Seeking Life as We Know It
Albert Einstein once famously wondered whether God had a choice in how he created the universe. His unanswered question drives physics to this day.
The same question could be asked about the biological universe -- especially now that the rover Opportunity has found signs of ancient standing water on Mars.
NASA’s search for alien life is based on the strategy “follow the water,” and for obvious reasons.
The only life we know is built on a scaffolding of carbon that floats in bags of water. Bacteria or brontosaurus, we’re all made from the same basic recipe.
But did life have a choice? Could it have evolved from entirely different ingredients? In looking for water-based life in worlds beyond, are we making the mistake of peering into a mirror?
Why not life in ethanol? suggested Cornell University’s Roald Hoffmann, a Nobel laureate in chemistry. Or ammonia?
“Now life in liquid ammonia, that would be colorful,” said Hoffmann, explaining that metals can dissolve in ammonia, “giving bright blue solutions.”
And why does the scaffolding have to be carbon?
Why not silicon, its neighbor on the periodic table of elements?
“We’re so dumb about what life is because we only have one example,” said astrobiologist Chris McKay of NASA Ames Research Center at Moffett Field, near the Bay Area city of Mountain View. “It may be true that we sail through the universe and everything we find is carbon and water, but I would hesitate to conclude that based on the one example we have.”
As a practical matter, NASA’s strategy of following the water makes good sense.
“We don’t know how to do anything better,” McKay said. “We’re too stupid to look for things if we don’t know what they are.”
At $820 million, the twin rover missions have to look at what’s most likely. “If you had to bet, what would you bet on?” asked Stanford chemist Richard Zare.
Still, one has to wonder what else might be out there.
The search is complicated by the fact that scientists aren’t even sure what life is exactly. Bizarre new species are discovered on Earth all the time in the most unlikely places.
“We even have trouble understanding what’s alive and what’s dead,” Zare said. “People still wonder what a virus is.”
All life as we know it is spun from carbon-based threads swimming in water solutions. Both carbon and water have unique -- some say magical -- properties. Indeed, physics and chemistry strongly suggest that life might not have had a choice.
Water is the most eccentric of liquids. “It’s this elusive, magical, mystery molecule,” said James Garvin, lead scientist for the Mars exploration program at NASA headquarters in Washington.
On the face of it, water seems a rather silly molecule -- two hydrogen atoms attached to an oxygen atom in a way that looks like the head of Mickey Mouse. Even children know its chemical formula: H2O.
But the bonds it forms with itself and other molecules are anything but ordinary.
Atoms normally bond by sharing the negatively charged electrons that buzz around their positively charged nuclei, like people sharing popcorn at a movie.
In water, the oxygen shares one electron with each of its hydrogens, leaving four extras. These clump together as “lone pairs” that can grab onto other molecules like prehensile feet.
At the same time, the two positive hydrogen nuclei stick out the other side like arms. The “feet” of one water molecule grab the “arms” of the other, forming abnormally strong networks. Where one water molecule goes, the others tend to follow. Thus, water can climb tall trees -- hand over foot, as it were -- in defiance of gravity, carrying nutrients from the soil to the leaves.
Chemists say they would expect water to be a gas at room temperature because it’s made up of just a few light atoms. But the strong bonds make the molecules stick together in a liquid form.
Luckily, the bonds aren’t so sticky that they form a viscous gel -- something that Boston University physicist Eugene Stanley initially found perplexing. Water flows freely, he and others discovered, because water molecules stick to each other only briefly, let go, grab another partner -- whirling an ever-changing cast of partners around in a molecular square dance.
The upshot is that water stays watery over a remarkable range of temperatures (32 to 212 degrees Fahrenheit, to be exact).
This is a liquid bonanza for life, which seems to need some form of fluid to transport things from place to place. In solids, molecules stick together and can’t go much of anywhere. In gases, the molecules don’t get close enough to interact.
Water’s unbalanced geometry -- positive charges on one side, negative on the other -- also gives it a distinctively schizophrenic personality (although chemists, like psychiatrists, prefer the term bipolar). This makes it an excellent solvent.
One side of a molecule grabs on to negative charges; the other side grabs the positive. This pulls most things apart, so water can dissolve almost anything. (If things didn’t dissolve, they’d sink to the bottom, or rise to the top -- not good for a free flow of chemical reactions.)
Why doesn’t life just disintegrate altogether in water then? While water is one of the most strongly bipolar molecules, it is not the most reactive -- meaning it can make things fall apart (dissolve) without changing their composition (react). So the parts can be endlessly rearranged.
And as it turns out, the few things water doesn’t dissolve are equally important in assembling life’s building blocks. Water hates fat. “It won’t dissolve a spot of grease on my nice silk tie,” Stanley said.
Water herds these hydrophobic (water-hating) and hydrophilic (water-loving) molecules into structures such as cells. The hydrophobes point away from each other, while the hydrophiles look inward. “It’s like circling the wagons,” McKay said.
Water, in other words, gives living things outsides and insides. The hostile outside is kept at bay, while inside, the proteins behind nearly all of life’s mechanisms go about their business.
“You have 3,000 proteins, minimally, in every cell,” said University of Massachusetts biologist Lynn Margulis, “and every reaction requires water. Everything else is negotiable.”
What’s the water doing with the proteins exactly? “Everything,” Margulis said. “It’s like a loom that you can do the weaving in. It’s the matrix that’s holding things in place. Nothing can go on without it.”
The magical molecule does a whole lot more: For example, it absorbs heat slowly, and holds on to it for a long time. This stabilizes temperatures not only in the oceans, but also inside living things -- which, lest we forget, are made mainly of water.
Finally, water expands when it freezes, contrary to nearly every other substance known. That’s why ice floats, allowing it to form an insulating blanket on lakes and ponds for life beneath. Without it, fish would freeze before they hit the grocer’s shelves.
