SCOTT TURNER lives in a old farmhouse in out-of-the-way Tully, New York. Step
outside Turner鈥檚 kitchen door, and you step into a gumdrop-green valley. His
backyard is open and roomy, with haphazard apple trees and wild flowers that
teeter towards the valley below. The breeze smells like mint, and it sets
everything swaying. In the midst of all this outdoor action, Turner stops to
choose a surprising subject for a lesson in thermodynamics.
In the elbow of an apple tree, a gauzy brown web holds about 400 little
worms. Webworms may look like tan paper clips come to life, but they鈥檙e smart.
Rather than constantly generating their own energy to stay warm, the worms weave
a silky bag to do the job for them. By capturing sunlight and blocking the wind,
the web creates a climate as steady as Los Angeles. It鈥檚 a convenient means of
energy conversion. But Turner argues that the webworms鈥� web is something far
more: it is alive.
In his new book, The Extended Organism, Turner suggests that animal
structures like webs, nests, hives, burrows and mats are physiological
extensions of their animal creators. They take energy and materials from the
environment, including sunlight, water and oxygen, and funnel them to the
organisms inside. This makes these structures as much a part of a living animal
as more conventional organs such as livers, lungs, kidneys and hearts. Although
the web is outside the webworm鈥檚 skin, it鈥檚 doing the same job as the tissues
inside other animals that break down glucose to release energy and keep them
warm. 鈥淪uddenly,鈥� says Turner, 鈥渢he boundary between the living and non-living
seems very arbitrary.鈥�
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These days, many biologists define life simply in terms of genes鈥攁n
organism is something with replicating DNA, or at the very least RNA. But over a
century ago, when ecologists first began studying the environment, some took a
more holistic view, and physiology was central. Turner, who fits squarely into
this camp, sees a living thing as a continuous flow of energy and matter鈥攁
kind of thermodynamic system. 鈥淯ntil now, this back-and-forth between animals
and their structures has, in many circles, been ignored,鈥� says Clive Jones of
the Institute of Ecosystem Studies in Millbrook, New York.
Turner doesn鈥檛 dismiss the role of genes. Indeed, his ideas build on the
now-famous 鈥渆xtended phenotype鈥� proposed by Oxford University biologist
Richard Dawkins 20 years ago. Just as genes direct an animal鈥檚 colour or shape,
Dawkins suggested, they also direct building behaviour. A beaver鈥檚 carpentry
genes help it build just the right dam, for instance, and as a result the beaver
finds more food and increases its survival chances. In this way, the beaver dam
is really an extension of the beaver鈥檚 expressed genes, or phenotype.
Although Dawkins gave us the philosophy of the extended phenotype, he had few
examples of animal architecture to back it up. Two decades later, Turner is
picking up where Dawkins left off. 鈥淭alking about genes is fine, but you鈥檝e got
to have the machinery that makes the matter and energy, that carry genes into
the future,鈥� says Turner. 鈥淚 want to put the flesh and bones on the extended
phenotype.鈥� He鈥檚 off to a flying start鈥攂ut there could be controversy
ahead.
Turner stumbled, quite literally, on the extended organism. A decade ago he
was in Mafikeng, South Africa, studying the energy flow between an ostrich
embryo and its mother, and when the project ended he took a teaching position.
Tall and quiet, Turner has the owlish look and patient ways of a teacher. And he
notices things. From his office, he could see the tops of a bunch of termite
mounds: dirt chimneys jutting a metre above the patchy grass, like baked-mud
monoliths, with termites living far in the ground below.
Southern African termites are known for their elegant mounds鈥攑alatial
networks of tunnels, galleries, cellars and fungus gardens鈥攚hich the
insects keep warm and well-ventilated by regulating the flow of oxygen and
carbon dioxide. How do they maintain such a steady environment? Curious, Turner
gave a gift of beer to a local villager, who took a backhoe to the dry dirt
surrounding some of the underground mounds, exposing them. Then Turner stuck a
syringe filled with a tracer gas inside to see how the gas flowed through the
interior.
When the residents of Mafikeng, and later those of Outjo in Namibia, saw
Turner on hands and knees inspecting termite mounds, they assumed he was some
kind of exterminator, there to tear down the towers of dirt. He became known as
the 鈥渢ermite doctor鈥�. 鈥淭hey were quite disappointed when they realised I was
interested in keeping the termite mounds going, not clearing them away,鈥� he
grins. Instead, Turner carried out a series of studies that have shown how
termite metabolism and wind outside the mound work together to drive gasses in
and out of the chambers, in a flow that鈥檚 remarkably similar to the movement of
air in lungs.
