WHEN David Baum was an undergraduate at the University of Oxford, he pondered, as students are wont to in the small hours after the bar has closed, one of biology’s most basic questions. It was this: how did eukaryotic cells (the complex sort that make up every plant and animal as well as lots of unicellular creatures like amoebas) evolve from prokaryotic ones (bacteria and their kin) which are much simpler? He had an idea, but he thought it so obvious he assumed that other, more established minds must surely be working on it. It turned out, though, that they were not. So, 30 years after his tyro musings, Dr Baum (he is now a botanist at the University of Wisconsin) joined forces with his cousin Buzz Baum, a cell biologist at University College, London, to work them up into a scientific paper. The result has just been published in BMC Biology.

The division between eukaryotic and prokaryotic life is one of the most profound in biology. Eukaryotic cells have a nucleus containing several elongated DNA-bearing chromosomes. This nucleus is surrounded by a membrane punctured by pores through which molecules can leave and enter. Beyond the nuclear membrane, in the part of the cell called its cytoplasm, is an array of specialised organelles such as mitochondria (which generate energy) and chloroplasts (which photosynthesise), and also further membranes, known as the endoplasmic reticulum, that carry out a range of tasks from making proteins to disposing of waste. Prokaryotes, by contrast, have no nucleus (their genes are carried on a circular DNA molecule without a surrounding membrane), no organelles and no endoplasmic reticulum. It was the shift to eukaryotism, sometime between two billion and one billion years ago, that opened the path to complex, multicellular life and thus, eventually, to human beings.

The consensus is that the first eukaryote was a prokaryote which engulfed, but failed on several occasions to digest, other prokaryotes. One of these undigested meals was a bacterium ancestral to mitochondria. Even today mitochondria have their own genes separate from those in the nucleus. These genes, which are carried on circular DNA molecules like those in bacteria, resemble those in a group of bacteria called Rickettsiales, suggesting they and mitochondria have a common ancestor. A similar explanation for chloroplasts, which seem to be the descendants of photosynthetic bugs called cyanobacteria, is also widely accepted. But this does not explain where the nucleus and the endoplasmic reticulum came from, and the suggestion that these, too, are somehow remnants of captured prokaryotes stretches credulity, as they resemble no modern free-living creature. The model proposed by the Drs Baum, however, covers more or less everything.

They imagine the original host prokaryote creating small protrusions, known to microbiologists as blebs, that poked out of it, as the diagram shows, like tiny fingers. Blebs like this are known to form in certain sorts of archaea, a group of prokaryotes distinct from bacteria proper that biochemical evidence suggests were involved in the formation of eukaryotes. The job of blebs is unclear, as archaea are not a well-studied group, but they may be feeding structures. The Drs Baum suggest that, in the case of the ancestral eukaryote, the blebs grew bigger and bigger, pinning proto-mitochondria (and, on a subsequent occasion, proto-chloroplasts), into the intervening spaces.

This joining up of the blebs would create a network of membranes—the precursor of the endoplasmic reticulum—but would also leave behind a central membrane (the original outer membrane of the host prokaryote) that would contain the host cell’s DNA. And that membrane would necessarily have pores in it. The host cell would thus, in other words, have become the nucleus of something larger and more complicated.

This is all speculation. But it is testable speculation. It predicts, for example, how the nuclear pores in a modern cell should form, assuming the ancestral machinery is still cranking away in the background. They should start from the inside, folding the membrane outward and then poking through it. Whether that happens is currently unknown, but imaging techniques are just getting to the stage where this will change and researchers will be able to watch the process unfold.

One important piece of the puzzle is still missing, of course. This is how the circular DNA molecule of the original host turned into the multiple, linear chromosomes of modern eukaryotes. But perhaps there is, even now, an insomniac undergrad out there who is working on it.