At first, the stem group may have had an advantage. Oxygen levels in the atmosphere were significantly lower than they are today. Because building protosterols requires less oxygen and energy than modern sterols require, stem-group eukaryotes were likely more successful and abundant.
Their influence declined when the world hit a critical transition known as the Tonian Period. Between 1 billion and 720 million years ago, oxygen, nutrients and other cellular raw materials increased in the oceans. Fossils of modern eukaryotes, like algae and fungi, start to appear in the rock record, and modern steroids start to outnumber protosteroids in fossilized biomarkers — evidence that suggests crown-group eukaryotes had begun to thrive, increase in number and diversify.
Why would sterols become more complicated over time? The authors suggested that the more complex sterols bestowed some evolutionary advantage on their owners — perhaps related to dynamics in the creatures’ cell membranes. Whatever the reason, the sterol shift was evolutionarily significant. The makeup of modern sterols likely gave crown-group eukaryotes a boost over the stem group. Eventually, “this lost world of ancient eukaryotes was replaced by the modern eukaryotes,” Brocks said.
A Bacterial Wrinkle
The researchers’ evolutionary sterol story is compelling, but it’s not rock solid.
“I wouldn’t be surprised” if their interpretation is correct, Gold said. However, there is another possibility. Although scientists tend to associate sterols with eukaryotes, some bacteria can also make them. Could the molecular fossils in the study have been left by bacteria instead?
Gordon Love, a geochemist at the University of California, Riverside, thinks the bacterial scenario makes more sense. “These protosteroids turn up in rocks of all ages,” he said. “They don’t just disappear, which means that something other than stem eukaryotes is capable of making those.” He argued that bacteria, which dominated the sea at the time, could have easily produced protosteroids.
The authors can’t rule out that possibility. In fact, they suspect that some of their fossil molecules were made by bacteria. But the possibility that their vast collection of fossilized protosteroids, stretching for hundreds of millions of years, was made entirely by bacteria seems unlikely, Brocks said.
“If you look at the ecology of these bacteria today, and their abundance, there is just no reason to believe that they could become so abundant that they could have produced all these molecules,” he said. In the modern world, bacteria produce protosterols only in niche environments such as hydrothermal springs or methane seeps.
Cohen, the Williams College paleontologist, agrees with Brocks. The interpretation that these molecules were made by eukaryotes “is consistent with every other line of evidence,” she said — from the fossil record to molecular clock analyses. “I’m not as worried” about that possibility, she said.
Either interpretation presents more questions than answers. “Both stories would be absolutely crazy weird,” Brocks said. They are “different views of our world,” he added, and it would be nice to know which one is true.
Lacking a time machine, the researchers are searching for more evidence to improve their certainty one way or the other. But there are only so many ways to reconstruct or perceive ancient life — and even scientists’ best guesses can never completely fill the gap. “Most life didn’t leave any traces on Earth,” Nettersheim said. “The record that we see is limited. … For most of Earth’s history, life might have looked very different.”
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