In the initial two billion years of Earth’s existence, our planet was dominated by a combination of bacteria and their relatives, the Archaea. This period can be described as “slimeball Earth,” marked by a critical merger that shaped the future of life on our planet.

One of these ancient cells engulfed a bacterial cell, and remarkably, the bacterium survived. Together, they replicated, leading the engulfed bacteria to evolve into mitochondria, which serve as the energy source for these early cells.

Nick Lane from University College London discovered that mitochondria enabled these cells to express an extraordinary 200,000 times more genes, fostering growth and the emergence of varied life forms. This new combination eventually evolved into complex eukaryotic cells, resulting in nearly every organism observable without a microscope, including humans.

Coexistence is fundamental to our existence, a factor that continues to sustain us today. Over 80% of terrestrial plant species engage in symbiotic relationships with mycorrhizal fungi, which provide essential nutrients while plants supply the fungi with carbohydrates. Without this interaction, oxygen as we know it would be nonexistent. The soil itself is a product of symbiosis among fungi, bacteria, and plants—an ecological partnership that took root when life transitioned from sea to land roughly 500 million years ago.

When many think of “symbiosis,” they often envision entities coexisting peacefully, like the clownfish and anemone or the vibrant ecosystems of coral reefs. Lichens, too, symbolize the intimate connections among distinct life kingdoms. Generally, we perceive symbiosis as a benevolent arrangement characterized by mutual benefit.

However, experts suggest viewing symbiotic relationships on a spectrum, ranging from parasitism to mutualism. Katie Field from the University of Sheffield, UK, points out that reciprocity isn’t always altruistic; partners often give in hopes of future benefits.

To illustrate this spectrum, consider the diverse strategies employed by orchids. Their minuscule seeds contain very few resources and must parasitize mycorrhizal fungi to access the sugars and nutrients needed for germination. As they develop leaves, some species begin to establish a more reciprocal relationship with the fungus, shifting from parasitism to mutual benefit.

Conversely, older orchids might provide sustenance for younger ones, while certain species may remain parasitic indefinitely, never developing photosynthetic leaves. “There’s a whole cycle of different stages of symbiotic interactions,” Field remarks.

Another significant perspective on symbiosis is its potential as a key to a sustainable future. Leguminous plants such as pulses, beans, and lentils utilize symbiotic bacteria to convert atmospheric nitrogen into fertilizer. Recent studies indicate that these plants have adapted mechanisms from existing cellular structures for this purpose.

This revelation could pave the way for other crops, notably grains like wheat and corn—staples that account for half of human caloric intake—to produce their fertilizers. Giles Oldroyd from the Crop Science Center at Cambridge University is exploring this avenue, with hopes of significantly reducing the reliance on chemical fertilizers in agriculture.

Oldroyd is conducting field trials using modified crops to harness the power of symbiosis, with a clear mission to minimize the use of chemical fertilizers. “I’m committed to this goal,” he states.