Recently discovered biomarker signatures reveal a variety of previously unknown organisms that were the dominant complex life forms on Earth around one billion years ago. These organisms differed from the complex eukaryotic life we know today, such as animals, plants, and algae, in terms of their cell structure and metabolism, which was adapted to a world with much less oxygen in the atmosphere than we have today. The international team of researchers who made this discovery describe it as a significant advancement for the field of evolutionary geobiology. The research has been published in the Nature journal.
Meet “protosteroids”
During Earth’s Middle Ages, “protosteroids” were discovered to be abundant. These molecules were produced in an earlier stage of eukaryotic complexity. This extends the record of fossil steroids beyond 800 million years ago up to 1,600 million years ago. Eukaryotes are organisms that include all animals, plants, and algae. They have a complex cell structure that includes a nucleus and a more complex molecular machinery, which sets them apart from bacteria.
GFZ geochemist Christian Hallmann, said:
“The highlight of this finding is not just the extension of the current molecular record of eukaryotes. Given that the last common ancestor of all modern eukaryotes, including us humans, was likely capable of producing ‘regular’ modern sterols, chances are high that the eukaryotes responsible for these rare signatures belonged to the stem of the phylogenetic tree”.
An unprecedented glimpse of a lost world
This “stem” refers to the lineage that all living branches of eukaryotes descended from. Although the representatives of this lineage no longer exist, understanding their nature can provide insight into the conditions surrounding the evolution of complex life. The discovery of new molecules known as protosteroids, which may have had a rare bacterial source, not only reconciles the geological record of traditional fossils with that of fossil lipid molecules but also provides a rare glimpse into a lost world of ancient life. The competitive demise of stem group eukaryotes, marked by the first appearance of modern fossil steroids some 800 million years ago, may reflect one of the most significant events in the evolution of increasingly complex life. More research is needed to evaluate what percentage of protosteroids may have had a rare bacterial source.
Benjamin Nettersheim from the University of Bremen, said:
“Almost all eukaryotes biosynthesize steroids, such as cholesterol that is produced by humans and most other animals due to potentially adverse health effects of elevated cholesterol levels in humans, cholesterol doesn’t have the best reputation from a medical perspective. However, these lipid molecules are integral parts of eukaryotic cell membranes where they aid in a variety of physiological functions. By searching for fossilized steroids in ancient rocks, we can trace the evolution of increasingly complex life”.
What the Nobel laureate thought impossible…
Nobel laureate Konrad Bloch speculated about a biomarker in an essay almost 30 years ago. He believed that short-lived intermediates in the modern biosynthesis of steroids may not always have been intermediates. He thought that lipid biosynthesis evolved in parallel with changing environmental conditions throughout Earth history. Nettersheim, in contrast, started searching for protosteroids in ancient rocks that were deposited at a time when those intermediates could actually have been the final product.
To find such molecules in ancient rocks, scientists had to first convert various modern steroids to their fossilized equivalent using a combination of techniques. They discovered that dozens of other rocks, taken from billion-year-old waterways across the world, were also oozing with similar fossil molecules. The oldest samples with the biomarker are from the Barney Creek Formation in Australia and are 1.64 billion years old. The rock record of the next 800 million years only yields fossil molecules of primordial eukaryotes before molecular signatures of modern eukaryotes first appear in the Tonian period.
According to Nettersheim, “the Tonian Transformation emerges as one of the most profound ecological turning points in our planet’s history “.
Hallmann adds that “both primordial stem groups and modern eukaryotic representatives such as red algae may have lived side by side for many hundreds of millions of years.”
During this period, the Earth’s atmosphere saw a significant increase in oxygen, which was produced by cyanobacteria and the first eukaryotic algae. While toxic to many other organisms, this oxygen enrichment persisted. However, global “Snowball Earth” glaciations eventually occurred, leading to the decline of the protosterol communities. The last common ancestor of all living eukaryotes is estimated to have lived between 1.2 to 1.8 billion years ago. Its descendants were likely better equipped to survive harsh conditions, such as heat, cold, and UV radiation, and displaced their primordial relatives.
As all stem group eukaryotes are long extinct, we can never be completely sure of the appearance of most of our early relatives. Nonetheless, attempts have been made to create tentative visualizations of these organisms, and future studies on primordial steroids may provide further insight into their biochemistry and lifestyle.
The discovery of protosterols now brings us one step closer to understanding how our earliest ancestors lived and evolved.
Image Credits
In-Article Image Credits
Assemblage of primordial eukaryotic organisms of the ‘Protosterol Biota’ via Orchestrated in MidJourney by TA with usage type - Creative Commons LicenseFeatured Image Credit
Assemblage of primordial eukaryotic organisms of the ‘Protosterol Biota’ via Orchestrated in MidJourney by TA with usage type - Creative Commons License
