Have you ever wondered how life as we know it developed? How living organisms extended beyond the constraints of the microscopic realm? 

 A rendering of Fractofusus misrai, a soft-bodied sea animal from the Ediacaran period. Courtesy of Wikimedia Commons

The Ediacaran Period, the era directly preceding the Cambrian explosion, ranging from 635 to 539 million years ago, witnessed a massive evolution of macroscopic life. This rapid development in fossil production led to the rise of Darwin’s dilemma and the seeming inability of his theory of gradual evolution to address this large shift in biological life.

In November 2021, researchers Daniel J. Condon, Xian-Hua Li, Dmitriy V. Grazhdankin, Fred T. Bowyer, Francis A. MacDonald, Chuan Yang, Alan D. Rooney and Maoyan Zhu from UC Santa Barbara’s Department of Earth Science shared their efforts in closing the gap of knowledge that surrounds the period with research published in Science. 

An interview with MacDonald revealed that prior to the development of macroscopic organisms, the beings existing on Earth were overwhelmingly microscopic and soft-bodied. 

“There’s actually quite a rich record there but it’s mostly micro and it’s mostly soft-bodied, and the micro stuff in the soft-bodied material does not preserve as well [in] the geological record,” MacDonald said. For this reason, there was an incomplete model of the fossil records. MacDonald and his colleagues joined together to map an accurate time-calibrated model of the Ediacaran Period through the use of new methods to date and analyze sediment. 

In the past, it was difficult to obtain radioisotopic age constraints, given that specific material created from volcanoes was needed in order to accurately retrieve information. Unfortunately, the sediment created from the eruptions was not always present when important changes to evolution occurred. MacDonald stated that many of the most important successions — changes in the composition of the sediment — “may have things like fossils on them, they don’t happen to have a volcano going in there all the time.” 

Fortunately, through the research project, MacDonald and Rooney were able to extract accurate dates from the use of a rhenium-osmium chronometer. In the interview, MacDonald explained the processes by which this form of dating functions. “The rhenium is actually trapped within carrageen and organic matter. And then, if that is trapped, you can actually count the rhenium and count the daughter osnium and actually get a date on a sediment.” 

Rooney and MacDonald experimented in southern China and were able to accurately extract dates from the sediment. “We’ve been kind of calibrating it and a lot of different successions,” MacDonald noted, “and it’s been proving to be at least accurate … What we did here is we found successions.” 

With the new data, the researchers are a step closer in creating a full model of the Ediacaran Period. “These changes to the carbon cycle that we’re seeing in carbon isotopes … reassess what the real record is and what the relationships are between perturbations in the carbon cycle and turnovers of these major fossil groups.” 

This research is only the beginning, according to MacDonald. “[The project] is coming at the beginning of a large, international drilling project that we’re involved with … we are drilling sedimentary cores in several regions around the world through this interval, going into the Cambrian explosion to try to create … a modern age model,” he said. 

While progress has been made, scientists continue their pursuit in mapping out Earth’s biological history. The answers to all scientific questions remain unknown, and in MacDonald’s words, the uncertainty is alright. “The beauty of the scientific method is that we can keep exploring these details,” he said. “We don’t have it all solved and that’s that’s okay. We still haven’t sorted out Darwin’s dilemma … we are, I think, making progress.”