When scum ruled the Earth

Artist's impression of Ordovician landscape

An artist's impression of an eroded Ordovician (490 to 443 million years ago) volcano and shore line near Builth Wells, Powys

29 December 2008

Newly-discovered fossils are shedding light on the organisms that lived on land before plants came on the scene. Charles Wellman explains.

A dramatic diversification of multicellular marine life began around 540 million years ago. The so-called "Cambrian explosion" witnessed the appearance and diversification in the oceans of abundant, diverse and disparate multicellular animals, fungi and algae. Surprisingly, though, while the oceans teemed with complex life, the land surface is thought to have been essentially barren.

It wasn't until the Mid Ordovician, some 75 million years later, that the first evidence for land plants appears. Why was there such a lag before the invasion of the land by complex multicellular organisms (plants) - and what, if anything, lived on land before the Mid Ordovician? Newly-discovered fossils from ancient continental deposits from southern Arabia are shedding light on this intriguing question.

Why was there such a lag before the invasion of the land by plants?

Scientists have long suspected that before the origin of land plants the continents may have harboured a simple biota often referred to, rather disparagingly, as an "algal scum".

It has been postulated that this biota consisted of photosynthesizing organisms such as cyanobacteria and green algae (unicellular and possibly also multicellular) that were perhaps accompanied by parasites, grazers and decomposers - viruses, bacteria, archaea and maybe also fungi and lichens.


Cambrian-Early Ordovician microfossils from southern Arabia. Note the small dark patch located in the centre of many of these. Scale bar = 40 microns.

These organisms probably inhabited a variety of environments including bodies of freshwater, within and on the surface of rudimentary soils (soil crusts), or even within rocks (as endoliths). Sadly this is mostly speculation. There is very little firm evidence for such terrestrial life, barring extremely rare but tantalising reports of simple fossils and faint geochemical signals.

Whilst working on a NERC-funded grant searching for the earliest fossil land plants, I discovered some highly interesting and exquisitely preserved microfossils in extremely old rocks.

These were found in Cambrian to Early Ordovician continental deposits from southern Arabia. The fossils were recovered from shales that represent either river deposits or possibly ponds or small lakes. They consist of simple, spherical, sac-like structures (sphaeromorphs), tubes and filaments, and various other organic microfossils.

The land before plants

The microfossils probably represent organisms that lived underwater, but may also come from those inhabiting the surrounding land surface that were washed into the aquatic environment. They clearly indicate the presence of early terrestrial life. But the challenge is understanding what types of organism they represent.

More microfossils

Miscellaneous microfossils from the Cambrian-Early Ordovician of southern Arabia. These include various tubes and filaments and a sac-like structure with attached tubes (see arrow on lower right image).

The fossil structures are simple, so comparative anatomy is of limited help. More detailed analyses of these structures is currently underway, including geochemical research and ultrastructural analysis using an electron microscope, in the quest to unravel their affinities. We hope this will confirm that they are the remains of various algal groups and possibly also fungi.

These discoveries have prompted the search for further sites that yield fossil remains from ancient non-marine strata. They are rare, but there are some out there. For example, in the early 1960s Charles Downie from the University of Sheffield briefly described fossil remains from the terrestrial Torridonian deposits from northwest Scotland.

These reside in the collections of the University of Sheffield, and reinvestigation shows them to be very similar to the Arabian material. Amazingly, they occur in various formations throughout the Torridonian Group, varying in age up to a staggering 1,150 million years old. This suggests a very long history of terrestrial life before land plants appeared.

Analysis of fossils recovered from ancient terrestrial deposits is greatly increasing our understanding of the nature of pre-land plant terrestrial biotas. Clearly an 'algal scum' inhabited the land surface for a very long time. This has immense evolutionary and palaeoecological significance, as well as enormous implications for understanding the nature of the environment and global change prior to the origin of land plants.

Terrestrial vegetation now makes up some 90% of the planet's living biomass, and has a major impact on the environment - it affects climate, atmospheric composition and patterns of sedimentation through carbon sequestration, soil formation, weathering and so on.

Just how significant were the oft-neglected organisms that lived on land before it was invaded by plants - and how much did they influence the environment through early biogeochemical cycles?

Charles Wellman is a senior lecturer in palaeobiology at the Department of Animal & Plant Sciences of the University of Sheffield.