Earth looked quite different billions of years ago. There were no forests, animals, or even oxygen in the air. In fact, oxygen, which we now depend on for survival, was once a harmful gas for ancient living organisms. So, how did these early life forms survive, and how did they evolve into the complex life we see today?

A new study led by Fatima Li-Hau, a former graduate student at the Earth-Life Science Institute (ELSI) in Tokyo, Japan, along with Associate Professor Shawn McGlynn, provides some insights. The researchers examined Japan's natural hot springs, and discovered clues about what life might have been like before Earth's atmosphere became rich in oxygen. The study was published in Microbes and Environments. 

A Glimpse into Earth's Ancient Past

Earth underwent a major transformation known as the Great Oxygenation Event (GOE) around 2.3 billion years ago. Before this event, the air contained almost no oxygen. Then, microscopic organisms called cyanobacteria began producing oxygen through photosynthesis. Over time, this oxygen accumulated in the atmosphere, forming the air we now breathe, about 21% oxygen and 78% nitrogen.

While this was beneficial for the development of plants and animals, it posed a problem for the early life forms that had never been exposed to oxygen. To explore how these early microbes adapted, Li-Hau and her team turned to five iron-rich hot springs in Japan. These springs are unique because they have low oxygen levels, high amounts of ferrous iron (Fe2+), and nearly neutral pH – conditions similar to the early Earth's oceans.

Nature's Time Capsules

The research team studied hot springs in Tokyo, Akita, and Aomori prefectures in Japan. In four of the five springs, they found that the dominant microbes were microaerophilic iron-oxidizing bacteria. These are tiny organisms that survive in low-oxygen environments and get energy by converting ferrous iron into ferric iron. Interestingly, one spring in Akita was different, as it contained microbes that did not rely on iron at all.

Using advanced genetic techniques (metagenomics), the team analyzed the DNA of these microbes and constructed more than 200 high-quality microbial genomes. This helped them understand the functions of each microbe and how they interacted with one another. They found that these ancient-like microbial communities had a way of recycling waste into useful energy.

For example, some microbes used small amounts of oxygen released by cyanobacteria, along with iron, to generate energy. Others continued to live in oxygen-free environments. Together, they supported a complex ecosystem.

Surprisingly, the researchers also found signs of a sulphur cycle, even though there was not much sulphur in the water. This suggests that microbes might be using sulphur in ways we don't yet fully understand.

Why Is It Important

Li-Hau explains that despite differences in water chemistry and microbial makeup across the five springs, a similar pattern emerged: wherever there were ferrous iron and a little oxygen, iron-oxidizing bacteria, oxygen-producing cyanobacteria, and anaerobic microbes (those that avoid oxygen) could coexist in balance. This shows that before oxygen dominated the planet, life was already beginning to adapt to it.

Understanding how early life adapted to changing Earth conditions not only helps us learn about our planet's past but may also help in the search for life on other planets. If we find worlds with similar geochemical conditions, low oxygen, iron-rich waters, they might also be capable of supporting simple life forms.

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