1.6-billion-year-old steroids may be traces of earliest complex life

Molecules found in 1.6-billion-year-old Australian rocks are believed to be remnants of a “lost world” of ancient organisms that comprised the earliest complex life.

“We say they are a lost world, firstly because they are extinct, and secondly because we know virtually nothing about them,” says Jochen Brocks at the Australian National University in Canberra, who led the study.

The earliest life forms on Earth were simple single-celled organisms like bacteria that emerged more than 3 billion years ago. At some point – we don’t know exactly when – came more complex organisms called eukaryotes, whose cells have a nucleus. These eventually evolved into all the animals, plants, fungi and algae alive today.

The oldest confirmed eukaryote fossils are red and green algae found in 1-billion-year-old rocks in Canada and China. Eukaryote fossils are confirmed by their shape and the detection of steroids – molecules that eukaryotes use to build cell membranes – co-located in the same rocks.

Older fossils that look like they might be eukaryotes have been found in rocks dating back 1.6 billion years, but scientists haven’t been able to find the tell-tale steroid molecules that should be associated with them, which has created a conundrum.

Now, evidence collected by Brocks and his colleagues suggests these older fossils are in fact eukaryotes, but a more primitive form that produced simpler steroid molecules known as “protosteroids”.

When modern eukaryotes make steroids, including cholesterol, they follow a 12-step biochemical pathway. The molecules made in the first few steps are protosteroids.

Brocks and his colleagues found examples of these protosteroids in 1.6-billion-year-old rocks collected from the McArthur river mine near Borroloola in Australia’s Northern Territory, an area that once formed an ancient seabed. They did this by grinding up the rocks and then chemically analysing them.

The researchers then studied a range of other rocks from around the world and also found abundant protosteroids in those aged between 1 billion and 1.6 billion years old.

This finding suggests that the earliest eukaryotes emerged at least 1.6 billion years ago and made protosteroids. Around 1 billion years ago, eukaryotes evolved the ability to make modern steroids, which presumably conferred advantages that gave them the “upper hand” and allowed them to thrive while the earlier eukaryotes died out, says Brocks.

These more sophisticated eukaryotes later evolved into algae, fungi, plants and animals that all produce modern steroids. The first animal eukaryotes, for example, emerged about 600 million years ago.

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The research supports the late biochemist Konrad Bloch’s hypothesis that, in the deep past, “maybe there were organisms that only produced the first step and then the second step and then the third step of the cholesterol biosynthetic pathway”, says Brocks. “They’re the molecules we found – those ones in the early steps.”

“I think this may turn out to be another important piece in the puzzle of early eukaryotic evolution,” says Birger Rasmussen at the University of Western Australia.

At this stage, we don’t know what the earliest protosteroid-producing eukaryotes would have looked like, but Brocks believes they were marine organisms that were much bigger and more complex than bacteria. “Bacteria are just simple blobs, but eukaryotes have protein skeletons called cytoskeletons that mean they can have all sorts of protrusions and ornamentations,” he says.

If the 1.6-billion-year-old eukaryote-like fossils are found to be co-located with protosteroids, this would confirm that they were these primitive eukaryotes and give some clues about their appearance, says Brocks.

One remaining question is whether anything else other than eukaryotes could have produced the protosteroids found in the ancient rocks. Some bacteria are known to make protosteroids, although they are unusual. “That is the biggest uncertainty in the study and will definitely be the focus of our future research, to see if bacteria played any role,” says Brocks.