A New History of Life (5 page)

There have been a number of models specifically derived to deduce past O
₂
and CO
₂
levels through time, with the set of equations referred to as GEOCARB being the oldest and most elaborate.
¹⁶
This model, used for calculating levels of carbon, was devised by Robert Berner of Yale University. In addition to GEOCARB, separate models have been developed by Berner and his students for calculating O
₂
. Together, the models show the major trends in O
₂
and CO
₂
through time. This work represents one of the great triumphs of the scientific method. The importance of the rise and fall of oxygen and carbon dioxide over time is really one of the newest and most fundamental of understandings about life’s history on Earth.

Some believe that by 4 billion years ago, conditions and materials on Earth were correct for life to form. But the fact that a planet is habitable does not automatically mean that it will ever be inhabited. The formation of life from nonlife, the subject of the next chapter, appears to have been the most complex chemical experiment of all time. While astrobiologists seem to constantly refer to how “easy” it must have been to start life on Earth, a more nuanced look implies anything but.

Almost more than any other aspect, it has become clear that the interplay and concentrations of the various components of the Earth’s atmosphere have been dominant determinants of not only what kind of life (or there being any life at all) on our Earth, but the history of that life. The increasing acceptance of the dominant roles of oxygen and carbon dioxide levels in understanding not only large-scale patterns but nuances of life’s progression on our planet is in many respects a twenty-first-century innovation in interpreting Earth history. As is the understanding that two other important gases have played dominant roles in the story of life, and in the pages to come: hydrogen sulfide, or H
₂
S, and methane (CH
₄
). Their stories are written in rock, life, and death as well.

In 2006, word began to leak out into scientific circles of a most curious set of experiments dealing with life, death, and what appeared to be a strange and unsettling mixture of the two. First germinating as rumors among colleagues, then slowly maturing at successive talks at various scientific conferences, these findings came into full flower in a series of brilliant papers written by an until-then-unremarkable biologist. Mark Roth was not to remain unremarkable for long, especially after the MacArthur Foundation awarded him a so-called genius grant in 2010 for this work. He is a pioneer, entering a far country, one that could tell us a great deal not only about what “life” is, but what “living” is, and if there could be one without the other not only now, but during the long-ago time when life on Earth first came alive.

Roth had discovered that sublethal doses of hydrogen sulfide put mammals into a state that can only be described as suspended animation.
¹
While there is a great deal of popular-culture baggage attached to this appellation (mainly from the science fiction world), in fact these two words quite nicely describe what took place in these gassed animals. Animation, or movement, stopped not only in the observable aspects of the study animals—they no longer moved, had a greatly slowed down respiration rate and heart rate—but also at even more fundamental levels. Normal tissue and cellular functions were greatly reduced in rate. And then even something more surprising occurred: the mammals lost their ability to thermoregulate. They stopped being endothermic, or warm-blooded, and reverted to the more primitive chordate state: ectothermy, or cold-bloodedness. But they were neither dead nor truly alive, for in one of the most basic of mammalian characteristics, they were as if dead. But that death was temporary. It was suspended for a finite amount of time, for when the application
of the gas ceased, all normal functions returned. Beyond the obvious medical applications, this new understanding says much about what life is—and is not.

Roth’s hunch was simple—that there exists a state between life and death that is both unexplored and of potential medical interest—and it also provided clues to why certain organisms survived mass extinctions. Perhaps death is not so final as generally assumed.
²
His hope was to be able to take organisms to this place and then bring them back. In fact there is no English word that accurately captures the essence of this place. Moviemakers call it zombie land or some such, and maybe stiff-necked science will eventually adopt that term. But we doubt it.

Here was one of his critical experiments. He took flatworms, simple animals, but animals nonetheless. Yet compared to any microbe, no animal can be called simple. He lowered the oxygen content that the flatworms were respiring. Like all animals, flatworms need oxygen, and lots of it. So down went the oxygen content in the closed vessel with the confined worms, and gradually they slowed and then ceased motion. No poking or prodding could get any sort of reaction. But Roth did not conclude the experiment there. In fact, he kept dropping the oxygen content of the worm’s water, and they came back to life.
³
The flatworms had entered the state of “dormancy” that is neither alive nor dead. Life and death seem to be far more complicated states than most of us currently believe.

Mammals are among the most complex of all animals. Even in these experiments, interesting as they are, the test subjects were obviously alive: their hearts still beat, blood continued to flow in veins and arteries, nerves fired, and the ion transport necessary for life continued to function, if at slower rates. But questions remain about the workings of life in much less complicated and smaller organisms, such as bacteria and viruses, especially when they are put into environments without gas, or in very cold environments. These are not theoretical questions,
because every day microbes are flung skyward into the highest reaches of the Earth’s atmosphere by violent storms, and find themselves so high that the Earth’s protective ozone layer—our major defense against ultraviolet radiation coming from space—can no longer screen them. This is the second frontier in the study of life and death: the study of the Earth’s highest life.

After spending days or weeks in the upper atmosphere, these members of the most newly discovered ecosystem on Earth, one not so subtly named “high life” by the scientists who now study this tropospheric biota, come back to Earth.
⁴
But when they are in space, are they alive?

