University of Washington
Life as we know it requires phosphorus. It’s one of the six main chemical elements of life, it forms the backbone of DNA and RNA molecules, acts as the main currency for energy in all cells and anchors the lipids that separate cells from their surrounding environment.
But how did a lifeless environment on the early Earth supply this key ingredient?
“For 50 years, what’s called ‘the phosphate problem,’ has plagued studies on the origin of life,” said first author Jonathan Toner, a University of Washington research assistant professor of Earth and space sciences.
The problem is that chemical reactions that make the building blocks of living things need a lot of phosphorus, but phosphorus is scarce. A new UW study, published Dec. 30 in the Proceedings of the National Academy of Sciences, finds an answer to this problem in certain types of lakes.
The study focuses on carbonate-rich lakes, which form in dry environments within depressions that funnel water draining from the surrounding landscape. Because of high evaporation rates, the lake waters concentrate into salty and alkaline, or high-pH, solutions. Such lakes, also known as alkaline or soda lakes, are found on all seven continents.
The researchers first looked at phosphorus measurements in existing carbonate-rich lakes, including Mono Lake in California, Lake Magadi in Kenya and Lonar Lake in India.
While the exact concentration depends on where the samples were taken and during what season, the researchers found that carbonate-rich lakes have up to 50,000 times phosphorus levels found in seawater, rivers and other types of lakes. Such high concentrations point to the existence of some common, natural mechanism that accumulates phosphorus in these lakes.
Today these carbonate-rich lakes are biologically rich and support life ranging from microbes to Lake Magadi’s famous flocks of flamingoes. These living things affect the lake chemistry. So researchers did lab experiments with bottles of carbonate-rich water at different chemical compositions to understand how the lakes accumulate phosphorus, and how high phosphorus concentrations could get in a lifeless environment.
The reason these waters have high phosphorus is their carbonate content. In most lakes, calcium, which is much more abundant on Earth, binds to phosphorus to make solid calcium phosphate minerals, which life can’t access. But in carbonate-rich waters, the carbonate outcompetes phosphate to bind with calcium, leaving some of the phosphate unattached. Lab tests that combined ingredients at different concentrations show that calcium binds to carbonate and leaves the phosphate freely available in the water.
“It’s a straightforward idea, which is its appeal,” Toner said. “It solves the phosphate problem in an elegant and plausible way.”
Phosphate levels could climb even higher, to a million times levels in seawater, when lake waters evaporate during dry seasons, along shorelines, or in pools separated from the main body of the lake.
“The extremely high phosphate levels in these lakes and ponds would have driven reactions that put phosphorus into the molecular building blocks of RNA, proteins, and fats, all of which were needed to get life going,” said co-author David Catling, a UW professor of Earth & space sciences.
The carbon dioxide-rich air on the early Earth, some four billion years ago, would have been ideal for creating such lakes and allowing them to reach maximum levels of phosphorus. Carbonate-rich lakes tend to form in atmospheres with high carbon dioxide. Plus, carbon dioxide dissolves in water to create acid conditions that efficiently release phosphorus from rocks.
“The early Earth was a volcanically active place, so you would have had lots of fresh volcanic rock reacting with carbon dioxide and supplying carbonate and phosphorus to lakes,” Toner said. “The early Earth could have hosted many carbonate-rich lakes, which would have had high enough phosphorus concentrations to get life started.”
Another recent study by the two authors showed that these types of lakes can also provide abundant cyanide to support the formation of amino acids and nucleotides, the building blocks of proteins, DNA and RNA. Before then researchers had struggled to find a natural environment with enough cyanide to support an origin of life. Cyanide is poisonous to humans, but not to primitive microbes, and is critical for the kind of chemistry that readily makes the building blocks of life.
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The research was funded by the Simons Foundation’s Collaboration on the Origins of Life.
