A new evolutionary theory 

could explain the origins of eukaryotic cells

Scientists have long pondered the question of how simple "prokaryotic" cells, like bacteria, which are little more than a membrane-bound sack, evolved into more complex eukaryotic cells, which contain numerous internal membrane compartments. These compartments include the nucleus, which holds genetic information in the form of DNA; the endoplasmic reticulum, which shunts proteins and lipids around the cell; and mitochondria which act as the cell's powerhouse. The mitochondria also contain their own distinct DNA, which is one good indicator of their once having been separate organisms. The trouble is that no one has identified eukaryotic cells that are intermediate in complexity, making it much harder to know how they evolved. [1]

At present, the most widely accepted theory is that mitochondria derive from a bacterium that was engulfed by an archaeon (plural = archaea), a kind of prokaryote that looks similar to a bacterium but has many molecular differences. The eukaryotic membrane systems, including the nuclear envelope, then formed within the boundaries of this archaeal cell through the invagination of the outer membrane. This fits with much current data, but a few problems remain. Most significantly, no archaeal cells are known that invaginate membranes [1]

In his paper published in BMC biology, David Baum from the University of Wisconsin explains:

'' we invert the traditional interpretation of eukaryotic cell evolution. We propose that an ancestral prokaryotic cell, homologous to the modern-day nucleus, extruded membrane-bound blebs beyond its cell wall. These blebs functioned to facilitate material exchange with ectosymbiotic proto-mitochondria. The cytoplasm was then formed through the expansion of blebs around proto-mitochondria, with continuous spaces between the blebs giving rise to the endoplasmic reticulum, which later evolved into the eukaryotic secretory system. Further bleb-fusion steps yielded a continuous plasma membrane, which served to isolate the endoplasmic reticulum from the environment '' [2]

David Baum explains the differences between the outside-in and inside-out theories using a metaphor: "A prokaryotic cell can be thought of as a factory composed of one large, open building in which managers, machinists, mail clerks, janitors, etc. all work side by side. In contrast, a eukaryotic cell is like a factory complex, composed of a several connected work spaces: a single control room and specialize rooms for receiving, manufacturing, shipping, waste disposal, etc. The traditional theories propose that the factory complex arose when partitions were built within a single hangar-like building. The inside-out theory, in contrast, imagines that a series of extensions were added around an original core building -- now the control room -- while others functions moved out into new, specialized quarters. [1]

The inside-out theory provides an alternative framework by which to explore and understand the dynamic organization of modern eukaryotic cells. 

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References

1 How did complex life evolve? The answer could be inside out | Science Daily | October 2014 | http://bit.ly/1wLN1En

2 David Baum, Buzz Baum: An inside-out origin for the eukaryotic cell, BMC biology, 12:76 (2014)

 |doi:10.1186/s12915-014-0076-2

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