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  Lovelock argued that it was because life on Earth was actively regulating the temperature by means of various bio-feedback mechanisms. One such example is the massive blooms of algae that form on the surface of the ocean as temperatures rise. They pull heat-trapping carbon dioxide out of the air, thereby lowering the global temperature. Similar methods keep a whole range of environmental factors, such as the alkalinity and pH of the ocean, within parameters that are optimum for life.

  Lovelock didn’t come up with all of this entirely on his own. The microbiologist Lynn Margulis collaborated with him to refine the biological details of the argument, while his neighbour, the author William Golding, coined the name Gaia, referring to the Greek goddess of the Earth.

  The concept of Gaia fascinated the general public. After all, it was awe-inspiring to think that there was a force operating on a global scale that was safeguarding the interests of life. It was also comforting to imagine that a principle of harmony resided at the heart of nature, that all life on Earth (with the exception, perhaps, of human beings) functioned together for the common good.

  Biologists, however, weren’t quite as taken with it. They agreed that it was a nice fairy tale, but as science, they insisted, the hypothesis simply didn’t work. There was no mechanism for it to do so. It required organisms to be doing things not for their own direct benefit, but rather for the benefit of the global ecosystem – and that, they argued, ran completely counter to evolution, which they considered to be the organizing principle of life.

  Richard Dawkins and Stephen Jay Gould were among Gaia’s most vocal critics. They hammered away on the point that evolution works by means of natural selection, which operates entirely on the selfish individual level and is entirely without compassion. Whatever traits best allow an organism to pass on its genes get selected. This happens ruthlessly, no matter the cost to anything else.

  It’s true, they conceded, that often an individual organism will have the best chance of passing on its genes by sacrificing its own interests for the good of the group it belongs to. But the good of the entire planet was too far removed and distant to exert any kind of selective pressure on an organism. As a result, there was no way for Gaia to emerge by means of natural selection. And, as far as the biologists were concerned, this meant there was no way for Gaia to emerge, period.

  They also argued that it was meaningless to claim that Gaia maintained an environment favourable for life, because there is no best overall environment. Life tries to adapt itself to whatever setting it finds itself in. Some organisms live in Arctic ice; others live in thermal hot springs. Those conditions are optimum for them.

  But what really drove the biologists to fits of rage was the Gaian idea that the Earth itself was a living organism. Lovelock frequently spoke about the Earth as if it were alive, because he said it was useful to compare the way organisms regulate their internal environment with the way similar forms of self-regulation could be found operating, on a far grander scale, across the Earth as a whole. When challenged on this point, he always insisted that he was speaking metaphorically. But many of his readers took him literally, and this horrified biologists. Making the planet out to be a living being seemed to them to be neo-pagan Earth worship, not science. The evolutionary biologist John Maynard Smith denounced Gaia as ‘an evil religion’, while the microbiologist John Postgate warned of ‘hordes of militant Gaia activists enforcing some pseudoscientific idiocy on the community.’

  So, the biologists did everything in their power to suppress the Gaian heresy, and they were quite effective. Lovelock complained that it became almost impossible to publish anything about the hypothesis in scientific journals.

  All these criticisms remain perfectly valid, and Gaia remains something of a dirty word in biological circles. So how is it possible to say that the hypothesis is now considered to be partially true? It’s because, while biologists could only see in it the looming spectre of pseudoscience, geophysicists had a very different reaction. They found the idea to be enormously intellectually stimulating. They weren’t that concerned about whether Gaia played nice with evolution. Instead, they were focused on the big planet-wide picture of how the Earth functioned, and, on this scale, Gaia offered an exciting new perspective.

