Life: is it inevitable or just a fluke?

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Life: is it inevitable or just a fluke?

• 25 June 2012 by Nick Lane
• Magazine issue 2870. Subscribe and save
• For similar stories, visit the Evolution Topic Guide

If life arises wherever conditions are right, why haven't we heard from aliens?

See gallery: "Prime locations for life in our solar system"

UNDER the intense stare of the Kepler space telescope, more and more planets similar to our own are revealing themselves to us. We haven't found one exactly like Earth yet, but so many are being discovered that it appears the galaxy must be teeming with habitable planets.

These discoveries are bringing an old paradox back into focus. As physicist Enrico Fermi asked in 1950, if there are many suitable homes for life out there and alien life forms are common, where are they all? More than half a century of searching for extraterrestrial intelligence has so far come up empty-handed.

Of course, the universe is a very big place. Even Frank Drake's famously optimistic "equation" for life's probability suggests that we will be lucky to stumble across intelligent aliens: they may be out there, but we'll never know it. That answer satisfies no one, however.

There are deeper explanations. Perhaps alien civilisations appear and disappear in a galactic blink of an eye, destroying themselves long before they become capable of colonising new planets. Or maybe life very rarely gets started even when conditions are perfect.

If we cannot answer these kinds of questions by looking out, might it be possible to get some clues by looking in? Life arose only once on Earth, and if a sample of one were all we had to go on, no grand conclusions could be drawn. But there is more than that. Looking at a vital ingredient for life - energy - suggests that simple life is common throughout the universe, but it does not inevitably evolve into more complex forms such as animals. I might be wrong, but if I'm right, the immense delay between life first appearing on Earth and the emergence of complex life points to another, very different explanation for why we have yet to discover aliens.

Read more: "Timeline: The evolution of life"

Living things consume an extraordinary amount of energy, just to go on living. The food we eat gets turned into the fuel that powers all living cells, called ATP. This fuel is continually recycled: over the course of a day, humans each churn through 70 to 100 kilograms of the stuff. This huge quantity of fuel is made by enzymes, biological catalysts fine-tuned over aeons to extract every last joule of usable energy from reactions.

The enzymes that powered the first life cannot have been as efficient, and the first cells must have needed a lot more energy to grow and divide - probably thousands or millions of times as much energy as modern cells. The same must be true throughout the universe.

This phenomenal energy requirement is often left out of considerations of life's origin. What could the primordial energy source have been here on Earth? Old ideas of lightning or ultraviolet radiation just don't pass muster. Aside from the fact that no living cells obtain their energy this way, there is nothing to focus the energy in one place. The first life could not go looking for energy, so it must have arisen where energy was plentiful.

Today, most life ultimately gets its energy from the sun, but photosynthesis is complex and probably didn't power the first life. So what did? Reconstructing the history of life by comparing the genomes of simple cells is fraught with problems. Nevertheless, such studies all point in the same direction. The earliest cells seem to have gained their energy and carbon from the gases hydrogen and carbon dioxide. The reaction of H₂ with CO₂ produces organic molecules directly, and releases energy. That is important, because it is not enough to form simple molecules: it takes buckets of energy to join them up into the long chains that are the building blocks of life.

A second clue to how the first life got its energy comes from the energy-harvesting mechanism found in all known life forms. This mechanism was so unexpected that there were two decades of heated altercations after it was proposed by British biochemist Peter Mitchell in 1961.

Universal force field

Mitchell suggested that cells are powered not by chemical reactions, but by a kind of electricity, specifically by a difference in the concentration of protons (the charged nuclei of hydrogen atoms) across a membrane. Because protons have a positive charge, the concentration difference produces an electrical potential difference between the two sides of the membrane of about 150 millivolts. It might not sound like much, but because it operates over only 5 millionths of a millimetre, the field strength over that tiny distance is enormous, around 30 million volts per metre. That's equivalent to a bolt of lightning.

Mitchell called this electrical driving force the proton-motive force. It sounds like a term from Star Wars, and that's not inappropriate. Essentially, all cells are powered by a force field as universal to life on Earth as the genetic code. This tremendous electrical potential can be tapped directly, to drive the motion of flagella, for instance, or harnessed to make the energy-rich fuel ATP.

