Animal fossils may have emerged 200 million years earlier than rocks show
The first animals on Earth may have originated 800 to 700 million years ago, pushing back the timeline of early animal evolution by up to 200 million years. Previously, the earliest definitive fossil evidence placed animal origins at around 574 million years ago during the Ediacaran period, leaving a massive evolutionary gap in the rock record according to a study published October 2, in the journal Science Advances.
Researchers examine the Kheseen Biota in Mongolia
To investigate why rocks from certain periods lack animal fossils, a research team led by Ross Anderson, an associate professor of natural history at the University of Oxford, studied more than 140 rock samples from the Kheseen Biota in Mongolia. livescience.com reported that this site is more than 40 million years younger than the roughly 590 million-year-old Weng’an Biota deposit in China. Using electron scanning microscopy, the scientists found microscopic fossils preserved in extraordinary detail, including spherical organisms and embryo-like structures containing internal cells. Despite this exceptional preservation, the team could not identify any of the microfossils as animals, even though animal life existed elsewhere on Earth at that time.
Molecular clocks date the origin of animal life
Because the fossil record lacks definitive animal remains from older strata, the researchers turned to molecular clocks to estimate when evolutionary lineages diverged. news.yale.edu reported that the study’s authors calibrated these molecular clock analyses by using three exceptionally preserved fossil sites dating from roughly 850 to 730 million years ago: the Bitter Springs Group in Australia, the Svanbergfjellet Formation in Norway, and the Chuar Group in Arizona. All three locations possessed conditions suitable for fossilizing early animals, yet yielded no animal fossils. By combining genetic differences among living organisms with rates of genetic change, the analysis calculated that animal life began 800 to 700 million years ago.
Ice ages complicate the timeline of early evolution
This revised timeline places the emergence of animals deep within the Neoproterozoic era, potentially predating a series of extreme ice ages known as “Snowball Earth” that began about 720 million years ago. During this period, global ice sheets covered the planet’s surface for thousands or millions of years. Paleontologists continue to debate whether environmental pressures from Snowball Earth slowed down or otherwise shaped early animal evolution.
Questions about the dawn of the animal kingdom
Why do some well-preserved ancient rocks lack animal fossils?
Fossils can be absent from rocks because scientists are looking in the wrong geographic locations or because local environmental conditions were unsuitable for creating fossils, according to study co-author Ross Anderson.
What methods do scientists use when fossils are missing?
Researchers use molecular clocks, which estimate when evolutionary lineages diverged by comparing genetic differences among living organisms and combining those differences with measured rates of genetic change.
Which sites were used to calibrate the molecular clock analysis?
The research team focused on three ancient sites with well-preserved fossil assemblages dating from 850 to 730 million years ago: the Bitter Springs Group in Australia, the Svanbergfjellet Formation in Norway, and the Chuar Group in Arizona, as reported by news.yale.edu.
The earliest undisputed animal fossils previously dated to the Ediacaran period, which spanned from 635 million to 538 million years ago.
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