Rocks collected in South Africa's Barberton Greenstone Belt reveal evidence for microbial life around hydrothermal vents 3.42 billion years ago. The discovery has implications both for the question of how life first appeared on Earth, and the chances of finding it elsewhere in the solar system.
One of biology's great unsettled debates is whether life formed in a warm little pond, as Darwin proposed (perhaps assisted by meteorites) or on the seafloor around hydrothermal vents. Evidence exists for both, and no one discovery is likely to settle the question. However, a paper in Science Advances gives a boost to the vent side, by at least establishing the presence of life nearby a very, very long time ago.
One problem with studying the history of life around hydrothermal vents is we seldom have the records. The deep ocean crust is recycled into the Earth every few hundred million years, so while we know life is abundant there now, its history is largely hidden. This makes the discovery by Professor Barbara Cavalazzi of the University of Bologna immensely significant.
“We found exceptionally well-preserved evidence of fossilised microbes that appear to have flourished along the walls of cavities created by warm water from hydrothermal systems a few meters below the seafloor,” Cavalazzi said in a statement. “Sub-surface habitats, heated by volcanic activity, are likely to have hosted some of Earth's earliest microbial ecosystems and this is the oldest example that we have found to date."
Although the conditions around this vent, pressure aside, would have resembled those in the deep ocean, its shallowness means it was on a continental shelf, which allowed its preservation.
The microfossil filaments Cavalazzi found not only have shapes indicative of living origin but are so well preserved she can identify carbon-rich cell walls and a distinct core. The high nickel concentrations found within the fossils indicate they were Archaea prokaryotes, lifeforms that survive today in oxygen-free environments, processing methane to power their metabolisms.
The area around the vents would have provided lifeforms with energy in the form of heat, and a rich mixture of nutrients. The cooling seawater would have created a gradient along which they found their place of comfort.
"Although we know that Archaea prokaryotes can be fossilised, we have extremely limited direct examples. Our findings could extend the record of Archaea fossils for the first time into the era when life first emerged on Earth," Cavalazzi said.
Old as these fossils are, we know life on Earth is older still. Molecular clocks date it to at least 4 billion years, and the oldest fossils (albeit contested) are 3.7 billion years old. Darwin's pond could still have been right, with life then spreading to the vents.
Nevertheless, the fact that these locations were occupied so long ago does no harm to the idea of them as life's birthplace.
Meanwhile, Cavalazzi noted; "As we also find similar environments on Mars, the study also has implications for astrobiology and the chances of finding life beyond Earth." The find is perhaps even more important for the prospects of life on Europa and Enceladus, both of which have produced intriguing findings in the last week. Neither of these ever had warm still ponds, but both are suspected of having environments quite similar to hydrothermal vents.
Fossils have been found within the Barberton belt previously, but have inspired debate as to the conditions in which they lived. The rocks in which Cavalazzi and co-authors found these filaments were formed in the steep temperature and pH-gradients only found around hydrothermal vents.