However, the way in which this force field is generated and tapped is extremely complex. The enzyme that makes ATP is a rotating motor powered by the inward flow of protons. Another protein that helps to generate the membrane potential, NADH dehydrogenase, is like a steam engine, with a moving piston for pumping out protons. These amazing nanoscopic machines must be the product of prolonged natural selection. They could not have powered life from the beginning, which leaves us with a paradox.

Life guzzles energy, and inefficient primordial cells must have required much more energy, not less. These vast amounts of energy are most likely to have derived from a proton gradient, because the universality of this mechanism means it evolved early on. But how did early life manage something that today requires very sophisticated machinery?

There is a simple way to get huge amounts of energy this way. What's more, the context makes me think that it really wasn't that difficult for life to arise in the first place.

The answer I favour was proposed 20 years ago by the geologist Michael Russell, now at NASA's Jet Propulsion Laboratory in Pasadena, California, who had been studying deep-sea hydrothermal vents. Say "deep-sea vent" and many people think of dramatic black smokers surrounded by giant tube worms. Russell had something much more modest in mind: alkaline hydrothermal vents. These are not volcanic at all, and don't smoke. They are formed as seawater percolates down into the electron-dense rocks found in the Earth's mantle, such as the iron-magnesium mineral olivine.

Olivine and water react to form serpentinite in a process that expands and cracks the rock, allowing in more water and perpetuating the reaction. Serpentinisation produces alkaline - proton poor - fluids rich in hydrogen gas, and the heat it releases drives these fluids back up to the ocean floor. When they come into contact with cooler ocean waters, the minerals precipitate out, forming towering vents up to 60 metres tall. Such vents, Russell realised, provide everything needed to incubate life. Or rather they did, four billion years ago.

Back then, there was very little, if any, oxygen, so the oceans were rich in dissolved iron. There was probably a lot more CO₂ than there is today, which meant that the oceans were mildly acidic - that is, they had an excess of protons.

Just think what happens in a situation like this. Inside the porous vents, there are tiny, interconnected cell-like spaces enclosed by flimsy mineral walls. These walls contain the same catalysts - notably various iron, nickel and molybdenum sulphides - used by cells today (albeit embedded in proteins) to catalyse the conversion of CO₂ into organic molecules.

Fluids rich in hydrogen percolate through this labyrinth of catalytic micropores. Normally, it is hard to get CO₂ and H₂ to react: efforts to capture CO₂ to reduce global warming face exactly this problem. Catalysts alone may not be enough. But living cells don't capture carbon using catalysts alone - they use proton gradients to drive the reaction. And between a vent's alkaline fluids and acidic water there is a natural proton gradient.

Could this natural proton-motive force have driven the formation of organic molecules? It is too early to say for sure. I'm working on exactly that question, and there are exciting times ahead. But let's speculate for a moment that the answer is yes. What does that solve? A great deal. Once the barrier to the reaction between CO₂ and H₂ is down, the reaction can proceed apace. Remarkably, under conditions typical of alkaline hydrothermal vents, the combining of H₂ and CO₂ to produce the molecules found in living cells - amino acids, lipids, sugars and nucleobases - actually releases energy.

That means that far from being some mysterious exception to the second law of thermodynamics, from this point of view, life is in fact driven by it. It is an inevitable consequence of a planetary imbalance, in which electron-rich rocks are separated from electron-poor, acidic oceans by a thin crust, perforated by vent systems that focus this electrochemical driving force into cell-like systems. The planet can be seen as a giant battery; the cell is a tiny battery built on basically the same principles.

I'm the first to admit that there are many gaps to fill in, many steps between an electrochemical reactor that produces organic molecules and a living, breathing cell. But consider the bigger picture for a moment. The origin of life needs a very short shopping list: rock, water and CO₂.

Water and olivine are among the most abundant substances in the universe. Many planetary atmospheres in the solar system are rich in CO₂, suggesting that it is common too. Serpentinisation is a spontaneous reaction, and should happen on a large scale on any wet, rocky planet. From this perspective, the universe should be teeming with simple cells - life may indeed be inevitable whenever the conditions are right. It's hardly surprising that life on Earth seems to have begun almost as soon as it could.

Then what happens? It is generally assumed that once simple life has emerged, it gradually evolves into more complex forms, given the right conditions. But that's not what happened on Earth. After simple cells first appeared, there was an extraordinarily long delay - nearly half the lifetime of the planet - before complex ones evolved. What's more, simple cells gave rise to complex ones just once in four billion years of evolution: a shockingly rare anomaly, suggestive of a freak accident.

