Strains of life on Earth may have existed about 700 million years earlier than previous thought and possibly before oxygen was present in Earth’s atmosphere, according to a new study by a Colorado State researcher and his collaborators.
The research findings, which challenge the current understanding of ancient Earth’s climate conditions, imply that important steps in the evolution of life took place before the dramatic rise in oxygen in the atmosphere and suggest that these early life forms are responsible for the change.
John Ridley, a geologist in the Department of Geosciences at Colorado State, and his team of researchers from the University of Sydney and Macquarie University in Australia and the University of Washington believe that some important early life forms on Earth, including cyanobacteria and primitive eukaryotes, existed long before the oxidation of the Earth’s atmosphere.
"We suspect that life has not only adapted to environments but also has exerted controls on Earth’s environment. But we need to establish a timeline of major steps in Earth’s history before we can do better than simply speculate on a suspicion," Ridley said.
Cyanobacteria are bacteria that produce oxygen through photosynthesis. Eukaryotes, organisms with nuclei in their cells, are the domain of life that includes all complex life forms on Earth. Prior to the team’s research, scientists used microfossil evidence to date eukaryotes to about 1.8 billion years old and the earliest cyanobacteria to 2.1 billion years old. Now new evidence suggests that both organisms existed as early as 2.45 billion years ago.
Implications of this revelation allow researchers to place the existence of eukaryotes and cyanobacteria to 150 million years before oxygen existed in the earth’s atmosphere – thus supporting suggestions that cyanobacteria were responsible for the change in oxygen levels.
Additionally, the findings suggest that eukaryotes and cyanobacteria survived through changes in the Earth’s oxygen levels which challenge the Snowball Earth theory. The well-known theory suggests that Earth was covered in a sheet of glacial ice more than 800 meters deep.
Some scientists think that melting glaciers after the Snowball Earth period created an environment that gave way to radiation of life. Ridley and his team’s finding gives strong support to recent suggestions that cyanobacteria may have instead helped bring about the glaciation by contributing oxygen to the atmosphere.
"We also consider the more extreme viewpoints held by the Snowball Earth theorists – that the Earth was completely covered by ice during glaciations that took place about 150 million years later – are challenged in view of our evidence that all major strains of life survived these glaciations," Ridley said.
Ridley’s team discovered the evidence of early life by studying droplets of oil trapped in vacuoles within quartz grains of 2.45-billion-year-old rocks at Elliot Lake in Ontario, Canada. Oil found in minerals such as quartz is rare, but it is a reliable trace of ancient life. The oil is trapped without being exposed to water or bacteria.
The oil is a result of organic matter that decayed before becoming trapped as the rocks were cemented. Biomarkers, or molecules that are produced by specific life forms, were analyzed from the oil extracted from the vacuoles. The researchers found biomarkers for both eukaryotes and cyanobacteria from the Elliot Lake rocks.
Ridley and his team continue to look for other mineral sources with uncontaminated biomarkers to continue to study and date early life forms on ancient Earth.