Did primitive earthly life begin from 3.3 billion years ago?

Mar 10, 2017

By Denis Sergent

What exactly did the researchers discover?
An international team has found evidence of microfossils in the “Nuvvuagittuq Greenstone Belt” on the coast of Hudson Bay in north-eastern Quebec in Canada. Today, it consists of an outcrop of an ancient oceanic crust that, over the course of time, was buried and then raised again by tectonic movements of the Earth’s crust until it arrived at the current surface.

Geologically, these are iron-rich sedimentary rocks that were scattered around hydrothermal vents known as “black smokers.” These rocks, which have been dated by robust geophysical methods to 4.28 billion years ago, are reputed to be the oldest known to date. They developed soon after the formation of our planet some 4.567 billion years ago.

Geochemists have found evidence within these rocks of inclusions harbouring tubes, filaments and other mineralised granules made of carbonates, haematite, magnetite and apatite. These range from 0.5 millimeters in length to 30-40 millimeters in diameter.

The granules appear identical in form to micro-organisms living in hydrothermal sources today. Yet these structures, which were analysed by spectroscopic technique, have been dated to 3.77 billion years ago. This is around 300 million years older than the Australian site of Strelley Pool, which has been dated to 3.4 billion years ago.

Is this genuinely the oldest evidence of life on Earth?
While the age of the rocks (4.28 billion years) is very probably correct, it is much more difficult to be certain about the inclusions enclosing the vestiges of life. In effect, supposing that it really was organic matter formed from ancient bacteria, these vestiges have undergone enormous changes in terms of depth (30-40 km), pressure (up to 10 times atmospheric pressure), and temperature (400-500°C) over a period of millions of years.

Why is there debate among scientists?
“There are two debates involved,” explains Sylvain Bernard, a geochemist at the Institute of Mineralogy (CNRS/ MNHN/UPMC).

“The first is to be able to certify that the vestiges of organic matter that have been found are really biological and are not simply abiotic pseudofossils of chemical origin,” Bernard says.

“The second is to be able to ensure that the traces of life are the same age as the rock that contains them and that they did not arrive later,” he adds.

“This work is very interesting but the researchers have not carried out a very advanced mineralogical study, e.g. with a synchrotron (a large cyclic particle accelerator.) This would have enabled them to confirm whether these structures are truly of biological origin,” Bernard notes.

“Life developed on our planet from the moment when Mars and the Earth had liquid on their surfaces,” observes Matthew Dodd from University College in London, the primary author of the study.

“Consequently, we can expect to find evidence of past life on Mars some four billion years ago. Unless the Earth is an exception.”