I’m a geologist and have worked in mineral exploration for four decades. For the last several years I have worked in northern Ontario where the rocks are around 2 billion years old. I call them earths ‘mid life crisis’ because they were deposited during the Huronian glacial events which represented several glaciations or ‘snowball earth’ events which saw ice cover over most of earths surface in several glacial events. It also was the first sign of significant oxygen in the atmosphere. These rocks contain widespread fossils of bacteria mounds and beds called stromatolites. Bacteria clearly ruled the world at this time and stromatolites are abundant in this cold dark paleoproterozoic environment.
Bacteria were clearly well developed by 2 billion years and I suspect earlier bacterial life in the Archean were developing between 4-3.5 billion years in what I believe would have been a very different environment than what we have today. We do not know the composition of the sea which would likely be iron rich, saline and acidic.
Archean earth atmosphere composition is also unknown and although it lacked oxygen and would not have protective ozone to block the suns lethal UV light it would have formed a much thinner blanket than the atmosphere today. As a result the sun would have been very much hotter than today. Day-night and seasonal variations would have been much more pronounced. Very hot days and freezing nights, hot summers and ice cover in winter.
The most stable and protective environment for microbes, possibly bacteria to evolve would be beneath polar ice and protected from damaging UV light.
We now have the technology to examine the environment in Antarctic lakes covered by hundreds of metres of ice for hundreds of thousands of years. These lakes have been found to contain thriving bacterial communities with many microbes using oxygen-free metabolic processes for their energy. These are the equivalent environments to the anoxic Archean systems in which life commenced and these are the communities we need to examine to determine early life forms.
Chemosynthesis likely preceded photosynthesis, and operated in an O2 free environment, but with plenty of hydrogen sulfide and methane.
Compare chemosynthesis
CO2 + 2 H2S -> CH2O + 2S + H2O
and photosynthesis
CO2 + H2O -> CH2O + O2
Carbohydrate biochemistry is supported by chemosynthesis and photosynthesis. Life does not require free O2. Life exists now underground, and may have originated, or persisted underground after meteor bombardment. There may have been different competing biochemistries on Earth, but only one emerged. All living things on Earth have more or less the same genetic code, the same encoding of amino acids by a series of 3-base codons in a DNA sequence specifying which amino acid should come next in the chain of a protein.
We may never find evidence of the true origin of life if it started deep underground billions of years ago. Deposits of sulfur in cracks of rocks that held running water? Or, someday future generations may find evidence of life on Earth having a different biochemistry, but went extinct. If life exists on Mars, it is likely underground also, where there is warmth and liquid water, and abundant chemicals to support whatever biochemistry Mars may have developed. Don’t expect DNA, and even if DNA, not the same genetic code.
There is some speculation life may have originated on Mars, which was more habitable 4+ billion years ago. The story goes martian life was blasted off the surface by meteorite impact, and traveled frozen to Earth. The argument is also interesting in terms of RNA biochemistry and phosphate. Fun to think about.
https://www.nationalgeographic.com/news/2013/9/130905-mars-origin-of-life-earth-panspermia-astrobiology/
Chemosynthesis definitely preceded photosynthesis, as practiced by cyanobacteria and plants.
Cyanobacteria evolved from chemosynthetic ancestors.
Yes, I’m used to writing “likely” to tone down my opinionated self, and I should not have used it here.
That’s usually the best “science communication” policy.
There might have been ozone, thanks to photodisassociation of water.
High UV light flux in the Late Hadean and Early Archaean might actually have helped life develop.
Boron on earth is nothing unknown. most you find in Turkey, Mojave Desert Argentinia. What you find in deeper earth is unknown. If you follow T. Gold, that sholdt be the place of lifs start.
Fluid water will be to find there too, very hot, so that bacteria still survive around 100°C and live near volcanoes until today.
https://en.wikipedia.org/wiki/20_Mule_Team_Borax
PS
Boron (B) you find in nearly all you eat, so why should that be of Mars origin ?