Gaia's evil twin: Is life its own worst enemy?
by Peter Ward
17 June 2009 from New Scientist Magazine issue
THE twin Viking landers that defied the odds to land on Mars in 1976 and
1977 had one primary goal: to find life. To the disappointment of nearly all
concerned, the data they sent back was a sharp dash of cold water. The Martian
surface was harsh and antibiotic and there was no sign of life.
James Lovelock and Dian Hitchcock, this came as no surprise - in fact, they would have been
amazed to see any evidence of life on Mars. A decade before Viking, Lovelock and
Hitchcock, both atmospheric scientists, had used observations of the Martian
atmosphere to deduce that there could be no life on the planet.
their research arose one of the most influential, ground-breaking scientific
ideas of the 20th century - the Gaia hypothesis, named after the ancient Greek
goddess of the Earth, a nurturing "mother" of life. But is it correct? New
scientific findings suggest that the nature of life on Earth is not at all like
Gaia. If we were to choose a mythical mother figure to characterize the
biosphere, it would more accurately be Medea, the murderous wife of Jason of the
Argonauts. She was a sorceress, a princess - and a killer of her own children.
The Gaia story starts in the 1960s, when Lovelock and Hitchcock showed that the
Martian atmosphere was in a state of chemical equilibrium - a stagnant pool of
carbon dioxide with a dash of nitrogen but very little oxygen, methane or
hydrogen. They contrasted it with our own, which they recognized as being in
chemical disequilibrium, with CO2 and oxygen levels in constant flux.
The key driver of this flux is life: photosynthesis exchanges CO2
for oxygen, and aerobic metabolism does the opposite. Without life, our
atmosphere would radically change from the oxygen-rich and life-sustaining
gaseous mix we breathe to one in chemical equilibrium - one that, like the
Martian atmosphere, would be inimical to life.
Earth's atmosphere is not
only in flux, it is welcoming to life, and has been for billions of years.
Similarly, Earth's surface temperature, acidity and ocean chemistry seem to have
been stable for billions of years, hovering around mean values that allow
continued habitability. Pondering these implications, Lovelock began piecing
together a novel view of life and its interaction with the planet that hosts it.
Although he focused on Earth, his ideas have implications for any habitable
planet, and he has spent the rest of his career honing them.
briefly, the Gaia hypothesis is that life as an aggregate interacts with the
physical environment in such a way that it not only keeps the Earth habitable
but continually improves the conditions for life. It does this through a series
of feedback systems similar to biological homeostasis, the mechanism by which
living organisms maintain a stable internal environment. Those aspects that most
affect the habitability of the planet - temperature, the chemical composition of
the oceans and fresh water, and the make-up of the atmosphere - are not just
influenced by life, they are controlled by it.
Lovelock's concept has
evolved over time, and Gaia has speciated into several different hypotheses
(see "The many faces of Gaia"). Within a decade of his first writings he
elevated his hypothesis to the scientifically stronger Gaia theory. In the
mid-1970s he described his view of things as follows: "The Gaia theory says that
the temperature, oxidation state, acidity and certain aspects of the rocks and
waters are kept constant, and that this homeostasis is maintained by active
feedback processes operated automatically and unconsciously by the biota."
Lovelock eventually began to refer to the planet itself as some kind of
superorganism. "The entire range of living matter... from whales to viruses and
from oaks to algae could be regarded as constituting a single living entity
capable of maintaining the Earth's atmosphere to suit its overall needs and
endowed with faculties and powers far beyond those of its constituent parts," he
wrote in his 1979 book
Gaia: A new look at life on Earth. In other words, the Earth is not simply a
planet that harbours life, it is itself alive.
The idea was simple and elegant, and quickly attracted many adherents, both
scientists and non-scientists. Some researchers saw in Gaia a new way of
thinking about the cycles of organic components and elements. Some followed
Lovelock's lead in searching for scientific support for the idea that life
regulates conditions on the planet. Some, mainly non-scientists, saw in it a new
view of how humans should relate to the planet and the rest of life. Some even
found the face of god in the concept.
Gaia continues to generate
scientific interest and debate: there have been three international conferences
devoted to the hypothesis, the
most recent in 2006.
