Ancient species may have evolved more slowly and lasted longer, but the pace of evolution picked up after global ice ages, according to a new study from Virginia Tech. Published in the journal Science, the research traces the rise and fall of ancient life—long before the age of dinosaurs.

If we think of the Earth as a stage and species as actors, their entrances and exits can be seen in the fossil record. Fossils of skeletons and shells clearly show how evolution and extinction have played out over the past 500 million years. But Virginia Tech researchers have extended this record back nearly 2 billion years.

This extended record reveals the ups and downs in species numbers, providing scientists with new insights into the origins, diversification, and extinction of ancient life. The study focuses on life forms from the Proterozoic eon, which spanned from 2.5 billion to 539 million years ago. Proterozoic life was mostly small and soft, like sponges without hard skeletons, leaving behind fewer fossils.

Ice ages evolution
Geobiologist Shuhai Xiao (at left) and colleague in the field in Canada. Credit: Danielle Fitzgerald

Virginia Tech geobiologist Shuhai Xiao and his team conducted a high-resolution analysis of global fossil data from the Proterozoic. They specifically studied marine eukaryotes, organisms with cells that have a nucleus. These early eukaryotes eventually evolved into multicellular organisms, leading to animals, plants, and fungi.

This is the most comprehensive analysis of this period to date, said Xiao, who was recently elected to the National Academy of Sciences. More importantly, we used a specialized tool to achieve a higher level of time resolution. Some patterns observed and ideas suggested by the analysis are:

  • Early Eukaryotes: These organisms appeared at least 1.8 billion years ago. Between 1.45 billion and 720 million years ago—known as the “Boring Billion”—evolution was slow, and species tended to stick around longer.
  • Snowball Earth: Between 720 million and 635 million years ago, the Earth went through at least two global ice ages, known as “Snowball Earth” events, when temperatures plummeted, and the planet was covered in ice. After the ice melted, evolutionary activity surged, and life became much more dynamic.
Ice ages evolution
The simplified summary diagram shows the relative diversity of eukaryotic fossils throughout the Proterozoic Eon. Credit: Qing Tang / Shuhai Xiao / Virginia Tech

Ice ages were a major reset button for evolution, Xiao explained. We see rapid species turnover immediately following these glaciations. That’s a key discovery.

This research raises intriguing questions, such as: Why was eukaryotic evolution so slow during the “Boring Billion”? What triggered the rapid increase in evolutionary activity after the ice ages? Were environmental factors like climate change or rising oxygen levels the main drivers? Or was it competition among organisms that spurred faster evolution?

Future scientists can use the patterns identified in this study to explore these questions and better understand the complex relationship between life on Earth and the planet itself.


SOURCES

Virginia Polytechnic Institute and State University

Qing Tang, Wentao Zheng, et al., Quantifying the global biodiversity of Proterozoic eukaryotes. Science, 2024; 386 (6728) DOI:10.1126/science.adm9137


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