Pattern of life30 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.
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