By Curtis Polk
Until quite recently, it has been assumed by everyone literate in science that intelligent life, which I will refer to as Life As We Know It (LAWKI) is common in the cosmos. That proposition is becoming less likely with every scrap of new knowledge that we acquire.
We have now detected thousands of extra-solar planets, that is planets that orbit stars other than our own star, the sun. The overwhelming majority of the new discoveries are gas giants, similar to Jupiter and Saturn in our own neighborhood. Most of them are close to their mother stars, making them "hot giants."
There are a few rock-like planets that could be like Earth, but the number that resemble Earth in mass and proper distance from their stars is close to zero at this point. Were the mother star even slightly unstable, it would emit flares that would not permit life to develop. Several of the planetary candidates are tidally locked to their star. That means that the length of the day is the same as the length of the year. On one of those planets, the star's position in the sky would never change. No sunrise, no sunset. So, the circadian rhythm, which all multi-cellular life on Earth times its cycles to, would not exist. It might be possible for life forms to develop without the rhythm, but not LAWKI.
The development of life itself on Earth seems straightforward because we know it happened. But consider the possibility that the emergence of living molecules is like a game similar to Jeopardy that requires answering every question on the the board correctly, with questions so difficult that the chances of getting them all correct is one in a million. On a million planets minus one, life does not arise. But on the one planet where it does arise, the process of evolution might eventually become known, and it would seem to the intelligent inhabitants that the origin of life is an obvious development, when what has really happened is that the planet has won an astonishingly difficult version of Jeopardy.
An important barrier to the development of life is chirality. All molecules from living cells have handed-ness, or chirality. Molecules that have chirality have mirror-image analogs, referred to as enantiomers. If you were in the hospital and they fed you with an IV of reverse glucose (if there were such a thing), you would starve to death, because your metabolism only works with normal glucose. Chemical reactions depend on the geometry of the reactants, and if one of them is turned inside out, the reaction won't work. You can separate a mixture of opposite-handed enantiomers, a racemic mixture, only with advanced chemical techniques, usually requiring an enzyme that is itself chiral. That poses a question. How did the first living molecules, which are chiral, arise from the random mixtures on a primitive Earth? They obviously did, but the probability of this happening as a random event is immeasurably low. It seems likely there was a combination of chemical and/or nuclear agents present when life arose that are no longer present.
Another barrier is the development of multi-cellular organisms. For half the time that life has existed on Earth, 1.75 billion years, the only life was single-celled. It must be extraordinarily difficult for multi-cellular life to develop from single-celled life if it took that long.
A significant barrier to LAWKI is the development of intelligence, which relies on the capability for symbolic thought. Symbolic thought is the ability to connect the name of an object with the object itself. For example, when a child learns the word "cat" and connects it with an actual cat, and realizes that the word applies to all cats, she has acquired the symbolic power. The ability to communicate in symbols distinguishes humanity from other earthly species. Symbolic communication makes possible the creation of knowledge, whether spoken, written, sung, or pictured. The raw probability of symbolic thought is so low that attempting to measure it makes no sense.
Our expanding knowledge of the cosmos illuminates how rare intelligent life is. The secular paradigm gives us a true appreciation of how precious are the simplest everyday acts, such as walking on two feet, carrying on a conversation, or listening to music. With our focus off of an imaginary afterlife, we can learn to appreciate even the smallest act of kindness, the sound of a gentle breeze moving through the leaves of a willow tree, or the aroma of baking bread. In our secular world, we can imbue every tick of the clock with meaning.