Paul Strother on the early evolution of life on land
Organisms with complex cells that would eventually go on to colonize land may have emerged much earlier than thought.
There’s evidence of them in the rocks around Loch Torridon in western Scotland, where scientists have found exquisitely preserved microfossils dating as far back as one billion years. Their findings were published in the April 13, 2011, issue of Nature. Previous to this, the oldest non-marine organisms preserved as microfossils were thought to be 540 million years old.
EarthSky asked the paper’s lead author, Dr. Paul Strother, how these delicate microscopic organisms, as old as one billion years, remained well-preserved as fossils.
We actually looked at two completely different kinds of preservation. Microfossils preserved in nodules composed of the mineral phosphate retain extremely fine detail – down to the level of individual cells.
Most of our finds come from shales which can preserve cell walls, but with multicellular organisms, typically preserve as degraded organic matter. So in this latter case, we see the general shapes of organisms that were trapped in rock layers, but we do not see internal cellular detail.
Strother also said the fossils were dated to about one billion years using radiometric dating of the same shales that contained the microfossils. This work had been done by specialists over the past 20 years.
According to research done in the 1970s, early vegetation on land most likely evolved from freshwater organisms, not marine organisms. The Loch Torridon fossils provide evidence to support this, and point to an earlier-than-thought milestone in the evolution of life on land, when simple bacteria (prokaryotes) evolved to become complex eukaryotic cells capable of photosynthesis and sexual reproduction.
These ancient cellularly-complex organisms may have evolved to become lichens, mosses, and liverworts that, according to the fossil record, were established on land by 500 million years ago. The first ferns appeared about 400 million years ago, followed by the first flowering plants about 120 million years ago. Meanwhile, some lifeforms from the sea evolved towards a life on land as early as 450 million years ago, drawn to land vegetation as a source of food.
EarthSky asked Strother what the world was like when those ancient organisms were alive a billion years ago.
At one billion years ago, there were probably higher levels of CO2 in the atmosphere, and [also] in the oceans which would have been more acidic than today’s oceans. Life in the oceans would have been mostly single-celled plankton (with organic cell walls) and bottom-dwelling microbial mats; but we do have some simple red algae so perhaps you would have seen some true algae growing along the seashore.
As to the terrestrial landscape, it was dominated by cyanobacteria, which form microbial mats today – quite extensive in areas like the Trucial Coast or Andros Island (Bahamas). These mats would have occupied the bottoms of streams, rivers, and lakes in addition to much of the surface landscape, depending upon the availability of water. I think you would have observed more or less a layer of dark green to brownish paint over the rock surfaces, perhaps a bit like desert varnish seen in today’s deserts.
He also told EarthSky that mud cracks and raindrop impressions on the rocks, features found only on surface sediments, were evidence that the organisms came from a freshwater aquatic environment.
Our understanding of early life on Earth comes from studying the geologic record. Before the details of these microfossils were revealed, what was the state of knowledge about life, at land and sea, a billion years ago?
The record of life is a combination of direct and indirect evidence. Direct fossil evidence of this age is somewhat rare, but the best data comes from microfossils preserved in the mineral chert. Also known as flint, chert was formed primarily in shallow marine environments, which means that most of our direct knowledge about life one billion years ago comes from marine settings, and geologists have had to guess about what kinds of organisms occupied terrestrial settings.
We have known since 1907 that there were microfossils in the Torridonian rocks. There have been reports in the scientific literature of microfossils from the early 1960s to the 1980s. However, many of the earlier workers used thin sections of rock to look for microfossils.
How, then, did he and his team extract the specimens presented in the paper?
We used a different technique, called palynology, in which we dissolve rock samples in acids and extract the remaining (acid-resistant) organic matter. This technique allowed us to concentrate the microfossils and sort through many thousands of specimens, rather than the more limited numbers viewed by prior workers.
What about the diversity of the organisms? Were some more abundant than others? Were they primarily photosynthetic? Based on their morphology, what did he and his team learn about the organisms? Strother said,
Most populations of microfossils we recovered contain at least a few cells that are over 200 microns in diameter. Some are up to 475 microns. (For reference, a typed period on a printed page is about 300 microns.) The biggest specimens are a bit less than 1 millimeter.
Diversity is only an estimate, but my conservative guess is that we have identified about 50 different species. Most of the microfossils are simple spherical cells. The more elaborate types are rare. We prepared a chart showing this information in the Suppplementary Information Table 2 of the paper that will give you a better feel for the diversity distribution throughout the assemblage.
We assume they were primarily photosynthetic, but this is an assumption based on the simple fact that ecosystems today contain more autotrophic  biomass than heterotrophic  biomass (the food pyramid thing).
Based on morphology we have concluded that these organisms were largely confined to the sediment-water interface and did not occupy diverse niches in the water column. Also, we concluded that these organisms are not related to fungi or to lichens, because they lack filamentous growth forms.
What are his future plans in this line of research? Said Strother,
This report is only one-half of the story. We have another ancient lake deposit from Michigan (USA) of similar age which contains an amazing array of microfossils. There is some overlap in species composition between Michigan and Scotland, but these newer finds will add considerably more information about biology on Earth at one billion years old. We hope to use these fossils to help date some of the fundamental branches in the eukaryotic tree of life. Right now we are looking for funding to support the continuation of this research.
Ancient microfossils, preserved for as long as a billion years in the rocks around a remote loch in western Scotland, have revealed evidence that the development of complex organisms that would eventually evolve to colonize land may have occurred much earlier than previously thought. These tiny organisms lived at the bottom of freshwater lakebeds, and are believed to be primarily photosynthetic.
Back to post 1 Autotrophs are organisms that can synthesize their own food, creating complex organic compounds from simple inorganic molecules using energy from light or chemical reactions.
Back to post 2 Heterotrophic organisms cannot make their own food, and depend on complex organic compounds for nutrition.