Fossil remains of a cell membrane component have been identified and preserved in rocks dated to 1600 million years ago, opening a window into what scientists call the ‘lost world’ of organisms that were before – keeps on planet Earth fungi, algae, plants and animals – including humans.
These traces, announced by scientists in this week’s edition of the journal
naturedate back to a period of time, the Proterozoic Eon, which was crucial in the evolution of complex life, but which has been shrouded in mystery due to an extremely rare fossil record of microscopic organisms that inhabited the marine world at that time.
The fossils now identified are of a rudimentary form of a steroid – a fat molecule that was an indispensable ingredient in the cell membranes of the then-pioneer and now-dominant organisms we call “eukaryotes”.
Eukaryotes have a complex cellular structure, including a nucleus that functions as a command and control center and subcellular structures called “mitochondria” that supply cells with energy. Eukaryotes were “crashers” in a world teeming with bacteria, simpler single-celled organisms without a nucleus. Today, eukaryotes include fungi, algae, plants, and animals.
The fossils now described do not really include the “bodies” of the organisms, but rather their molecular remains, making it unclear what size, appearance, behavior and complexity they would have – including whether they were all single-celled or whether some were multicellular.
“We have no leads,” says geobiologist Jochen Brocks, from the Australian National University in Canberra, and lead author of the scientific paper published in
Scientists suspect they were far from docile. “Despite their particularly small size, there could already be formidable predators among them that fed on smaller bacteria or even other eukaryotes”, explains Benjamin Nettersheim, geobiologist at the University of Bremen (Germany) and co-author of the study.
There are fossils of ancient eukaryotic “bodies” over 1600 million years old, but their rarity compared to the abundant remains of bacteria from that time suggests they were minor players in a larger story.
Researchers have now found that molecular fossils that indicate the presence of these early eukaryotes are common in rocks from 1600 million years ago to 800 million years ago.
Researcher Jochen Brocks examines 1600 million year old sedimentary rocks in Australia
Australian National University Why a lost world
“It’s a lost world in the sense that we have no way of seeing it or detecting it – even if there was a whole world full of it. They weren’t rare and they lived for hundreds of millions of years,” says Jochen Brocks.
It is still a lost world because these life forms are now all extinct, adds Jochen Brocks. Their disappearance paved the way for the spread of modern eukaryotic forms around 800 million years ago. To put these time intervals into perspective, our eukaryotic species, the
homo sapiensappeared about 300,000 years ago.
When the first eukaryotes existed, vast tracts of land were barren rock, while much of the sea floor was covered in mats of microbial life, and ocean waters were steeped in hydrogen sulfide, a gas that smelled of rotten eggs.
Until now, it was thought that the oceans were mainly a breeding ground for bacteria, eukaryotes being rare or confined to marginal habitats, such as coastal areas or rivers. Instead, fossils of steroid molecules trapped in sedimentary rocks deposited at the bottom of ancient seas reveal that eukaryotes would have been surprisingly abundant.
The oldest rocks containing these fossils were unearthed in the remote outback of northeast Australia near Darwin.
Scientists have long been puzzled by the lack of
apparent molecular fossils of early eukaryotes for this time interval. After all, what was happening was that scientists were looking for steroids that were biologically more complex than what these organisms had.
Biochemist Konrad Block, who won the Nobel Prize in 1964 and died in 2000, had theorized that early eukaryotes produced such steroids, but doubted they would ever be discovered. “I wish I could tell you we found them,” says Konrad Block.