The ground is shifting, though. A number of recent discoveries have cast
serious doubt on the Gaia hypotheses. Two lines of research are especially
damning: one comes from deep time - the study of ancient rocks - and the other
from models of the future. Both overturn key Gaian predictions and suggest that
life on Earth has repeatedly endured "Medean" events - drastic drops in
biodiversity and abundance driven by life itself - and will do so again in the
a timeline of Medean events).
Let us turn first to the deep-time discoveries. One of the most powerful
arguments made by Gaia proponents is that planetary temperatures remain steady
and equable thanks to feedbacks that are caused, or at least abetted, by life.
The single most important of these various "thermostats" is the
carbonate-silicate weathering cycle. Because of the constant volcanic activity
that is a feature of our planet, there is an unceasing but variable input of CO2
into the atmosphere. CO2
is a potent greenhouse gas. Without some way of scrubbing it out, it would
build up to the point where the Earth would experience runaway warming that
would ultimately cause the oceans to boil away - the fate of Venus some 4
billion years ago.
That scrubbing is provided largely by chemical
weathering of silicate-rich rocks such as granite. This weathering drives a
chemical reaction with CO2 that removes the gas from the atmosphere
and locks it away as limestone (calcium carbonate).
The rate of this reaction is increased by land plants, whose roots break up
rock and allow water and CO2 to penetrate. Plants also directly
remove CO2 from the atmosphere through photosynthesis.
So far, so Gaian. But as scientists have made ever more precise estimates of
past global temperatures, the constancy predicted by Gaia theory has been found
wanting. In fact there has been a rollercoaster of temperatures, caused by the
evolution of new kinds of life (see
a timeline of Medean events).
Around 2.3 billion years ago, for example, Earth endured a gigantic episode
of glaciation that lasted 100 million years. It was so intense that the oceans
froze completely, creating a "snowball Earth". The cause was life itself.*
Around 200 million years earlier, evolution had come up with a novel way to make
a living: photosynthesis, the process that uses the energy in sunlight to
convert inorganic CO2 into sugars. Photosynthetic microbes sucked so
much heat-trapping CO2 out of the atmosphere that the planet was
plunged into the freezer.
A second episode of snowball Earth, brought about by the evolution of the
first multicellular plants, happened 700 million years ago. Much later on, the
evolution of land plants gave the climate a double whammy. As well as reducing
CO2 by photosynthesis, their deep roots dramatically increased
weathering rates. The result was that soon after the appearance of forests near
the end of the Devonian period (416 to 360 million years ago), the Earth entered
an ice age that lasted 50 million years. The warm, verdant planet cooled rapidly
and vast swathes of life died out. Not a very Gaian result.
In fact, for
as long as life has existed it has been well able to devastate itself. Charles
Darwin envisaged newly evolved life forms entering the world like a wedge,
easing into a narrow vacant niche then expanding it gradually. Some do. But
others enter like a sledgehammer, smashing away entire branches of the tree of
life as they arrive.
This has been the way since the very earliest life.
Around 3.7 billion years ago, we think a "methane crisis" nearly wiped life off
the face of the Earth almost as soon as it had got going. Methane-belching
microbes filled the atmosphere with a hazy smog that all but blocked out the sun
a timeline of Medean events).
Perhaps the worst Medean event of all was precipitated by the same
biological innovation that led to the first snowball Earth: the evolution of
photosynthesis and the concomitant rise of atmospheric oxygen. Until that time,
living things could not tolerate oxygen - it was a deadly poison to the microbes
that constituted life before 2.5 billion years ago. With the evolution of
photosynthesis a weapon of mass destruction was unleashed, creating the first,
and perhaps the most extreme, of all mass extinctions. Life was devastated. All
that survived were photosynthesisers and microbes that evolved rapidly to
Even more damning to the Gaia hypothesis are new results
from the study of the mass extinctions that have occurred since the evolution of
animals 565 million years ago, of which there have been five big ones and about
10 more minor ones.
When in 1980 geologists made the ground-breaking
discovery that the Cretaceous/Tertiary mass extinction of 65 million years ago
was caused by an asteroid hitting the Earth, it soon became orthodoxy that all
mass extinctions had been caused by extraterrestrial events: either impacts or,
in the case of the Ordovician extinction 443 million years ago, a gamma-ray
burst. These events are termed "Gaia neutral", because life has no way of
preparing for them.
