Scientists have long questioned how complex life first evolved. All multicellular life found on Earth today came from single eukaryotic cells, which contain a nucleus and other cellular machinery not found in simpler, ‘prokaryotic’ organisms. Until recently, the most widely accepted theory for how these arose had been the endosymbiotic theory. This is the notion that eukaryotic cells were first formed when an archaeon – a kind of prokaryote separate from bacteria – engulfed a primitive bacterial cell. Our own mitochondria – the energy factory of our cells – derive from these bacterial cells, even containing their own DNA separate from the cell nucleus. However, no cells have ever been observed to carry out a similar process, and this hypothesis fails to explain how other cellular components – such as the nucleus – came to be.
However, scientists at the University of Wisconsin have proposed an alternative hypothesis which overcomes several of the problems raised by the previous theory. Published in BMC Biology, the ‘inside out’ theory instead proposes that the symbiosis that formed eukaryotic cells began with archaeal cells that formed protrusions, or ‘blebs’, in their outer membranes. These blebs trap free living bacteria nearby, allowing them to further expand as materials are exchanged. The fusion of these membranous blebs then eventually formed the various cell organelles found in eukaryotes, such as endoplasmic reticulum, and the plasma membrane.
As the new theory is radically different to previous models, the scientists who carried out the research hope that this will stimulate further research into the topic and provide new insights into cellular biology. Buzz Baum, University College London, says: “Even if the hypothesis or parts of it are refuted, we are optimistic that the effort to evaluate it will spawn new cell biological discoveries and, in so doing, will improve our understanding of biology of eukaryotic cells as they grow and divide. Although students studying cell biology may come to think that it’s too late for them to contribute to a field where almost everything is known, this simply isn’t the case. As the model helps to make clear, there is still much to be discovered about the basic logic of eukaryotic cell organisation”.