An international team of researchers published a high-resolution analysis of the global diversity of life forms spanning 2 billion years based on a global compilation of fossil data.
The fossil record of organisms with hard parts (like bones and shells) shows how evolution and extinctions unfolded over the past half a billion years. But life on Earth arose much earlier, at least 3 to 2 billion years ago. This early life was generally smaller and squishier, dominated by microbial communities, algae, sponges, worm-like creatures and jellyfish, leaving few traces to fossilize in the first place. Proterozoic fossils, dating from 2,500 million to 539 million years ago, include body imprints in fine sediments, stromatolites, trace fossils, chemical and isotopic remains.
A new study based on available fossil data compiled a curve of life’s diversity spanning 2,000 million years, showing how around 600 million years ago a series of super ice ages spurred a whole new era for life on Earth.
“This is the most comprehensive and up-to-date analysis of this period to date. And more importantly, we’ve used a graphic correlation program that allowed us to achieve greater temporal resolution,” explains Virginia Tech geobiologist Shuhai Xiao, coauthor of the study.
The research team looked specifically at records of ancient marine eukaryotes — organisms whose cells contain a nucleus. Early eukaryotes later evolved into the multicellular organisms credited for ushering in a whole new era for life on Earth, including animals, plants, and fungi.
The first eukaryotes arose no later than 1.8 billion years ago and gradually evolved to a stable level of diversity from about 1,450 million to 720 million years ago, a period sometimes referred by paleontologists as the “boring billion,” as species seemingly evolved slower and lasted longer than those that came later.
Between 720 million and 635 million years ago, Earth plunged into a series of super ice ages. Ice covered almost the entire planet, leaving behind glacial debris at the equator. When the ice eventually thawed, evolutionary activity picked up, and things weren’t so boring anymore. The first complex and large organisms appear in the fossil record shortly after, around 580 million years ago.
“The ice ages were a major factor that reset the evolutionary path in terms of diversity and dynamics,” Xiao summarizes the study’s most intriguing conclusion. “We see rapid turnover of eukaryotic species immediately after glaciation. That’s a major finding.”
How exactly the glaciations boosted evolution is not yet clear. One hypothesis suggests that ice-shields eroded the continents and glaciers deposited nutrient-rich sediments in the oceans, providing a fertile environment for complex life to evolve.
The study, “Quantifying the global biodiversity of Proterozoic eukaryotes,” was published in the journal Science and can be found online here.
Additional material and interviews provided by Virginia Tech.