He concludes that termite mounds are really an extension of the termites
themselves, an integral and active part of their physiology. The same goes for
the homes of similar social insects, like bees and ants. Thomas Seeley, a bee
researcher from Cornell University, agrees. 鈥淎 lot of biologists see the outer
portion of the skin or skeleton as the limit of the effects of genes, and that鈥檚
pretty clearly not the case,鈥� he says. Seeley believes that the honeycomb is an
extension of the bee鈥檚 phenotype because, like termites, bees work together
using their accumulated body heat to keep the comb at a constant temperature.
鈥淵ou can think about ecosystems as being massive flow pipes of matter and
energy,鈥� says Turner, 鈥渁nd start looking for ways that organisms modify the
environment to help them live.鈥�
Consider the diving bell spider. All spiders must breathe air, but these
particular spiders can live underwater because they construct an external
鈥渓ung鈥�, a web that traps a bubble of air below the surface. In fact, many
aquatic insects tote a 鈥渂ubble gill鈥� around. Turner points out that there is
some marvellously subtle physics at work. Without the insect attached, a
submerged bubble of air will gradually collapse as the gases move out into the
water under a combination of hydrostatic pressure and surface tension. But with
an insect breathing from the bubble, the partial pressure of oxygen falls below
its value in the water. Oxygen then flows into the bubble, delaying its collapse
and replenishing the insect鈥檚 reserves.
But Turner does not see extended organisms as mere oddballs and anomalies. He
argues that every animal has two physiologies. While conventional biology
recognises the flow of energy inside the body, there is a second external
physiology whenever an organism alters its environment to make nature work for
it. How energy flows, and how it can be made to do work in the process is, of
course, the subject of thermodynamics, which is why Turner frames his
description of the extended organism in the language of physics. To explain how
an animal鈥檚 physiology can reach beyond its living cells, he starts by
describing classic physiology.
Work to order
Take a freshwater fish, says Turner. Its internal fluids contain a higher
concentration of salts and other small solutes than the surrounding water. As a
result, water tends to flow into the fish, through its skin, by osmosis.
Expressed according to the second law of thermodynamics, entropy increases as an
ordered system (the fish and its surrounding water) becomes disordered. To
maintain order, the fish鈥檚 kidneys and gills are constantly at work, flushing
out excess water while storing up valuable solutes. 鈥淧hysiology is essentially
how animals use energy to do order-producing work,鈥� notes Turner.
Now he whips out a murkier example. Corals and their tiny colourful house
guests, zooxanthellae, build a tapestry of undersea reefs from the mineral
calcium carbonate. But a crucial part of the physiological process occurs
outside the animal. Corals expend energy moving hydrogen ions into their cells
and calcium ions out. Inside, the hydrogen ions allow the zooxanthellae to
photosynthesise, producing glucose to power the process. Outside, on the surface
of the reef, the removal of hydrogen ions forces hydrogen carbonate to form
carbonate, which then reacts with the calcium to give calcium carbonate. The
system doing the work is a combination of coral, zooxanthellae and the reef
itself.
Turner finds extended organisms everywhere he looks, from microbes to
earthworms and from crickets to mosses. 鈥淣ature is full of energy ready to be
tapped, and I see engines everywhere,鈥� he says. But not everyone is convinced.
鈥淭urner has identified a very interesting problem,鈥� says Michael Hansell, an
ecologist at the University of Glasgow, 鈥渂ut I think he鈥檚 heading off in the
wrong direction.鈥� Hansell takes the more conventional view that mounds, hives,
nests and other ordinary animal dwellings are merely architecture鈥攗seful
artefacts that often get left behind by their creators or tapped by other
animals in the neighbourhood. So how can such structures be extensions of the
creatures that make them, he asks?
Turner responds that an extended physiology may indeed be temporary, as an
animal鈥檚 give-and-take with some sliver of the environment changes over time.
