Did complex life form from the outside-in or the inside-out? Scientists may have cracked the code on the formation of multi-celled organisms. Biologists and cousins, David Baum of the University of Wisconsin and Buzz Baum of University College London, have developed a new theory about how life evolved from early prokaryotic cells into the eukaryotic cells that make up multi-cellular organisms. They propose that complex cells formed from the inside-out, generating cellular membranes as they incorporated new organelles.
Eukaryotes appeared on earth between 1.6 and 2.1 billion years ago. It was one of the most important milestones in the creation of life on the planet, and paved the way for plants, animals and fungi to develop. Determining the exact date or the exact path to their evolution remains a mystery for biologists. Traditional explanations for the move from cells without a nucleus or other membrane-bound organelles into eukaryotic cells focus on the outside-in model. Biologists generally assume that one bacterium subsumed or incorporated another bacteria or virus into itself. A bacteria could pull in, or engulf, another cell which becomes the nucleus or the mitochondria. In this way, multiple membrane-bound organelles begin to work together to perform the life processes much more effectively than in a prokaryotic cell. For example, a mitochondrion produces 36 ATP of energy during cellular respiration but respiration that takes place in the cytoplasm only produces 2 ATP. The Baums’ theory suggests that complex life formed from the inside-out.
Prokaryotic cells can be compared to a factory composed of open space where the various employees work side by side. Managers mingle with mail handlers and material processors. A eukaryotic cell is like a factory with departments operating from connected but separate work spaces. A control room directs the factory activities, with boiler rooms, mail rooms and storage rooms performing tasks. The outside-in theories suppose that these separate rooms were built within an existing structure. The Baums’ inside-out model imagines that the the factory was built outward from the control room to enclose new departments. This may have helped them crack the code on the formation of complex life.
The Baums theorize that some cells had extensions they call blebs. These protrusions may have trapped other early bacteria between them. They were able to use the energy from mitochondria-like bacteria, or capitalize on whatever the other cells they encountered had to offer. Unlike pseudopods, one bacteria did not engulf the other, but grew around it instead. The original cell membrane became the nuclear membrane and the blebs eventually fused to form a new cell membrane. Inside were now mitochondria and endoplasmic reticulum with their own lipid membranes. David Baum explains, “The inside-out theory … imagines that a series of extensions were added around an original core building – now the control room – while other functions moved out into new specialized quarters.”
It is still impossible to know exactly how bacteria merged to form eukaryotic cells, though biologists agree they must have arisen out of some sort of symbiotic relationship. The Baums believe their ideas explain more of the complexities of multicellular organisms. Even if they are wrong, they believe that their theory will advance understanding because it provides new questions to test.
The Baums’ paper, An Inside-out Origin for the Eukaryotic Cell concludes that the inside-out theory is “consistent with diverse kinds of data” and provides a new framework for understanding the “dynamic organization of modern eukaryotic cells.” Does it matter whether complex life evolved from the outside-in or the inside-out? A more complete understanding of the cell could radicalize medical treatments and healthcare, and now, scientists may have cracked the code on the formation of complex life. Understanding how life developed and how it functions is crucial to forming a more complete picture of how all living organisms interact.
By: Rebecca Savastio