop somewhere else. Abiogenesis occurs and then evolution takes over.
To sum up
The truth is it’s almost impossible to say for sure whether hydrothermal vents are the source of abiogenesis. That’s because there are so many theories and no real way to indefinitely prove any of them without jumping in a time machine and going to find out. Maybe one day we will solve the abiogenesis problem but honestly the amazing thing is that we have evolved to a point where we can ask the question at all. As a science lover thinking about where we come from and whether or not we are alone is the pinnacle of what it means to be human. But one things for certain whether or not life sprang forth from hydrothermal vents or not they are incredibly interesting and vitally important to marine life today. They are also a reminder that we are far more connected to the oceans than we realise.