Scientists have discovered the remains of a “lost world” of mysterious lifeforms that thrived on Earth some 1.6 billion years ago and may be the oldest known ancestors of the lineage that eventually produced plants and animals, including humans, reports a new study.
The breakthrough detection of microscopic creatures called “protosterol biota” in ancient Australian rocks fills a major gap in our understanding of the early evolution of eukaryotes, a family that includes all lifeforms with nucleated cells. These organisms thrived in watery habitats across our planet about a billion years before the emergence of animals and plants, but they have managed to remain hidden in the fossil record until now.
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“Our findings illustrate that most life that ever existed is now extinct and therefore often overlooked, while these organisms may have played important roles in the evolution of complex life and may have shaped ecosystems for much of Earth history,” said Benjamin Nettersheim, a geobiologist at the University of Bremen’s Center for Marine Environmental Sciences (MARUM), who co-led the work, in an email to Motherboard.
For decades, scientists have worked to uncover the roots of our eukaryotic lineage in Earth’s “middle age,” a period that started about 1.7 billion years ago and lasted for a billion years. The simplicity of organisms at this time, combined with the scarcity of rocks with such ancient origins, has presented a huge challenge in the effort to reconstruct bygone ecosystems that ultimately led to all complex life on Earth.
Early eukaryotes produced sterols, a type of steroid compound, that are found in fossils, but these biomarkers seem to taper off in the record around 800 million years ago. Konrad Bloch, a Nobel-Prize-winning biochemist, predicted in the 1990s that the oldest eukaryotes might have produced primordial versions of these sterols, though he was doubtful that they could ever be identified in ancient rocks.
Now, Nettersheim and his colleagues have validated Bloch’s suspicions with the discovery of proto-sterols in rocks from Australia’s Barney Creek Formation, which date back more than 1.6 billion years.
These molecules represent “early stages of eukaryote evolution that did not yet possess a complete sterol biosynthetic pathway,” making them “witnesses of a lost world of ancient stem-group eukaryotes that were widespread and possibly abundant during Earth’s middle age” according to a study published on Wednesday in Nature.
“One of the great enigmas in geobiology was the absence of fossil sterols in all rocks older than ca. 800 million years,” Nettersheim said. “Konrad Bloch hypothesized that early in evolutionary history, now short-lived intermediates in sterol biosynthesis may have been fully-functional end-products, so we wondered if such primordial sterols may be preserved in these older strata.”
“While we knew Bloch’s hypothesis and had well-preserved rock samples from geological periods where the modern intermediates may have been biosynthetic end-products, it was still very surprising to discover how widespread and abundant these compounds are in these ancient rocks,” he continued. “It was extremely exciting to realize that these ancient rocks contained steroids after all. Realizing that 1.64 billion-year old rocks contained fossil proto-steroids was a true eureka moment for us.”
The detection of the primordial steroids exposes the existence of early microscopic eukaryotes that may have dominated many aquatic ecosystems during Earth’s middle age, potentially becoming Earth’s first predators. These organisms likely belonged to the “stem group” of eukaryotes, a term that refers to all the extinct relatives of a “crown group” that includes the last common ancestor of a family, and all its living relatives.
In other words, the protosterol biota may be the direct ancestors of modern eukaryotes, including humans, or they may have been closely related rivals to our microbial progenitors in the deep past.
“They were probably direct or indirect ancestors (more like cousins) of the eukaryotes alive today and probably competitors of modern groups of eukaryotes (crown groups),” Nettersheim explained. “Since they probably evolved earlier and likely already occupied most ecological niches, they may be responsible for the late expansion of modern eukaryotes. It may have taken additional evolutionary innovations or changing environmental conditions (such as increased oxygen concentrations) to eventually allow modern eukaryotes to outcompete their primordial relatives in most environments.”
Given that the protosterol eukaryotes are only known from their chemical byproducts, it’s difficult to speculate about what they looked like or how they proliferated across our planet. It’s possible that these primordial forebears were better suited to environments with lower-oxygen and energy requirements, giving them an advantage in Earth’s middle age when deoxygenated habitats were more common. However, as Nettersheim mentioned, the protosterol biota may have lost this edge some 800 million years ago as oxygen levels rose, a shift that facilitated the emergence of the crown group eukaryotes that persist to this day.
The new study has illuminated this ancient lost world by extending the timeline of steroids in the fossil record back by hundreds of millions of years. The results not only vindicate predictions by Konrad Bloch, who died in 2000, they also chart a new path forward to follow the threads of our eukaryotic lineage into the deep past.
For instance, Nettersheim said his team plans to use MARUM’s advanced instruments to “zoom into the cradle of eukaryotic life in unprecedented resolution to further improve our understanding of our early ancestors and their co-evolution with changing environmental conditions in the future.”
“Bloch was skeptical these molecules could be preserved in the ancient rock record. We wish we could tell him that we found them,” he concluded.
