Did Life Start out at the Bottom of the Ocean?
New Research Points to Alkaline Vents as Life's Hatchery
February 12, 2010
How life started on Earth is shrouded in mystery. It is difficult to imagine how something so complicated could have appeared.
Life needs an energy source, lots of different chemicals, something like DNA that can store information and be passed on, something to speed up chemical reactions and a cell membrane to keep it all in. This is a lot! And surprisingly, alkaline vents at the bottom of the ocean may have been able to provide all of this and more.
Is this where life began?
Alkaline vents form under the ocean at places where tectonic plates separate. Large carbonate mounds spring up that are bathed in hydrogen gas and carbon dioxide. A few billion years ago, there would also have been a lot of dissolved iron and sulfur too because there was little or no free oxygen around. And all of these chemicals would have been concentrated in small cell sized pores within the mounds.
Finally, because oceans were almost certainly acidic when life started 3.5 billion years ago, something called a proton gradient would have been set up within the mound. This gradient would provide the energy to create a primitive form of ATP to drive the creation of life.
These features would have provided the perfect pot in which life could brew. The mounds have the building blocks for life's chemicals, premade cell-sized pores, ways to concentrate chemicals and speed up their reactions, and an energy system to get it all started.
And this isn't all. A close look at some of the processes all life shares shows that there still may be a bit of that vent chemistry around today. Processes like the Krebs cycle, protein with iron and sulfur as key parts, and many other aspects of current life all could have come from these vents. Alkaline vents are the most compelling scientific possibility for the start of life so far.
A word of caution"Šnote the word possibility. None of this is proved and it is only an idea at this point. But it certainly presents a plausible way that life may have gotten started all those billions of years ago.
The Concentration Problem
Most origin of life theories are undone by concentration problems. How do chemicals get concentrated enough to react with one another? And if an interesting chemical does get made, what's to keep it from floating away?
Life needs many chemicals to be created, stay around, and work together. Alkaline vents as the start of life is the only theory so far that provides for a way to corral these chemicals and concentrate them enough to get them to react.
Modern cells need
Alkaline vents came
with premade ones.
One way the mounds accomplish this is with their premade cell sized pores. These pores could act like primitive cell membranes keeping any chemicals confined.
In fact, this is one of the important things cell membranes do today. They keep good stuff in and bad stuff out.
Another concentration problem is that as complicated chemicals get made, the chemicals they were made from disappear. Not only do the more complicated chemicals stop getting made, but some of them can even fall apart too.
For example, most scientists agree that one of the early chemicals necessary to get life going is RNA. Without it, you just end up with various pots of soup that can't pass on any successful strategies.
Why RNA? Because it is like a hybrid between DNA and proteins. It can both store and pass on information and make chemical reactions go faster. In fact, it is thought to be so important that early life is said to have lived in an RNA world.
Let's get back to the concentration problem. At high concentrations, bits of RNA will form long strands. But when the individual units of RNA become scarce, the longer RNAs break up into smaller pieces. Not a great way to store information!
The vents help get around this problem through thermal currents and diffusion. The waters around the mounds are being mixed constantly and because of the way the mounds are built, chemicals concentrate in some parts of them.
So some of the pores would have a constant supply of chemicals to keep building up RNAs and other chemicals. As long as the chemical reactions could happen fast enough, that is.
The Reaction Speed Problem
Most reactions that make life's chemicals are incredibly slow. Any origin of life theory has to come up with a way to speed them up.
Today's life uses proteins to speed things up. Each protein is suited for a single chemical reaction. Life inside a cell is like a choreographed dance of thousands of proteins each speeding up the appropriate reaction at the right time.
Of course, this wasn't an option 3.5 billion years ago since there weren't any proteins around just yet. But this is probably OK. In the beginning, there is no need for staged choreography. Wild dancers doing their own thing were just fine.
Without proteins like this,
pre-life reactions were
accelerated directly by
iron and sulfur.
At life's beginning, the most important thing is to get lots of reactions going constantly and quickly. So how can alkaline vents accomplish this? With a little help from iron and sulfur.
Before there was a lot of free oxygen, the oceans are thought to have contained lots of iron and sulfur. These chemicals would have almost certainly decorated the walls of the cell-sized pores.
Both iron and sulfur are very good at speeding up many of life's key chemical reactions. The idea is that the iron and the sulfur on the inside walls of these pores would accelerate lots of different reactions within the pores. In fact, many proteins still use iron and sulfur today.
Of course without proteins specifying which reactions to speed up, every reaction that can happen faster will. And this is how things must have been in these alkaline vents. Chemical reactions furiously happening as a seemingly endless supply of simpler chemicals washes into the pores.
An Energy Source
The lack of free oxygen not only affected what chemicals were dissolved in the ocean but it also made the oceans acidic. And that is where the energy might have come from to create all of these wonderful chemicals.
The acidic ocean and the alkaline vents would have set up a protein gradient. Basically, this is similar in many ways to how modern cells make energy.
Life today uses this sort of process to create ATP. And ATP is used for energy to drive most everything every living thing's cells do.
A primitive form of ATP called acetyl phosphate could theoretically have been made under the conditions described so far. If true, it would have provided the energy for these protocells to churn out lots of different chemicals.
Putting it All Together
Did all of this start out in an
alkaline vent under the sea?
Now we have energy molecules to power chemical reactions that are sped up in cell sized pores awash in useful starter chemicals. From this, RNAs would be made that could make copies of themselves. Those RNAs that can help in creating more primitive ATP and precursor molecules would become more common than those that couldn't. The start of natural selection!
All well and good but now more experiments need to be done. What chemicals actually get made when scientists mimic these conditions? If nothing useful gets made, what else needs to be added? Is it conceivable that that extra thing could have been around back then? I can't wait to see the results of these kinds of experiments!
This may or may not be how life started. But it is definitely a plausible way that it might have.
Solving Chicken and Egg Problems
One of the great things about this idea is that it gets scientists past many of the chicken and egg problems that plague any theories about the start of life. Here are four that no other model so far deals as well with:
How can life get started without a cell membrane? How can a cell membrane get made without DNA and proteins to make it?
How can organic chemicals be made without the proteins to make them? How do proteins get made without organic chemicals?
How can DNA form since it can't do anything on its own? How can proteins get made and the information for making them be passed down without DNA?
How can ATP be made without proteins? And how can proteins be made without ATP?