  In the 1970s, the dominant paradigm of how the Earth’s environment worked was that it was governed by a combination of two forces: geology (such as plate tectonics and volcanoes) and astronomy (the sun and asteroid impacts). The assumption was that these two forces were so overwhelmingly powerful that life could do little but passively adapt itself to them. But the Gaia hypothesis drew attention to what a powerful role life actually played in shaping Earth’s environment. Once this was pointed out to them, geophysicists recognized that it was obviously true, although it hadn’t been evident to them before. With the benefit of this new perspective, they began seeing all kinds of ways in which life had radically transformed the Earth: not just its oceans and atmosphere, but also its rocks and minerals, and perhaps even its crust as well.

  Geologists now recognize that most of the types of minerals on Earth wouldn’t exist if it wasn’t for life. This is because they need oxygen to form, and it was life that put sufficient levels of oxygen in the atmosphere to allow this to happen. In fact, the Earth boasts far more types of minerals than any other planet in the solar system. Mineral diversity seems to be a signature of life.

  Life also enormously accelerates the production of clay by causing rocks to erode more quickly, and this then plays numerous roles in geophysical processes, by trapping and burying huge amounts of carbon biomass. The clay also acts as a lubricant, softening and hydrating the crust of the Earth, which facilitates the sliding of tectonic plates. Some geologists speculate that, without life, the process of plate tectonics (and therefore the movement of the continents) might have shut down long ago.

  So, geophysicists came to realize that life hadn’t merely adapted to geology; it had altered it as much as it had been altered by it. The two had co-evolved together. By the mid-1980s, this Gaia-inspired study of the interlocking series of relationships between the geosphere and biosphere had developed into the branch of geophysics known as Earth system science.

  Admittedly, Earth system science is a weaker version of Gaia. It makes no claim that life deliberately maintains conditions favourable for itself. Sticklers might argue that, for this reason, it’s not really Gaia. But Lovelock certainly viewed the two as being equivalent, and many practitioners of Earth system science have been happy to acknowledge and defend the influence of the Gaia hypothesis on their work.

  So, this is the compromise that’s been worked out. According to biologists, Gaia is dead, and they take credit for killing it. But if you talk to geophysicists, the hypothesis is still going strong. In fact, it serves as a fundamental paradigm in their discipline. They just call it Earth system science rather than Gaia.

  What if we’ve already found extraterrestrial life?

  Does life exist elsewhere in the universe, or is the Earth its only home? This is one of the great questions people have pondered for centuries. Making contact with an extraterrestrial civilization would provide the most satisfying answer to this mystery, but, failing that, many scientists would settle for just finding plain old microbes on another planet. This discovery, if it were made, could address basic questions about the place of life in the universe, such as whether it’s a one-in-a-trillion chance event, or if it tends to arise wherever possible. It could also enormously advance biological knowledge by giving us something to compare Earth-based life against. For these reasons, the search for extraterrestrial life has always been a major focus of space agencies like NASA.

  But, according to one theory, the question of whether life exists elsewhere has already been definitively answered, in the affirmative. This theory has nothing to do with little green men who ride around in flying saucers abducting people and occasionally creating crop circles. Instead, it focuses on the two American Viking landers that, in 1976,
became the first crafts to land successfully on Mars. They carried equipment designed to test for the presence of microbial life in the Martian soil. According to NASA’s official statements, and the consensus view of most scientists, the landers found no conclusive evidence of such life.

  Dr Gilbert Levin, however, strongly disagrees. For several decades, he’s been waging a campaign to convince the scientific community that the landers actually did conclusively detect life, but that NASA, for various reasons, has been unwilling to admit this. Levin is in a position to address this issue authoritatively as he was one of the researchers who designed the life-detection equipment carried by the probes.

  Levin got his start as a sanitary engineer. A sewage specialist. This may seem far afield from the world of NASA, and it was, but it meant that he spent a lot of time thinking about microbes because one of his first job responsibilities was to test things such as swimming pools for bacterial contamination. Back in the early 1950s, the way of doing this was time-consuming, taking several days to complete. Frustrated by this slowness, Levin invented a faster way to do it. He called it his Gulliver test, since, like Jonathan Swift’s Gulliver, it found tiny beings.

