Ask a group of blind men to touch an elephant and one might declare its tail a snake, while another might call its tusk a spear. Every cell in your body is a bit like that elephant. If you ask a bunch of scientists about a cell¿and specifically, how it gained a nucleus¿you'll get a variety of answers, depending on which part of it they point to.

Image: Courtesy of BILL MARTIN

MERESCHKOWSKY'S VISION is depicted in this chart, which shows where he thought symbiotic unions had enriched the evolutionary history of eukaryotic life. Click here to enlarge.

One idea that has gained popularity over the past two decades originated with the Russian botanist Constantin Mereschkowsky. He first tentatively suggested in 1905 that the nuclei of today's complex cells once roamed free but were absorbed by larger cells because each had something to offer the other. Eventually they became dependent on each other, and their fates intertwined. In biological lingo, he proposed that the nucleus began its career as an endosymbiont.

But those who study the dawn of life still strongly disagree over the details of this scenario, and so the question of how the nucleus arose seems far from being answered. "It's one of the big, big unsolved mysteries: where does the nucleus come from?" says Bill Martin, chair of botany at Heinrich-Heine University in D¿sseldorf.

Finding the answer would fill a major gap in the history of eukaryotes (literally, cells with a "true nucleus"), which in the space of two billion years have populated the world with everything from singled-celled amoeba and plankton to pine trees, scientists and, of course, elephants.

The Nuclear Divide

Though eukaryotes get their name from the nucleus, where the genome resides, they differ from nucleus-free prokaryotes¿run-of-the-mill bacteria and the more exotic archaea, which tend to live in extreme habitats¿in other ways. First, they have a sophisticated set of internal fibers, arrayed like pick-up sticks, that gives them shape and allows them to engulf food. Prokaryotes have versions of the proteins that make up this cytoskeleton but form nothing so elaborate.

Second, eukaryotes contain various organelles, internal compartments partitioned off from the rest of the cell by membranes. Such organelles as mitochondria, for instance, generate fuel; in plant cells, chloroplasts manufacture sugars to break down into that fuel. They were also endosymbionts in the past, as it turns out. Though a controversial idea when first proposed, molecular comparisons made the argument stick: both organelles have genomes that retain similarities to two distinct types of bacteria that persist today.

The nucleus also has some superficial similarities to these organelles: a two-layered membrane surrounds them all; they each have their own genome; and they are capable of reproducing. "It's easy to get [these features] when you're essentially swallowing a symbiont," says Hyman Hartman, a research scientist at the Massachusetts Institute of Technology who studies the origin of life. "It doesn't take a rocket scientist to [suggest] the nucleus had an endosymbiotic origin."

Image: K. LEUTWYLER after JAMES LAKE and MARIA RIVERA

TWO CONTENDERS for the birth of the nucleus appear above. The sequence on top could have occurred if an early mitochondrion (not pictured) gave its host a gene for producing membrane building blocks. The endosymbiotic model diagrammed in the lower sequence shows a bacterium surrounding an archaeal cell called an eocyte.