How did cholesterol evolve? Oil trapped in ancient rocks hides clues

Ancient life forms may have left traces of oily molecules in rocks more than 1 billion years ago, providing new insights into the evolution of cholesterol. The molecular fossils, described today in Nature, suggest early organisms that relied on precursors of cholesterol were widespread on ancient Earth. Later, rising levels of oxygen allowed organisms to make the more sophisticated version of the molecule we know—and sometimes hate—today.

Cholesterol gets a bad rap for its role in heart disease, but animal cells can't live without it. Our cell membranes are roughly 30% cholesterol; the molecule keeps membranes flexible over a range of temperatures and plays a key role in receiving signals from other cells. Cholesterol belongs to a family of similar molecules called sterols. Animal cells make cholesterol in a complex, 37-step process. Other eukaryotes—organisms with complex cells—produce their own sterols, including stigmasterol in plants and ergosterol in fungi.

Geochemists and paleontologists look to fossilized traces of these sterols as evidence for the presence of eukaryotes in ancient ecosystems. Researchers commonly find them in rocks as old as 800 million years. But in older rocks, remains of sterols seemed to be missing. That was puzzling, because at least some fossil and genetic evidence suggests eukaryotes had evolved by 1.6 billion years ago.

An old idea from biochemist Konrad Bloch provided one potential explanation. Bloch, who was awarded the 1964 Nobel Prize for his work deciphering the chemical pathway cells use to make cholesterol and other sterols, speculated in the 1990s that as this pathway evolved over time, earlier life forms might have used the intermediate chemical products in their cell membranes in place of today's sterols. He called these compounds "protosteroids" or "ursterols," but, given the technology available at the time, he didn't think it would be possible to find evidence for them.

Since then, however, geochemical techniques have advanced. In the new study, geochemist Jochen Brocks at the Australian National University and his colleagues went looking for the fossilized remains of some of these protosteroids. First, they synthesized protosteroids called lanosterol, cycloartenol, and 24-methylene cycloartenol in the lab. Then, they mimicked the fossilization process by exposing them to heat and pressure. In doing so, the scientists identified dozens of derivatives of these molecules that could distinguish them from other steps in the cholesterol pathway.

The researchers then searched for those compounds in ancient rocks. In sediments that formed 1.64 billion years ago, they found chemicals that matched the derivatives of lanosterol and cycloartenol. And in 1.3-billion-year-old rocks, they found derivatives that matched the pattern produced by 24-methylene cycloartenol, which is one step further along the sterol pathway than cycloartenol. The ancient rocks "were oozing with these molecules," Brocks says. "I wish I could call Konrad Bloch and tell him, ‘We found them!’" (Bloch died in 2000 at the age of 88.)

In younger rocks, formed between 800 million and 720 million years ago, the researchers found a mix of old and new: as expected, they found traces of cholesterol and other modern sterols. But they also found significant quantities of fossilized protosteroids, suggesting the organisms that relied on them had not yet gone extinct. The proportion of protosteroids decreased over time, with modern sterols dominating in rocks younger than 600 million years.

"The data are beautiful," says organic geochemist Fabien Kenig at the University of Illinois Chicago. "We move from a protosterol world to a [modern] sterol world."

The work "provides the first clear example of the evolution of [sterols] over time," says James Sáenz, an expert in membrane biochemistry at the Dresden University of Technology. The final steps of cholesterol synthesis are expensive for cells, he notes, requiring plenty of energy and oxygen, but they are apparently worth it.

Brocks and his colleagues propose that when oxygen was relatively scarce on early Earth, eukaryotes that used protosterols in their membranes dominated our planet's seas, rivers, and lakes. As oxygen became more abundant about 800 million years ago, some eukaryotes started to modify those compounds to make new sterols, which gave their cells an evolutionary advantage. Eventually, the authors say, the organisms that relied on protosterols went extinct.

That's possible, says paleontologist Susannah Porter, who studies early eukaryotes at the University of California, Santa Barbara. But there are other explanations as well, she says, including that it was bacteria instead of early eukaryotes that made the protosterols. "It's a little bit of a jump to conclude that it had to be eukaryotes." Still, "It's great to be thinking about these molecules as having an evolutionary history," she says. "We may never know who made what, but it's exciting to try to think about this."