  His invention exploited the fact that all microbes need to eat and excrete. They take in nutrients, process them and then expel them as gaseous waste. Levin realized that it would be possible to detect the presence of microbes by testing whether a liquid nutrient was being converted into a gas. He did this by lightly lacing a nutrient broth with a radioactive isotope, and then squirting the broth onto or into a sample of whatever needed to be tested. If there were microbes present, they would eat the nutrients and expel them as a gas, and, because the nutrients had been radioactively labelled, a Geiger counter would detect the atmosphere above the sample growing more radioactive – a sure sign of metabolic activity, and therefore of microbes.

  His Gulliver device worked like a charm. It sniffed out bacterial contamination in mere minutes or hours, rather than days. It was also exquisitely sensitive, able to detect even the slightest contamination.

  In 1954, Levin heard that NASA was looking for equipment capable of detecting life on Mars, so he submitted his invention and, to his delight, and against fierce competition, it was eventually chosen. It took many more years to make it lightweight and compact enough to fit on a spacecraft. NASA also renamed it the ‘labelled response’ (LR) test, because that sounded more scientific. But, when the two Viking landers successfully descended onto the surface of Mars in 1976, Levin’s test was part of their on-board package of biological experiments designed to look for extraterrestrial life.

  Before the missions left Earth, the NASA scientists had established strict criteria about what would count as the successful detection of life. They decided that, if a biological experiment generated a positive result, it would then need to be validated by a second control experiment, in which the same test was performed on Martian soil that had been sterilized by heating it to 160 degrees Celsius for three hours. If the sterile sample produced no response, this would be interpreted as compelling evidence that a biological agent had caused the positive result in the first experiment and that life had been detected.

  A few days after landing, the first Viking lander shovelled a sample of soil into the test chamber. Levin’s automated equipment then squirted it with radiated nutrient broth and everyone back at NASA waited with baited breath to see what would happen. Soon, the Geiger counter recorded a rapidly rising level of radiation in the chamber. It was an unambiguously positive response. But, next, the control experiment had to be run. In another chamber, Martian soil was sterilized and then tested. This time, the Geiger counter recorded no change in the atmospheric radiation level. The conclusion seemed clear. The pre-mission criteria had been met. Life had been detected.

  The second Viking lander, which carried identical equipment, had descended onto the Martian surface 4,000 miles away. When it subsequently ran the same sequence of experiments, it produced the same data. With these results in hand, Levin and the other NASA scientists began popping champagne to celebrate. It seemed like a momentous occasion in the history of science. The Earth was no longer the only known home to life in the universe.

  But the celebration was to be short-lived. A few days later, the NASA scientists changed their minds and decided that no life had been detected, after all. The problem was that the other on-board experiments had produced results far more ambiguous than Levin’s LR test.

  The Viking landers carried two other life-detection experiments on board. The gas-exchange experiment looked for possible metabolic activity by measuring whether Martian soil, when wetted, would produce oxygen. The tests indicated that it did, but so rapidly that the response seemed more chemical than biological. And, when sterilized, the Martian soil still produced oxygen. This suggested no life.

  Then there was the pyrolytic-release test. This measured whether anything in Martian soil would respond to artificial sunlight by absorbing radioactively tagged carbon from the air. If it did, this would indicate the possible synthesis of organic compounds by microbes. The equipment measured a small positive response, which was intriguing, but the designer of the test, Norman Horowitz, eventually decided that it just wasn’t enough of a response to indicate life. Perhaps a peculiarity of the soil had caused the result.

  The really damning result, however, came from a fourth experiment, called the gas chromatograph–mass spectrometer (GCMS), designed not to test directly for life, but rather for the presence of organic compounds, which are the carbon-based building blocks out of which all known living organisms are fashioned. Its results came back entirely negative, which surprised everyone. The assumption had been that at least a few organic compounds should have been present in the soil. But the GCMS indicated there were none at all.

