Veterinarian Finds Evolutionary Link Between Humans and Bacteria
We higher forms of life have always distinguished ourselves from our single-celled microscopic friends. After all, we have a brain and a more interesting sex life. And we have organelles, tiny structures that are the basic workhorses inside complex cells. Organelles have their own membranes and perform such functions as producing energy or gobbling up foreign invaders. Such luxuries don’t normally go to a single cell.
Or so we thought. This year Roberto Docampo, a professor of veterinary pathobiology at the University of Illinois at Urbana-Champaign, found that the bacterium Agrobacterium tumefaciens, which causes crown gall disease in plants, does indeed have organelles. His discovery muddies the distinction between eukaryotes (organisms with nuclei in their cells) and prokaryotes (single-celled life-forms without nuclei). It also sheds light on how eukaryotes and prokaryotes evolved.
Docampo was hardly the first to notice that prokaryotes have mysterious black specks floating in their cytoplasm. Yet he is one of the few ever to study them. When he focused an electron microscope on the specks, he discovered that they were actually pouchlike compartments with membranes. He then discovered that they contained pyrophosphatase, an enzyme that can shuttle proteins in and out of a structure to maintain its acidity. On further examination, it turned out that the black specks are a type of organelle—called an acidocalcisome—that is also found in certain single-celled eukaryotes.
Although the precise function of acidocalcisomes is not understood, Docampo says their discovery implies that organelles may have been in cells before our evolutionary ancestors and those of today’s bacteria diverged. “They are really the same organelle that probably originated in bacteria and was conserved during evolution,” he says. “Bacteria are more similar to eukaryotes than we thought.”
Scientists Play God, Creating New Life
Although nature has created more than 100 amino acids, almost every living creature on this planet uses the same 20 to build proteins. To find out why, Peter Schultz, a chemist at the Scripps Research Institute in California, began tinkering with the genes of the Escherichia coli bacterium a few years ago. In 2001 he announced he had built a brand-new organism in the lab that uses 21 amino acids. But, the extra amino acid had to be forced into the bacterium. So Schultz moved ahead, further engineering E. coli. This year he announced a new E. coli that can manufacture its own 21st amino acid, making it the world’s first truly synthetic, man-made form of life—an autonomous bacterium that would survive quite nicely on its own, even if it were thrown in the garbage.
To find out if the extra amino acid gives the bacterium an evolutionary advantage, Schultz and his team are adding random mutations to the genetic code of the tweaked E. coli as well as to that of a regular 20-amino-acid control group. “You’d almost be surprised if more wasn’t better,” says Schultz. “If you’re cooking dinner, having access to more ingredients may lead to more interesting things, right?” While waiting to find out, Schultz has constructed five different strains of 21-amino-acid yeast, and he is trying to build a 21-amino-acid Caenorhabditis elegans—a worm. “Ultimately, we want to go to complex, multicellular organisms, like a mouse,” he says. Yet a startling question remains for which Schultz has no answer: “Why didn’t nature do it to begin with?”
Biologists Reverse History
In a major breakthrough, scientists may have turned one species into another. By manipulating the genes of yeasts, molecular biologist Stephen Oliver and his colleagues at the University of Manchester in England have also shown it is possible to turn back the clock of evolution.
Oliver’s team began with two species of yeast called Saccharomyces cerevisiae (common baker’s yeast) and Saccharomyces mikatae. The species can mate, but they typically produce infertile offspring, just as a horse and a donkey produce an infertile mule. (A species is usually defined as an organism that can reproduce fertile offspring like itself.) Scientists reversed some of the chromosomes on S. cerevisiae to match those of S. mikatae, a process Oliver compares to “fishing in your sock drawer and lining up a blue sock with another blue sock, and a brown sock with another brown sock.” It seems straightforward, but Oliver says problems arise “when parts of the chromosomes have been swapped around. Do you pair a sock that is half blue and half brown with a blue sock or a brown sock?”
The biologists were able to achieve fertility rates between the two species that were as high as 30 percent. Because the two species have the same distant ancestor, researchers declared they had essentially reversed evolution. The implications could be profound. For example, Oliver believes the discovery could be used to prevent genetically engineered crops from obliterating wild species.
“This is quite exciting,” says Michael Travisano, an evolutionary biologist at the University of Houston. “We’ve observed many hybrid species develop naturally, but this is the first time someone was able to directly intervene on a sophisticated molecular level.”
To those who tremble at the thought of manipulating the chromosomes of living creatures, Oliver says, “We’re only applying our techniques to yeast at the moment, not monsters.”