Darwin was a brilliant observer and described everything he could perceive with the naked eye. However, the micro-organisms from the beginning of evolution remained hidden from him. He came unsuspectingly close to them in his essay on reefs.
A modern laboratory with unusual aquariums: they are open at the top and have a larger area than conventional ones although they are slightly less deep. There are no fish swimming in them; instead, rocks known as stromatolites lie in crystal-clear water. The rocks are covered with a dark green gelatinous mass about a centimeter thick. The aquariums are located in the Geomicrobiology Laboratory of the Department of Earth Sciences (D-ERDW) of ETH Zurich. Until now, it is one of the few laboratories to have successfully “cultivated” living stromatolites.
Crisogono Vasconcelos, director of the laboratory, and his colleague Rolf Warthmann, spared no trouble in bringing the sensational “living rocks” from Brazil to Zurich for ETH Zurich’s 150th anniversary celebrations. What began as a small adventure has now become a long-term research project.
Stromatolites are inhabited by organisms that were probably the first to colonize the planet. The jelly-like mass consists of micro-organisms. Cyanobacteria (formerly called blue-green algae) live on the very top. The oxygen-free lower regions are populated by other micro-organisms such as those that consume methane and sulfate ions, living together in symbiosis. They metabolize either by photosynthesis or by reducing sulfate ions, and excrete carbonate as a by-product, so to speak, of their metabolism. The lower layers of the microbial mat gradually die off, while new organic material forms continuously at the surface. Thin layers of carbonate are left behind to form the laminated rocks or stromatolites, which grow very slowly: by only about a few centimeter in thirty years.
Ecological niches as protection from predators
Stromatolites are virtually living fossils and are found nowadays only at specific locations, such as the Hamelin Pool of Shark Bay in Western Australia. The “living rocks” in the ETH Geomicrobiology Laboratory originate from the Lagoa Vermehla in Brazil. The salinity of the lagoon water there is two to three times higher than normal seawater. Today, stromatolites have retreated into ecological niches that are apparently hostile to life and in which they need fear no natural enemies. In the ocean, they would be grazed upon by fish, crabs or snails.
Predators such as the trilobites, which were among the first arthropods in the Paleozoic era, are probably the reason why stromatolites were driven away into ecological niches of this kind. Stromatolites formed entire reefs and were the first ecosystems in the Precambrian era more than 540 million years ago. They record the early stages of evolution beginning at 3.45 billion years ago. In Darwin’s publication “On The Structure and Distribution of Coral Reefs” of 1842, in which he describes the different types of coral reefs and their distribution and sketches theories about coral reef development, he came – indirectly – very close to the origin of life.
This is because, billions of years ago, the reef-forming stromatolites were inhabited exclusively by micro-organisms that are regarded as our planet’s first forms of life. The microbial fossils, invisible to the naked eye, and the so-called Ediacaran fauna, which were still unknown at that time, led to “Darwin's Dilemma”, namely the question of why fossils suddenly appeared on the Earth in rocks less than 540 million years old.
The cradle of evolution
In Western Australia, there are large fossilized stromatolite reefs in rocks about 3.5 billion years old, known as the Apex Chert. Judith McKenzie, emeritus Professor of Sedimentology at ETH Zurich, who together with Crisogono Vasconcelos has built up the stromatolite laboratory, enthuses that “One can walk across them for several dozen meters.” For a long time the Apex Chert was regarded as the cradle of evolution because, in 1987, the paleontologist William Schopf from the University of California, Los Angeles, described fossil micro-organisms in the Apex Chert.
He even suspected that they could involve cyanobacteria, which, at 3.465 billion years old, would have been the oldest traces of life. In the spring of 2002, the Oxford University paleontologist Martin Brasier refuted Schopf’s study and described the supposed cyanobacteria as artifacts. Similar studies on other findings followed, opening up a new discussion about the beginning of evolution. More stringent criteria were applied from then on in order to identify fossilized traces of life with certainty – above all the detection of biomarkers.
Since then, experts have become cautions and skeptical about new discoveries. This became clear once again at a specialist conference in September 2005, when leading authorities met to resolve the question as to when and how microbes began to change the world. Even during this intensive exchange of information, the experts were unable to agree on which fossil findings were indisputable and which were the oldest traces of life. Or, as to when the first oxygen-producing organisms appeared on the evolutionary stage and whether there was any life at all on our planet before then.
As a result, the studies relating to stromatolites, which are considered to have first appeared about 3.5 billion years ago, came under attack. There was discussion as to whether their mushroom-like conical shapes could have formed on the young Earth either as a result of other types of organisms unknown today or in purely chemical ways. Scientists counter this by saying that, up to now, it has been impossible to observe in nature any structures of this kind that could have formed by chemical means.
Learning to understand through analogies
To interpret particular structures as traces of early life, Judith McKenzie and Crisogono Vasconcelos compare ancient stromatolites with recent analogous systems, for example by exposing the living stromatolites to different conditions: they vary the salinity, temperature, light input or oxygen content. In a glass box in the laboratory, the scientists can also simulate conditions like those on the early Earth. Vasconcelos says, “This may make it possible to find new stages in evolution.”
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