Experiments led by Nicolas Dauphas of the University of Chicago and Chicago’s
Field Museum have validated some controversial rocks from Greenland as the
potential site for the earliest evidence of life on Earth.
“The samples that I have studied are extremely controversial,” said Dauphas, an
Assistant Professor in Geophysical Sciences at the University of Chicago and a
Field Museum Associate. Some scientists have claimed that these rocks from
Greenland’s banded iron formations contain traces of life that push back the
biological record of life on earth to 3.85 billion years ago. Others, however,
dismiss the claim. They argue that the rocks originally existed in a molten
state, a condition unsuitable for the preservation of evidence for life.
“My results show unambiguously that the rocks are sediments deposited at the
bottom of an ocean,” Dauphas said. “This is an important result. It puts the
search for life on the early Earth on firm foundations.”
Dauphas will announce his findings in the Dec. 17 issue of the journal Science.
His co-authors are Meenakshi Wadhwa and Philip Janney of Chicago’s Field Museum,
Andrew Davis of the University of Chicago, and Mark van Zuilen and Bernard Marty
of France’s Centre de Recherches Petrographiques et Geochimiques.
The oldest-known microfossils, which come from Australia and are themselves
disputed, are more than 3.4 billion years old. Scientists have now turned their
attention to Greenland for evidence of even older biological activity.
The controversy over the Greenland rocks stemmed from changes they underwent
over the long history of the Earth. “During burial they were cooked under high
pressure and temperature, which completely modified the chemistry and mineralogy
of the rocks,” Dauphas said. Consequently, scientists found it difficult to
determine whether the rocks were igneous (those that had cooled from a
once-molten state) or sedimentary (eroded and deposited by wind or water). Only
sedimentary rocks would be able to preserve evidence of life.
That question was finally answered by a state-of-the-art mass spectrometer in
Wadhwa’s laboratory at the Field Museum. The spectrometer was among the
resources that led Science co-authors Davis, Dauphas, Wadhwa and others earlier
this year to form the Chicago Center for Cosmochemistry.
The center is a collaboration between the University of Chicago, the Field
Museum and Argonne National Laboratory to study the elements and their many
atomic variations in meteorites and other materials from Earth and space.
Dauphas used the spectrometer to measure with high precision the subtle atomic
variations in the composition of iron, called isotopes, preserved in rocks on
the southwest coast of Greenland and Akilia Island. The variations in these
isotopes told them what type of process formed the rock, Wadhwa said.
“From the standpoint of these isotopes, there’s very convincing evidence that
these rocks cannot be of igneous origin,” she said.
Unlike igneous rocks, the Greenland samples contained a considerable range of
isotopic variation in iron isotopics, said Davis, Director of the Chicago Center
for Cosmochemistry and Senior Scientist at the University of Chicago’s Enrico
Fermi Institute. “All igneous rocks on the Earth have pretty much the same iron
isotopic composition, so it was really a pretty simple test.”
The question that remains is whether the Greenland rocks actually contain
evidence for early life. Circumstantial evidence suggests that they do. These
ancient rocks have been oxidized, meaning that they have chemically reacted with
oxygen. But the atmosphere of the early Earth contained much less oxygen than it
does today. Where did the oxygen come from?
Photosynthesis, a chemical process signaling the presence of bacteria, might be
the answer. It’s a question that Dauphas intends to pursue in his new Origins
Lab at the University of Chicago.
“We can’t claim at this stage that there is unequivocal evidence for biological
activity four billion years ago,” Davis said. “There are more experiments that
need to be done.”
