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Extended chart of life shows ancient species may have evolved slower and lasted longer

If all the world's a stage and all the species merely players, then their exits and entrances can be found in the rock record. Fossilized skeletons and shells clearly show how evolution and extinction unfolded over the past half a billion years, but a Virginia Tech analysis extends the chart of life to nearly 2 billion years ago. The study is in the journal Science.

The chart shows the relative ups and downs in species counts, telling scientists about the origin, diversification, and extinction of ancient life.

With this new study, the chart of life now includes life forms from the Proterozoic Eon, 2,500 million to 539 million years ago. Proterozoic life was generally smaller and squishier—like sea sponges that didn't develop mineral skeletons—and left fewer traces to fossilize in the first place.

Virginia Tech geobiologist Shuhai Xiao and collaborators published a high-resolution analysis of the global diversity of Proterozoic life based on a global compilation of fossil data, which was released Dec. 20 in the journal Science.

Xiao and his 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.

"This is the most comprehensive and up-to-date analysis of this period to date," said Xiao, who was recently inducted into the National Academy of Sciences. "And more importantly, we've used a graphic correlation program that allowed us to achieve greater temporal resolution."

The choreography of species offers critical insights into the parallel paths of the evolution of life and Earth.

Observed patterns and insights suggested by the analysis:

• 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 aptly known as the "boring billion," when species turnover rates were remarkably low.
• Eukaryotic species in the "boring billion" may have evolved slower and lasted longer than those came later.
• Then cataclysm: Snowball Earth, a spiral of plunging temperatures, sealed the planet in ice at least twice between 720 million and 635 million years ago. When the ice eventually thawed, evolutionary activity picked up, and things weren't so boring anymore.

"The ice ages were a major factor that reset the evolutionary path in terms of diversity and dynamics," Xiao said. "We see rapid turnover of eukaryotic species immediately after glaciation. That's a major finding."

The patterns, Xiao said, raise a lot of interesting questions, including:

• Why was eukaryotic evolution sluggish during the "boring billion"?
• What factors contributed to the increased pace of evolution after snowball ice ages?
• Was it environmental, such as climate changes and increases in atmospheric oxygen level?
• Was it an evolutionary arms race between different organisms that could drive creatures to evolve quickly?

Future scientists can use the quantified pattern to answer these questions and better understand the complex interplay of life on Earth and the Earth itself.