This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: The earliest animals may have evolved hundreds of millions of years before their first unmistakable fossils appear in the fossil record, according to a new study led by the University of Oxford. The findings, published in Science Advances, suggest animals may have originated deep in the Neoproterozoic Era, even before some of the most extreme ice ages in Earth's history.
The findings could transform our understanding of one of evolution's greatest mysteries: why animals seem to appear relatively suddenly in the fossil record just before the Cambrian Period (539 to 487 million years ago), despite other evidence suggesting that their evolutionary history began much earlier. At the heart of the study is a problem with the way scientists have recently used the fossil record to set a maximum age for the origin of animals. A roughly 590-million-year-old fossil deposit known as the Weng'an Biota in rocks of the Ediacaran age in China preserves microscopic organisms in extraordinary detail, none of which are definitively identified as animals.
Scientists have therefore inferred that the first animals must have originated after Weng'an, assuming that this exceptional fossil deposit would have preserved them if they existed at that time. The research team, which included members from the University of California, Berkeley, ETH Zürich and Yale University, carried out a comprehensive survey of exceptionally well-preserved microfossils from the Kheseen Biota in Mongolia. The Kheseen Biota is more than 40 million years younger than Weng'an, even though they share some species.
By this time, animal fossils are known from sites elsewhere around the world, including Namibia and South China. The researchers studied over 140 samples, including some from previously undocumented localities, and examined them using scanning electron microscopy. This revealed exquisite new microfossil species, including acritarchs (tiny spherical organisms with spines and branching projections) as well as embryo-like fossils containing internal cells.
Yet, despite this remarkable level of preservation, none can be confidently identified as an animal. Senior author and associate professor Ross Anderson (Museum of Natural History, Oxford University) said, "The Kheseen Biota breaks the argument that the exceptional microfossils of Weng'an mean we would have seen animal fossils in the assemblage had they existed at the time. The Kheseen microfossils are just as well preserved, yet animals continue to be absent—despite the fact we know at that point they existed." Animals must have lived in different environments from those that preserved the fossils in the Kheseen Biota, or the chemical conditions there failed to preserve animal remains.
Given that the Kheseen Biota was preserved when animals were present but did not preserve them as fossils, the researchers argue that Weng'an does not automatically qualify as evidence that no animals existed at that time and that they appeared only later than 590 million years ago. Instead, they looked much further back, to several deposits dating from roughly 850 to 730 million years ago. These included the Svanbergfjellet Formation (Norway), Bitter Springs Group (Australia) and Chuar Group (Arizona, U.S.)—all fossil-rich deposits with the potential to preserve animals, even though animal fossils have eluded discovery.
Extract — continue reading at the source.
