When did animals first appear on Earth? The question has been troubling naturalists since Charles Darwin, and a team from Oxford University appears to have taken an important step toward solving the storied mystery.
As explained in an article published in the magazine Trends in Ecology
Many scientists (including Darwin himself) believed that the first multicellular eukaryotic organisms in the animal kingdom evolved long before the Cambrian period, but they cannot explain why they are nowhere to be found in the fossil record.
The 'molecular clock' method, for example, suggests that animals first evolved 800 million years ago, during the early part of the Neoproterozoic era (between 539 and 1 billion years ago). This approach uses the rates at which genes accumulate mutations to determine when two or more living species last shared a common ancestor.
But while early Neoproterozoic rocks contain fossil microorganisms, such as bacteria and protists, no animal fossils have been found, ultimately posing a dilemma for paleontologists: perhaps animals from this period were too soft and fragile to be preserved?
To clarify this point, researchers at the University of Oxford carried out the most comprehensive evaluation. "The earliest animals presumably lacked shells or mineral-based skeletons, and would have required exceptional conditions to fossilize," says Dr. Ross Anderson, lead author of the study.
Certain Cambrian shale (sedimentary rock) deposits demonstrate exceptional preservation, even of soft and fragile animal tissues. From here, experts believe that if these conditions also occurred in Neoproterozoic rocks, then the lack of fossils suggests "a real absence of animals at that time."
They analyzed samples of Cambrian shale deposits from nearly 20 different sites and found that the exceptionally preserved fossils were particularly enriched in an antibacterial clay called berthierine. Samples with a composition of at least 20% berthierin produced fossils in about 90% of the cases.
"The presence of these clays was the main predictor of whether the rocks would harbor fossils," adds Dr. Anderson in a statement. "This suggests that the clay particles act as an antibacterial barrier that prevents bacteria and other microorganisms from breaking down organic materials," he says.
The researchers applied these same techniques to analyze samples from numerous Neoproterozoic shale deposits. The analysis revealed that the majority did not have the necessary compositions for the preservation of fossils. That is why three deposits found in Nunavut (Canada), Siberia (Russia) and Svalbard (Norway) had compositions almost identical to the rocks of the Cambrian period.
"The similarities in the distribution of clays with fossils in these rare early Neoproterozoic samples and with exceptional Cambrian deposits suggest that, in both cases, the clays were attached to decaying tissue and that conditions conducive to preservation were available at both times." says Ross Anderson.
This provides the first "evidence of absence" and supports the view that animals had not evolved by the early Neoproterozoic era, contrary to some estimates of the molecular clock.
With these data in hand, the researchers suggest that the maximum possible age for the origin of animals is around 789 million years ago. The experts now want to continue looking for more recent Neoproterozoic deposits that serve to confirm that the lack of animals in the fossil record because they really were absent, and not because the conditions did not allow fossilizing them.