If simple cells had slowly evolved into more complex ones over billions of years, all kinds of intermediate cells would have existed and some still should. But there are none. Instead, there is a great gulf. On the one hand, there are the bacteria, tiny in both their cell volume and genome size: they are streamlined by selection, pared down to a minimum: fighter jets among cells. On the other, there are the vast and unwieldy eukaryotic cells, more like aircraft carriers than fighter jets. A typical single-celled eukaryote is about 15,000 times larger than a bacterium, with a genome to match.

The great divide

All the complex life on Earth - animals, plants, fungi and so on - are eukaryotes, and they all evolved from the same ancestor. So without the one-off event that produced the ancestor of eukaryotic cells, there would have been no plants and fish, no dinosaurs and apes. Simple cells just don't have the right cellular architecture to evolve into more complex forms.

Why not? I recently explored this issue with the pioneering cell biologist Bill Martin of the University of Düsseldorf in Germany. Drawing on data about the metabolic rates and genome sizes of various cells, we calculated how much energy would be available to simple cells as they grew bigger (Nature, vol 467, p 929).

What we discovered is that there is an extraordinary energetic penalty for growing larger. If you were to expand a bacterium up to eukaryotic proportions, it would have tens of thousands of times less energy available per gene than an equivalent eukaryote. And cells need lots of energy per gene, because making a protein from a gene is an energy-intensive process. Most of a cell's energy goes into making proteins.

At first sight, the idea that bacteria have nothing to gain by growing larger would seem to be undermined by the fact that there are some giant bacteria bigger than many complex cells, notably Epulopiscium, which thrives in the gut of the surgeonfish. Yet Epulopiscium has up to 200,000 copies of its complete genome. Taking all these multiple genomes into consideration, the energy available for each copy of any gene is almost exactly the same as for normal bacteria, despite the vast total amount of DNA. They are perhaps best seen as consortia of cells that have fused together into one, rather than as giant cells.

So why do giant bacteria need so many copies of their genome? Recall that cells harvest energy from the force field across their membranes, and that this membrane potential equates to a bolt of lightning. Cells get it wrong at their peril. If they lose control of the membrane potential, they die. Nearly 20 years ago, biochemist John Allen, now at Queen Mary, University of London, suggested that genomes are essential for controlling the membrane potential, by controlling protein production. These genomes need to be near the membrane they control so they can respond swiftly to local changes in conditions. Allen and others have amassed a good deal of evidence that this is true for eukaryotes, and there are good reasons to think it applies to simple cells, too.

So the problem that simple cells face is this. To grow larger and more complex, they have to generate more energy. The only way they can do this is to expand the area of the membrane they use to harvest energy. To maintain control of the membrane potential as the area of the membrane expands, though, they have to make extra copies of their entire genome - which means they don't actually gain any energy per gene copy.

Put another way, the more genes that simple cells acquire, the less they can do with them. And a genome full of genes that can't be used is no advantage. This is a tremendous barrier to growing more complex, because making a fish or a tree requires thousands more genes than bacteria possess.

So how did eukaryotes get around this problem? By acquiring mitochondria.

About 2 billion years ago, one simple cell somehow ended up inside another. The identity of the host cell isn't clear, but we know it acquired a bacterium, which began to divide within it. These cells within cells competed for succession; those that replicated fastest, without losing their capacity to generate energy, were likely to be better represented in the next generation.

And so on, generation after generation, these endosymbiotic bacteria evolved into tiny power generators, containing both the membrane needed to make ATP and the genome needed to control membrane potential. Crucially, though, along the way they were stripped down to a bare minimum. Anything unnecessary has gone, in true bacterial style. Mitochondria originally had a genome of perhaps 3000 genes; nowadays they have just 40 or so genes left.

For the host cell, it was a different matter. As the mitochondrial genome shrank, the amount of energy available per host-gene copy increased and its genome could expand. Awash in ATP, served by squadrons of mitochondria, it was free to accumulate DNA and grow larger. You can think of mitochondria as a fleet of helicopters that "carry" the DNA in the nucleus of the cell. As mitochondrial genomes were stripped of their own unnecessary DNA, they became lighter and could each lift a heavier load, allowing the nuclear genome to grow ever larger.

