Discovery Unearths Primordial Organisms’ Remains from an Extinct Epoch

Picture an assortment of primeval eukaryotic organisms, the ‘Protosterol Biota,’ dwelling within a bacterial mat on the seabed. Drawing from molecular fossils, these organisms, believed to be our earliest known progenitors, thrived in oceans roughly 1.6 to 1.0 billion years ago. The artistic rendering is credited to ‘Orchestrated in MidJourney by TA 2023.’

Until the present, certain biomarkers, known as “protosteroids,” were often neglected as primordial life’s fossil testimonies.

Recent detection of biomarker patterns introduces us to a range of hitherto unknown organisms that were the dominant complex life on Earth around a billion years ago. Their cell structure and probable metabolism, adapted to an atmosphere with significantly lower oxygen levels than we have now, differed from known complex eukaryotic life like animals, plants, and algae. A multinational research group, which includes Christian Hallmann, a GFZ geochemist, shares this groundbreaking revelation for evolutionary geobiology in the Nature journal.

The hitherto unknown “protosteroids” were found to be surprisingly widespread during Earth’s Middle Ages. These primitive molecules were a product of a former stage of eukaryotic sophistication, expanding the existing record of fossil steroids from 800 million years ago to 1.6 billion years ago. The word “eukaryotes” refers to a life kingdom that includes all animals, plants, and algae, and is distinguished from bacteria due to its intricate cell structure, including a nucleus, as well as more complex molecular mechanics.

Hallmann notes the importance of the finding, stating, “The significance of this discovery is not solely the extension of the eukaryotes’ current molecular record.” He suggests that if the last common ancestor of all current eukaryotes, including humans, was likely capable of producing ‘modern’ sterols, it’s highly probable that the eukaryotes responsible for these unique signatures were part of the phylogenetic tree’s stem.

Eukaryotes, which possess cells with a nucleus containing their genetic material, are one of the three recognized cellular life domains, the others being bacteria and archaea. The term “eukaryote” stems from the Greek words “eu” meaning “true” and “karyon” meaning “nut” or “kernel,” referring to the nucleus.

Unveiling an Extinct Era

This “stem” denotes the mutual ancestral lineage that led to all extant eukaryote branches. While its representatives are long extinct, their characteristics may provide more insight into the circumstances surrounding complex life’s evolution. Although further investigation is required to ascertain the proportion of protosteroids potentially derived from rare bacterial sources, the discovery of these new molecules reconciles the geological record of traditional fossils with fossil lipid molecules. This offers a unique and unprecedented peek into a lost world of ancient life. The competitive decline of stem group eukaryotes, signaled by modern fossil steroids’ first appearance around 800 million years ago, could represent one of the most pivotal moments in the evolution of increasingly complex life.

Benjamin Nettersheim from the University of Bremen, the study’s first author, explains, “Almost all eukaryotes biosynthesize steroids, such as cholesterol produced by humans and most other animals.” He highlights the importance of these lipid molecules, which are integral to eukaryotic cell membranes, and contribute to various physiological functions. By hunting for fossilized steroids in ancient rocks, we can trace the evolution of increasingly complex life.

An Impossible Notion Turned Reality…

Almost three decades ago, Nobel laureate Konrad Bloch proposed the existence of such a biomarker in an essay. Bloch hypothesized that transient intermediates in modern steroid biosynthesis might not always have been intermediate. He suggested that lipid biosynthesis evolved along with the Earth’s changing environmental conditions throughout its history. Unlike Bloch, who doubted that these ancient intermediates could ever be found, Nettersheim began searching for protosteroids in ancient rocks deposited at a time when these intermediates might have been the final product.

How do you locate such molecules in ancient rocks? Jochen Brocks, a professor at the Australian National University and co-first author of the study with Nettersheim, explains, “We used a mix of techniques to first transform various modern steroids to their fossilized counterpart; otherwise, we wouldn’t even know what to look for.” Scientists had missed these molecules for decades because they don’t fit the usual molecular search patterns. “Once we recognized our target, we found that dozens of other rocks, sourced from billion-year-old waterways across the globe, were seeping similar fossil molecules.”

The oldest samples containing the biomarker are from the Barney Creek Formation in Australia, dating back 1.64 billion years. The next 800 million years of rock records only yield fossil molecules of primordial eukaryotes before molecular signatures of modern eukaryotes first appear in the Tonian period. “The Tonian Transformation emerges as one of the most profound ecological turning points in our planet’s history,” observes Nettersheim. Hallmann adds that both primordial stem groups and modern eukaryotic representatives such as red algae might have coexisted for several hundred million years.

During this period, however, the Earth’s atmosphere became progressively oxygen-rich, a metabolic product of cyanobacteria and the first eukaryotic algae that would have been toxic to many other organisms. Later, global “Snowball Earth” glaciations occurred, and the protosterol communities largely vanished. The last common ancestor of all living eukaryotes might have lived 1.2 to 1.8 billion years ago. Its descendants were likely better adapted to survive heat, cold, and UV radiation, outcompeting their primeval relatives.

We will never definitively know how most of our early ancestors appeared as all stem group eukaryotes are long extinct. However, artistic renderings offer tentative visualizations, and the primeval steroids may eventually provide more insights into their biochemistry and lifestyle. “Earth was predominantly microbial for much of its history and left few traces,” Nettersheim concludes. Research at ANU, MARUM, and GFZ continues to delve into tracing our existence’s roots. The discovery of protosterols brings us one step closer to understanding how our earliest ancestors lived and evolved.

To delve deeper into this research:

Scientists Uncover “Lost World” of Our Early Ancestors in Billion-Year-Old Rocks

Reference: “Lost world of complex life and the late rise of the eukaryotic crown” by Jochen J. Brocks, Benjamin J. Nettersheim, Pierre Adam, Philippe Schaeffer, Amber J. M. Jarrett, Nur Güneli, Tharika Liyanage, Lennart M. van Maldegem, Christian Hallmann and Janet M. Hope, 7 June 2023, Nature.
DOI: 10.1038/s41586-023-06170-w

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More about Protosteroids Discovery

• Lost world of complex life and the late rise of the eukaryotic crown – Nature
• The University of Bremen
• Australian National University
• GFZ German Research Centre for Geosciences