Of course, it’s hard to ignore one obvious reason life may depend on water. Hydrogen is the most abundant element in the universe. Helium is the second, but it’s inert -- so standoffish it doesn’t bond with other atoms at all. Oxygen comes third. Maybe life is made of water simply because it’s there.
But some otherwise habitable worlds just don’t have water. Are they out of luck?
Not necessarily. “Water’s a wonderful molecule,” McKay said, “but there are other wonderful molecules.”
Ethanol, or grain alcohol, would probably work, concurred UCLA chemist Ken Houk. Proteins and nucleic acids are soluble in ethanol. But the liquid is rare in nature because the chemistry needed to produce it is complicated.
In contrast, water “is the easiest fluid to make,” Garvin said.
As for ammonia (used in smelling salts), it’s scarce on Earth, but “you could easily have an ocean of ammonia,” Houk said. In fact, scientists speculate that Saturn’s moon Titan could have such an ocean. Life could certainly exist at the cold temperatures at which ammonia is liquid (between minus 28 degrees and minus 108 degrees on Earth). Like water, ammonia is polar, and an excellent solvent.
Even if water does turn out to be the beverage of choice for quenching life’s insatiable thirst, does that mean carbon has to be in the mix too?
Many scientists think it does.
“I feel more strongly about carbon than about water,” said David Des Marais, an astrobiologist at NASA Ames Research Center.
Again, there’s an abundance argument. Carbon is the fourth-most common element. And life grabs the ingredients at hand.
Carbon also has unique properties that allow it to form long chains and rings easily.
Think of carbon as a small atom with four Velcro (actually electronic) attachment points. One, two or three of these can form links with other atoms, giving carbon enormous versatility.
Almost anything can find a way to attach. So carbon just naturally makes the kinds of complex molecules life needs.
Like water, carbon is a Goldilocks substance: It forms strong, stable bonds, but not so strong that those bonds can’t break off and attach to something else. “You have this kind of texture,” Margulis said, “a range of properties that change in very subtle ways.”
Carbon’s closest competitor, silicon, is not so subtle. Sitting right below carbon on the periodic table of elements, it also has four attachment points, but it’s heavier and has different chemical properties.
It can make long chains if you add oxygen, for example. But then everything it touches turns to stone. “It locks on to things, and folks, it’s over,” Zare said. “It’s very hard to break the bonds. It’s like rigor mortis.” So virtually any attempt at metabolism as we know it would produce something solid.
Solid silicon compounds are already familiar -- as rocks, glass, gels, bricks and, of course, medical implants.
Life seems to have ignored silicon, except here and there as structural material in rice, grasses and microscopic algae. How ironic, Hoffmann noted, that the silicon worlds we build ourselves (computers, electronics) now dominate our lives. “This is silicon’s revenge!”
If there were such a thing as silicon life, it would have to be built on an entirely different biological model. It probably would be stiff -- unable to breathe, for example, as we do.
“You’d have to give up not just carbon but the whole pattern,” McKay said. “We live as bags of liquid. A better model [for silicon life] is more like computers, a rigid life form that gets its energy from some electrochemical means directly.”
Just because we do our chemistry on the inside, he said, doesn’t mean all life does. Silicon life might do its chemistry on the surface.
But if silicon life appeared on ancient Earth along with carbon life, as some speculate (rather wildly) that it might have, it wouldn’t stand a chance from an evolutionary perspective.
“You might be able to make living things out of different materials,” said UCLA planetary scientist David Paige. “But I’m comfortable with the idea that the life we are is the best that we could do given the constraints of our environment and the laws of physics and chemistry.”
Those laws of physics and chemistry apply to the entire universe, so life elsewhere, Paige speculates, might well look familiar. “If we find a planet that’s covered with water, the life forms are likely to look like fish, because there’s a good reason fish look like fish and dolphins and submarines.”
Of course, life can’t spring from carbon and water alone.
At a minimum, life also needs some form of energy -- the kind we use from the sun, or the heat of radioactive decay from deep inside the Earth, or tidal friction that comes from being a large moon (like Titan) orbiting a large planet.
Life, at its essence, is a mechanism for turning energy into order.
Many purely physical processes do that as well: Gravity herds stars into galaxies. The late Columbia University physicist Gerald Feinberg and New York University biochemist Robert Shapiro speculated that what they called “physical life” could exist in solid hydrogen, in neutron stars, even in interstellar clouds, living on the energy of radiation. This “radiant life” would consist of individual beings they called “radiobes.”
“It may be difficult to think of such systems of being alive,” they acknowledged in an article included in the collection “Extraterrestrials: Where Are They?” But our own biochemistry -- based on proteins and nucleic acids -- does little “to convey the wonders, such as elephants and Sequoia trees, that ultimately arise from it.”
Would we recognize these alternative life forms if we saw them? Probably not.
“Our imagination is biased by what we’re able to see,” Paige said. “We can’t be as clever as the universe. So we have to be careful.”
One of the mistakes of the 1976 Viking missions to Mars, Paige said, was looking for life that was “too lifelike.” Life, for example, that eats familiar kinds of food, thrives in similar environments.
Since that time, scientists have discovered bizarre new biological worlds of so-called extremophiles on Earth, thriving in places where life was thought to be impossible -- such as boiling-hot vents at the bottom of the ocean, shut off from sunlight, subsisting on hydrogen sulfide.
These life forms (giant tube worms, for example) came as a complete surprise. Now, many scientists believe they may be our earliest ancestors.
More surprises are certainly in store. “We still don’t understand how life works,” Houk said. “It’s utterly miraculous. Even though it’s sitting there and staring us in the face, we don’t understand it.”