Saturday, 1 November 2014

A New Theory On How Complex Life Evolved

A diagram showing the bare bones of Margulis's
endosymbiosis model of the origins of the complex cell
The endosymbiont theory created by Lynn Margulis in 1966 is one of the most remarkable and compelling scenarios as to how complex cells evolved.

Simply put it suggests that an aerobically respiring bacterium was engulfed by a larger host cell. Yet instead of digesting its captive, the two formed a mutually beneficial relationship, the captor provided protection and the captive energy. Over time the two organisms fused into one, devolving into the mitochondrion - the so called power house of the complex cell.

This fusion is agreed upon by all. Yet David Baum from the University of Wisconsin, departs from this received wisdom. 'All agree that eukaryotes arose from a symbiotic relationship between two cell types: bacteria that became mitochondria and a host cell, archaea, or a close relative of archaea, that became the cytoplasm and nucleus,' said Baum. 'This symbiosis explains the origin of mitochondria, but what about other eukaryotic structures, most notably the nucleus?'

The issue with this version of endosymbiosis is that archaea do not have the ability to invaginate their membranes in order to engulf (properly referred to as phagocytosis) foreign particles. What is more, phagocytosis requires the extension of the cell membrane which involves the synthesis of extra lipids, lipids which are derived from the mitochondria. Phagocytosis also requires a large input of energy which in complex cells is also provided by the mitochondrion; in short engulfing a captive cell would require the host to have mitochondria to begin with -  a chicken and egg conundrum. To get around this problem, Baum proposed an inside out sequence of phagocytosis rather than the outside in.

A cell displaying blebs stained in red
The inside out process begins with an archaeal cell developing protrusions known as blebs. Blebs are known to exist in a multitude of creatures, including bacteria and archaea.

The importance of the blebs leads into the hypothetical part of the theory. Free living bacteria would have become trapped in between closely placed blebs. Close contact between the two would have provided the bleb cell with the energy and bacterial lipids needed to expand. Over time, the blebs would have grown around the captive bacteria and fused around them.

Mitochondria have an inner and an outer membrane; bacteria and archaea have just one. Phagocytosis provided the second membrane when the blebs fused and pinched off, providing further evidence to support the endosymbiosis scenario.

In Baum's modified scenario, the blebs also gave rise to the endoplasmic reticulum - a structure responsible for protein and lipid synthesis - something which Margulis's original theory could not explain. '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.' continued Baum. 'In contrast, a eukaryotic cell is like a factory complex, composed of a several connected work spaces: a single control room and specialist 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 other functions moved out into new, specialized quarters.'

The inside out version of events is a fantastic variation of a theory which is central to biology since it was first proposed nearly 50 years ago. While it has yet to be tested or backed up by evidence from the fossil record (unlikely) science progresses in the wake of new evidence or old discrepancies. As the erstwhile Sherlock Holmes said, 'when you have eliminated the impossible, whatever remains, however improbable, must be the truth.'