The desert lizard eventually leaves its burrow, for instance, and when it does,
the burrow becomes simply a physical anomaly in the environment. But when the
lizard creates the burrow, it does so to maintain body temperature and fluid
levels. When it uses the burrow for these purposes, it is a bodily extension, of
sorts. Seeley agrees. 鈥淵es, structures have secondary users. But the design or
properties of a built structure are what they are to serve the fitness or
physiology of the builder.鈥�
Taken to its logical conclusion, does that make our homes and cars extensions
of our physiology? Are these structures, in a sense, alive? To distinguish a BMW
from a burrow, Turner falls back on tight links to physiology, energy
transactions of a primal sort. 鈥淭he BMW, in moving us from place to place, is
doing work for us that would ordinarily be done with internal physiology,鈥�
concedes Turner. But, he adds, the structures that we build are one step removed
from those of other animals because we power them with fossil fuels rather than
the Sun, wind, gravity, osmosis and other 鈥渘atural鈥� energy sources.
Here Turner鈥檚 line of reasoning may be sticky, but his ideas really become
controversial at the grand scale. If a termite colony can regulate its
environment, creating a kind of natural balance, he asks, why can鈥檛 whole
ecosystems? For Turner, the extended phenotype naturally marches up the scale of
nature, to include Earth itself. He is an advocate of the Gaia
hypothesis鈥攖he idea that Earth acts like a superorganism, with all its
biological and physical systems cooperating to keep it healthy
(快猫短视频, 30 May 1998, p 28).
Turner acknowledges that many scientists
still see Gaia as 鈥渁 big crackpot idea鈥�. Even so, he says, there are many
physiology lessons to be found in work by Gaia theorists.
Most of his colleagues, however, believe that this is where he goes astray.
鈥淚f you stay within the realm of single organisms, where Turner starts, you can
clearly measure the energy and material going between the organism and its
environment,鈥� says Jones. 鈥淏ut when you talk globally, it becomes very difficult
to sum up the specific energy transactions going on.鈥� Steven Vogel from Duke
University adds: 鈥淭urner鈥檚 insights are very useful. But where do they take us,
in a global sense? That鈥檚 hard to say.鈥�
If Turner can鈥檛 quite build the big picture yet, he鈥檚 content to continue
finding extended organisms one at a time. 鈥淲hat I鈥檓 trying to do is bring a
perspective to biology that we鈥檝e lost sight of in the race to sequence genes
and see life at the molecular level,鈥� he says. While others look inward for
answers, Turner鈥檚 gaze is decidedly turned out.
LIVING things evolve. And so do the structures that animals build. In his
book The Extended Phenotype, Richard Dawkins pointed out that a construct such
as the beaver鈥檚 dam is heritable in the sense that there are genes for building
behaviour, and natural selection favours individuals that build the best dams.
But some biologists go much further than this. They argue that the way living
things alter their environment鈥攖heir so-called niche
construction鈥攑lays a central role in evolution.
For a start, by building a home an animal influences the selection of a great
number of genes that might be beneficial for life in that constructed
environment, not just those responsible for building behaviour. Many creatures
that live in burrows or nests, for example, have evolved behaviours to defend
and maintain their property. And, after countless generations of web building,
some spiders have evolved the ability to create dummy spiders in their webs to
divert the attention of birds that might prey on them.
At a deeper level, living things don鈥檛 just adapt to their environmental
niche, they also create it. 鈥淭here are two routes to adaptation, since organisms
can adapt to become suited to their environments or modify their environments to
suit themselves,鈥� says Kevin Laland from the University of Cambridge. What鈥檚
more, organisms can modify the selection pressures acting on their distant
descendants via niche construction. Worms, for example, have dramatically
changed the structure and chemistry of soil by their burrowing. Over time, they
will also have adapted to this worm-made environment, perhaps through changes in
their skin and mucus secretions.
Laland even argues that human culture is just a special form of niche
construction. In a paper published earlier this year (Behavioral and Brain
Sciences, vol 23, p 131), he and colleagues John Odling-Smee from the University
of Oxford and Marcus Feldman from Stanford University in California suggest that
by seeing culture in this way we can understand how it leads to adaptation at
the level of the genes. Cultural traits such as the use of tools, fire and
language have modified the selection pressures we face and led to genetic
change. 鈥淥ne reason why our species thrives is that we are such potent niche
constructors we can render even the most hostile environments benign,鈥� says
Laland. 鈥淣o organism, least of all humans, adapts to a pre-existing niche.鈥�
Kate Douglas