A study led by the University of Oxford has brought mankind one step closer to solving a mystery that has baffled naturalists since Charles Darwin: when did animals first appear on the Earth? The study, “Fossilization processes and our reading of animal antiquity,” has been published in Trends in Ecology & Evolution.
Animals first appear in the fossil record about 574 million years ago. Their arrival seems like a sudden “explosion” in rocks from the Cambrian period (539 million years ago to 485 million years ago) and is not in sync with typically gradual pace of evolutionary change. Many scientists (including Charles Darwin himself) believe that the first animals actually evolved long before the Cambrian period, but they simply cannot provide explanation for their absence from the fossil record.
The “molecular clock” approach, for instance, proposes that animals first evolved 800 million years ago, during the early part of the Neoproterozoic era (1,000 million years ago to 539 million years ago). This approach utilizes the rates at which genes accumulate mutations to determine the point in time when two or more living species last shared a common ancestor. But although rocks from the early Neoproterozoic contain fossil microorganisms, no animal fossils have been found.
This posed a dilemma for palaeontologists: does the molecular clock method overestimate and misjudge the point at which animals first evolved? Or were animals in fact present during the early Neoproterozoic, but too soft and fragile to be preserved?
To investigate this, a team of researchers led by Dr. Ross Anderson from the University of Oxford’s Department of Earth Sciences have conducted the most thorough assessment to date of the preservation conditions that would be expected to capture the earliest animal fossils.
Dr. Anderson said, “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.”
Dr. Andersons’s team found that similarities in the distribution of clays with fossils in some rare early Neoproterozoic samples and with exceptional Cambrian deposits suggest that, in both cases, clays were attached to decaying tissues, and that conditions conducive to BST preservation were available in both time periods. This provides the primary ‘evidence for absence’ and supports the view that animals had not evolved by the early Neoproterozoic era, contrary to some molecular clock estimates.”