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.<\/p>\n

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 \u2014 evidence that suggests crown-group eukaryotes had begun to thrive, increase in number and diversify.<\/p>\n

Why would sterols become more complicated over time? The authors suggested that the more complex sterols bestowed some evolutionary advantage on their owners \u2014 perhaps related to dynamics in the creatures\u2019 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, \u201cthis lost world of ancient eukaryotes was replaced by the modern eukaryotes,\u201d Brocks said.<\/p>\n