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Pattern of life

30 May 2000

PHILIP BALL

Order and regularity of shape and form is something that we associate with the non-living world. Crystals have shapes governed by a strict geometry, all flat faces and sharp angles. Living things, on the other hand, are rounded and irregular, constantly changing their shape.

But the living world is full of pattern, if you know where to look for it. The leaves on a plant stem are regularly arranged, often alternating in left-right and forward-backward pairs up the stem. And the stripes on a zebra or a tiger are patterns of a sort. But few natural patterns are so ornate or 'geometric' as those seen by nineteenth-century biologists when they started to inspect the living world of the oceans under the microscope.

Several microscopic marine organisms make themselves shells -- strictly speaking, external skeletons (exoskeletons) that enclose their soft bodies. Single-celled organisms called radiolarians and diatoms make these protective structures from silica, the mineral in sand and quartz. So-called coccolithophores, which live in warm tropical seas, use calcium carbonate instead, the fabric of chalk and marble.

These exoskeletons are as elaborate as they are diverse. Many are elegant cages fashioned from struts that link up into geometric shapes. Some sport spines like tiny stars. Others are shaped like cylinders, peppered with holes like a salt shaker. Coccolithophores typically make their shells from a collection of little disks, like the overlapping plates of medieval armour, fantastically grooved and decorated.

When the biologist Christian Gottfried Ehrehberg first saw the 'bones' of coccolithophores in 1836 while studying chalk under the microscope, he thought that they must be crystalline mineral formations. Ehrenberg spent 14 years recording thousands of these forms, all the time under the impression that he was drawing curious crystals. Not until the 1860s was their true organic origin understood.

Then in the 1870s the British research vessel HMS Challenger embarked on a cruise to probe the secrets of the mud at the sea floor, which brought to light the fossil shells of radiolarians long since dead. The German biologist Ernst Haeckel made meticulous, exquisite drawings of hundreds of these structures, and published them in a book called Art Forms in Nature.

Our understanding of the formation of patterns in nature was pioneered by the Scottish zoologist D'Arcy Wentworth Thompson in the early twentieth century. As far as radiolarians are concerned, Thompson proposed that the organisms blow bubbles, and then set them in stone. The radiolarian surrounds itself with foam, made from tiny bubble-like membranes called vacuoles. It then secretes the mineral along the borders where the bubble walls meet, like a builder squeezing filler along the join between two walls. Once the vacuoles are removed, a fine mineral mesh is left behind. Because bubbles tend to pack together geometrically (each one surrounded by six others in a layer) the mesh has a geometric shape -- like chicken wire. The vacuoles act as a template for making the microscopically patterned mineral.

Radiolarians are the biological equivalent of snowflakes. But as the eminent biologist Karl von Frisch has pointed out, nature is indifferent to aesthetics. "I do not want to wax philosophical about so much 'useless' beauty scattered over the oceans," he says. "Nature is prodigal."

Another common patterning method in nature is exemplified by the stripes or spots of animal markings. In 1952 the British mathematician Alan Turing proposed a theory for how such patterns can appear spontaneously in a mixture of chemicals in which the molecules are simultaneously reacting with one another and diffusing through the mixture. Turing showed that if one type of molecule produced in the reaction inhibits the reaction from taking place, while another encourages ('activates') it, the mixture can separate into regions of different chemical composition. These regions were later shown to form regularly spaced spots or stripes.

A real chemical mixture that develops Turing's 'activator-inhibitor' patterns was not discovered until 1990. And it was not until five years later that good evidence was found of the process happening in nature -- in the striped body of the angelfish. But it is widely believed that, during the growth of embryos of patterned animals like the zebra, molecules called morphogens diffuse and react under the skin to generate the markings. These become 'frozen' in place on the animal's skin and subsequently grow with the animal.

Turing's mechanism is a very general one. Something similar is thought to give rise to the banded structure of certain kinds of minerals, such as agate. The key feature of all nature's patterns is that they are 'self-organized' -- there is no guiding hand. This is the characteristic that engineers are now seeking to emulate.


Nature News Service / Macmillan Magazines Ltd 2001