Billion-year-old fat tells of a long history of complex cells

All organisms around us—plants, animals, and fungi—are eukaryotes, which consist of complex cells. Their cells have many internal structures surrounded by membranes that keep energy production separate from genetic material, etc. Even single-celled organisms on this branch of the tree of life have membrane-encased structures that move and rearrange to feed.

Some of this membrane flexibility comes from steroids. In multicellular eukaryotes, steroids have a variety of functions; among other things, they are used as signaling molecules such as estrogen and testosterone. But all eukaryotes insert various steroids into their membranes, increasing their fluidity and changing their curvature. Thus, the evolution of complex steroid metabolism may have been important in enabling complex life.

Now, researchers have traced the origins of eukaryotic steroids back to billions of years ago. The results suggest that many branches of the eukaryotic family tree once produced early versions of steroids. But our branch has developed the ability to produce more detailed versions – this has helped us compete with our relatives.

A messed up timeline

To some extent, the new work involves testing an idea proposed decades ago by biochemist Konrad Bloch. Bloch won the Nobel Prize for discovering the biochemical pathways that allow cells to produce steroids from simple precursors. In 1994, Bloch proposed that the chemical intermediates in the pathways he identified are end products at some point in our evolutionary pathways. Cells make uncomplicated steroids that play an important role in their survival; over time, our branch has developed enzymes and modified them to be more efficient.

It was an opportunity to make sense out of a variety of evidence that otherwise did not fit together very well. We found microfossils from 1.6 billion years ago that appear to show complex cells with surface processes usually restricted to eukaryotes. This works well with DNA evidence, which suggests that all modern eukaryotes can be traced back to a common ancestor at least 1.2 billion years ago, probably 1.8 billion years ago.

But we can also look for steroids in old rocks because the molecules are very stable. But steroids in modern eukaryotes do not appear until about a billion years ago – much later than eukaryotes themselves. If early eukaryotes used Bloch’s biochemical intermediates, this gap can be clearly explained.

It is here that Bloch, although he was right, made a big mistake. He hypothesized that the intermediates would be chemically unstable, so they wouldn’t survive long enough in sediments for us to find them. There was no point in looking at it.

Long

An international team of researchers decided it would be worthwhile to test Bloch’s hypothesis about the strength of these molecules. So a group of researchers synthesized and exposed the molecules to conditions of heating and accelerated aging and looked at what happened. Although they lost a few atoms on the sides of the ring structures, most of the molecule survived. And, most importantly, other steroids are not known to produce the same molecules when broken down, so these old intermediates can serve as tracers of steroid production.

With this information, researchers took samples of oil and bitumen from sediments from various points in the Earth’s past. Even the oldest specimen, 1.6 billion years old, had abundant remains of these steroid intermediates. The researchers isolated dozens of relatives of steroid intermediates, but found none of the molecules you’d expect from modern steroids.

Eukaryotes also seem to be ubiquitous. “These protosteroids have been identified in deep and relatively shallow water environments, microbial mats and pelagic habitats, shales and carbonates, as well as marine and possibly lacustrine basins,” the researchers write.

Again, the first signs of modern steroids don’t appear until a billion years ago, suggesting that eukaryotes—both our ancestors and other branches of the evolutionary tree—have been growing for nearly a billion years using molecules that are now merely chemical intermediates. Different classes of modern steroids also appear slowly in the geologic record, suggesting that there was no burst of innovation.

Surviving emergencies

Researchers propose an intriguing idea that would put the emergence of modern eukaryotes into the geological record. Eukaryotes appear to have originated during a period of geological time known as the “curious billion,” which occurred about 1.8 billion to 0.8 billion years ago. During this time, as the name suggests, not much happened. During much of this time, geology saw Earth’s continental plates assemble into supercontinents, helping to maintain a seemingly constant climate. Life seems to have responded to the relative stasis by forming the same stable ecosystems that have persisted for most of this time.

Although the ancestor of all modern eukaryotes evolved in the boring billions, the lack of ecological disruption may mean that it had a hard time finding an empty ecological niche. Given this challenge, the researchers hypothesize that the evolution of modern steroids may have given them the resilience needed to conquer extreme environments such as mudflats dominated by cold or high temperatures or periodically desiccated. This may mean that modern steroids were made, but only at levels that are unlikely to be detected.

The curious billion ended with an increase in tectonic activity and global glaciations, which could have caused the microbial equivalent of a mass extinction. Tolerating the environmental extremes allowed by modern steroids in the established turbulent environment may have allowed our ancestors to drive all other branches of the eukaryotic tree to extinction.

Nature, 2023. DOI: 10.1038/s41586-023-06170-w (About DOI).