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.