Where life on land began: just off a mountain road outside Barberton
Scientists forced to change their ideas about when the earliest organisms crawled out of the ocean
Evidence from SA’s latest world heritage site has pushed back the start of terrestrial life by 500 million years.
Fossilised remains of microbial life found on the Barberton Greenstone Belt date back 3.22 billion years, forcing scientists to change their view that the earliest primitive organisms crawled out of the ocean 2.7 billion years ago.
These marine organisms also came from Barberton, but the new fossils — which yielded carbon and nitrogen isotopes that were analysed at the Institute of Earth Physics in Paris — are the first definitive evidence of ancient terrestrial life.Former Wildlife and Environment Society of South Africa CEO Tony Ferrar, who played a key role in having the Barberton Makhonjwa Mountains declared a world heritage site by Unesco this year, was also central to the latest discovery, taking lead researcher Martin Homann to a farm 4km south-east of Barberton on the road to Swaziland.
Homann, who at the time was studying for his doctorate in sedimentology, said he met Ferrar in 2012. “[He] guided us to some beautiful outcrops well hidden behind a locked gate on private property,” he said.
This was Dycedale farm on the Saddleback Pass, and it was where Homann discovered unique “fossil microbial mats”. He described them as “multi-layered organic ‘carpets’ mainly formed by bacteria and archaea”. Archaea are micro-organisms that resemble bacteria and are considered by some scientists to be the earliest forms of cellular life.While living on the early Earth, these microbial communities became interlayered and packed together with sedimentary rock made of rounded stones of different sizes that geologists call a conglomerate.Writing in the journal Ecology & Evolution about the story behind his discovery, Homann said fossil microbial mats usually contained medium- to coarse-grained tidal marine sandstones.
But on Dycedale “they were interbedded with fluvial conglomerates and gravelly sandstones, once deposited in an ancient river bed. I knew that these outcrops were clearly unique, but their significance had yet to be fully uncovered.”
The team first analysed and described the rocks’ positions in detail and compared them to other rock formations to understand how they moved, formed and were preserved. They concluded that the microbes were indigenous to the host rock and part of what was once an ancient river delta.In 2016, Homann began post-doctoral research at the European Institute for Marine Studies in France, and started to analyse organic carbon isotopes from the sandstones in the Barberton Greenstone Belt, also known as the Moodies Group.
“We even managed to acquire hard-to-obtain nitrogen isotope data from our South African samples,” he said.
The researchers analysed the isotopes and found they were distinct from marine samples, suggesting that the microbes had developed different metabolisms to live on land.Kurt Konhauser, a professor of earth and atmospheric sciences at the University of Alberta in Canada, who was not involved in the research, told The Scientist this week: “The structures and isotopic composition of the microbial mats certainly seem to suggest the presence of photosynthetic microbes already existing on land.
“This work represents the oldest and least unambiguous work that we have so far that life existed on land already 3.2 billion years ago.”
Homann published his findings on Monday in the journal Nature Geosciences, saying: “The colonisation of emergent continental landmass by microbial life was an evolutionary step of paramount importance in Earth history.“Already at 3.2 billion years ago, we see evidence of differences in mat-forming microbial communities suggesting that some were likely better adapted for life in the ocean versus on land.”A study in 2017 suggested that microbes found on a 3.5-billion-year-old volcano in the Pilbara region of Western Australia may have also have lived on land. It included bubbles trapped in a sticky microbial fluid and layers of rock formed by ancient micro-organisms. But the findings were not conclusive.Before three billion years ago, most of Earth is believed to have consisted of oceans interspersed with volcanic islands. Evidence for life on land has so far been hard to come by, partly because ancient marine rocks appear to be better preserved than terrestrial sediments.
Another issue, according to Homann, is that very old terrestrial sediments are also difficult to distinguish from marine sediments because so-called index fossils – which help to determine the environment and to date rocks – do not exist from this early period of Earth’s history.
Apart from supplying proof of the earliest life on Earth, the new study proves there was extensive exposure of continental crust on Earth’s surface 3.2 billion years ago.The local hero
Ecologist and wildlife management specialist Tony Ferrar guided scientists to an outcrop in the Saddleback Pass outside Barberton which was critical to their findings.
Ferrar, 76, who lives in Barberton, was centrally involved in efforts to have the natural features of the Makhonjwa Mountains (also known as the Barberton Greenstone Belt or Barberton Mountain Land) documented for Unesco approval as a geological world heritage site.The Makhonjwa is a range of small mountains and hills that covers an area of 120km by 60km, about 80% in Mpumalanga and the remainder in Swaziland. Tentative world heritage listing was achieved in 2008 and final approval was granted this year.
Ferrar has been involved in several tourism development projects in the region, including the Barberton Makhonjwa Geotrail, completed in 2013. Having co-authored the Geotrail’s guidebook, Ferrar conducts groups up the 40km roadside Geotrail through the mountains to Swaziland.