Stemonitis slime mould under magnificationCredit: Dan Molter. Licensed under the Creative Commons Attribution-Share Alike 3.0 Unported licence.

The name might sound off-putting, but slime moulds are remarkable life-forms that provide a direct link to the earliest forms of life on Earth that existed more than two and a half billion years ago.

Slime moulds were among the earliest examples of “eukaryotic” cells. These differ from “prokaryotic” cells in that the latter do not have cell nuclei, merely loose strands of DNA contained within a membrane. Eukaryotic cells, on the other hand, have quite a complex structure with the DNA contained in a central nucleus plus other “organelles” that perform a variety of functions, all surrounded by an outer membrane.

The basic structure of the eukaryotic cell allowed for much greater functional variety than that of prokaryotic cells, which are represented down to the present day by bacteria. Just about every other living thing is a conglomeration of eukaryotic cells.

A Famous Experiment

Slime moulds demonstrate one of the most important abilities of eukaryotic cells, namely that of being able to work together. This was demonstrated in a remarkable experiment that was carried out in 2000 by a Japanese scientist, Professor Toshiyuki Nakagaki.

Slime moulds hatch as single cells from spores. They feed on bacteria which they ingest by surrounding and digesting them. This process, called phagocytosis, is vital for the survival of all higher species because that is precisely what white blood cells do when a living organism, such as a human being, is threatened by bacteria.

However, it is what happens when food is in short supply that is worthy of note, and which was central to Professor Nakagaki’s experiment.

He set up a maze – similar to what you might see in a puzzle magazine – that measured three centimetres square. It had plenty of dead ends, but there was a route from any part of it to the two entrances on either side, and there was a shortest route from one side to the other and three others that were not so short. The maze was set in a sterile area but a quantity of “slime mould food” was placed just outside the maze next to the two entrances.

Dr Nakagaki then dropped small pieces of slime mould into the maze at various places. None of these pieces touched each other. Some were quite close to the food but others were remote from it and some were at the ends of blind alleys. He then sat back and waited to see what would happen.

Within a few hours the blobs had elongated as they produced “pseudopodia” (false feet) and soon all the blobs were connected with each other, leaving no part of the maze “unslimed”.

However, after eight hours the appearance of the slime was very different, because it had by then formed itself into a continuous worm-like column that stretched from the food at one end to the food at the other, the column being along the shortest route with the others left empty.

The slime mould cells, despite having started off with most of them being separate from each other, had managed to work out the best configuration for them all to obtain food. There was no central organiser to direct operations, but merely the instinct of lack of food that led them to send out chemical signals that were picked up by nearby cells which then came together to, in effect, solve the problem.

What We Can Learn From Slime Moulds

This behaviour has been observed many times, both in the laboratory and in the field. Up to 100,000 slime cells have been seen to come together in a cooperative effort to find food.

Another action that has been observed in slime moulds is the sacrifice of some cells to form fruiting bodies that create spores which are then blown away on the wind to where they might later germinate into new slime cells. Again, there is no central direction here, just an inbuilt “intelligence” that sets the action in motion.

These studies, based on a very primitive life-form that is neither a plant nor an animal, have given scientists a great deal of information about how multi-celled creatures evolved. In particular, such studies have shown how complex intelligent systems can arise without the need for central direction.

Even more fascinating are the parallels between such “automatic” behaviour and the development of modern computer and communication systems. What is the Internet if not a massive example of “artificial life” that works according to rules first worked out by slime moulds hundreds of millions of years ago?

Source: “What on Earth Evolved?” by Christopher Lloyd. Bloomsbury, 2009.

Stemonitis slime mould under magnification
Credit: Dan Molter. Licensed under the Creative Commons Attribution-Share Alike 3.0 Unported licence.