Did animals evolve 800 million years ago?

A recent study conducted by the University of Oxford has made significant progress in unraveling a longstanding enigma that has confounded naturalists, including Charles Darwin: the emergence of animals in Earth’s history. The study, titled “Fossilization processes and our understanding of the ancient origins of animals,” has been published in the journal Trends in Ecology & Evolution.

The fossil record indicates that animals made their first appearance approximately 574 million years ago. This occurrence is characterized by a sudden “explosion” of animal fossils found in rocks from the Cambrian period, spanning 539 to 485 million years ago. This seemingly rapid emergence contradicts the typically gradual pace of evolutionary changes observed in other organisms. Scientists, including Darwin himself, have speculated that animals evolved long before the Cambrian period, but their absence in the fossil record has posed a perplexing challenge.

One approach used to estimate the timing of animal evolution is the “molecular clock” method, which suggests that animals originated around 800 million years ago during the early Neoproterozoic era (1,000 to 539 million years ago). This method relies on the mutation rates in genes to determine when different species last shared a common ancestor. However, despite the existence of fossilized microorganisms like bacteria and protists in rocks from the early Neoproterozoic, no animal fossils have been discovered.

This dilemma has presented paleontologists with two potential explanations: either the molecular clock method overestimates the timeframe of animal evolution, or animals existed during the early Neoproterozoic but were too delicate and soft-bodied to be preserved in the fossil record.

The study led by the University of Oxford aims to shed light on this perplexity by exploring the fossilization processes and their impact on the preservation of ancient animals. By examining the factors that influence fossilization and the likelihood of animal remains being conserved over millions of years, researchers hope to gain a better understanding of why the earliest animal fossils have proven elusive.

Dickinsonia, one of the oldest animal fossils from the Ediacara Biota, Ediacaran Rawnsley Quartzite Formation, Australia. 560–550 million years old. Credit: Lidya Tarhan

Dr. Ross Anderson, leading a team of researchers from the University of Oxford’s Department of Earth Sciences, has conducted a comprehensive investigation into the conditions necessary for the preservation of early animal fossils. This study represents the most thorough assessment to date in this field.

According to Dr. Anderson, the first animals likely lacked mineralized shells or skeletons, making their fossilization a challenge. However, certain deposits of Cambrian mudstone exhibit exceptional preservation, even of delicate animal tissues. The researchers hypothesized that if these conditions, known as Burgess Shale-Type (BST) preservation, also existed during the Neoproterozoic era, the absence of fossils in those rocks would indicate a true lack of animals during that time.

To explore this hypothesis, the research team employed various analytical techniques on samples of Cambrian mudstone from nearly 20 sites. They compared deposits that contained BST fossils with those preserving only mineral-based remains, such as trilobites. The methods utilized included energy dispersive X-ray spectroscopy and X-ray diffraction at the University of Oxford’s Departments of Earth Sciences and Materials, as well as infrared spectroscopy at Diamond Light Source, the UK’s national synchrotron facility.

The analysis revealed that BST fossils were particularly enriched in an antibacterial clay called berthierine. Samples with at least 20% berthierine composition yielded BST fossils in approximately 90% of cases. Furthermore, microscale mineral mapping of BST fossils indicated that another antibacterial clay called kaolinite directly bonded to decaying tissues at an early stage, forming a protective halo during fossilization.

Dr. Anderson explained that the presence of these clays served as the primary predictor of whether rocks would contain BST fossils. The clay particles acted as a barrier against bacteria and microorganisms, preventing the breakdown of organic materials.

Next, the researchers applied these techniques to analyze samples from various fossil-rich mudstone deposits in the Neoproterozoic era. The analysis demonstrated that most of these deposits did not possess the necessary compositions for BST preservation. However, three deposits in Nunavut (Canada), Siberia (Russia), and Svalbard (Norway) exhibited compositions almost identical to BST rocks from the Cambrian period. Surprisingly, none of the samples from these three deposits contained animal fossils, despite conditions being favorable for preservation.

Dr. Anderson emphasized that the similarities in clay distribution between these rare early Neoproterozoic samples and exceptional Cambrian deposits indicated that clays were associated with decaying tissues during both time periods. The findings provide the first evidence suggesting the absence of animals during the early Neoproterozoic era, which contradicts some molecular clock estimates and supports the notion that animal evolution had not yet taken place during that time.

In conclusion, Dr. Anderson and his team’s research has offered valuable insights into the preservation conditions required for the earliest animal fossils, shedding light on the mystery surrounding their appearance in the fossil record.

Reconstruction of Charnia, a candidate for the first animal fossil from the Ediacaran Period as old as 574 million years ago. Credit: Oxford University Museum of Natural History / Mighty Fossils

The researchers involved in the study have proposed a potential maximum age for the origin of animals, estimating it to be approximately 789 million years based on the youngest age of the Svalbard formation. Moving forward, their plan is to search for even younger Neoproterozoic deposits that possess the conditions necessary for BST preservation. By doing so, they aim to confirm whether the absence of animal fossils in certain rock formations is due to their actual absence or the inability to fossilize under prevailing conditions. Additionally, the team intends to conduct laboratory experiments to investigate the mechanisms underlying clay-organic interactions in BST preservation.

Dr. Anderson expressed excitement about the microscale compositional mapping of rocks, as it provides unprecedented insights into the nature of exceptional fossil preservation. This knowledge could potentially reveal biases in the fossil record, shedding light on which species and tissues are more likely to be preserved. Ultimately, this has the potential to reshape our understanding of biodiversity throughout different geological eras.

Source: University of Oxford

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