Wethink this is the wrong way round. In a recent paper, we propose instead thateukaryotic cells evolved from the inside out – that a prokaryote extruded blebs of outer membranethrough its cell wall, and these fused to form the peripheral parts of theeukaryotic cell that contain the ER and mitochondria (BMC Biology, vol 12, p 76). Like the famous optical illusion in which onecan see either two faces or a candlestick, when theeukaryotic cell is viewed afreshfrom this perspective, many things look different. Now the outer membrane ofthe eukaryotic cell is an evolutionary novelty,while the nuclear envelope corresponds to the boundary of the original prokaryotic ancestor – the opposite of what is assumed by traditionalhypotheses.

Although our inside-out modelwas published last year, the idea was born some 30 years ago when David wasstudying botany at the University of Oxford. Looking at an image of a largeeukaryotic cell next to a much smaller prokaryotic cell, David wondered why itwas always assumed that the boundaries of the two types of cell wereequivalent, when it was easy enough to imagine that the prokaryote cellcorresponded to the nucleus of the eukaryote. His essay on the topic, writtenin 1984, described this basic idea. While it got a respectable mark, it did notseem very compelling at the time.For a start, no prokaryote was then known to extrude membrane outwards. Davidsat on the idea, always thinking that somebody else would come forward with theconcept, and his research turned in other directions.

Thirty years later, when Davidstarted to think again about the origin and early evolution of life, he wassurprised to see that, in the interim,nobody had suggested that complex cells arose in this way. Perhaps it requiredthe naiveté of an undergraduate toquestion dogma? So David dusted off hisinside-out model and wrote up a short piece to explain how it might work andwhy it ought to be considered as an alternative explanation for the origin ofeukaryotes. And it was much more compelling now it is known that theprokaryotes most closely related to eukaryotes, the archaea, often produce extracellularprotrusions. As well as friends and colleagues, David sent the essay to hiscousin, Buzz.