The Phosphate Problem for the Origin of Life May be Solved
And it may help us decide whether to search for life on ocean worlds or lake worlds.
In a new paper just published in Proceedings of the National Academy of Sciences, Jonathan Toner and David Catling of the University of Washington in Seattle lay out their case that carbonate-rich lakes might have been a critical requirement for the origin of life.
Phosphorus is essential for creating life’s building blocks, such as DNA, ATP, and lipids. But a long-standing problem has been where early life would have gotten its supply of phosphorus molecules, which typically are found only in tiny amounts in the natural environment. In fact, phosphorus is much more enriched in terrestrial organisms—compared to its natural background abundance—than carbon is. That’s why I have argued in the past that we should label life on Earth as being phosphorus-based rather than carbon-based.
Considering that it’s so rare, and that most of it usually converts into the mineral apatite (which life can’t access directly), how could phosphorus have reached high enough concentrations for life to construct cells?
Toner and Catling provide an elegant solution with their finding that the concentration of phosphorus in carbonate-rich lakes is up to 50,000 times higher than in seawater. And when lake water evaporates, as in a desert environment, the enrichment can be even much higher. The main reason is that phosphorus in these lakes is not turned into apatite. Calcium in the water takes up the carbonate, and the phosphorus stays available.
Carbonate-rich lakes, like the Searles Lake in California or Lake Magadi in Africa, exist today. But they would have been much more common at the time life originated on Earth. Increased volcanic activity would have produced an abundance of phosphorus-containing rocks, which would have weathered more readily under Earth’s early, carbon-dioxide-rich atmosphere. Large amounts of phosphorus would have been set free. Since there were no organisms to scoop it up right away, it would have accumulated in lakes to even higher concentrations than in today’s carbonate-rich lakes.
The phosphate problem is critical for understanding the origin of life, and the authors may have solved it. If they’re right, their work suggests that life originated in surface-water lakes. Other people, such as David Deamer, have suggested that life began in hydrothermal fields in freshwater environments on Earth’s surface. But most scientists still favor an origin at suboceanic hydrothermal vents. Will this new research by Toner and Catling turn the tide of opinion? Only follow-up work and time will tell.
Furthermore, if an enrichment of phosphate in carbonate-rich lakes is needed for the origin for life—or at least for phosphorus-based life as we know it—we may search in vain for life on so-called ocean worlds like Jupiter’s moon Europa. On the other hand, Mars is carbonate-rich, has plenty of carbon dioxide, and would have been a prime location for the kind of lakes that may have given rise to the first living things on Earth.