by Ben Bova
Long-chain carbon molecules grew in that rich organic soup. Following ordinary laws of chemistry, carbon-rich compounds will link together to form ever more complex molecules. Biochemist Melvin Calvin called this process autocatalysis, the method by which carbon molecules spontaneously combine to form more complex assemblages. This process is also called chemical evolution. More recently, astrobiologists have used the term prebiotic chemistry: that is, chemical reactions that led to the creation of living organisms.
At the University of Chicago in 1953, Nobel laureate Harold C. Urey (1893–1981) and his student Stanley L. Miller tested the idea of chemical evolution. They filled a glass flask with the chemicals then thought to predominate in Earth’s primitive atmosphere: methane, ammonia, water vapor, and hydrogen. They applied energy to the mixture in the form of an electric spark. The gaseous mixture was then circulated through a water bath.
After a week, the flask had collected a dark brown gunk at its bottom. Chemical analysis showed that the goo contained several long-chain carbon-based molecules called amino acids, the basic components of proteins. Proteins are what living creatures are made of. Prebiotic chemistry! What the scientists were able to accomplish in a week with a few quarts of chemicals, nature could easily have done over millions of years with whole oceans for a laboratory.
Although amino acids could be created in the laboratory easily enough, no one has yet been able to produce the next step, combining amino acids into actual proteins. Moreover, geologists and astronomers have come to the conclusion that the Earth’s atmosphere was not what Urey and Miller thought it to be: There was little or no free hydrogen in it, even 4 billion years ago. The hydrogen that was originally part of our atmosphere either combined with heavier elements or quickly boiled away once the Sun began to shine.
So perhaps life did not originate in Darwin’s warm little pond, after all. Or even in an ocean full of organic soup. Remember, prebiotic chemistry can take place in ice grains in space. The question remains, however: How did these organic chemicals (regardless of where they first formed) come together to create living organisms?
As David Wolfe emphasizes in his book Tales from the Underground, the building blocks of life may have come together not in water, but in the wet layers of clay minerals at the water’s edge. Clays have “an incredibly high surface-to-weight ratio. A single gram—a pinch—of clay powder can have the surface area of a baseball diamond!” Wolfe’s point is that moist clays could have made an ideal substrate where carbon-chain molecules could have met and mingled, undergone autocatalytic prebiotic chemistry, and evolved into ever more complex organic molecules until the first living molecules were assembled.
Remember that our planet was still being pounded by asteroids and comets when the first steps toward life were taken. The rocks were hot, nearly the boiling point of water. Harsh, unfiltered ultraviolet light bathed the surface. It might well have been that life began below the surface, underground, where it could be protected from the dangers of the surface.
How biblical. In Genesis, God creates Adam out of clay. Wolfe points out that Adam’s very name is derived from the Hebrew word for soil or clay, adama, and the Latin name for man, homo, comes from humus, which means loosely, “of the soil or earth.”
WHAT IS LIFE?
How do we make the distinction between living organisms and nonliving chemicals? What is the basic difference between a baseball and a bacterium?
Fundamentally, living things can reproduce themselves out of simpler building materials. That is the difference between living creatures and nonliving things. A molecule of a living creature can reproduce itself from molecules that are simpler than itself.
A fire eats and grows and eventually dies, but it is not alive. It breaks down the stuff it feeds upon; it turns candles into melted wax and heated air, forests into charred, lifeless stumps. Life builds. Life grows. Life can metabolize simple inputs such as sunlight and water and carbon to create an orchid. Only life can do that.
In that organic soup of some 4 billion years ago, or perhaps in banks of clay at the water’s edge, chemical evolution of ever more complex molecules finally led to a form of molecule that could reproduce itself out of the simpler molecules around it.
Life began.
It was molecular life at first, microscopic collections of atoms, no bigger than a few thousandths of a centimeter. But life had taken root on Earth. It began to evolve, to adapt itself to the conditions in which it existed. And it began to change those conditions, subtly at first, but eventually in ways that have vastly altered our world.
As the Hungarian-American mathematician John von Neumann (1903–1957) aptly put it, life actually consists of two things: metabolism and reproduction. Living organisms—even though they are no more than molecular-sized—must be able to take in the energy they need to drive their life processes, and they must be able to reproduce themselves.
To accomplish these two goals, life on Earth consists of proteins and nucleic acids: DNA and RNA.13
The nucleic acids handle reproduction. DNA is the master blueprint; it is what genes are made of. Your DNA, your genes, determine your physical characteristics: eye color, body form, brain size, every physical trait that is uniquely you. At the heart of every cell of every creature on Earth are molecules of DNA—genes—that carry the blueprint for every protein of that creature. Every moment of your life your genes are producing exact copies of the thousands of proteins that make up your body. When your DNA fails to reproduce these proteins exactly, the result is disease, debilitation, aging, and death.
Inside your cells, RNA takes the blueprint information from the DNA in the nucleus and brings it to specialized units outside the nucleus called ribosomes. The ribosomes make proteins in accordance with the DNA blueprint. And proteins do all the work of metabolism: eating, growing, sensing the environment, eliminating wastes.
