by Aimee Levitt
Depending on which source you look at, the human body contains anywhere from 50 to 75 trillion cells. Somehow they all manage to work together, carrying out their various functions to keep the whole body alive. How the hell is this possible?
A pair of biologists at Washington University have a theory: It's all because all humans start off as a single cell. That cell divides and multiplies, and its descendants develop different specialties, but they cooperate because they're all related. (If only this logic worked with human families.)
Joan Strassmann and David Queller developed their theory, which you can read about in more detail in Science, based on a series of experiments with a social amoeba called Dictyostelim discoideum, or Dicty for short. Yes, you are remembering your basic biology right: amoebae are one-celled creatures. But when Dictys' lives are threatened -- if they lack warmth or light or food (Dicty eat E. coli bacteria) -- they band together into multi-celled colonies that function as a single organism.
And here's where it gets interesting.
This is a picture of the evolution of a colony of Dicty. When E. coli are scarce, the Dicty secrete chemicals, the amoeba version of a distress call. It's very effective: 10,000 Dicty, mostly descended from one original cell, come together and pile into an amorphous mound. Gradually, as the colony gets better organized, the mound solidifies and grows until it looks like a bowling pin, the second image from the left. Like a bowling pin, the colony topples and stretches out into the long, slug-like thing in the bottom left corner. (It's actually only a couple of millimeters long, but when you're dealing with single-celled organisms, everything is relative.) The slug crawls through the dirt until it finds its happy feeding grounds.
Then the colony starts to break up. It rearranges itself into the shape you see on the far left, the one that resembles either a sombrero, a baby pacifier or a rolled-up condom, depending on your point of view. The front end of the slug turns into the crown of the hat (or whatever) and eventually grows into a stalk that comprises about twenty percent of the colony. The remaining 80 percent climb up the stalk and rearrange themselves into a ball of spores called a fruiting body, the dandelion-like thing on the right. The spores float away and reproduce. The amoebae in the stalk have only the consolation that they have sacrificed their genetic material for the greater good.
OK, it's probably a bit far-fetched to say that amoebae have a sense of altruism, but when Strassmann and Queller started working with Dicty in 1998, they combined two colonies of unrelated amoebae. When the spore formed, they noticed that one of the original colonies had contributed a disproportionate amount of amoebae to the spore. In the words of the scientists, they had "cheated" in the game of reproduction.
It's equally far-fetched to say those Dicty had a sense of self-preservation, so Queller has a more scientific explanation: "They might have a mutation that makes an adhesion molecule less sticky, for example, so that they slide to the back of the slug, the part that forms spores."
The scientists started a series of experiments wherein they created colonies of unrelated Dicty and colonies of the self-preserving mutants and their descendants. They discovered that Dicty colonies that were not formed from close "relatives" were less cohesive and more likely to contain "cheaters." They also discovered that the descendants of the original cheaters went on to cheat their ancestors, and that a third of the all-cheater colonies were unable to form fruiting bodies at all. Instead, if the cheaters wanted to reproduce, they would have to freeload on colonies of more cooperative amoebae.
But colonies like the artificial ones set up in the lab don't occur in nature. Ever.
In order to figure out why, Strassmann and Queller took a lone amoeba and let it multiply into a colony of clones. Since there were no competing genetic strains, each mutation was reproduced faithfully. The scientists created 90 colonies this way.
"At the end," Queller reported, "we found that among those 90 lines not a single one had lost the ability to fruit. So that's almost 100 lines, almost a thousand generations, so 100,000 opportunities to lose fruiting, and none of them did."
In a final experiment, the scientists made a series of calculations using formulas originally developed for population genetics. They determined that no matter how big the colony grows, to 50 trillion cells, or even if it becomes the size of a blue whale, the world's largest animal, it's highly unlikely that there will be spontaneous mutations of non-cooperative cells, so long as all 50 trillion cells are related.
Which is why, it appears, that all life forms originate from a single cell. Or, as Queller puts it, "It's the single-cell bottleneck that generates high relatedness among the cells that, in turn, allows them to cooperate."