The windswept coastline of northwest  Scotland today is a seemingly barren place.

Sandstone cliffs dip gently into the  sea, encrusted by seaweed and lichens.

There isn’t an animal in sight.

But if you pick among the boulders,  and clear that seaweed from the rock   surfaces, there is a miraculous microscopic  ecosystem frozen in time, inside the rocks.

These fossils are easy to miss – they were  tiny organisms, a once-thriving microbial   ecosystem of prokaryotic bacteria and eukaryotic  algae, living together in minuscule harmony.

And among them, there even seem  to be mysterious collections of   cells that look like they could be  multicellular – like simple animals.

The preservation of this ecosystem  is remarkable enough in itself,   but the real marvel comes with the realization  that these rocks are almost a billion years old.

And this goes against much of what  we think we know about the evolution   of animals and our definition of geologic time.

Because the eon we’re in today, the  Phanerozoic, literally means ‘visible life.’ And its beginning is marked by the moment  that animals seem to arrive on the scene,   around 539 million years ago,  in the Cambrian explosion.

So what are animal-like fossils doing  in rocks that are almost twice that old?

And what does that mean for our understanding  of their evolution and geologic time itself?

Turns out, there might’ve actually been a long,   slow-burning fuse that ultimately  ignited the Cambrian explosion.

The mystery of animal ancestry  really began in 1859, when Charles   Darwin published his theory of  evolution by natural selection.

He hypothesized that organisms gradually  adapt and transform in response to changes   in the environment, and that evolution is the  result of incremental changes to existing forms.

While this nicely explained the variety of  living organisms across the world, as well as   parts of the fossil record, the very early fossil  record didn’t play nicely with natural selection.

Because, as far as scientists could tell,  animal life appeared quite suddenly,   explosively even, near the  beginning of the Cambrian period.

Now, scientists define animals as multicellular,  eukaryotic organisms with cells that don't   have rigid cell walls, and which, for the most  part, eat instead of producing their own food.

We find their fossils all through  the Phanerozoic, from skeletons of   dinosaurs to the shells of ancient sea  creatures, and everything in between.

But at the time there were no animal  fossils in the rocks from periods   before the Cambrian, and no evidence of  what animals could have evolved from.

It was a dilemma for Darwin, and has been a  preoccupation for paleontologists ever since.

But over the last 150 years, we’ve found some   clues from that seemingly dark  period before the Phanerozoic.

See, in 1946, a prospector called  Reginald Sprigg found curious,   jellyfish-like impressions in rocks  in the Ediacara hills of Australia.

And while there wasn’t much  interest in them at the time,   that changed in the early 1950s  when schoolchildren in England   uncovered intricate leaf-like fossils  in the rocks of an abandoned quarry.

These fossils predated the Cambrian, and were  the first in a series of discoveries that defined   a brand new collection of multicellular  organisms, known as the Ediacaran biota.

Today, macroscopic Ediacaran organisms - that is,  the ones that can be seen with the naked eye - are   known from all over the world, including  Namibia, Russia, China, Brazil, and Canada.

Whether they’re actually ancestors of the  animals we have today is open for debate,   because they don’t seem to share many of  the features that define modern animals.

But their size and complexity  does extend the fossil record   of multicellularity back to the Precambrian.

However, Ediacarans only take us so far.

The  oldest of these fossils dates back to 574   million years ago, a mere 35 million  years before the Cambrian explosion.

Much of the Precambrian is still resoundingly  empty of multicellular macrofossils.

But long before those macroscopic  Ediacaran organisms were discovered,   scientists in Scotland were finding microscopic  traces of Precambrian life that were much older.

Lots of life on Earth today is microscopic, and,   under the right conditions, microscopic  life can also be preserved in the rocks.

In the early 1900s, geologists were mapping  the rocks of the Torridon Formation in   northwest Scotland, to understand their  age and the environments they formed in.

And while using hydrofluoric acid to  dissolve the siltstones and sandstones,   they recovered organic-walled  microfossils from that ancient time.

But they don’t look like much.

Just  flattened circles and oblongs of carbon.

But they turned out to be one billion years old.

They were a window into Precambrian life,   and an insight into the microscopic  life that reigned before the Ediacaran.

But they were too simple and too ravaged by time   to shed any light on the origin of  animals or resolve Darwin’s dilemma.

Many more organic-walled microfossils  have been found in Precambrian rocks   around the world since then, but  they are still simple, single cells.

The Precambrian record consists of these,   then a few weird Ediacaran experiments,  and then suddenly all the animals.

And those ragged Scottish microfossils were just  bad examples of an underwhelming fossil record,   consigned to a footnote and all but forgotten.

