In the vast expanse of time, life on Earth began with a single-celled microbe. However, the transition to the multicellular world we inhabit today is owed to a vital chemical process known as biomineralization. It is through this process that living organisms produce hardened mineralized tissue, such as skeletons. Not only did biomineralization give rise to the diverse array of body plans we see today, but it also had a profound impact on the planet’s carbon cycle.
Unearthing Clues in Namibia
In the Tsau Khaeb National Park of Namibia, researchers have made a remarkable discovery. Fossilized skeletons of cloudinids, specifically Cloudina, have been found dating back to approximately 551-550 million years ago in the Ediacaran period, which spans from 635-538 million years ago. These tubular structures, comprised of carbonate cones up to ~1.5cm in length, provide a valuable opportunity to study the origins of biomineralization and understand its significance.
To shed light on the location, timing, and reasons behind the initiation of biomineralization on Earth, a research team led by Dr. Fred Bowyer from the University of Edinburgh conducted a comprehensive study. Their research, recently published in Earth and Planetary Science Letters, combined sediment analysis with geochemical data in the form of carbon and oxygen isotopes.
Their investigation focused on limestones in the Kliphoek Member of the Nama Group, a geological formation. These sediments were believed to have been deposited in a shallow sea during a period of lowstand before transitioning into open marine conditions. The Nama Group rocks, often referred to as the “Biological Big Bang,” are highly significant for understanding the radiation of life during the Cambrian period, approximately 538-485 million years ago.
The Story Written in Rocks
During fieldwork in Namibia, the researchers closely examined the layered structure of the rock units. Within these bedding planes, traces of ancient activity known as ichnofossils provided glimpses into the history of life. These structures were believed to be created by soft-bodied microbes and were found in the lower part of the study site (Mara Member) before the onset of biocalcification. Moving upward, the researchers began to observe the first signs of Cloudina in the Kliphoek Member, characterized by distinctive conical fossils with nested cone structures.
To unravel the secrets hidden within the rocks, the researchers analyzed the carbon and oxygen isotopes present in the calcium carbonate limestone that encased the fossils. These isotopes reveal information about the marine environment and the planet as a whole.
Carbon isotopes, specifically the ratio of lighter 12C to 13C, are influenced by various factors such as photosynthesis, respiration, and upwelling zones. Generally, greater ocean productivity by photosynthesizing organisms leads to the use of lighter 12C, resulting in an ocean enriched in 13C. On the other hand, oxygen isotopes, the ratio of 16O to 18O, provide insights into environmental conditions. Warmer global temperatures, for example, encourage evaporation of seawater, leaving the ocean enriched in heavier 18O.
The dataset from Namibia exhibited a range of carbonate-derived 12C/13C ratios, from -7.24‰ to +2.91‰, and 16O/18O ratios from -12.14‰ to -0.78‰, increasing up the stratigraphic section. Notably, Cloudina-bearing units displayed lower mean 12C/13C ratios and oscillating 16O/18O ratios. This suggests that Cloudina originated in a semi-restricted environment connected to the open marine conditions but with limited access, characterized by lower oxygen levels.
The Role of Marine Transgression
The isotopic data further revealed the importance of high carbonate concentrations in the ocean for the formation of Cloudina’s calcified structures. A period of marine transgression, where the shoreline moved landward, resulted in shallow intertidal conditions within the study site’s evaporite basin during the Mara Member. This was followed by a rise in sea level, leading to the deposition of sandstones and calcitic sediments in the shallow open marine conditions of the Kliphoek Member.
During subsequent sea level fall, open marine carbonates were deposited above a redoxcline, a layer characterized by significant differences in water oxygenation. It is within this dynamic environment that Cloudina biomineralization took place. The research team suggests that the appearance of skeletonization was driven by the opportunistic colonization of short-lived periods of oxygenation in otherwise relatively anoxic conditions, coupled with oscillations in sea level.
Building upon previous research, this study highlights the evolutionary novelty of skeletonization and its close association with the instability of the marine environment. The origins of biomineralization can be traced back to a low oxygen environment with intermittent periods of higher oxygenation. This research provides valuable insights into the ancient history of Earth, shedding light on the emergence of multicellular life and the profound impact of biomineralization on our planet’s carbon cycle.