While it has been known since the dawn of the space age that bacterial and fungal spores could be found at some of the highest altitudes achievable by aircraft, there was very little appreciation of just how many different species can be found in this largest of Earth’s habitats, a volume of space utterly dwarfing the volume of the second-largest habitat, the top to bottom of the oceans. But work begun in 2010 demonstrated that at any given time there might be thousands of
species
of bacteria, fungi, and untold viral taxa. It was also discovered by a University of Washington team, sniffing air high atop a mountain in the Cascades of Oregon, that Chinese dust storms routinely drop fungi, bacteria, and viruses onto the West Coast of North America.
⁵

Yet beyond an intrinsic biological interest that microbes can be found so high in the atmosphere (or that the atmosphere could be a transport system sending us intercontinental, weaponized viruses), there is a new fundamental understanding that is part of the story of this book: atmospheric transport of life may be how the first life on Earth dispersed away from its site of origin. Why slowly float in an ocean, captive of capricious waves and current, when one can jump from continent to continent through the air in less than a day. Later we will return to the implications of high life for the history of life on Earth; here the issue is whether they are constantly alive during their atmospheric, intercontinental travel, or if they are in dormancy. Here, at the fundamentally basic kind of life, we are finding that the
categories of life and death are rather incomplete, if not disingenuous concepts.

High life is collected in three ways: from retired U.S. military high-altitude spy planes, from high-altitude balloons, and when great storms lift off Asia and pass over the Pacific Ocean, and sufficiently “dent” the atmosphere so that air “sniffers” on high mountains can catch a whiff of a descended troposphere. In that air is a zoo full of microbial life. When collected from the immense atmospheric altitudes where cells and viruses are now known to commonly occur, the bacteria are dead. But when brought back to Earth and given some time to react to the altitude they presumably evolved at, they come back to life.

Most of us would agree that for mammals, and perhaps all animals, dead is
dead
. But in simpler life, such is not the case. It turns out that there is a vast new place to be explored between our traditional understanding of what is alive and what is not. And this newly discovered region has important implications about the first chapter in the history of life on Earth, telling us whether “dead” chemicals, when correctly combined and energized, could become alive. Life, simple life at least, is not always alive. But now science seeks to find out if there is a place in between. It could be that the first life on Earth came from the place we call death, or from someplace closer to being alive.

The question “What is life?” is the title of several books, the most famous by the early twentieth-century physicist Erwin Schrödinger.
⁶
This short book was a landmark, not just for what was written, but also because of the scientific discipline of its author. Schrödinger was a physicist, and before and during his life, the study of biology had been scorned by physical scientists as not worthy of study. Schrödinger began to think of organisms as a physicist would, in physical terms: “The arrangement of the atoms in the most vital parts of an organism and the interplay of these arrangements differ in a fundamental way from all those arrangements of atoms which physicists and
chemists have hitherto made the object of their experimental and theoretical research.” While much of the book dealt with the nature of heredity and mutation (for this book was written twenty years before the discovery of DNA, when the nature of inheritance was still a perplexing mystery), it is late in the book that Schrödinger considered the physics of “living,” when he wrote: “Living matter evades the decay to equilibrium,” and life “feeds on negative entropy.”

Life does this through metabolism, overtly by eating, drinking, breathing, or the exchange of material, which forms the root of the word from its original Greek definition. Is this the key to life? Perhaps, to a biologist, at least. But Schrödinger, the physicist, saw something much more profound: “That the exchange of material should be the essential thing is absurd. Any atom of nitrogen, oxygen, sulfur, etc. is as good as any other of its kind; what could be gained in exchanging them?” What then is that precious “something” that we call life, contained in our food, which keeps us from death? To Schrödinger, that is easily answered. “Every process, event, happening that is going on in nature means an increase of the entropy of the part of the world where it is going on. Thus a living organism continually increases its entropy.” This, then, was his secret of life: life was matter that created an increase in entropy, and in this, a new way of comparing living to nonliving was made.

To Schrödinger, then, life is maintained by extracting “order” from the environment, something that he called (with the self-avowed awkward expression) “negative entropy.” Life was thus the device by which large numbers of molecules maintain themselves at fairly high levels of order by continually sucking this orderliness from their environment. Schrödinger suggested that organisms not only created order from disorder but order from order.

Is that all that life is—a machine that changes the nature of disorder and order? From the physics point of view, life could be understood as a series of chemical machines, all packed together and somehow integrated, maintaining order by expending energy to do so. For decades this view was the most influential of all concerning the definition of life. But a half century later, others began to question and
amend these views. Some were, like Schrödinger, physicists, such as Paul Davies and Freeman Dyson. But others were trained biologists.

Paul Davies, in his book
The Fifth Miracle
,
⁷
approached the question “What is life?” by using a different question: what does life
do?
It is
actions
that define life, according to his argument. These main actions are as follows:

Life metabolizes
. All organisms process chemicals, and in so doing bring energy into their bodies. But of what use is this energy? The processing and liberation of energy by an organism are what we call metabolism, and they are the way that life harvests the negative entropy that is necessary to maintain internal order. Another way of thinking about this is in terms of chemical reactions. If the organism moves from this state of performing chemical reactions on their own (not in the body of the organism) to a state where the reactions stop, the organism has ceased to be alive. Not only does life maintain this unnatural state, but it also seeks out environments where the energy necessary to stay in this state can be found and harvested. Some environments on Earth are more amenable to life’s chemistry than others (such as a warm, sunlit ocean surface of a coral reef or a hot spring in Yellowstone National Park), and in such places we find life in abundance.

Life has complexity and organization.
There is no really simple life, composed of but a handful of (or even a few million) atoms. All life is composed of a great number of atoms arranged in intricate ways. It is organization of this complexity that is a hallmark of life. Complexity is not a machine. It is a property.