IF YOUR TWO parents hadn』t bonded just when they did—possibly to the sed, possiblyto the nanosed—you wouldn』t be here. And if their parents hadn』t bonded in a preciselytimely manner, you wouldn』t be here either. And if their parents hadn』t done likewise, andtheir parents before them, and so on, obviously and indefinitely, you wouldn』t be here.
Push backwards through time and these aral debts begin to add up. Go back just eightgeions to about the time that Charles Darwin and Abraham Lin were born, andalready there are over 250 people on whose timely couplings your existence depends.
tinue further, to the time of Shakespeare and the Mayflower Pilgrims, and you have nofewer than 16,384 aors early exgiic material in a way that would,eventually and miraculously, result in you.
At twenty geions ago, the number of people procreating on your behalf has risen to1,048,576. Five geions before that, and there are no fewer than 33,554,432 men andwomen on whose devoted couplings your existence depends. By thirty geions ago, yourtotal number of forebears—remember, these aren』t cousins and aunts and other ialrelatives, but only parents and parents of parents in a line leading iably to you—is overone billion (1,073,741,824, to be precise). If you go back sixty-feions, to the time ofthe Romans, the number of people on whose cooperative efforts your eventual existencedepends has risen to approximately 1,000,000,000,000,000,000, which is several thousandtimes the total number of people who have ever lived.
Clearly something has gone wrong with our math here. The answer, it may i you tolearn, is that your line is not pure. You couldn』t be here without a little i—actually quitea lot of i—albeit at a geically discreet remove. With so many millions of aors inyour background, there will have been many occasions when a relative from your mother』sside of the family procreated with some distant cousin from your father』s side of the ledger. Infact, if you are in a partnership now with someone from your own rad try, theces are excellent that you are at some level related. Indeed, if you look around you on abus or in a park or café or any crowded place, most of the people you see are very probablyrelatives. When someone boasts to you that he is desded from William the queror orthe Mayflower Pilgrims, you should a once: 「Me, too!」 In the most literal andfual sense we are all family.
We are also unily alike. pare yenes with any other human being』s and onaverage they will be about 99.9 pert the same. That is what makes us a species. The tinydifferences in that remaining 0.1 pert—「roughly one ide base ihousand,」
to quote the British geicist a Nobel laureate John Sulston—are what endow uswith our individuality. Much has been made i years of the unraveling of the humangenome. In fact, there is no such thing as 「the」 human genome. Every human genome isdifferent. Otherwise we would all be identical. It is the endless rebinations of enomes—eaearly identical, but not quite—that make us what we are, both as individualsand as a species.
But what exactly is this thing we call the genome? And what, e to that, are genes?
Well, start with a cell again. Ihe cell is a nucleus, and inside eaucleus are theosomes—forty-six little bundles of plexity, of which twenty-three e from yourmother and twenty-three from your father. With a very few exceptions, every cell in yourbody—99.999 pert of them, say—carries the same plement of osomes. (Th