Science & Technology, UK (Commonwealth Union) – A groundbreaking study conducted by the University of Oxford has taken us one step closer to unraveling a mystery that has intrigued naturalists since the time of Charles Darwin: the timing of the first appearance of animals in Earth’s history. The research findings recently appeared in the journal Trends in Ecology & Evolution.
According to the fossil record, animals first emerged approximately 574 million years ago, marked by a sudden proliferation in rocks during the Cambrian period (539 million years ago to 485 million years ago). This remarkable “explosion” of life contradicts the typically gradual pace of evolutionary change. However, scientists, including Darwin himself, believe that animals may have evolved much earlier than the Cambrian period. Yet, the absence of animal fossils in the earlier rock layers remains an enigma.
The “molecular clock” method, which relies on the rates of genetic mutations to estimate the time when two or more species shared a common ancestor, suggests that animals originated around 800 million years ago, during the early Neoproterozoic era (1,000 million years ago to 539 million years ago). Nevertheless, even though fossil microorganisms like bacteria and protists have been discovered in rocks from the early Neoproterozoic, no animal fossils have been found.
This presented a perplexing question 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 soft to be preserved as fossils?
To address this intriguing matter, a team of researchers led by Dr. Ross Anderson from the University of Oxford’s Department of Earth Sciences has conducted the most comprehensive evaluation to date of the preservation conditions that would potentially capture the earliest animal fossils.
“The first animals presumably lacked mineral-based shells or skeletons, and would have required exceptional conditions to be fossilised. But certain Cambrian mudstone deposits demonstrate exceptional preservation, even of soft and fragile animal tissues. We reasoned that if these conditions, known as Burgess Shale-Type (BST) preservation, also occurred in Neoproterozoic rocks, then a lack of fossils would suggest a real absence of animals at that time,” explained Dr Ross Anderson.
To delve into this mystery, the research team conducted a comprehensive investigation utilizing various analytical techniques on samples of Cambrian mudstone deposits from nearly 20 sites. They compared the sites containing Burgess Shale Type (BST) fossils with those preserving only mineral-based remains, such as trilobites. The University of Oxford’s Departments of Earth Sciences and Materials conducted energy dispersive X-ray spectroscopy and X-ray diffraction analyses, while infrared spectroscopy was carried out at Diamond Light Source, the UK’s national synchrotron facility.
Through their analysis, the researchers made a significant discovery. Fossils exhibiting exceptional BST-type preservation were found to be notably enriched in an antibacterial clay known as berthierine. Samples containing at least 20 percent berthierine exhibited BST fossils in approximately 90 percent of cases.
Further investigation using microscale mineral mapping revealed another antibacterial clay, kaolinite, which seemed to interact directly with decaying tissues at an early stage. This interaction led to the formation of a protective halo during the fossilization process.
The researchers’ findings from the study propose a potential upper limit for the emergence of animals at approximately 789 million years, based on the youngest estimated age of the Svalbard formation. Moving forward, the team plans to explore progressively younger Neoproterozoic deposits that have suitable conditions for preserving BST fossils. This endeavor aims to verify whether the absence of animal fossils in certain rock layers is due to their actual absence or if the conditions did not allow for fossilization.
Additionally, the researchers intend to conduct laboratory experiments to delve into the mechanisms underlying clay-organic interactions in BST preservation.
