Эртний амьтдын үүслийн хугацааг тогтоох судалгаанд Монголын Хэсээн формацаас олдсон бичил олдворууд шинэ эргэлт авчирч, молекул цагийн тооцооллыг эргэн харах шаардлагатайг харууллаа.
Виржиниа Тек их сургуулийн палеонтологич Орин Лоле Дурбин болон түүний багийнхан амьтны аймгийн үүслийн цаг хугацааг нарийвчлан тогтоохын тулд молекул цагийн аргыг ашиглан судалгаа хийжээ. Одоогийн байдлаар маргаангүй батлагдсан хамгийн эртний амьтны олдворууд 574 сая жилийн настай гэж үздэг ч химийн шинжилгээгээр олдох липидийн биомаркерууд амьтад 635 сая жилийн өмнө ч байсныг илтгэдэг. Энэхүү зөрүүтэй байдал нь эрдэмтдийн дунд маргаан дагуулсаар ирсэн юм.
Судлаачид Монголоос олдсон, 550-526 сая жилийн настай Хэсээн формацын фосфатжсан бичил олдворуудыг судалсан байна. Эдгээр олдвор нь амьтны аймгийн үүслийн хязгаарыг тогтооход ашиглагддаг Хятадын Вэн-ань бүлэг олдвортой ижил төстэй хадгалалтын нөхцөлтэй боловч амьтны ул мөр илрээгүй нь анхаарал татаж байна. Оксфордын их сургуулийн судлаач Росс Андерсоны дүгнэснээр, энэхүү олдвор нь тухайн үед амьтад байгаагүй гэсэн таамаглалыг үгүйсгэж, олдвор олдоогүй нь амьтад үүсээгүй байсны баталгаа биш гэдгийг харуулж байна.
Судалгааны баг хуучин ашиглагдаж байсан он цагийн тооцооллын аргачлалыг өөрчилж, илүү эртний давхаргын олдворуудыг харьцуулалтад оруулснаар амьтны аймаг 800-700 сая жилийн өмнө үүссэн байж болзошгүй гэсэн дүгнэлтэд хүрчээ. Гэсэн хэдий ч энэ нь амьтад 800 сая жилийн өмнө байсныг шууд нотлохгүй гэдгийг судлаачид онцолж байна. Цаашид амьтны аймгийн үүслийн нарийн хугацааг тогтоохын тулд дэлхийн өнцөг булан бүрээс олдсон биеийн олдвор, химийн биомаркер болон амьтны үйл ажиллагааны ул мөрийг цогцоор нь судлах шаардлагатай байгаа юм.
Дэлгэрэнгүйг эх сурвалжаас харах
↓Эх сурвалжийг нээх ↓
The oldest undisputed animal fossils are about 574 million years old, but a new study suggests the animal kingdom’s real origins may lie much deeper in time. Fossils from Mongolia are undermining a key assumption behind dating methods, and when that assumption is swapped out, molecular-clock estimates jump back to roughly 800 to 700 million years ago.
The question of when animals first appeared has puzzled scientists for centuries, in the researchers’ own telling. The oldest undisputed fossils come from the Ediacaran biota, and the microfossil record doesn’t offer definitive animals before that period either.
Chemical evidence tells a different story. Lipid biomarkers, traces left behind by sponges, hint that animals were around before 635 million years ago. To try to settle the matter, Virginia Tech paleontologist Orin Lole Durbin and colleagues turned to molecular clocks, which use genetic differences between living species to estimate when lineages split.
A Stubborn Gap Between Rocks And Chemistry
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.
According to the study’s authors, recent analyses indicate that sponge-specific signatures are preserved in Neoproterozoic rocks in sufficient abundance to support the presence of sponges. They describe the clash between fossils and chemistry as “a stark dichotomy.”
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’t necessarily mean animals hadn’t yet evolved.
Mongolia’s Fossils Test the Clock
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.
The Kheseen fossils are preserved in a manner comparable to those of China’s Weng’an 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 of Oxford University’s Museum of Natural History:
“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 logic follows neatly. Animals were already evolving when the Kheseen sediments accumulated, so their absence can’t simply be read as proof they hadn’t appeared. That calls into question the use of the Weng’an Biota as a maximum calibration. The authors suggest that ecological or fossilization conditions could explain why animals are missing from both assemblages.
A Caution on Older Estimates
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.
Lole Durbin was careful, though. Pre-Ediacaran animal body fossils still elude researchers, he said, and the analysis does not prove that animals existed 800 million years ago. What the Mongolian evidence does, he added, is undermine one of the main arguments for restricting animal origins to the Ediacaran interval.
The team’s recommendation is to look further afield: fossil deposits from different parts of the world, representing different environments and modes of fossilization. Any such search should weigh every available kind of evidence, including body fossils, traces of animal activity and chemical biomarkers. Until then, Anderson noted that:
“Until that evidence becomes available, the precise birth date of the animal kingdom remains uncertain.”
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