Fossils show ‘lost world’ of early life – 6/9/2023 – Science

Fossil remains of a cell membrane component identified in rocks dating back around 1.6 billion years are opening a window into what scientists call the “lost world” of microscopic organisms that were the primordial precursors of fungi, algae, Earth’s plants and animals—including humans.

These remains, researchers said Wednesday, date to a time period during what’s called the Proterozoic eon, which was crucial in the evolution of complex life but shrouded in mystery because of an erratic fossil record. of the microscopic organisms that inhabited Earth’s marine kingdom.

The newly identified fossils are of a rudimentary form of steroid — a fatty molecule that was an indispensable ingredient in the cell membranes of early members of a now-dominant kingdom of organisms called eukaryotes. Eukaryotes have a complex cell structure, including a nucleus that acts as a command and control center and subcellular structures called mitochondria, which power the cell.

They were “crashers” into a world teeming with bacteria—simpler single-celled organisms without nuclei. Today eukaryotes include fungi, algae, plants and animals, but none of them had yet evolved.

The newly described fossils do not include the organisms’ actual bodies, but rather their molecular remnants, leaving their size, appearance, behavior and complexity uncertain — including whether they were all unicellular or some were multicellular. “We have no idea,” said geobiologist Jochen Brocks of the Australian National University in Canberra, lead author of the study published in the journal Nature.

Researchers suspect they weren’t tame.

“Despite their small size, they may have included ferocious predators that fed on smaller bacteria or perhaps even other eukaryotes,” said geobiologist and study co-author Benjamin Nettersheim of the University of Bremen in Germany.

There are some fossil “bodies” of early eukaryotes that date back more than 1.6 billion years, but their sparseness compared to the plentiful bacterial remains from that era suggests that they were small players in a larger drama. The researchers found that molecular fossils that indicate the presence of these early eukaryotes were common in rocks from about 1.6 billion years old to 800 million years ago.

“It’s a lost world in the sense that we haven’t been able to see them or detect them — although there was a whole world of them. They weren’t rare and they lasted for hundreds of millions of years,” Brocks said.

It’s a lost world too because these forms are totally extinct, Brocks added. Their disappearance paved the way for modern eukaryotic forms to spread around 800 million years ago. To put these time intervals into perspective, our eukaryotic species, the homo sapiensappeared about 300 thousand years ago.

When early eukaryotes existed, Earth’s land expanses were barren rock, while large parts of the sea floor were covered in thick microbial mats, and ocean waters were incurred by toxic hydrogen sulfide gas, which smells like rotten eggs.

Until now, these oceans were thought to be largely a bacterial stew, with eukaryotes rare or restricted to marginal habitats such as coasts or rivers. Fossil steroid molecules found trapped in sedimentary rocks deposited on ancient seabeds reveal that eukaryotes were surprisingly abundant.

The oldest of the rocks with these fossils was unearthed in Australia’s remote northern Outback near Darwin.

Scientists have long been puzzled by the apparent absence of molecular fossils from this time period indicative of early eukaryotes. Turns out they were looking for more biologically advanced steroids than these organisms had.

Biochemist Konrad Block, who won the Nobel Prize in 1964 and died in 2000, had hypothesized that early eukaryotes produced equally primitive steroids, but doubted that they would ever be discovered.

“I wish I could tell him we found them,” Brocks said.

^{Translated by Luiz Roberto M. Gonçalves}