An Old Hypothesis on the Origin of Life Gains New Momentum With the Solution to the ‘Chicken-and-Egg’ Problem

Over the past two billion years, life on Earth has evolved from simple single-celled organisms to complex life forms in which billions of cells coordinate their activities. This evolution was driven by a critical change within those simple cells: the appearance of organelles.

Organelles are a series of structures within cells. The best-known organelles are mitochondria, which supply energy to the cell; chloroplasts, which perform photosynthesis in plant cells; and nuclei, where most of the cell’s genetic information is stored.

Organelles in a cell can be compared to the organs of an animal. Scientists once thought that organelles had clearly defined structures and functions that enabled the cell to function. However, they’ve gradually realized that organelles may be more diffuse than originally thought.

One factor challenging the conventional view of organelles is the discovery of unusual types known as biomolecular condensates.

The key lies in the membrane. Until a few years ago, experts believed organelles had a specific structure bounded by a membrane, which separates the inside of the organelle from the rest of the cell.

As Allan Albig, anassociate professor of biological sciences at Boise State University, explains in an article for The Conversation, this idea has faded over the past few decades. This shift is due to the discovery of biomolecular condensates—groups of molecules, such as proteins and RNA chains, that cluster into small bubbles or droplets to perform their functions.

If they don’t have a membrane, how are they organized? Like conventional organelles, biomolecular condensates condense their constituent molecules without needing a membrane to confine them. Similar to oil in water, these compounds naturally condense and remain distinct from the liquid in which they float.

The bubbles can move, split, or merge within the cell, but the condensate remains compact as small droplets—a liquid within a liquid. This structure has advantages, such as facilitating interactions between molecules both inside and outside the organelle.

Since the discovery of these organelles, the scientific community has identified about 30. This is especially remarkable considering that only about a dozen membrane-bound organelles are known. “Although easy to identify once you know what to look for, it’s difficult to figure out exactly what biomolecular condensates do,” Albig said.

Revealing Secrets

Albig says that discovering these organelles may help answer some questions about the inner workings of cells. However, it has also introduced new unknowns and reopened a debate about how scientists classify cells.

Organelles were crucial to the development of complex cells, or eukaryotic cells, which are distinguished from simpler prokaryotic cells by the presence of organelles.

However, some bacteria contain “unstructured” proteins, also called “intrinsically disordered proteins,” with segments that vary in structure, unlike conventional proteins, whose structure is critical to their function. These proteins are also found in biomolecular condensates.

This discovery has led experts to believe that prokaryotic organisms such as bacteria might have organelles, even without a membrane. This challenges the idea that the interior of these cells is merely a shapeless mass of proteins and genetic material. The distinction between eukaryotes and prokaryotes may lie more in the presence of a membrane than in the presence of organelles.

Albig concludes that this could affect how the scientific community views the evolution of cells and complex life on Earth. Cell membranes are so fundamental to life that scientists assumed they must have been present when nucleotides—the “building blocks” of DNA and RNA—began to assemble.

The problem is that the lipids that form these membranes hadn’t yet emerged, so how could nucleotides have come together and intertwined to form RNA strands? Without an egg, there could hardly be a chicken. For Albig, the answer may lie in biomolecular clusters.

He points to a recent study in Molecular Cell showing how RNA molecules could create conditions for forming a biomolecular condensate. This discovery supports the RNA world hypothesis, which suggests that life originated from the emergence of RNA chains.

Image | Jennifer Lippincott-Schwartz (NIH)

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