The early Earth may have hosted many carbonate-rich lakes, which contained high enough phosphorus concentrations to get life started on our planet, according to a study. Phosphorus is one of the six main chemical elements of life, and forms the backbone of DNA and RNA molecules.
The element acts as the main currency for energy in all cells and anchors the lipids that separate cells from their surrounding environment. "For 50 years, what's called 'the phosphate problem,' has plagued studies on the origin of life," said Jonathan Toner, an assistant professor at the University of Washington (UW) in the US.
The problem is that chemical reactions that make the building blocks of living things need a lot of phosphorus, but the element is scarce. The new study, published in the journal Proceedings of the National Academy of Sciences, has found an answer to this problem in certain types of lakes.
The research focused on lakes rich in carbonate -- a salt of carbonic acid -- 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 said. They first looked at phosphorus measurements in existing carbonate-rich lakes, including Mono Lake in California, US, 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, the researchers said.
Today these carbonate-rich lakes are biologically rich and support life ranging from microbes to Lake Magadi's famous flocks of flamingoes, they said. The researchers carried 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 noted that the reason these waters have high phosphorus is their carbonate content. In most lakes, calcium, which is much more abundant on the Earth, binds to phosphorus to make solid calcium phosphate minerals, which life can't access, according to the researchers.
However, in carbonate-rich waters, the carbonate outcompetes phosphate to bind with calcium, leaving some of the phosphate unattached, they said. 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. It solves the phosphate problem in an elegant and plausible way," Toner said.
The researchers explained that 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 David Catling, a professor at UW.
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, the researchers said. Carbonate-rich lakes tend to form in atmospheres with high carbon dioxide, which dissolves in water to create acid conditions that efficiently release phosphorus from rocks, they said.
"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.