New study sheds light on the evolution of animals

Key points:

• Scientists have been mystified as to why animals are missing in much of the fossil record;
• Researchers have developed a new method to determine if animals really were absent during certain geological eras, or if they were present but too fragile to be preserved;
• The findings establish a new maximum point at which animals first evolved on Earth.

Newswise — A research team from the University of Oxford has made significant progress in unraveling a longstanding enigma that has intrigued naturalists ever since the time of Charles Darwin: the origin of animals in Earth's history. Today, the findings of their study have been published in the journal Trends in Ecology & Evolution.

According to the study, the earliest evidence of animals can be traced back approximately 574 million years ago, as indicated by fossils in the geological record. Interestingly, their emergence during the Cambrian period (539 million to 485 million years ago) appears to have transpired abruptly, defying the typically gradual pace of evolutionary transformations. This phenomenon has puzzled scientists, including Darwin himself, who hypothesized that animals evolved long before the Cambrian era. Nevertheless, the absence of their fossils during earlier periods remains unexplained

.The "molecular clock" technique, which utilizes the mutation rates of genes to estimate when different species last shared a common ancestor, suggests that animals originated around 800 million years ago, during the early phase of the Neoproterozoic era (1,000 million to 539 million years ago). Surprisingly, while microorganism fossils like bacteria and protists have been discovered in rocks from the early Neoproterozoic, no animal fossils have been found.

This presented a dilemma for paleontologists: does the molecular clock method overestimate the timing of animal evolution, or were animals indeed present during the early Neoproterozoic but too delicate and fragile to be preserved?

To shed light on this issue, Dr. Ross Anderson and a team of researchers from the University of Oxford's Department of Earth Sciences conducted an extensive investigation. Their goal was to comprehensively evaluate the conditions necessary for the preservation of the earliest animal fossils, representing the most thorough assessment conducted thus far.

The application of the "molecular clock" technique, which utilizes gene mutation rates to estimate the shared ancestry of different species, suggests that animals emerged approximately 800 million years ago, during the early phase of the Neoproterozoic era (1,000 million to 539 million years ago). Interestingly, while fossils of microorganisms like bacteria and protists have been discovered in rocks from the early Neoproterozoic, the absence of animal fossils has presented a puzzling situation.

This dilemma has posed two possibilities for paleontologists: either the molecular clock method overestimates the timing of animal evolution, or animals did exist during the early Neoproterozoic but were too delicate and fragile to be preserved as fossils.

To provide clarity on this matter, Dr. Ross Anderson and a team of researchers from the Department of Earth Sciences at the University of Oxford conducted a comprehensive investigation. Their primary objective was to thoroughly assess the necessary conditions for the preservation of the earliest animal fossils, resulting in the most extensive evaluation undertaken thus far.

Through the analysis, it was discovered that fossils exhibiting exceptional preservation known as BST (Bacteria-Sized Terrestrial) fossils were notably abundant in a specific antibacterial clay called berthierine. Notably, samples containing at least 20% berthierine demonstrated BST fossils in approximately 90% of cases.

Further examination through microscale mineral mapping of BST fossils revealed that another antibacterial clay, known as kaolinite, seemed to directly adhere to decaying tissues during the early stages of fossilization, forming a protective halo.

Dr. Anderson elaborated, stating, "The presence of these clays emerged as the primary indicator of whether rocks would contain BST fossils. This suggests that clay particles serve as an antibacterial barrier, hindering the decomposition of organic materials by bacteria and other microorganisms."

Subsequently, the researchers applied these methodologies to examine various samples extracted from Neoproterozoic mudstone deposits known for their abundant fossils. The analysis unveiled that the majority of these deposits lacked the necessary compositions for BST preservation. However, three specific deposits situated in Nunavut (Canada), Siberia (Russia), and Svalbard (Norway) exhibited nearly identical compositions to BST-bearing rocks from the Cambrian period. Despite the favorable conditions for preservation in these three deposits, none of the samples contained animal fossils.

Dr. Anderson further explained, "The resemblance in clay and fossil distribution patterns between these rare early Neoproterozoic samples and exceptional Cambrian deposits suggests that clays were indeed associated with decaying tissues in both cases. The availability of conditions conducive to BST preservation during both time periods is evident. This provides the initial 'evidence for absence' and supports the notion that animals had not yet evolved during the early Neoproterozoic era, contradicting certain estimations derived from molecular clock studies."

Based on the study findings, the researchers propose a potential upper limit for the emergence of animals at approximately 789 million years ago, based on the youngest estimated age of the Svalbard formation. Moving forward, the team aims to explore Neoproterozoic deposits of progressively younger ages that possess favorable conditions for BST preservation. This investigation will confirm whether the absence of animal fossils in certain rock records is due to their actual absence or the inability to fossilize under prevailing conditions. Additionally, laboratory experiments will be conducted to investigate the mechanisms underlying the interactions between clay and organic matter in BST preservation.

Dr. Anderson emphasized, "By mapping the compositions of these rocks at the microscale, we are gaining unprecedented insights into the unique fossil record. This knowledge has the potential to illuminate how the fossil record may exhibit biases in preserving certain species and tissues, thereby influencing our understanding of biodiversity across different geological eras."

*Note: "Animals" refer to multicellular, eukaryotic organisms in the Animalia kingdom. In general, animals consume organic matter, respire oxygen, engage in sexual reproduction, possess specialized sensory organs and a nervous system, and exhibit rapid responses to stimuli.