The structures, built by microbes, were found on a newly exposed outcrop of rock that had been buried under snow for millions of years.
Allen Nutman (left) and Vickie Bennet (right) with a specimen of 3,700 Myr-old stromatolites from Isua, Greenland. Credit: Yuri Amelin/Nature
Scientists have discovered the oldest fossils known to humankind, and they predate the existing record by 220 million years. The newly discovered fossils are stromatolites found in the rock formations of Isua, a belt of ancient rocks in southwest Greenland. They are 3.7 billion years old, displaying evidence of microbial mat formations.
The origin of life has always been shrouded with mystery. There are several unanswered questions when we talk about the earliest lifeforms. Where did life first evolve? What was the environment where it evolved? Why did life evolve? One of the questions the new study published in the journal Nature this week helps us understand better is when life evolved.
The tedious journey to finding an answer has thrown out several bits of valuable knowledge, such as what defines a fossil. When we think of fossils, we typically picture a rock with the imprint of a large animal embedded in it. This is the most obvious kind of fossil. There are also trace fossils – evidence of an animal’s footprints or faeces. Animal and plant secretion, such as amber, is also evidence of life and therefore a fossil.
In the same vein, life also secretes chemicals whose signatures are preserved in rocks. These are physical manifestations of animal remains, like petroleum and natural gas, as well as things like carbon isotope ratios in rocks that could have been modified only by biological processes. The further back in time we go, the harder it is for us to pinpoint the exact reason for changes in chemical composition. The chemical fossil evidence for the oldest life on earth currently points to 4.1 billion years ago, according to a study published last year. However, the paper is contested as there could be alternate explanations for these chemical anomalies.
What cannot be rebutted, however, is direct evidence of life in the form of structures that can only be biological in origin. An example of such formation is a coral. The newly discovered fossil, called a stromatolite, is also such a bio-chemical structure built by microbes, most notably cyanobacteria, in shallow water. The bacteria stick together side by side and form mats. These mats then expand to form layers tied closely with sand grains in the water. The bacteria secrete chemicals that trap and bind together all the grains to form a complex, rock-like structure. After the bacteria die, the rock is essentially a shell of a huge thriving colony of microscopic living creatures.
Stromatolites in the Soeginina Beds (Paadla Formation, Ludlow, Silurian) near Kübassaare, Saaremaa, Estonia. Credit: Wilson44691/Wikimedia Commons
Until last week, the oldest known fossils were also stromatolites, from 3.5 billion years ago, but located in Shark Bay, Australia. They were discovered in the 1950s and dated by the early 1990s. Indeed, Australia, Greenland and Canada tend to have the some of the oldest surviving fossils in the world because they’re made up of some of the longest surviving pieces of land. The latest discovery incidentally was in the oldest ever rock formation on Earth, called the Isua greenstone belt, believed to be between 3.7 and 3.8 billion years old.
The newly discovered stromatolites were found to be one to four centimetres thick, considered high. The team behind the discovery was led by Allen Nutman, a geologist at the University of Wollongong.
The findings are exciting because, for the longest time, we believed that life could not have formed in the volcanic hell that Earth was soon after it formed some 4.5 billion years ago. In fact, we’ve gone from thinking today that the planet was too hot to hold water to thinking that between 4.4 and 4.0 Ga, vast oceans could have existed for long periods, allowing simple organic compounds to come together. Perhaps it was during this warm, tranquil period that key steps in the origin of life occurred.
The fossil discovery provides proof for a proposal first advanced way back in 1996, about geochemical signatures on these rocks as having been that of bacteria’s. No evidence had been uncovered for this theory till today, as there is always a tiny possibility that even very strong bio-geochemistry could continue to be explained by natural, non-biological processes. The Isual fossil opens up the prospect of confirming at some time in the future that the chemofossils dating to 4.1 billion years might indeed be bacteria as well. This could very well be true as the stromatolite-forming bacteria were already sophisticated and diverse, which would mean simple life originated much earlier.
The new paper also seemingly lends credibility to the theory of Genetic Molecular Clocks, according to Nutman and his colleagues. The molecular clock is a dating technique that uses DNA sequences to determine the point in time when one lifeform diverged from another. The current time for the split of prokaryotes (cells without a nucleus, including bacteria) from eukaryotes (cells with nucleus and organelles) by molecular clock is estimated to be at 3.97 billion years, which is quite close to the 3.7 billion year number on our hands.
There is an even exciting prospect. These findings take us one inch closer to legitimately using “aliens” in a headline one day. If life could form under unimaginably violent conditions on Earth, it is not picky. Life seems to have been hardy, fighting for opportunities to flourish and thrive under the harshest of conditions. We know that 3.8 billion years ago, Mars was very like Earth, teeming with oceans and an atmosphere. Now, there has been a marked improvement in the odds that any of our current or future missions could discover microscopic evidence of life on the now-barren red planet.
Pushing the record back for the oldest fossils on earth is next to impossible now. There is no rock on the face of Earth today that is older than Isua’s. And any rock older than those at Isua would have undergone such extensive metamorphosis due to plate tectonics that it would not have retained any evidence of life or anything else at all. The discovery of these fossils is perhaps the only positive thing to have come out of human-made global warming, as the stromatolites were found on a newly exposed outcrop of rock that had been buried under snow for millions of years.
Sandhya Ramesh is a science writer focusing on astronomy and earth science.