But, in the late 1990s, scientists began to  realize that certain minerals and certain   types of clay can form so quickly that they can  preserve incredibly fine-scale fossil structures.

So paleontologists started looking for  these kinds of minerals around the world.

And so, in the early 2000s,   researchers turned their attention back  to the rocks of northwest Scotland,   where those minerals had been reported alongside  the ragged microfossils 100 years earlier.

And within the rock, researchers found  microscopic cellular structures preserved in 3D.

These included isolated cells,  clusters of up to 20 cells,   and other evidence of a  dynamic microbial community.

But there were also fossils that hinted  at something more than just bacteria.

Some of the bigger spheres had double-layered  walls, or walls marked with a complex pattern.

This level of biological embellishment  is beyond the abilities of the simple   metabolism of a prokaryote - an organism  whose cells don’t have a nucleus.

And so these large fossils are interpreted as   being eukaryotes – organisms whose cells  do have nuclei – and not as bacteria.

They are algae, a more advanced  member of the microbial community.

One billion years ago, they were  relatively new to the scene,   having probably evolved at some point  in the last few hundred million years.

Together, the Scottish fossils point to  a diverse and interconnected microbiota   similar to the one that you’d  find in a modern-day pond.

There were simple bacteria, photosynthesizing  cyanobacteria, and more advanced eukaryotic   algae, with smaller bacteria helping to  break down dead cells and recycle them.

What’s more, there were signs  that the little community did   inhabit a pond or a lake, rather than the ocean… Like mud-cracks that suggested the  ancient sediments had periodically   dried out, but no evidence of salt or other  minerals that would form from salt water.

This new analysis of the Scottish rocks  revealed that microbial ecosystems,   including relatively novel eukaryotic  cells, were well-established in   non-marine environments as far  back as one billion years ago.

It was a huge discovery that pointed  to complex life finding its feet in all   of Earth’s waterways much earlier  than paleontologists had thought.

But it turns out these rocks  had even more to offer.

When scientists looked more closely  at some of those eukaryotes,   to try to tell the story of  algal evolution and diversity,   one microfossil in particular had  features that didn’t match any known alga.

It was a spherical mass of  cells about 30 microns across.

The cells on the inside were pressed together so   tightly that they weren’t spheres  anymore, but had straightish edges.

And they were surrounded by an outer  shell of longer, sausage-shaped cells.

The fossil was named Bicellum braiseri,   after the two different types of cells,  and the scientist who discovered it.

Now, the interesting thing about  Bicellum is that all of the cells   are squished together with no gaps between  them and with no constraints on their shape.

That means that, unlike plants, algae,   and bacteria, they probably  didn’t have rigid cell walls.

Instead their cells were naked, with only  cell membranes.

Like the cells in our bodies.

So this fossil was multicellular,   and had no cell walls.

Those are two of  the major characteristics of animal life.

But it was still very small and simple, so it  wasn’t like any animal we’re familiar with today.

Instead, researchers think that  it’s likely to be one of the very   earliest ancestors of animals, the holozoans.

Holozoans are a big group  that contains all animals,   as well as their closest relatives -  the steps towards true animal life.

Members of the group might not  always look like true animals,   but their cell structures and behaviors are closer  to animals than to any other multicellular group.

Some of the simplest holozoans spend  most of their life as single cells,   but also have a multicellular  phase to their life cycle.

So they’re among the simplest creatures to show  both multicellularity and cell differentiation.

And Bicellum shows all the characteristics  expected of a holozoan, a billion years ago.

If life was beginning to experiment  with multicellularity that far back,   and do it well enough for those new  lifeforms to thrive in lakes as well   as oceans, then this could just be  the solution to Darwin’s dilemma.

It provided a long, microscopic  fuse to the Cambrian explosion.

The idea goes that these early holozoans  evolved the ability to be multicellular,   and to have different cell types.

But they stayed microscopic for  a few million years while they   figured out how to coordinate  more than one cell together.

In that time, they weathered a few  global glaciations, before bursting   onto the scene with an experiment  in being big during the Ediacaran.

And while that ultimately didn’t work  out, that long-burning microscopic fuse   had given animal ancestors the  tools they needed to try again.

The next time, they hit on something  successful, producing resilient   macroscopic multicellular forms that could  adapt into different ecological niches.

The diversity of this new animal life  blew up in the Cambrian explosion.

And the rest, as we know from the  visible fossil record, is history.

So those ancient rocks of northwest Scotland  helped us solve a 150-year old mystery.

The evidence of animal precursors in rocks that  are a billion years old finally addresses Darwin’s   dilemma and opens up a whole new field of  Precambrian paleontological possibility.

While there is more work to be done to understand  the microscopic origins of animal life,   the Precambrian is no longer the dark,  lifeless eon that Darwin once imagined.