An Ancient Partnership: Co-Evolution of Earth Environments and Microbial Life
An artist interpretation of the hazy atmosphere of Archean Earth - a pale orange dot.
NASA’s Goddard Space Flight Center/Francis Reddy
NASA-supported scientists have examined the long and intricately linked history of microbial life and the Earth's environment. By reviewing the current state of knowledge across fields like microbiology, molecular biology, and geology, the study looks at how microorganisms have both shaped and been shaped by chemical properties of our planet's oceans, land, and atmosphere. The study combines data across multiple fields of study and discusses how information on the complicated history of life on our planet from a single field cannot be viewed in isolation.
The first life on Earth was microbial. Today the vast majority of our planet's biomass is still made up of tiny, single-celled microorganisms. Although they're abundant, the history of microbes can be a challenge for astrobiologists to study. Microbes don't leave bones, shells or other large fossils behind like dinosaurs, fish or other large organisms. Because of this, scientists must look at different evidence to understand the evolution of microbial life through time.
In order to study ancient microbes on Earth, astrobiologists look for isotopic fingerprints in rocks that can be used to identify the metabolisms of ancient communities. Metabolism refers to the conversion of food into energy, and happens in all living things. Many elements (think carbon (C), nitrogen (N), Sulfur (S), iron (Fe)) are involved in microbial metabolism. As microbes process these elements, they cause isotopic changes that scientists can spot in the rock record. Microbes also help to control how these elements are deposited and cycled in the environment, affecting geology and chemistry at both local and global scales (consider the role of microbes in the carbon cycle on Earth today).
Another way to study ancient microbial life is to look back along the evolutionary information contained in the genetics of life today. Combining this genetic information from molecular biology with geobiological information from the rock record can help astrobiologists understand the connections between the shared evolution of the early Earth and early life.
For an example of geological evidence of microbial metabolism, we can consider the formation of banded iron formations (BIFs) on the ancient seafloor. These colorful layers of alternating iron- and silicon-rich sediment were formed from 3.8 billion to 1.8 billion years ago and are associated with some of the oldest rock formations on Earth. The red colors they exhibit are from their high iron content, showing us that the ocean of Earth was rich in iron during the 2 billion years in which these rocks were forming.
The study also has wider implications in the search for life beyond Earth. Understanding the co-evolution of life and the environment can help scientists better understand the conditions necessary for a planet to be habitable. The interconnections between life and the environment also provide important clues in the search for biosignature gases in the atmospheres of planets that orbit distant stars.
The study, “Co‐evolution of early Earth environments and microbial life,” was published in the journal Nature Reviews.