These huge genomes provided the genetic raw material that led to the evolution of complex life. Mitochondria did not prescribe complexity, but they permitted it. It's hard to imagine any other way of getting around the energy problem - and we know it happened just once on Earth because all eukaryotes descend from a common ancestor.

Freak of nature

The emergence of complex life, then, seems to hinge on a single fluke event - the acquisition of one simple cell by another. Such associations may be common among complex cells, but they are extremely rare in simple ones. And the outcome was by no means certain: the two intimate partners went through a lot of difficult co-adaptation before their descendants could flourish.

This does not bode well for the prospects of finding intelligent aliens. It means there is no inevitable evolutionary trajectory from simple to complex life. Never-ending natural selection, operating on infinite populations of bacteria over billions of years, may never give rise to complexity. Bacteria simply do not have the right architecture. They are not energetically limited as they are - the problem only becomes visible when we look at what it would take for their volume and genome size to expand. Only then can we see that bacteria occupy a deep canyon in an energy landscape, from which they are unable to escape.

So what chance life? It would be surprising if simple life were not common throughout the universe. Simple cells are built from the most ubiquitous of materials - water, rock and CO₂ - and they are thermodynamically close to inevitable. Their early appearance on Earth, far from being a statistical quirk, is exactly what we would expect.

The optimistic assumption of the Drake equation was that on planets where life emerged, 1 per cent gave rise to intelligent life. But if I'm right, complex life is not at all inevitable. It arose here just once in four billion years thanks to a rare, random event. There's every reason to think that a similar freak accident would be needed anywhere else in the universe too. Nothing else could break through the energetic barrier to complexity.

See graphic: "Other worlds"

This line of reasoning suggests that while Earth-like planets may teem with life, very few ever give rise to complex cells. That means there are very few opportunities for plants and animals to evolve, let alone intelligent life. So even if we discover that simple cells evolved on Mars, too, it won't tell us much about how common animal life is elsewhere in the universe.

All this might help to explain why we've never found any sign of aliens. Of course, some of the other explanations that have been proposed, such as life on other planets usually being wiped out by catastrophic events such as gamma-ray bursts long before smart aliens get a chance evolve, could well be true too. If so, there may be very few other intelligent aliens in the galaxy.

Then, again, perhaps some just happen to live in our neighbourhood. If we do ever meet them, there's one thing I would bet on: they will have mitochondria too.

Nick Lane is the first Provost's Venture Research Fellow at University College London. His research on the origin of life is funded by the Leverhulme Trust

• From issue 2870 of New Scientist magazine, page 32-37.
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Studies suggest that microbes can control our minds

Studies suggest that microbes can control our minds

By David Ferguson
Tuesday, June 19, 2012 15:04 EDT

Topics: bacteria ♦ Scientific American

An increasing amount of evidence is coming to light suggesting that human moods, emotions and perceptions can be influenced by the type and number of microscopic life forms inhabiting our gut, according to an article in Scientific American magazine. Scientists say a time may even come when we treat mental illness and depression with probiotic supplements, and that the bacteria, viruses and fungi that inhabit our gut — making up what is called the gut’s microbiome — may be an indicator of our ability to handle stress and what diseases we will be vulnerable to over the course of our lives.

It has been observed in the animal kindgom that some microorganisms can influence behavior in their hosts. The single-celled protozoan Toxoplasmosis gondii has been shown to make rats less frightened of cats and perhaps even cause them to be drawn to the smell of cats’ urine. This makes the rat more likely to be eaten by a cat, whose digestive tract is a necessary stop in the organism’s reproductive cycle.

Some scientists, like Jaroslav Flegr, who was profiled in the Atlantic in March of this year, have theorized that Toxo can have a similar effect in humans, producing the “crazy cat lady” effect, wherein people become inured to the stench of cat wastes.  Flegr, who is himself infected with Toxo, has shown that people carrying the disease suffer from slower reaction times and impaired judgment.  Men who are infected with Toxo were shown to be more introverted, suspicious of other people’s motives and less concerned with others’ perceptions of them.  Infected women, on the other hand, were more “outgoing, trusting, image-conscious, and rule-abiding than uninfected women.”

The fungus Cordyceps invades the brains of ants and other insects and fills them with an irresistible urge to climb to the highest place they can find.  Once they’re in position, they die and a mushroom sprouts from their head, enabling the fungus to spread its spores over a much greater area than if its host were at ground level.

But can microbes living in our bodies influence the normal everyday behaviors of average people?  The answer may well be yes.

