FRANKFURT. Bacteria are living organisms consisting of just a single cell, and at first, they only regulate themselves. Nevertheless, many types of bacteria can form colonies that behave like a complex organism. Within these, individual microbes suddenly take on different roles: some produce a slime that holds the colony together; others supply their “siblings” with nutrients and energy; still others are especially mobile and help the colony spread. Together, they achieve something that no single one could accomplish alone.

The sudden appearance of a new, unpredictable property in a system is a phenomenon researchers call emergence. “Emergence also exists in the world of molecules,” says Professor Harald Schwalbe from the Institute of Organic Chemistry and Chemical Biology at Goethe University Frankfurt. “Take water, for example: it consists of two hydrogen atoms and one oxygen atom. When these combine to form water, a molecule with entirely new properties emerges—properties that cannot be derived from those of the individual atoms.”

Water Shaped the Emergence of Life

Water is polar: the oxygen atom carries a slight negative charge, while the hydrogen atoms are slightly positive. Without the combination of these properties in water, life would not exist – at least not in its current form. The polarity causes water molecules to attract each other, like weak magnets. This cohesion is why water is liquid between 0 and 100 degrees Celsius rather than gaseous. This is the temperature range on Earth – determined by its distance from the Sun. Under the physical conditions on Earth, water is the liquid environment in which the molecules of life can form and in which chemical reactions in organisms are accelerated.

“This, in turn, is a prerequisite for DNA to store information and for proteins to adopt a specific structure,” explains the chemist. DNA consists of different molecular building blocks that formed even before life emerged; some are polar, while others are nonpolar. The polar components interact well with water and therefore orient outward in an aqueous environment, while the nonpolar ones are positioned inward. This is one reason why DNA adopts a double helix structure under natural conditions – similar to a spiral staircase, where the polar railings are on the outside and the nonpolar steps are stacked and twisted inside.

“The emergent properties of water thus impose a certain order on more complex molecules,” explains Schwalbe. “It’s like a conductor ensuring that musicians don’t just play randomly.” This order, in turn, forms the basis for these complex molecules to develop specific, unpredictable properties of their own. It is partly responsible, for example, for DNA consisting of two intertwined strands. These usually behave complementarily – like matching puzzle pieces. As a result, DNA has the ability to replicate, meaning it can create copies of itself. During this process, the DNA strands separate, and matching “puzzle pieces” attach themselves anew to each strand. The ability to replicate is central to the emergence of life.

The Evolution of Complex Systems Will Not Repeat Exactly

The publication identifies a total of 13 characteristics of complex systems. One of them is the phenomenon that such systems can reach critical states in which their properties fundamentally change through emergence. This suddenly enables new functions. Exactly when this happens cannot be predicted. These leaps were often key steps on the path to the emergence of life. For them to occur, systems require a constant input of energy – on Earth, this comes from the sun.

A driving force behind this development is evolutionary mechanisms, which began shaping molecules even before life emerged. Together with emergence, they have led to the diverse forms of life we observe on Earth today. Despite these forces, however, the exact course of this development was not predetermined, Schwalbe emphasizes: if we could turn back the clock four billion years, entirely different life forms would arise than those we know today.