"But if (and oh what a big if) we could conceive in some warm little pond with all sorts of ammonia and phosphoric salts, light, heat, electricity etcetera present, that a protein compound was chemically formed, ready to undergo still more complex changes [..] "
~Charles Darwin, in a letter to Joseph Hooker (1871)
All life on earth is related. Trace back the separate lines of descent of all organisms that ever lived, and they will converge to a single point of origin - the beginning of life. Charles Darwin was reluctant to publish his views on life's origin. His only speculations on the subject are known from a private letter to his friend and colleague Joseph Hooker, in which he speaks of a 'warm little pond' in which the first molecules of life could have formed.
A new and controversial study suggests Darwin's stab in the dark hit close to truth. In the article that was published earlier this week, researchers claim that the first cells evolved in volcanic pools. This new hypothesis brings the origin of life debate back from the depths of the oceans to the surface of the earth - other scientists believe hydrothermal vents in the deep sea are the most conducive environments for nascent life.
The researchers, led by Armen Mulkidjanian, presume that the chemistry of modern cells mirror the original environment in which life first evolved. Since oceans and cells are chemically dissimilar, they think it is unlikely life evolved there. The chemical nature of volcanic pools, or 'warm little ponds', resembles the cell's composition of its cytoplasm much more closely.
The researchers invented the term 'chemistry conservation principle' for their idea that organisms retain their chemical traits throughout time. They reason that the membranes of the first cells must have been simple and leaky. Metal ions could have flowed in and out unhindered, leading to a equilibrium between environment and protocell. As the cells adapted to the ion levels in their surroundings, they came to depend on them. Circumstance became necessity. Cells evolved ion pumps and iontight membranes to maintain the ion balance that was initially forced upon them - hence the assumption that cells themselves are reflections of their ancestral environment.
This is not a new approach. The Canadian biochemist Archibald Macallum applied it as early as 1926, when he noted that ion levels were similar between blood and sea water and concluded that animals must have evolved in the sea. "Maccallum was also the first to measure the concentrations of ions within cells", says Mulkidjanian. "He discovered that all modern cells contain more potassium than sodium."
This century old observation is one of the cornerstones of Mulkidjanian's argument: potassium outnumbers sodium in living cells, yet in oceans and lakes, sodium dominates. Other ions, like zinc, magnesium and phosphate are also present in much higher concentrations in modern cells than they are in oceans of past and present.
The same small set of ions is built into the core machinery of the cell, inherited from the last common ancestor of life. The backbone of DNA is made of phosphate, many ancient proteins require zinc, and the cell needs potassium ions to solder amino acids together in the manufacture proteins, one of the most important chemical reactions in life.
From these observations, Mulkidjanian and his colleagues conclude that it is unlikely life evolved in the sea. They think terrestrial springs, like those in Yellowstone Park, are much better candidate environments for the earliest evolution of life. They argue that geothermally active pools are the only places on earth where potassium, zinc, magnesium and phosphate are found in high enough quantities to explain the ionic content of cells.
Aside from containing the right mix of ions, the researchers list several other features that make volcanic pools suitable cradles for early life, borrowing heavily from the theories that were developed to explain how life could have formed in hydrothermal vents. "In water that flows through hot rock, organic molecules are spontaneously produced through a process called serpentinization", says Mulkidjanian. "Michael Russell was the one who first brought this reaction under the attention of origin of life researchers. He also noted that hydrothermal vents are not solid, but porous. He suggested these pores could serve as hatcheries for the first cells."
Mulkidjanian incorporated both these ideas in the volcanic pond model. "Serpentinization can also proceed in continental rocks. Indeed geochemists find organic molecules in the vapour that escapes from the surface. And our geochemical analyses show that in the past, porous minerals would have been deposited on the bottom of geothermic pools rather than mud, because of the lower acidity at the time. Cells could have used these honeycomb structures to survive. In a sense we took these ideas that were developed by Russell for the origin of life in the deep sea, and brought them to the surface."
In their paper, the researchers write that 'the terrestrial scenario outlined here incorporates all the features of the hydrothermal vents that favour the origin and early evolution of life, and adds more.' But not everyone is convinced.
"The 'principle of chemistry conservation' is a postulate rather than a proven principle", says Jim Cleaves from the Carnegie Institution of Washington. "It may be true on short time scales, but who can say what has happened since the origin of life?"
"Overall, I think it is questionable that organisms would have kept their original composition, given the variability observed in present cells. Is it not at least equally likely that they have modified their cytosolic composition once they had control over this process? Any modern environment which matches this composition would then be purely coincidental. In summary, I don't get much from this paper that I would hang my hat on."
Jack Szostak, professor at Harvard Medical School and 2009 winner of the Nobel Prize, has similar doubts about the chemical conservation principle, but he does not dismiss volcanic pools entirely. " If there is a reason that a high potassium/sodium ratio is biochemically a good thing, then a prebiotic scenario that provided such a ratio might have been more favorable for the origin or early evolution of life", says Szostak. "But we can't rule out an origin in a low potassium environment followed by selection for high internal potassium."
"Independent of these arguments, I do not think the oceans were a favorable environment for the origin of life. Fresh water ponds seem more favorable due to the lower salt and ion concentrations, which would allow for fatty acid based membranes to form. The accumulation of organic compounds in ponds is also easier to imagine than in the ocean, and geothermally active areas provide numerous advantages, as expressed by the authors."
Michael Russell, one of the pioneers of the hydrothermal vent hypothesis, did not want to comment on the study.
Do I think life began in volcanic waters myself? I don't know. What I do know, is that the origin of life has always been a topic that has divided scientists and poisoned debates. This is not surprising. The questions about who we are and where we come from incite controversy, precisely because they are dear to us all. Still I think it's important to not dismiss new ideas outright, especially if they have been thought through. The wider our gaze, the higher our chance that we will one day find our warm, little pond.
Mulkidjanian, A., Bychkov, A., Dibrova, D., Galperin, M., & Koonin, E. (2012). PNAS Plus: Origin of first cells at terrestrial, anoxic geothermal fields Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1117774109
Photo of Mutnovsky Volcano copyright Anna S. Karyagina
Table adapted from reference
Figure from reference
Read Jack Cleaves' complete, unedited reply here.
Read Jack Szostak's complete, unedited reply here.