A NASA report has compiled decades of theoretical, laboratory and field research in order to give the most detailed picture so far on a popular theory of the origin of life on earth- the “water world” theory. The report has been published in the journal Astrobiology.
There are many theories of how life began on Earth some 3.5 billion years ago. One such theory is that life began from an RNA world. The idea is that since RNA can both store genetic information and catalyze chemical reactions, life may have begun as simple self-replicating molecules of RNA in primitive cells. Over time, DNA would have taken over as the genetic material within cells and proteins would have replaced RNA to catalyze reactions.
Another popular theory is that life began at deep sea hydrothermal vents, which are ancient chimney-like structures which spew out mineral-rich water at some 300oC due to tectonic activity. It is thought that the chemicals and energy found at these vents could have provided perfect conditions for primitive life to form. This theory is called the “submarine alkaline hydrothermal emergence of life” or “water world” for the sake of simplicity.
Contrary to earlier theories which suggested that life may have begun around a scorching hot and acidic type of deep sea vent called a black smoker, the NASA team believe evidence points to life originating in cooler, gentle springs with alkaline rather than acidic fluids. This was first theorized by Michael Russell in 1989.
Russell’s team believe that these more placid vents may have resulted in two critical chemical imbalances. First, when the hydrogen-rich alkaline water from the vents met the acidic ocean water, a natural proton gradient could have been generated within pores of the rocks, which could have been used as an energy source. Second, electron transfer could also have occurred when the hydrogen and methane-rich vent fluid met the carbon dioxide-rich ocean water, generating an electrical gradient.
Together, this means that in this location we have two potential systems at play which occur today in current life forms; proton gradients and electron transfer. For example, both take place in our own energy making factories- the mitochondria.
So we’ve got energy making systems. Is this alone sufficient for life? Probably not. But what these vents also provided was a source of minerals which could have behaved as enzymes, catalyzing reactions as chemicals came into contact with them. Two substrates are thought to have been important for these enzyme-like minerals; green rust and molybdenum. Green rust could have allowed the utilization of the proton gradient to generate molecules that contained phosphate, which in turn could have stored the energy produced. Molybdenum, which is found in our bodies, assists in electron transfer.
What’s also interesting is that two years ago, researchers investigating hydrothermal vents also came up with a similar proton gradient hypothesis. They also envisaged a situation where tiny pores existing in the walls of the vents that housed proton gradients could also have acted as catalytic “proto-cells”, concentrating simple carbon-containing molecules that could have formed from the reaction of CO2 and H2 with the enzyme-like minerals present. These molecules could then have potentially reacted with each other to produce larger, more complex molecules such as amino acids or nucleotides.
Although these hypotheses are very difficult to test in the lab, it hasn’t deterred dogged scientists yet. But the scientists also believe that they can apply this knowledge to searching for life on other planets if they can find liquid oceans, which is exciting.
Check out the image below for an example of the replica chimney structures NASA have been creating in their Icy Worlds lab.
Image credit: NASA.