A recent groundbreaking discovery in the field of evolutionary geobiology, published in the prestigious journal Nature, has unveiled the existence of previously unidentified organisms that thrived on Earth approximately one billion years ago. Biomarker signatures, newly uncovered by an international team of researchers led by GFZ geochemist Christian Hallmann, shed light on these enigmatic life forms that differed from the familiar complex eukaryotic organisms we know today, such as animals, plants, and algae.
These ancient organisms possessed distinctive cell structures and likely had metabolisms adapted to an environment with significantly lower oxygen levels than the present-day atmosphere. One of the key findings of the study is the revelation of “protosteroids,” primordial molecules that were remarkably abundant during Earth’s Middle Ages. These protosteroids represent an earlier stage of eukaryotic complexity and extend the existing fossil record of steroids back in time from 800 million years ago to as far as 1,600 million years ago.
Eukaryotes, a kingdom of life encompassing animals, plants, and algae, are distinguished from bacteria by their complex cell structure, including a nucleus, as well as a sophisticated molecular machinery. The significance of this discovery lies not only in the expansion of the molecular record of eukaryotes but also in the implications regarding the ancestral origins of these organisms. Hallmann suggests that since the last common ancestor of all modern eukaryotes, including humans, likely had the capacity to produce conventional sterols, it is highly probable that the eukaryotes responsible for these unique biomarker signatures belonged to the earliest branches of the evolutionary tree.
Unprecedented glimpse of a lost world
The “stem” lineage, which represents the common ancestor of all existing branches of eukaryotes, provides crucial insights into the evolution of complex life, despite its extinction. By studying the nature of these ancestral organisms, we can gain a better understanding of the environmental conditions that shaped the emergence of complex life forms.
While further research is necessary to determine the extent to which protosteroids may have originated from rare bacterial sources, the discovery of these novel molecules serves to bridge the geological evidence from traditional fossils with that derived from fossil lipid molecules. Consequently, it offers an extraordinary glimpse into a forgotten realm of ancient life.
The decline of stem group eukaryotes, evidenced by the appearance of modern fossil steroids approximately 800 million years ago, likely represents a pivotal event in the evolution of complex life. This competitive decline signifies one of the most significant turning points in the progression towards increasingly intricate forms of life.
Benjamin Nettersheim, the lead author of the study from the University of Bremen, emphasizes that the production of steroids is a common trait among eukaryotes. For instance, humans and the majority of other animals synthesize cholesterol, a well-known example of a steroid. While cholesterol may have acquired a negative reputation due to its potential health risks in humans, it plays a vital role in the composition of eukaryotic cell membranes and facilitates various physiological functions. Exploring the presence of fossilized steroids in ancient rock formations enables us to track the evolutionary trajectory of complex life forms over time.
What the Nobel laureate thought impossible
Nearly three decades ago, Nobel laureate Konrad Bloch postulated the existence of a biomarker similar to the recently discovered protosteroids. Bloch suggested that these biomarkers could represent final products rather than short-lived intermediates in the biosynthesis of steroids. He proposed that lipid biosynthesis evolved in parallel with changing environmental conditions throughout Earth’s history. Unlike Bloch, who believed these ancient intermediates would never be found, Benjamin Nettersheim took on the challenge of searching for protosteroids in ancient rocks that were deposited during a time when these intermediates could have been the end products.
To locate such molecules in ancient rocks, the researchers employed a combination of techniques. Initially, they converted various modern steroids into their fossilized equivalents to determine what they should be searching for. Once they identified the target, they discovered the presence of similar fossil molecules in numerous rocks sourced from billion-year-old waterways around the globe. These molecules had been overlooked for decades due to their deviation from typical molecular search images.
The oldest samples containing the biomarker were extracted from Australia’s Barney Creek Formation and date back 1.64 billion years. Over the subsequent 800 million years, the rock record exclusively contained fossil molecules from primordial eukaryotes, preceding the appearance of molecular signatures from modern eukaryotes during the Tonian period.
According to Nettersheim, the Tonian Transformation represents one of the most significant ecological turning points in Earth’s history. Hallmann suggests that both ancient stem groups and modern eukaryotic representatives, such as red algae, likely coexisted for hundreds of millions of years. However, the increasing oxygenation of the Earth’s atmosphere, driven by cyanobacteria and early eukaryotic algae, posed a threat to many organisms, as oxygen was toxic. Subsequently, global “Snowball Earth” glaciations occurred, leading to the decline of protosterol communities. The last common ancestor of all extant eukaryotes likely lived between 1.2 and 1.8 billion years ago. Its descendants were better equipped to survive extreme temperatures, UV radiation, and other environmental challenges, gradually supplanting their primordial relatives.
Since the stem group eukaryotes are extinct, the exact appearance of our early relatives remains a mystery. Nevertheless, artists have made efforts to visualize these organisms, providing tentative depictions. The study of primordial steroids may offer insights into the biochemistry and lifestyle of these ancestral organisms.
Nettersheim concludes that Earth predominantly existed as a microbial world throughout much of its history, leaving behind few traces. However, ongoing research at institutions like ANU, MARUM, and GFZ aims to uncover the origins of life. The discovery of protosteroids brings us closer to understanding the lives and evolution of our earliest ancestors.