Steroids discovered in 1.6 billion-year-old rock could help scientists solve a long-standing mystery about the evolution of single-celled life.
These compounds are produced by eukaryotic organisms, which are defined by having cells with membrane-bound nuclei and internal organelles. Modern eukaryotes include plants, fungi and animals. In contrast, prokaryotes – bacteria and archaea – lack these characteristics. Based on molecular data, researchers know that single-celled eukaryotes first evolved at least 2 billion years ago, but there are very few fossil records from their earliest days.
Particularly puzzling is that steroids produced by eukaryotes as part of their membranes do not appear in the fossil record until about 800 million years ago. The last common ancestor of modern eukaryotes, including today’s humans, lived about 1.2 billion years ago and must have produced these steroids, leading to confusion as to why they didn’t show up in ancient rocks.
Now, the researchers have discovered that they were looking for the wrong thing. Instead of looking for modern-looking steroid compounds, they discovered precursors to earlier stages in the metabolism of microbes. The team published the findings on Wednesday (June 7) in the journal Nature.
“It’s like passing something obvious every day but not ‘seeing’ it,” studies the first author Jochen Brock, a professor in the Research School of Earth Sciences at the Australian National University, told Live Science. “But once you know what it looks like, you suddenly see it everywhere.”
Once the researchers figured out which molecules to look for, they found them in all sedimentary rocks between 1 billion and 1.6 billion years ago. That changes the picture of what researchers believed about the original abundance of eukaryotes, Brocks said.
“We previously thought that eukaryotes were very low abundance or limited to marginal environments where we can’t find the molecular fossils,” he said. “It now appears that more primordial forms may also be quite abundant in open marine habitats.”
The compounds were first found in rocks that formed on the ancient ocean floor, which are now exposed on land in Australia’s Northern Territory. When the researchers extended their hunt for billion-year-old rocks globally, however, they found traces of steroids in ancient streams around the world, including West Africa, Scandinavia and China.
The oldest samples date back 1.64 billion years; scientists have yet to find older rocks that are preserved well enough for analysis. There’s also a gap in the record between 1 billion and 800 million years ago, Brocks said, because few marine rocks from that period still exist. That period is right at the peak of the emergence of modern eukaryotes, however, he said, so it’s important to fill those gaps.
The new study is a “significant step” in completing missing data on early eukaryotes, he said Laura Katza Smith College biologist who studies eukaryotic evolution but was not involved in the new study.
“This paper is helping us understand these early eukaryotes and what early eukaryotes might have looked like,” Katz said.
These organisms evolved in an environment very different from today’s, Andrew Roger, a molecular biologist at Dalhousie University in Canada who was not involved in the research, told Live Science. Earth’s atmosphere didn’t contain significant levels of oxygen until 2.4 billion years ago, and it didn’t reach modern oxygen levels until 650 million years ago, Roger said.
Atmospheric oxygen levels may have played a role in the timing of eukaryote evolution, given that most eukaryotes use oxygen in their metabolism, he said. It’s also possible that newly evolved steroids allowed these early eukaryotes to move into new oxygen-rich environments, Katz said.
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