Once the first sperm enters the egg, the egg closes off its outer wall so that no other sperm can enter.  The winner takes all (with the odd failure!).  This is the start of you;  and you are who you are, because of the specific egg and sperm that won the race.  Your unique genetic code resulted from the unique combination of sperm and egg that became you.

(I used to wonder,  who would I would have been if my mum had married someone else; what would I have been like.  But, that would not have been me.)

It doesn’t take long before that one cell splits into two almost identical cells, and that doubling keeps going until you become hundreds of trillions, a newborn baby that emerges from your mother’s womb about nine months later.  But let’s go back to the start of you.

When the sperm and egg combine combine, each comes with a single set of  23 chromosomes.  When they combine to create a new human, it starts with a single  cell, now called a zygote, with the full set of 23 pairs of chromosomes.

Doesn’t that word sound full of colour and meaning?  Chromosomes contain the ‘instructions’ that create each cell, coded in the 4-letter alphabet of DNA (molecules of De-oxy-ribo Nucleic Acid).  All life is made of cells, and DNA and its precursor RNA (Ribo Nucleic Acid) is central to making and controlling each cell.  Each one of our trillions of human cells has nearly exactly the same genetic code.

That genetic code is created form the unique pairing of sperm and egg.  And here we learn that 1+ 1 = 1 since 1 sperm and 1 egg make a single zygote that then becomes a baby in mothers’ womb.  For humans, as for all other mammals.  One could also argue that  1 + 1 = 3, for mummy and daddy create out of themselves a new baby.

It seems obvious here, but may be hidden: arithmetic demands you use numbers that refer to the same thing.  You cannot add up or do any other operations on number and expect a meaningful answer.  The arithmetic of reproduction is meaningful, but it is not useful for an arithmetic where the answer to 1 + 1 is not reliable. So, whenever you see two or number used or compared; ask yourself, what’s the unit in each case.  Is it the same; and a quick way to see that is to imagine what does single unit look like.

When you are a single cell and divide into two cells, are the two the same? They seem to be, yet one is destined to become embryo, the other the placenta. Each cell divides by first creating two sets of 23 paired chromosomes; so your cells enter the Noah’s ark of life two by two.  And the power of multiplication of just keeing dividing by two to create the trillions of cells that you became as a new baby.

This is the power of compound interest, the exponential factor that is behind biology as well as human society.  This is the power of mathematics.  When my father taught me chess, he told me of the story of the ruler who was so pleased to play the game that we wished to reward the person who taught him. He just asked for a single grain of rice to start, but to be doubled on every square of the chess board, which has 64 squares.  The ruler was happy to agree. A single grain multiplied a few times did not seem so much.  But before covering half the board, the number is over 4 trillion, more rice grains than the ruler had.

You grew from one cell that kept dividing. It takes a while to start after fertilisation, the moment that the sperm enters the egg, and the two cells to become one, the zygote.  It takes a little more than a day for zygote your to start dividing, first into two equal cells.  These two cells divide into four, the four into eight, then by the time you have become 16 cells, you are called a morula. As your cells keep dividing, you become a blastula that enters the lining of your mother’s womb, and into a gastrula, with three layers of cells that will make your inside, outside and middle (respectively), though not in exactly the way you might think.

Somehow equal cells have become different, and that difference continues to develop as your cells develop into at least 200 different types of cells that a human is made of.  All of them from one single cell, who at this stage in your story is a few hundred cells of each of these three types: endo-,  ecto-, and mesoderm.   Can you relate to yourself, when you were just a single cell or the three layers of cells?