We know that the health and variety or a person’s enteric flora play a role in the body’s susceptibility to diseases like irritable bowel syndrome (IBS) and stomach ulcers.  Evidence is showing, however, that our emotional and mental health and well-being could be inextricably linked with the contents of our guts.

In a Japanese study, tests with rats have shown that rats bred without a normal microbiome of enteric organisms were less able to handle stress or to return to an unstressed state when unpleasant stimulus was removed.  Within a few weeks of being given probiotic supplements, however, these rats handled stress as well as rats who were born with regular enteric flora.

Humans are more complex than rats, of course, but in a 2011 French study, women test subjects who consumed a daily portion of fermented dairy products (like yogurt) that contained the bacteria Lactobacillus and B. longum, two bacteria commonly found in normal enteric flora, showed greater decreases in signs of psychological and emotional stress than women who ate a non-fermented dairy product or those who took nothing.  According to Scientific American, when researchers “scanned each woman’s brain, they found that compared with the two control groups, the participants given probiotics had significantly less resting-state activity — the brain’s firing patterns when thinking about nothing in particular — as well as a dampened response in their arousal networks, which includes the amygdala, in response to emotional faces.”
The magazine conceded that, ultimately, to read too much into the gut-brain connection could prove to be too simplistic, as it does not take into account the chemical and hormonal influences of other organs in the system. Other factors, including the environment and genetics, can affect the type and number of organisms living in the gut as well, with genetics taking a particular role given that the people with the most similar combinations of organisms in their micobiome are identical twins.

The relationship between gut flora and brain function may go beyond mood and our ability to handle stress, however.  In 2005, the Journal of Medical Microbiology published a study saying that the enteric flora of children with autism is markedly different from that of their healthier siblings, opening a previously unexplored avenue of autism research.

As far back as 1930, dermatologists John Stokes and Donald Pillsbury put forward the “gut-brain-skin axis” hypothesis connecting stress and skin irritations like acne.  Stokes and Pillsbury theorized that emotional states alter the gut’s assortment of microbes, increasing the digestive tract’s permeability and triggering skin inflammation.  The doctors prescribed doses of  Lactobacillus acidophilus, a microbe commonly added to milk products, a practice that has recently found new supporting evidence.  Studies have shown that “Lactobacillus soothes skin inflamed by stress and restores normal hair growth in mice.”

John Bienenstock of the Brain-Body Institute at McMaster University in Ontario told Scientific American that we’re only at the beginning of this line of inquiry.  He theorizes that bacteria on our skin may communicate with bacteria in our gut to influence our behavior.  “Could we have some microbial ointment that improves health and well-being?” he asked, “The mind boggles at the possibilities.”

["A Young Man Suffering From Food Poisoning From Eating A Bad Corn Dog" on Shutterstock]

David Ferguson

David Ferguson is an editor at Raw Story. He was previously writer and radio producer in Athens, Georgia, hosting two shows for Georgia Public Broadcasting and blogging at Firedoglake.com and elsewhere. He is currently working on a book.

 

Features | Mind & Brain

See Inside

Microbes Manipulate Your Mind [Preview]

Bacteria in your gut may be influencing your thoughts and moods

By Moheb Costandi  | June 22, 2012

The thought of parasites preying on your body or brain very likely sends shivers down your spine. Perhaps you imagine insectoid creatures bursting from stomachs or a malevolent force controlling your actions. These visions are not just the night terrors of science-fiction writers—the natural world is replete with such examples.

Take Toxoplasma gondii, the single-celled parasite. When mice are infected by it, they suffer the grave misfortune of becoming attracted to cats. Once a cat inevitably consumes the doomed creature, the parasite can complete its life cycle inside its new host. Or consider Cordyceps, the parasitic fungus that can grow into the brain of an insect. The fungus can force an ant to climb a plant before consuming its brain entirely. After the insect dies, a mushroom sprouts from its head, allowing the fungus to disperse its spores as widely as possible.

 

Image: Brian Stauffer

In Brief

Moody Microorganisms

• Bacteria and viruses dwelling in our gut produce compounds that can interact with our nervous system in ways that appear to affect our anxiety and stress responses.
• Early clinical trials suggest that bacterial remedies, such as probiotic supplements, may be useful in treating several types of psychological distress.
• Eventually individual assessments of gut microbial communities could allow physicians and researchers to tailor treatments for mental disorders.