  The larger context also had to be taken into consideration. Mars just didn’t seem like the kind of place that could support life. Temperatures there were well below freezing, the atmosphere offered no protection against ultraviolet radiation, and the environment was bone dry.

  The positive results of Levin’s LR test were therefore eventually called into question. Yes, something in the Martian soil had definitely caused the liquid broth to transform into a gas, but many of the NASA researchers felt that the response had occurred too quickly to be biological. They hypothesized that, if Martian soil contained a chemical such as hydrogen peroxide, this could have produced the observed reaction.

  Given all these facts, the conclusion seemed disappointing, but unavoidable. There was no life on Mars. Gerald Soffen, the chief Viking scientist, announced this to the public at a press conference in November 1976, and it’s remained the official stance ever since.

  At first, Levin toed the party line. He sat quietly during the NASA press conference, even as Jim Martin, the Viking mission manager, elbowed him in the ribs and whispered in his ear, ‘Damn it, Gil, stand up there and say you detected life!’

  But, as the years passed, his dissatisfaction grew. He didn’t think it right that people were being told that Mars was a lifeless planet when, so he believed, his experiment had clearly indicated otherwise. He also felt that the no-life verdict had led to a loss of public interest in Mars. In 1997, he finally went public with his dissent, declaring outright, ‘the Viking LR experiment detected living microorganisms in the soil of Mars.’ Ever since then, he’s been a vocal thorn in the side of NASA.

  Levin raises a series of technical issues to cast doubt on the no-life theory. First and foremost, he insists that a chemical agent couldn’t have produced the results shown by his LR test. After all, heating the soil to 160 degrees Celsius stopped the reaction. This suggests that the heat killed whatever organism was producing the gas. Most chemicals, on the other hand, wouldn’t have been affected by that temperature. Hydrogen peroxide, which the official explanation attributed the positive result to, certainly wouldn’t have been. That was the entire point of heating the soil – to differentiate between a biologic
al and chemical agent.

  In 2008, the Phoenix lander did find perchlorate in the Martian soil. Like hydrogen peroxide, it’s a powerful oxidant that could have produced a positive result. But Levin notes that perchlorate also wouldn’t break down at 160 degrees Celsius. Therefore, its presence doesn’t rule out the possibility of microbial life.

  Levin also notes that it was the GCMS that tipped the balance in favour of the no-life conclusion by failing to find organic compounds, but subsequent experimentation on Earth has revealed that the GCMS had serious shortcomings. It failed to find organic compounds in both Chile’s Atacama Desert and in Antarctic soil, even though they were certainly there. And, in 2012, NASA’s Curiosity rover did detect organic compounds in Martian soil, further calling into question the GCMS results.

  Levin has even suggested that there may be visual evidence of life on Mars. As early as 1978, he pointed out that some of the pictures taken by the Viking landers appeared to show ‘greenish patches’ on Martian rocks. A few of these patches, he claimed, shifted position throughout the Martian year. Sceptics have dismissed this as just a trick of the light, but he maintains that there may be some kind of bacterial substance growing in plain sight on the Martian rocks.

  He also offers a broader argument, which focuses on the hardiness of life. Scientists once viewed life as being fragile, able to survive in only a limited range of environmental conditions. Given this belief, it wasn’t surprising that NASA scientists in the 1970s concluded that life didn’t exist on Mars – at least, not in the places where the Viking landers looked for it. But, since the 1970s, the scientific understanding of life’s toughness has changed dramatically. Researchers now realize that life exists just about everywhere on Earth. They’ve found microbes surviving in the coldest parts of Antarctica, high up in the stratosphere, at the lowest depths of the oceans and even kilometres deep beneath the ground. Life, we now know, possesses an amazing ability to thrive in even the most extreme conditions.