Chapter Three

The Genesis of Life


... extract from the middle of this chapter
:

A Mathematical Impossibility

In Chapter 1 we identified four distinct base types, namely adenine, cytosine, thymine and guanine. These are represented by the letters A, G, C and T respectively. In the section above, I established that the shortest DNA sequence that will support life is a string of 350,000 of these letters.

I have acknowledged that certain amino acids occur naturally, so let's presume for the moment that there was a sufficient concentration of the 20 amino acids needed for life, as well as the four base types above, somewhere in the waters of the early Earth.

The key question posed by this chapter, and indeed by this book, is how did 350,000 amino acid molecules assemble themselves in just the right order to code the information necessary for life? Remember that we have stripped out any gene without which the organism can still survive. Any explanation for the origin of life must suggest a plausible mechanism by which this set of instructions for assembling proteins could arise spontaneously.

To put that question in a mathematical context, let us imagine that you have a dump truck full of millions of Scrabble pieces. Each piece has one of the letters A, G, C and T printed on it.

Now let us recall the sample sequence for the human betaglobin gene provided in Chapter 1. The first 9 bases of that sample were:

TCCTAAGCC

Imagine you are sitting beside the dump truck, and you pick out your first Scrabble piece. What is the chance it is a T? Clearly, the chance is 1 in 4. You pick out a second piece. What is the chance it is a C? Assuming the supply of Scrabble pieces is virtually unlimited, the answer is again 1 in 4. You pick out a third piece. What is the chance it is also a C? The answer for this and every subsequent pick is 1 in 4.

Now imagine that you place the pieces you have picked in a line on a flat surface. What is the chance the first three letters are TCC, making up the codon that specifies the amino acid serine? (See Table 1 in Chapter 1.) If you studied basic probability in school, you will agree that the answer is 1 in 4 x 4 x 4, or 1 in 64. Another way of expressing this is 1 in 4 raised to the power of 3, or 1 in 43.

I think you see where I am leading. Suppose you stay beside that dump truck, picking out Scrabble pieces at the rate of one per second. Suppose the flat surface you place them on is a conveyor belt moving at just the right speed to allow you to keep adding letters to the end of the sequence without having to move. Suppose that you keep doing this without a break and without sleeping. In just over 4 days, you will have picked 350,000 pieces. As a standard Scrabble piece is an inch square, the sequence you have created on the conveyor belt is 350,000 inches, or around 5.5 miles (9 kilometers), in length.

Going back to my key question, what is the chance that these 350,000 Scrabble pieces, in precisely the right order, match the DNA of the simplest living organism? To get the answer, we just extend our previous calculations. The chance is 1 in 4 raised to the power of 350,000, or 1 in 4350,000. Scientists have a term for such extremely low probabilities. That say the probability is "vanishingly small". Lay people have a different word for it. They would say the probability is zero. In other words, the spontaneous assembly of a DNA sequence that will support life is a mathematical impossibility.

Note carefully my use of the word "spontaneous". You and I are a reality, so our existence cannot be said to be impossible. The explanation to this paradox offered by this book is that the assembly was not spontaneous. Life had help!

Before considering how scientists respond to this problem, let us leave the world of numbers, and consider an analogy some of you may be more comfortable with, such as English text. In Table 2 of Chapter 1, I showed how each letter of English (Roman) script could be represented by a codon of 3 nucleobases. A sequence of 350,000 bases can therefore be represented by around 117,000 letters of English, including the spaces between words. The average word length in English is 5 letters. Allowing for spaces and punctuation, it is a little over 6 letters per word. Using this average, we find that our sequence of bases can be represented by 19,000 words of English text. So how long is that? It is the length of a short novel, or about half the length of the book you have in your hand!

Personally, I like to use the books of the Bible for comparisons. Consider the gospel of John, which is considered one of the most profound religious texts. It takes me around 4 hours to read this account of the life and teaching of Jesus. The text contains a little over 20,000 words. What would you estimate is the chance of spelling out the entire gospel through picking Scrabble pieces at random? The probability that it could be assembled without the use of intelligence is similar to the probability of those 350,000 bases spontaneously assembling in the correct order. The probability is vanishingly small.

Human languages are codes in which each word is a symbol representing some object or concept. Computer software uses binary codes as symbols for the various functions of the central processing unit. DNA codons, each a sequence of 3 bases, provide 64 symbols for specifying the structure of proteins. It is the contention of this book that each system of symbols represents intelligence. The arrangement of the symbols used in human language and computer software reflect meaning. They are the product of human intelligence. DNA is a vast storehouse of information, and its symbols are no different. Their arrangement reflects meaning. They are the product, I suggest, of divine intelligence.

 

Back to Top | Full Screen