Putting together the history of life on Earth has a major stumbling block: Prior to about 540 million years ago, most life was squishy and microbial, which meant it rarely fossilized. This major blind spot makes it difficult for researchers to study ancient life at a key point in Earth’s evolutionary history, and even more difficult to potentially find evidence of ancient microbial life on Mars. But a recent study of kaolinite, an aluminum-rich clay, gives researchers some major hints about where to start looking.
The smallest needle in the biggest haystack
In 1859, Charles Darwin was at a loss. Plenty of fossils of multi-celled and large animals had been found, but there seemed to be no fossils of those animals’ more basic evolutionary ancestors. His theory of evolution, however, contended that life must have existed for eons before then. “To the question why we do not find records of these vast primordial periods, I can give no satisfactory answer,” he wrote in “On the Origin of Species.”
We now know the answer to his question: Fossilization of an organism with no shell or skeleton is incredibly rare. Before about 540 million years ago, when the age known as the Cambrian began and multicellular life rapidly diversified, the only living things on Earth were small -- microbes, bacteria and algae. They lacked the hard components that would have allowed their bodies to be more easily fossilized.
“When we try to reconstruct the study of life billions of years ago, we find only pieces of the puzzle,” said Emmanuelle Javaux, a geobiologist at the University of Liege in Belgium. “It’s important to understand what they really represent.”
A new study by researchers at Yale University in New Haven, Connecticut and the University of Oxford in the U.K. analyzed pre-Cambrian fossils to understand how they were created. The analysis showed “halos” of a specific type of clay, kaolinite, around microbial fossils, suggesting that kaolinite is particularly good at preserving these traces of life.
Researchers had long suspected clay could be an important material for preserving soft-bodied organisms. Ancient medical texts up to 5,000 years old mention that clays tend to aid in healing wounds, and scientists have found that clays with metallic ions -- like kaolinite with aluminum -- can work as antibacterial agents. This antibacterial property of kaolinite is likely what kept the ancient microbial life from being consumed by bacteria, allowing it to be fossilized instead of decaying.
Information about how soft-bodied organisms are fossilized can show how the early fossil record might be biased toward certain time periods or types of environments. “The point of this study was to understand some of the environmental conditions which are conducive to fossilization,” said Ross Anderson, a paleobiologist at Oxford and lead author of the study. “Because if we know that, we've got a much better idea of how to go find them.”
Finding microscopic, billion-year-old traces of life is, as one might expect, akin to “looking for a very tiny needle in a very, very big haystack," said Anderson. Any hints about likely places to find these fossils shrinks the haystack significantly.
A well-known blind spot of paleontology is that it can only know about life that was preserved. If the only fossils from a certain era were found in tropical climates, that doesn’t necessarily mean all life on Earth lived in the tropics at that time; it could mean that the tropics had the best conditions for fossilization. Anderson’s study suggests pre-Cambrian fossils may be biased toward environments that were rich in kaolinite, a discovery that helps put these fossils into context and potentially make them easier for paleontologists to find.
“We paleontologists like recognizable patterns,” said Elena Naimark, a paleontologist at the Russian Academy of Sciences in Moscow, Russia who was not involved in the study. “Patterns mean that we understand things correctly, they mean that we know where we should search for our precious fossils and how and why they are there. This is how we decipher the past.”
An explosive mystery of life
A key mystery for paleontologists to decipher is why the transition between simple microscopic life and complex macroscopic life, the lack of fossil evidence for which flummoxed Darwin, occurred. This event is called the Cambrian explosion: the crucial turning point in Earth’s evolutionary history. And because evidence of life before that transition is so difficult to find, researchers don’t know why it happened.
A popular idea is that pre-Cambrian oxygen levels were low to nonexistent, and a rise in the amount of oxygen in the atmosphere allowed animals to grow bigger and more diverse. There are theories based in developmental genetics as well, suggesting that minor genetic modifications to an animal’s development may have been able to cause very large changes when the animal grew to adulthood. Still more theories look at ecological explanations, such as changes to the food chain or “arms races” between predators and prey.
“If we want an answer to this question, we need an accurate timescale,” Anderson said. “We don’t have a very good one at the moment, because it’s such a spotty fossil record. It’s difficult to know when exactly things happened and in what sequence of events, or if our fossils are biased in some way.”
Finding billion-year-old microbes is particularly relevant to an upcoming Mars rover mission, Perseverance, which will be looking for evidence of ancient life on Mars. The rover can only take so many samples, so having some guidance about where to look raises the chances of a fruitful discovery.
“I think this is our best chance to find some trace of life on Mars,” said Javeaux, who is involved with a similar Mars rover project at the European Space Agency. That rover is set to launch in 2022. “This study will help us to target the best potential locations to find life.”