Looking for fossils in old rocks is a tough job. Body parts degrade over the years, and the older the rock, the less likely it will be that you will find any evidence that life was once there. One question facing scientists is: Just how far back in time can we go before the traces of life are completely lost?
A new study provides one answer to that question, and in doing so suggests the limits to looking for ancient life not only on Earth, but also on other rocky worlds like Mars.
The study, conducted by Daniel Brigel of the University of Bremen in Germany and colleagues and published in the July issue of the journal Geology, determined that 300 million-year-old limestone deposits in Namibia were formed by a community of ancient microbes. Methane-eating microbes known as methanotrophic archaea caused the formation of minerals that led to the limestone, and alongside the archaea were sulfate-reducing bacteria that aided in processing methane.
The researchers found evidence that the methanotrophic and sulfate-reducing organisms were present by searching for unique biomarkers in the limestone. These biomarkers are lipid compounds with names like archaeol, crocetane and pentamethylicosane (PMI). These lipids are made by living cells to form important structures like cell membranes.
Usually, after a cell dies, the lipids are degraded and used by other organisms. However, the organisms that produced the lipids found in Namibia also produced the minerals that make up the limestone. As the microbes produced minerals called carbonate, the limestone rock formed and engulfed the cells. When the organisms died, their cells' lipids were protected inside the limestone.
Lipid compounds like those found in the Namibian rocks can indicate that microbes use methane but not oxygen. The problem with the compound archaeol, however, is that it is easily broken down and often disappears over long periods of time. Crocetane and PMI are more durable, and are more likely to survive over geological time scales. Because of this, crocetane and PMI in particular are the best indicators in ancient rocks of methane being processed in the absence of oxygen.
Biomarkers from microorganisms have been identified before in limestones from the Cenozoic period (65 million years ago to the present day). PMI and crocetane also have been found in Mesozoic limestones (248 to 65 million years ago) in places like California. The limestone from Namibia dates back to the Paleozoic era, which lasted from 542 to 251 million years ago. The team found that if the lipids were any more degraded, they wouldn't be identifiable. This means that lipid biomarkers probably wouldn't be found in rocks older than the Paleozoic era.
Processes past and present
The authors of this study say that the processing of methane without oxygen is "the key metabolism at modern marine methane seeps" on Earth today. This important metabolic process produces carbonates that form structures around methane seeps in the ocean.
The microbes involved in modern methane processing in these environments are the same types that were present in the ancient Namibian rocks.
According to the research team, "In this study we provide robust biomarker and isotope evidence that methane was oxidized in the same manner in the Paleozoic as it is at modern marine seeps today." Additionally, the methods they used to study the rocks highlight "the potential of lipid biomarkers to unravel past microbial activity and biogeochemical cycles." This can help us understand how ancient microbes on Earth affected the planet's environment.
Using biomarkers to uncover information about past life on our planet is important for determining how the biosphere of Earth has evolved alongside our ever-changing planet. The rocks that remain from ancient times contain numerous clues about our planet's past climate and life.
Developing techniques to search for biomarkers from ancient organisms on Earth can also help us determine ways to search for signs of past life on other planets, such as Mars. Future missions to Mars will examine rocks, looking for the molecular remnants of ancient living organisms.
It is thought that such organisms may have lived on Mars 3.5 billion years ago, when the planet was warmer than it is today, and there were lakes and perhaps even oceans on the surface. It will be difficult to find fossils after all those eons, so biomarker studies may be the best way for explorers to determine whether or not Mars ever had life of its own.