Can answers to evolution be found in slime?

The pulsating networks that some slime molds form are giving other scientists clues to solving difficult mathematical problems. In 2000,Japanese researchers placed Physarum polycephalum—the name means “many-headed slime mold”—in a maze,along with two blocks of food. It extended its tendrils down the corridors of the maze,bending around curves,reaching dead ends and then backing out of them. After four hours,the slime mold was feasting on both blocks of food. Andrew Adamatzky,a researcher at the University of West England,has been watching slime molds since 2006,finding inspirations in their growth for designing computer software. In 2010 he and his colleagues placed a slime mold in the middle of a map of Spain and Portugal,with pieces of food on the largest cities. The slime mold grew a network of tentacles that was nearly identical to the actual highway system on the Iberian Peninsula. By analysing the DNA of different slime mold species,researchers are reconstructing their evolutionary history,which turns out to reach back about a billion years. Since all known slime molds live on land,that suggests that they were early pioneers,arriving hundreds of millions of years before animals or plants.

Slime molds first came to scientific fame in the mid-20th century with the work of the Princeton biologist John Tyler Bonner. Bonner learned of a North American species of slug-forming slime mold called Dictyostelium discoides and began to raise them in his lab,studying them as a simple analog of animal embryos. The organisms respond to starvation by rushing together by the thousands into a single blob. The blob stretches out into a slug-shaped mass about one millimetre long,which then crawls like a worm toward light. Once it reaches the surface of the soil,the slug undergoes another transformation: Most of the cells turn into a stiff stalk,while the others crawl to the top and form a sticky ball of spores. They stick to the foot of an animal and travel to a hospitable place. Inside the slug,about 1 per cent of the amoebas turn into police. They crawl through the slug in search of infectious bacteria. When the amoebas find a pathogen,they devour it. These sentinels then drop away from the slug,taking the pathogen with it. They then die of the infection,while the slug remains healthy. When the slug is ready to make a stalk,more amoebas must die so that others can live. They climb on top of one another and transform their insides into bundles of cellulose. Eighty per cent of Dictyostelium cells die this way,allowing the survivors to climb up their lifeless bodies and become spores.

Dictyostelium belongs to one of the two great branches of slime molds. Its branch is known as the cellular slime molds,because its spore and stalk are made out of many cells. By contrast,the so-called acellular slime molds do not form slugs. Instead,two cells merge,combining their DNA into a new single-celled organism that just keeps growing—extending tentacles that can extend as far as several yards. It pulsates to pump food from its extremities to its core,and can even crawl to search for food. Eventually,acellular slime molds also make spores. They produce tens of thousands of stalks,and the spores that cap them blow away in the wind. The Global Eumycetozoan Project,based at the University of Arkansas,has doubled the known species of slime molds. Biologists have found slime molds in Antarctica,in barren deserts,high in the canopies of jungles and even on the leaves of household plants. Slime molds are present in huge numbers everywhere. Their collective hunger makes them a powerful ecological player. When plants and animals die,microbes break them down; slime molds then devour many of the bacteria,releasing their nutrients for other organisms to grow on.

As scientists sequence the DNA of new species,they can start to figure out how slime molds evolved; genetic studies have confirmed,for example,that the two main groups of slime molds are each other’s closest relatives. Other studies have shown that the slime mold lineage is immensely old. In a paper to be published in the journal Genome Research,British and German scientists estimate that the cellular slime molds evolved 600 million years ago. Preliminary studies suggest that the common ancestor of all living slime molds is much older than that.