A "lost world" from a billion years ago discovered

Discovered by scientists from the Australian National University (ANU) and the Helmholtz Association of German Research Centers, the ancient creatures extend the current record of fossil steroids up to 1.6 billion years (from 800 million). Another remarkable achievement lies in just how the discovery was made. 

The scientists first had to visualize what they were looking for, utilizing a combination of techniques to “convert various modern steroids to their fossilized equivalent,” as explained Jochen Brocks, professor at the ANU, one of the study’s first authors, in a press release. This allowed them to find overlooked traces in fossil fat molecules inside a 1.6-billion-year-old rock located at the bottom of the ocean near Australia’s Northern Territory. 

Interesting Engineering reached out to Dr. Benjamin Nettersheim from the University of Bremen, the study’s other first author, for more insight on his team’s findings and the methods they employed. 

Making the discovery

Dr. Nettersheim elaborated further on how their team determined where to find the primordial organisms, sharing that their goal was to trace the molecular remains of early organisms as far back into Earth’s history as possible.

“From previous research we already knew how and where on Earth to find this precious molecular record of early life on Earth: there are only a few places in the world where ancient sediments have never been buried deeper than a few kilometres and never heated to a degree that diagnostic ecological information of original biomolecules is completely lost by geothermal heating and geological transformation processes,” he wrote.

One such place where diagnostic molecular fossils still exist is in the northern Australian Outback. As Dr. Nettersheim explained, sedimentary rocks there have suffered very little geological change over the past 1.64 billion years of geological history.

“But only molecular fossils of bacteria – so called hopanoids – where previously known from these and all other biomarker-bearing rocks that are older than ca. 800 million years," he shared. "The structurally very similar molecular equivalents of complex (eukaryotic) life, fossil equivalents of sterols such as cholesterol produced in abundance by most animals and red algae were enigmatically absent.”

To find these missing fossils, scientists had to create new search templates in the laboratory, after which they began finding fossil steroids in virtually all biomarker-bearing rocks in the time period between 800 million and 1.6 billion years. Of these, only the bacterial hopanoids were known, according to Nettersheim. 

“But these are not fossil cholesterols but biosynthetically more primitive version of it – early intermediates in the biosynthesis of modern sterols,” he explained. “Konrad Bloch, who received the Noble prize for deciphering the biosynthetic pathway of cholesterol, predicted that these modern intermediates where once, hundreds of millions of years ago, the final biosynthetic products employed by early eukaryotes, but he was skeptical that these molecules could be preserved in the geological record.

Now we find the fossil derivatives of exactly these molecules. We think that they probably derive, at least in part, from our very early and still quite primordial eukaryotic ancestors, which then must have been much more widespread and ecological important than previously thought.”

Changing our understanding of evolution

Asked about how this discovery impacts our knowledge of the evolutionary processes, Netterheim stated that “If our interpretation is correct that the molecular fossils largely derive from our early eukaryotic ancestors, it would imply that our primordial ancestors, that dominated most marine environments between [1.6 billion] and ca. 800 million years ago still belonged to the so called stem of the eukaryotic tree, lineages that diverged early in earth history before the last eukaryotic common ancestor (LECA) evolved. 

This would mean that ... it is only in the Tonian period some 800 million years ago that modern types of eukaryotes such as red algae and heterotrophic (still largely unicellular) eukaryotes, so called protists, such as ciliates or amoebozoans, became truly abundant and ecologically important on a global scale.”

What caused the extinction?

The researchers believe their findings solidify our understanding of a major change that took place during the Tonian period, as the age from around one billion to 720 million years ago is known. Nettersheim believes that as “both a substantial increase in eukaryotic fossil diversity and the first appearance of modern sterol fossils, following 800 million years of exclusively primordial sterols (so called protosterols)” took place in that timespan, a significant transformation of marine ecosystems on a global scale must have resulted and let to the end of the ancient organisms.

“It is possible that the primordial Protosterol Biota went extinct or, more likely, were marginalized through the ecological perturbations in this period,” he proposed, adding, “Their demise may have been caused, for example, by the rise of modern eukaryotes, changes in atmospheric oxygen concentrations or nutrients regimes.

Further analyses may eventually reveal the currently enigmatic drivers for the surprisingly late rise of modern complex lifeforms to ecological dominance that, according to the molecular fossil data, occurred hundreds of millions of years later than the first occurrence of our modern eukaryotic ancestors, as demonstrated by the presence of microscopic eukaryotic (body) microfossils.”

Searching for other ancient organisms 

Now that they have identified some microorganisms from a billion years ago, the scientists set their sights on finding more. As Dr. Nettersheim shared, they are “trying to reconstruct ancient ecosystems and the evolution of early life as recorded in the rock record in the greatest possible detail.”

In fact, he reminds us that most of the life forms that have ever existed are now extinct like the Protostrol Biota, which was dominant for a long period. This gives the scientists a lot to explore but also provides unique challenges. How do you know what you should be looking for?

As Dr. Nettersheim said, “By combining lipid profiles of modern organisms with genetic data and molecular clock estimates for the emergence of certain lineages and associated biosynthetic capabilties, we hope to better constrain non-actualistic (now extinct) sources of molecular fossils in the future."

Check out their study “Lost world of complex life and the late rise of the eukaryotic crown” in the journal Nature.

Abstract

Eukaryotic life appears to have flourished surprisingly late in the history of our planet. This view is based on the low diversity of diagnostic eukaryotic fossils in marine sediments of mid-Proterozoic age (around 1,600 to 800 million years ago) and an absence of steranes, the molecular fossils of eukaryotic membrane sterols1,2. This scarcity of eukaryotic remains is difficult to reconcile with molecular clocks that suggest that the last eukaryotic common ancestor (LECA) had already emerged between around 1,200 and more than 1,800 million years ago. LECA, in turn, must have been preceded by stem-group eukaryotic forms by several hundred million years3. Here we report the discovery of abundant protosteroids in sedimentary rocks of mid-Proterozoic age. These primordial compounds had previously remained unnoticed because their structures represent early intermediates of the modern sterol biosynthetic pathway, as predicted by Konrad Bloch4. The protosteroids reveal an ecologically prominent ‘protosterol biota’ that was widespread and abundant in aquatic environments from at least 1,640 to around 800 million years ago and that probably comprised ancient protosterol-producing bacteria and deep-branching stem-group eukaryotes. Modern eukaryotes started to appear in the Tonian period (1,000 to 720 million years ago), fuelled by the proliferation of red algae (rhodophytes) by around 800 million years ago. This ‘Tonian transformation’ emerges as one of the most profound ecological turning points in the Earth’s history.