Which came first, the nucleic acids or the proteins? It takes both to be alive, but biologists have been puzzled over how both could have come about at the same time. By the 1970s, several researchers had hit on the idea that perhaps RNA played both roles originally. The concept of an “RNA world,” where RNA alone accomplished both metabolism and reproduction, has gained favor among most biologists.
Not every researcher agrees with the “RNA world” concept, however. Andrew Pohorille, at the Ames Research Center, is constructing computer models of the chemical processes that may have led to the formation of living organisms. He believes that proteins came first, nucleic acids afterward. Animatedly intense, Pohorille finds that the whole organic soup concept is “silly.” There was no need to spend eons concentrating chemicals in the seas, he maintains. He prefers what he calls “one-pot chemistry.”
Researchers at Carnegie Institution of Washington have shown that when carbon-rich material from a carbonaceous chondritic meteorite is mixed with water, the mixture spontaneously forms microscopic spherical vesicles, which Pohorille believes are protocells, the precursors of true biological cells. Pohorille’s computer models show that when these microspheres come into contact with one another, they coalesce and create a stable internal environment in which the hydrocarbons, PAHs, and amino acids they contain can spontaneously interact to form the long carbon chains (polypeptides) of proteins: protein synthesis without DNA or RNA. When these proteinoid microspheres get too large, they divide to produce two spheres with slightly different proteinoid contents.
Thus Pohorille’s computer models show a form of primitive evolution, with different proteins coming into being without the need of DNA and RNA to guide their production.
That is how life began on Earth, Pohorille is convinced. Proteins arose first, out of simple nonliving chemicals. They produced the earliest form of metabolism (the first half of von Neumann’s definition of life), but they reproduced randomly, without the precise copying that DNA and RNA allow. In that primeval environment, the microspheres could reproduce wildly, constantly changing, multiplying within minutes, spr
eading wherever they found the basic chemicals and energy inputs they needed. It must have been like a natural, unplanned, unbridled biology experiment, an explosion of life into a pristine world.
This was the moment that Pasteur and later biologists could not envision when they decided that spontaneous generation was impossible. In that early dawn of life, nearly 4 billion years ago, life could and did arise out of nonliving chemicals. Once life established itself on Earth, it became impossible for spontaneous generation to occur again because the organisms already living in every available ecological niche quickly gobble up any organic precursors (i.e., food) they can find.
Only later were the nucleic acids developed. Once that happened, life could both metabolize and reproduce faithfully. At that point, life began to spread into every part of our world.
In a chemistry lab near Pohorille’s office, other Ames researchers are attempting to reproduce these early steps in the origin of life. They are starting with carbonaceous material and water and watching microspheres form. If they succeed in producing protein-based protocells, they will have generated living, metabolizing entities out of nonliving chemicals. “Life in a test tube” will become a reality. Astrobiologists will understand at last how life arose on this world. This will help them to understand what they should be looking for on other worlds.
OUR STABILIZING MOON
Even if life arose on Earth through completely natural processes, is there anything that makes Earth truly unique? Is there any trait that our world possesses that we might not expect to find on other worlds?
Remember the lesson of the Copernican Revolution: We are not the center of the universe. Our planet, our Sun, the very atoms of our bodies are all quite ordinary. We are not special.
But is that entirely true? Earth has one feature that most other planets lack. We have a Moon that is quite large in relation to the size of our planet. Our Moon is fully one-quarter of the Earth’s size and exerts a noticeable tidal influence on our oceans. Except for frozen Pluto, all the other planets of our solar system have moons that are minuscule in comparison to their own size, or there is no moon at all.
Yes, Jupiter and Saturn both have moons that are larger than our own, but in comparison to the sizes of those gas giant planets, even Ganymede and Titan are pygmies.
Our Earth has a relatively big Moon, and it does not merely tug at the oceans. Its gravitational pull affects our planet’s axial tilt. And that, in turn, affects our climate. Which, in the final analysis, is vital for the existence of life on our world.
All planets spin around on their axes, like tops. Each planet’s axis is tilted somewhat from the vertical: Jupiter is the most “stand-up guy” among the planets, with an axial tilt of a mere 3.08°; Uranus rolls around almost sideways—its axial tilt is 97.9°, which means that Uranus’ north pole points toward the Sun during part of its eighty-four-Earth-year-long year, then forty-two years later its south pole points sunward. This must produce a weird climate on Uranus!
Earth’s axial tilt is 23.4°, which causes the seasons we know so well. Like a spinning top, the axis of a planet’s spin tends to wobble, and Earth’s axial tilt slides slowly around this 23° angle once every 26,000 years. The star that hangs above the north pole today is Polaris, at the end of the Little Bear’s tail. When the great pyramids were built in ancient Egypt, the pole star was Thuban, in the constellation Draco (the Dragon); Thuban will be the pole star again in approximately 22,000 years.