Researchers quickly identified impact craters
apparently associated with mass extinctions, including the huge Permian/Triassic
event of 251 million years ago and the Triassic/Jurassic event 200 million years
Yet the evidence that impacts cause mass extinctions has not stood up to
scrutiny. Most are now seen as "microbial" mass extinctions, caused by huge
blooms of bacteria belching poisonous hydrogen sulphide gas
(New Scientist, 9 February 2008, p 40). These blooms thrive in the stagnant
oceans that arise during intense episodes of global warming, such as the one at
the end of the Permian, when prolonged volcanic activity vented vast amounts of
into the atmosphere. According to Gaia theory, life should have buffered these
events. But it did not. Far from being Gaian, their existence seems to strongly
support the Medean view, as do many other events in the history of life
including, arguably, the human-induced mass extinction that is going on around
us now (see
a timeline of Medean events).
What of the future? Here too we can refute Gaia, and this is perhaps the
most interesting - and shocking - of discoveries. Life seems to be actively
pursuing its own demise, moving Earth ever closer to the inevitable day when it
returns to its original state: sterile.
Life seems to be pursuing its own
demise, moving Earth ever closer to the day it returns to being sterile.
How so? The starting point is that the sun is getting hotter. It has increased
in brightness by about 30 per cent over the past 4.5 billion years and will
carry on doing so. As the sun continues to burn brighter it will cause global
warming, which will translate into increased weathering of silicate rocks - the
rate of weathering rises with temperature. This will remove CO2 ever
faster from the atmosphere, aided and abetted by photosynthesis and plant roots.
At first, this removal of CO2 will buffer the solar-induced
temperature increase. But there will come a time - possibly as early as 500
million years from now - when there is not enough CO2 in the
atmosphere to support photosynthesis. When that calamitous day arrives, a very
pronounced end of the world as we know it will begin.
The changes will be
dramatic and catastrophic to life. Plants will wither and die, shutting off the
main source of biological productivity and atmospheric oxygen. Animals will
quickly follow. The loss of plants will also lead to a renewed build-up of CO2
in the atmosphere, leading to a runaway greenhouse. Eventually, the temperature
of the Earth's surface will exceed that of boiling water, and the last microbe
will perish. Earth will be lifeless once more. This is very anti-Gaian, since
the theory states that the presence of life on a planet should extend its
habitability. The opposite is true.
If these models are correct, life on
Earth is already in its old age. The adventure that started 3.8 billion years
ago, and is still the only life we know of in the universe, has maybe another
billion years to run. The long-term, and terminal, decline of CO2 in
the atmosphere has already started - the effect of burning fossil fuels is just
a blip. Gaia is dying. Long live Medea. For now.
Medean extinctions throughout evolutionary history
The many faces of Gaia
There are at least three different variants of the Gaia hypothesis
This early interpretation remains one of the "strongest" versions of Gaia
theory. It implies that life actively controls environmental conditions,
including purely physical aspects of the biosphere such as temperature, oceanic
acidity and atmospheric gas composition, such that the Earth remains optimally
Self-regulating (or homeostatic) Gaia
A more recent and slightly weaker incarnation of the theory. Rather than life
actively optimizing conditions on the planet, it creates negative feedback
systems that keep life-constraining factors such as temperature, and more
recently atmospheric oxygen and carbon dioxide levels, within certain ranges.
The Earth isn't just a physical planet that supports life, it is itself alive.
This is the strongest interpretation of the theory and tends to be viewed as
Peter Ward is professor of biology at the University of
Washington in Seattle. This article is based on his new book
The Medea Hypothesis: Is life on Earth ultimately self-destructive?
(Princeton University Press)
17 June 2009 from
Scientist Magazine issue
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Medean extinctions throughout evolutionary history
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- James Lovelock
- Other Sources
- NSR Comment:
When I first read James Lovelock's "Gaia Hypothesis"
more than 20 years ago, I thought this idea sounded a little flaky and
suspected it would never be anything more than a teaching aid targeted at
students without the necessary prerequisites in science. After reading
numerous books on climate change from a systems analyst perspective, I think
a very good case could be made for promoting this idea from "hypothesis"
Kitchener - Waterloo - Cambridge,