Phospholipids: Tiny bubbles that may have been Earth's first cells

Imagine an early Earth billions of years ago, a young world where life as we know it was just beginning to take shape. Scientists have long pondered how the complex cells that make up plants, animals, and humans first emerged.

A study from the Scripps Research Institute sheds light on this mystery, focusing on tiny bubbles that might have been Earth’s first cells: phospholipid vesicles.

“At some point, we all wonder where we came from. We’ve now discovered a plausible way that phosphates could have been incorporated into cell-like structures earlier than previously thought, which lays the building blocks for life,” said study co-senior author Dr. Ramanarayanan Krishnamurthy. 

What are phospholipid vesicles?

Phospholipid vesicles are microscopic spheres formed by special molecules called phospholipids. These molecules are unique because one part of them is attracted to water (hydrophilic), while the other part is repelled by water (hydrophobic). 

When placed in water, phospholipids clump together in a specific way. The water-repellent parts huddle together in the center, avoiding the water, while the water-attracting parts form a double layer facing the water on either side. 

The spheres can capture other materials inside them. This characteristic makes them valuable for two purposes: mimicking the structure of cell membranes and delivering drugs within the body.

Dr. Krishnamurthy and his team investigate the chemical processes and formations that were present before life emerged on Earth. For the current study, the team set out to examine if phospholipids may have been involved during the formation of protocells. 

Steps to prepare phospholipid vesicles

The researchers began by utilizing prebiotically plausible materials, meaning substances likely present on early Earth. They followed the following procedure:

Formation of Cyclic-Phospholipids

The study began with short-chain fatty acids and glycerol, known to form vesicles that can encapsulate other molecules. The team introduced a phosphorylation step to the vesicles, transforming the starting materials into cyclic-phospholipids. 

Generation of heterogeneous vesicles

The study demonstrated that incorporating cyclic-phospholipids resulted in the formation of a diverse collection of vesicles. These vesicles exhibited varied shapes and structures (morphologies) and maintained stability under various conditions, including different concentrations of metal ions, temperatures, and pH levels. 

Transition to phospholipid vesicles

The phosphate group within the cyclic-phospholipids played a key role in the natural emergence of vesicles composed of diacyl-phospholipids. These diacyl-phospholipids closely resemble the phospholipid membranes of contemporary cells. 

So, how did life begin?

The research suggests that the formation of cells might have happened in stages, starting with simple molecules joining together to form more intricate structures. 

“The vesicles were able to transition from a fatty acid environment to a phospholipid environment during our experiments, suggesting a similar chemical environment could have existed 4 billion years ago,” noted study first author Sunil Pulletikurti.

Initial cell formation

The scientists propose that in ancient oceans, basic building blocks like fatty acids could have naturally formed vesicles. The simple molecules could have changed through chemical reactions, possibly caused by factors like lightning or sunlight. 

A crucial step was the creation of cyclic-phospholipids through the factors, which are more complex molecules compared to fatty acids and act as a stepping stone towards the phospholipids found in modern cell walls.

The advantage of stability in cells

Among the different types of vesicles formed, the ones that were more stable and efficient at hosting life-sustaining chemical reactions would have been favored. This process would eventually lead to a greater number of vesicles resembling the phospholipid-based membranes found in current cells.

Over time, these vesicles could have incorporated even more complex molecules, like DNA and RNA, which are crucial for storing and passing on genetic information. This step is essential for the transition from non-living chemical systems to living cells, marking the beginning of the evolution of life as we know it.

Future research

“This finding helps us better understand the chemical environments of early Earth so we can uncover the origins of life and how life can evolve on early Earth,” explained Professor Krishnamurthy. 

Overall, the study is an important step in understanding how life began, but there’s still more to learn. The scientists aim to figure out why some structures merged together while others split apart. This is important because merging and splitting are essential for cells to function and reproduce. Understanding these processes could help us see how early life forms interacted with their environment and became more complex.

Ultimately, the team’s goal is to not only understand the conditions needed for life to emerge on Earth, but also to help us find life on other planets. 

The study is published in the journal Chem.

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