The sad fate of the ancient, well-shelled sailors
In the Cambrian period, 500 million years ago, the armored suit ruled the seas. The soft-bodied animals secreted a mineral paste that hardened into protective shells of immense strength and decorative beauty, some shaped like rams’ heads or eagles’ wings, others like champagne flutes studded with dagger-sharp spines.
But by the Devonian period, some 70 million years later, most of these brachiopods, bryopods and related well-shelled mariners had died out, victims of theft and their own extravagant ways.
As researchers recently proposed in the journal Trends in Ecology and Evolution, the collapse of the brachiopod kingdom exemplifies the struggle that has defined life since its inception: the search for phosphorus. Scientists have long known that the element phosphorus is essential on many fronts, here holding DNA molecules together and there powering every cell movement. The new report highlights another way in which phosphate – a biochemically useful form of phosphorus – has shaped the course of evolution as an arbiter of nature’s hard parts, its shells, teeth and bones.
“Phosphorus was stolen by vertebrates, bony fish,” said Petr Kraft, a paleontologist at Charles University in the Czech Republic and author of the new report. “And when that happened, they quickly diversified and took over.” dr. Kraft collaborated with Michal Mergl from the University of West Bohemia.
The research is part of a renaissance in phosphate studies, an enterprise that spans disciplines and time frames. Chemists investigate how phosphates managed to flavor the prebiotic soup that gave rise to life, while materials scientists manipulate the element into stunning new colors and shapes.
“If you heat phosphorus under different conditions, different temperatures, different pressures, strange things start to happen,” said Andrea Sella, professor of inorganic chemistry at University College London. “You get red fibrous forms, metallic black forms, purple forms.” You can also stack layers of phosphorus atoms and then separate them into ultrathin and flexible layers called phosphorenes, all with the goal of controlling the flow of electrons and light particles on which the technology depends. “We’ve only scratched the surface of what this element can do,” said Dr. Sella.
Phosphorus was discovered at the end of the 17th century by the Hamburg alchemist Hennig Brand, who inadvertently isolated it while searching for the legendary “philosopher’s stone” that would turn base metals into gold. Experimenting with large quantities of the golden liquid he knew best—human urine—Brandt emerged with an eerie substance that lacked any Midas touch but glowed in the dark, prompting Brandt to christen it phosphorus, Greek for “bringer of light.”
This pure form of the element, called white phosphorus, turned out to be poisonous and flammable, and was used in warfare to make tracer bullets, smoke screens, and the Allied incendiary bombs that destroyed Brandt’s hometown during World War II.
White phosphorus also achieved dark Dickensian fame in the 19th century, when it was added to match tips to produce “hit anywhere” matches. Girls and women who worked in the poorly ventilated factories that produced the enormously popular product were sometimes exposed to so much phosphorus vapor that they developed “phosphorus jaw,” a horrific condition in which their gums receded, teeth fell out, and jawbones dissolved. According to historian Louise Raw, the matchmaker’s struggle for safer working conditions helped fuel the modern trade union movement.
Pure phosphorus does not exist in nature, but is bound to oxygen, as phosphate, and this molecular union, the phosphorus-oxygen bond, “is central to why biology works,” Matthew Powner, an organic chemist at University College London, said. The body stores and burns energy by constantly making and breaking phosphate bonds found in the cell’s little ATMs, its adenosine triphosphate molecules, better known as ATP. The phosphate recycling operation is so ruthless, said Dr. Powner, “you’re essentially converting your body weight into ATP every day.”
Phosphate joins with sugar to form the backbone of DNA, holding in meaningful order the letters of genetic information that would otherwise collapse into an alphabet soup. Phosphate colludes with lipid molecules to envelop each cell in an ever-vigilant membrane that dictates what gets in and what must be kept out. Proteins send messages to each other by exchanging phosphate packets.
Behind the spectacular, superior utility of phosphates lies a negative charge that prevents unwanted leakage. “You can put energy in and take it out only when you want to,” said Dr. Owner. “It will not leak into the environment.” In contrast, he said, the equivalent carbon-based molecule, called carbonate, dissolves easily in water: “If you put DNA together with carbonate and not phosphate, everything would fall apart.” dr. Powner joked that we should consider living on phosphate rather than carbon.
However, unlike the other main ingredients of life – carbon, nitrogen, oxygen, hydrogen – phosphate molecules do not have a gaseous phase. “They are too big to fly,” said Dr. Sella. Phosphates jump into the game of life through rock erosion, the breakdown of living organisms, or waste products like urine or guano. Understanding the impact of phosphate fluxes over time is a major research endeavor.
One long-standing mystery is how early life got phosphate to begin with. Given how essential phosphate is to every aspect of biology, the original aquatic environment in which the first cells arose must have been rich in phosphate. “However, most natural waters on Earth today are quite poor in terms of phosphate,” said Nicholas Tosca, a geochemist at the University of Cambridge. “We expected the same to be the case with the early Earth.” Iron, he explained, was thought to separate phosphates.
dr. Tosca and his colleagues from Cambridge tackled the puzzle of the origin of life in a study published recently in Nature Communications. The researchers decided to reexamine the assumption, asking: What was there in the beginning, when there was much less oxygen around? Oxygen, they knew, turns iron into a form that persistently accumulates phosphate. What would happen if oxygen were removed from the equation? The researchers created artificial seawater in a large glove compartment without oxygen and found that, sure enough, under those conditions, the dissolved iron left most of the phosphate alone, presumably available to all proto-cells in the neighborhood.
In the article Trends in Ecology and Evolution, Dr. Kraft similarly proposed that Cambrian seas were relatively supersaturated with phosphates. Animals could absorb so much, in fact, that they could form thick and durable shells, as hard as the hardest tissue in the human body – the phosphate enamel of our teeth.
“It is a great advantage to have these shells,” said Dr. Kraft. By comparison, the shell of a modern mollusk, made of calcium carbonate, cracks easily under the feet of beachgoers. But as the seas became crowded and bony fish appeared, phosphate supplies dwindled, and the brachiopods could no longer freely gather what they needed to build their expensive dwellings. Bony fish sensibly used phosphate as a building material: teeth, a few parts of the skeleton, and that was it. And because they are mobile, the fish were able to capture all the phosphates and other nutrients filtered from the land into the sea before they reached the bulky hard shells below.
The Vertebrates had taken control of the phosphate and nothing could stop them now.
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