As far as geologists can tell, Earth has maintained this tilt for eons. This is due to the Moon’s gravitational pull, which acts as an anchor on the axial wobble and prevents the Earth from tilting farther than about 23°. Mars, with its two diminutive moons, has no such brake on its axial tilt. Mars’ poles have wobbled hugely over geological time, which has produced wild swings in the red planet’s climate.
If a stable long-term global climate is necessary for the long-term survival of life and the eventual rise of intelligence on a planet, then perhaps only planets with relatively large moons will develop intelligent species.
In that respect, perhaps Earth is unique—or, at least, very rare.
11
Life’s Impact on Planet Earth
There is a grandeur in this view of life.
—Charles Darwin
ONCE LIFE GOT STARTED on Earth (no matter how it started) it began a chain of events that utterly changed our planet. For life is not merely a passive passenger on “spaceship Earth.” As life adapts to the environment in which it exists, it also changes that environment to make it more suitable for life to flourish.
FROM MOLECULES TO CELLS
Molecular life adapted to its constantly changing environment. That is one of the prime characteristics of life. It adapts. Although DNA strives to reproduce its blueprints exactly, errors do creep in from time to time. Such errors are called mutations. Most mutations are neutral, making no discernable difference in the organism’s ability to survive. Many are harmful and lead to the organism’s death. But some mutations actually help the organism to adapt to the environment in which it finds itself.
Since environments inevitably change over time, organisms that can adapt to these changes will survive; organisms that do not change will ultimately die off.
Life on Earth adapted. Life evolved.
Probably the biggest step in life’s evolution was to develop cells. Beginning with the proteinoid microspheres such as those produced spontaneously when carbonaceous meteoric material is mixed with water, living molecules wrapped themselves inside a protective membrane of fatty material, which maintained the optimum conditions for their own workings inside the membrane and allowed them to remain relatively aloof from the conditions outside.
Every living creature on Earth is composed of one or more cells. Bacteria are single-celled organisms. So are amoebas and paramecia and many of the thermophiles. Most of the life on Earth consists of single-celled organisms; they outweigh all the multicellular organisms, by far. The oldest rocks show that single-celled organisms have probably existed since the Earth’s surface cooled enough for water to remain liquid. Single-celled creatures have existed on our world for nearly 4 billion years. Multicellular organisms did not make their first appearance until about a mere 750 million years ago.
Since we are multicellular (the human body is composed of some 100 trillion cells) we tend to think of our fellow multicelled creatures as the dominant species on our planet. Not so. An objective alien observer of life on Earth would quickly come to the conclusion that single-celled organisms are the predominant form of life on this planet; they have been since the very beginning, and they still are today.
There are two types of cells:
Prokaryotic cells have no nucleus. The word prokaryote is derived from Greek and means, roughly, “before the seed.” Bacteria and archaea are prokaryotic: The DNA of their cells is not walled off into a nucleus, but floats freely within the cells like a raveled bit of twine.
Eukaryotic (“good seed”) cells have a nucleus. Moreover, they have other subdivisions within them that are specialized units for building proteins (the ribosomes), producing energy (mitochondria), removing waste products (lysosomes), and, in the case of chlorophyllic organisms, chloroplasts that convert sunlight, water, and carbon dioxide into carbohydrate foodstuffs.
IF YOU CAN’T EAT ’EM, JOIN ’EM
How did life go from prokaryotes to eukaryotes? A eukaryotic cell is much more complex than a prokaryotic cell. What caused this innovation?
Lynn Margulis of the University of Massachusetts at Amherst pioneered the idea that symbiosis14 was the way. Prokaryotes lived by eating their neighbors. Margulis’ idea was that some prokaryotes ingested a neighbor or two but did not digest them. Instead, the eater and the eaten began to work together, resulting in an organism that was better able to survive. For example, a plant cell’s chloroplasts were originally free-living cyanobacteria.
One of the best lines of evidence supporting Margulis’ idea of
symbiotic adaptation is the fact that eukaryotic cells’ mitochondria—which produce the chemical energy for the cell—contain their own DNA, separate and apart from the genetic DNA in the cell’s nucleus. The mitochondria must have once been free-living prokaryotes that were incorporated into other cells and resulted in an organism better able to survive and flourish.
Biologists have found living proof of symbiotic adaptation among bacteria. The mealybug is an insect that has bacteria in its gut that helps it to digest its diet of plant sap. (Don’t shudder, we have helpful bacteria in our guts, too. We can’t live without them.) A team of researchers at Utah State University found in 2001 that one type of the mealybug’s helpful bacteria has another type of bacterium living inside it. Active symbiosis, just as Margulis originally proposed.
THE MANY-BRANCHED TREE OF LIFE
Before we go any further, we need to understand the different forms of life that exist on Earth.
“Is it animal, vegetable, or mineral?” we ask in the game of Twenty Questions. Those three categories cover everything on Earth, right? All living things are either animal or vegetable, while nonliving things are mineral. Right?
Wrong.
Modern biologists recognize three distinct categories of life: the archaea, the bacteria, and the eukarya. Plants and animals, all the familiar species that we deal with in everyday life, are part—a fairly small part—of the domain of the eukarya.