If only a Few conditions were not just right, Life on Earth would not exist at all. Four Simple Facts Behind The Miracle of Life. - David H. Levy. David H. Levy, a Parade contributing editor, is the author of 20 books, most recently "Comets: Creators and Destroyers" (Touchstone). He is also the discoverer of 21 comets.

Have you ever in the thought about how truly amazing life is? It thrives everywhere on earth, so most of us rarely consider how fragile it is - how, if a few conditions were not just tight, life after the would not exist at all.

Thinking about these conditions was a hobby of sorts of two famous two famous scientists, both of whom were born in 1906 and died last year. The Nobel prize-winner George Wald believed the universe is designed for life. The evidence, he said, is clearly apparent in the elements carbon, hydrogen, oxygen and nitrogen. "They have unique properties that fit the job and are not shared by any elements in the periodic system," said Wald. Life everywhere has to be made of some combination of these four simple building blocks.

In Waid's universe, the miracle of life is based on only four simple conditions, the absence of any one of which would have made life absolutely impossible.

1) Ice floats.

2) The night sky is dark.

3) Protons and electrons have absolutely identical charges.

4) We have the right kind of Sun.

The other scientist was Clyde Tombaugh, the gentle and brilliant man who discovered the planet Pluto in 1930. Tombaugh spent a good deal of his 90 years thinking, like Wald, about the miracle of life. I talked with him about this a few months before he died in 1997. Whereas as Wald came up with factors needed for the emergence of life, Tombaugh suggested two things that are not quite right about life on earth:

1) The earth tilts too much.

2) Our sun emits too much ultraviolet (UV) radiation.

Because the earth is tilted at a sharp angle of 23.5 degrees (the result, some believe, of a cosmic impact with another world long ago), we have seasons. In January, when the tilt points our Northern Hemisphere away from the sun, the sun appears low in the southern sky, sunlight is brief and indirect, and we have winter. In July, with our hemisphere pointed toward the sun, we enjoy summer.

But while the changing seasons offer variety, they also breed dangerous storms like hurricanes in late summer or tornadoes in early spring. If the Earth tilt were much less, the weather would be more moderate, with fewer and less violent storms. However, if the tilt were much greater (a point Wald might have made), seasonal changes in weather would be so devastating-with bitter winters and broiling summers- that life might not have survived. (as for UV radiation, see the box.)

The foundations for life were laid at the birth of the universe. In the very beginning of time and space, the most popular theory goes, all mailer and energy was contained as a single point that blew up in an explosion that continues to this day (the big bang). Our universe is still expanding like a loaf of raisin bread: as the loaf rises, the raisins appear to move away from each other. By the end of the first second after the explosion, the first atoms already were formed. Some of the conditions that make life possible, in fact, already were set by this time, and the rest of the story was built on tiny changes. As George Wald loved to say, the building of life is a process of editing, not authorship. That the whole tapestry of life is based on a few fundamental principles, and that those principles have allowed life to go as far as it has, is a miracle.
 

#1 If ice didn't float, you wouldn't be reading this.

One fact is true of almost every molecule ever made: As it gets colder, it contracts. And when it freezes from its liquid form, its greater density allows it to sink to the bottom of that liquid.

Water is an exception. As it cools below 39°F, something extraordinary happens: it expands rapidly. It expands even more as it finally freezes at 32°F. Because it is now less dense it is now less dense -not more dense- than the water around it, ice rises to the surface. Every cube of frozen water since the beginning of time has floated atop that refreshing drink.

When ponds and lakes freeze, the ice forms a protective layer that shelters the liquid water below. Protected from further cold and wind, that water is able to maintain a steady 39°F temperature, so further freezing is slowed down. The life-forms in the water are allowed to survive for another season.

What if ice didn't float? As the temperatures of earth's early ponds dropped below freezing, the top layer of ice would have sunk, allowing the next layer to freeze quickly and also sink. As the pond gradually froze solid, life would have had a hard time making it past the first winter.

If ice didn't 'float, George Wald said, there no longer would be any ponds at all (or lakes or any other bodies of water): as a frozen pond warmed up after that first winter, a thin layer of liquid water would have floated atop the ice. The top of this layer would then have vaporized (somewhat like dry ice) into the atmosphere and space, while the lower layer would have frozen and sunk thus, as the pond thawed, it eventually would have all vaporized and never been a pond again.

 

#2 Billions of Stars, Yet the Night Sky is Dark

When the German physician Heinrich Olbers peered at the night sky almost 200 years ago, he wondered about a simple fact the rest of us take for granted: The night sky is dark. It shouldn't be, he wrote with so many stars and other things in the universe, why doesn't all their light come charging in on us at once? Their combined radiation should make the entire sky as bright as the sun. Olbers never knew that the conditions foil darkness were set in the instant the universe began with a titanic explosion. All the super-clusters of galaxies are still expanding from that explosion the darkness, it seems, is the result of the young age of our expanding universe-still so young that light from its most distant object hasn't even reached us yet if it were far, far older, from all the galaxies would indeed flood us with radiation, rendering life impossible. But , for now, the sky remains dark.

#3 Equal Charges Keep Things Together As anyone who has taken high school chemistry knows or has forgotten, atoms are made of charged particles called protons and electrons, and the positive charge of one counter balances the negative charge of the other. However, what we might not he aware of is that the two charges are precisely equal and have been since the first thousandth of a second of the universe, if they were not equal then a force other than gravity would have determined the large-scale structure of the Universe.

Physicists identify four forces that - acting separately or together - account for every event in the Universe. Two of these, the strong and weak nuclear forces, are only relevant at the level of an atom. The two other forces, capable of acting over distances, are gravity and the electromagnetic force. The electromagnetic force is much stronger than gravity, but it is usually canceled out because positive protons and negative electrons have equal charges. But at times they aren't equal and then we get fireworks. A slight difference cause a spark as we walk on a dry carpet; a bigger one in a humid atmosphere results in a dramatic lightning strike. These "sparks" are brief and temporary. But, when the Universe was about a second old, if either protons or electron had had a slightly higher charge than the other, the electromagnetic force would have overwhelmed gravity. There would be no galaxies, no stars, no planets and no life as we know it. Without the effect of gravity, our Universe might be an expanding shell of unformed matter and energy

#4 Our Single Sun is the Right Size

The earth orbits a small, single star in the boondocks of our galaxy. We're lucky with our choice of sun. More than half of the stars in our galaxy are double or even triple suns. What if our home world circled suns like these? As it moved near one sun and then another, temperatures would change catastrophically, probably rendering life impossible. What if we were a planet near the seething radiation at the center of the galaxy? Or what if our sun varied in size and brightness, the way many stars do? Then life as we know it would probably never have started.

If our sun were much more massive than it is, it would burn its fuel more quickly. Instead of being stable for so many billions of years, allowing life to develop slowly, it would have burnt itself in a cosmic flash in the pan. Life could have started, but it wouldn't have had the time necessary to advance to the stage where thinking life-forms could ponder these issues.

So we have the right sun -but not, as Cloyed Tombaugh noted, the perfect sun. If our sun were a little bit redder than it is, it would send out less ultraviolet radiation, and humanity would not have to fear the destruction of the ozone layer, which protects life from harmful UV rays. Life would be easier and more comfortable.