A Stubborn Gap Between Rocks And Chemistry<\/p>\n
The mismatch is the heart of the puzzle. Unambiguous macrofossil evidence stretches back only to the earliest examples of the Ediacara Macrobiota, dated to 574 million years ago. Biomarkers, meanwhile, point to animals during the final 10 to 20 million years of the Cryogenian, a period that ran from 717 to 635 million years ago.<\/p>\n
According to the study\u2019s authors, recent analyses indicate that sponge-specific signatures are preserved in Neoproterozoic rocks in sufficient abundance to support the presence of sponges<\/a>. They describe the clash between fossils and chemistry as \u201ca stark dichotomy.\u201d <\/p>\n The team set out to reexamine the geological deposits used to fix the maximum possible age of animals, arguing that some relatively young ones may be unreliable, because an absence of animal fossils doesn\u2019t necessarily mean animals hadn\u2019t yet evolved.<\/p>\n Mongolia\u2019s Fossils Test the Clock<\/p>\n To check one commonly used molecular-clock calibration, the researchers examined the Kheseen Biota, a collection of phosphatized microfossils from Mongolia. It dates to roughly 550 to 526 million years ago, a time when animals were already present elsewhere. Yet it holds no fossils that can be confidently identified as animals.<\/p>\n The Kheseen fossils are preserved in a manner comparable to those of China\u2019s Weng\u2019an Biota, which has served as a maximum calibration for the origin of animals. The two assemblages share several fossil species and show similarly detailed preservation. As reported by Dr. Ross Anderson<\/a> of Oxford University\u2019s Museum of Natural History: <\/p>\n \u201cThe Kheseen Biota breaks the argument that the exceptional microfossils of Weng\u2019an mean we would have seen animal fossils in the assemblage had they existed at the time.\u201d<\/p>\n The logic follows neatly. Animals were already evolving when the Kheseen sediments accumulated, so their absence can\u2019t simply be read as proof they hadn\u2019t appeared. That calls into question the use of the Weng\u2019an Biota as a maximum calibration. The authors suggest that ecological or fossilization conditions could explain why animals are missing from both assemblages.<\/p>\n A Caution on Older Estimates<\/p>\n When the team substituted older, Tonian-age deposits for the younger Ediacaran calibrations, the molecular-clock analyses produced much older estimates for animal emergence. Depending on the calibration, the origin of crown-group animals reached into the range of roughly 800 to 700 million years ago.<\/p>\n![\"Stratigraphic](\"https:\/\/www.newsbeep.com\/au\/wp-content\/uploads\/2026\/10\/Stratigraphic-and-geological-context-of-the-Kheseen-Formation-1200x683-1.jpg.webp\")
Stratigraphic and geological context of the Kheseen Formation. Credit: Science Advances<\/p>\n![\"Spiny](\"https:\/\/www.newsbeep.com\/au\/wp-content\/uploads\/2026\/10\/Spiny-microfossils-from-the-Kheseen-Biota-1200x1069-1.jpg.webp\")
Spiny microfossils from the Kheseen Biota. Credit: Science Advances<\/p>\n
![\"Small](\"https:\/\/www.newsbeep.com\/au\/wp-content\/uploads\/2026\/10\/Small-embryo-like-fossils-from-the-Kheseen-Biota-1200x1120-1.jpg.webp\")
Small embryo-like fossils from the Kheseen Biota. Credit: Science Advances<\/p>\n