Originally published 9 Oct 2003. Expressed entirely materially / genetically, whereas today I think of life primarily in phenomenological / experiential and consciousness / awareness terms. The genetics are the 'nature' component of the conditioning of experience, with the accumulated interaction with the environment (including other lives) being the 'nurture' component.
Let's celebrate life! Not yours or mine, but life in general. Life is not just, as some masters of understatement point out, 'better than the alternative'. It's pretty amazing.
What is life?
And I'm not asking that in a Woody Allan sort of way. As a working definition, let's say that life, as we know it, requires two things: auto-catalysis and replication. The first ensures that, with some food or fuel as input, chemical reactions are self-sustaining. The second ensures that the living thing, which has a finite life-span, can create reliable copies of itself as a next generation.
How was life born?
The detailed answer is way beyond me, but, sometime between 3-1/2 and 4 billion years ago, life sprang forth on the Earth. We don't know whether it has appeared anywhere else in the universe, but if so, it hasn't shown itself to us. One view says that the probabilities of life springing from inert matter are so slim that it is effectively (indeed some say actually) a miracle. Another reasons that with a sufficiently large pool of chemicals and some conditions in which they can be contained in some way, odds are that life will emerge. I tend toward the second view, but that doesn't mean I'm any less in awe of it.
Life's universal language
If we fast-forward to the present and look at a living human, we'll see things have come a long way since L-day. I have about 60 trillion cells in my body. There are over 200 different types of cells in there, and each cell is an impressive machine in its own right. Inside each of those cells, within the cell nucleus, are two sets (actually, the germ cells - sperm in men and eggs in women, only have one set) of 23 chromosomes, constructed of DNA, that constitute the hereditary contribution to who I am. They make up the instruction booklet for building and operating something like me.
Remarkably, this booklet is written using only four different 'letters' (they are usually referred to as 'bases', but I'm going to stick to a language metaphor). The shorthand for these is A, T, C and G. The two sets of 23 chromosomes are made up of 3 billion of these letters. More than 99% of that code is the same for all humans. As a matter of fact, we share 98.5% of it with our nearest non-human relatives, chimpanzees! These same four 'letters' are at the heart of all life on earth.
Segments of code that can differ among people are called genes, and the most recent estimate is that we have about 30,000 of these. If each gene has two possible values, then the number of different human gene combinations is 2 raised to the 30,000th power, or roughly 10 raised to the 10,000th power. To give you an idea of how big this number is, the number of seconds that have elapsed since the universe was born in the Big Bang is less than 10 raised to the 18th power! The upshot is that, aside from identical twins, whose genetic content is exactly the same, it is practically impossible for any two people who have ever lived (or will ever live) to have the same gene composition. Multiply this variability by the great range of different life forms on Earth, and you begin to get a feel for the seemingly infinite range of life's faces.
Passing life's baton
Exciting creatures like you and me, unlike boring asexual bacteria, pass our genes on via sexual reproduction. This process in itself, in addition to being great fun, is quite astounding. Each of those two sets of chromosomes I mentioned above comes from one of your parents. Your father's contribution was via a sperm cell, which, as I mentioned, has only one set of chromosomes. Well, that one set was created by a roughly random process of choosing, for each chromosome, from either his mother's or his father's contributed set. This mixing up is not confined to the chromosomal level, either. There is also crossing over of segments within chromosomes. The net effect is that your father's contribution for a given chromosome is an altered version of either his mother's or his father's contribution for that chromosome.
So, the fact that we (and most other large, complex animals) reproduce sexually contributes greatly to the genetic diversity within our species. This, in turn makes it less likely that the species as a whole will be wiped out by any particular parasitic (viral or bacterial). However, that's not the reason that we've come to produce sexually. Nothing happens for the good of the species, but rather for the 'good' of the genes being passed on. This mixing up evolved because it made it less likely that a parasitic disease that killed a father and or mother would also wipe out all the children.
A trivial line of thought flows from this that I find interesting. The random process I described above means that 50% of my genetic makeup (i.e. 1 of 2 sets of chromosomes) is contributed by my mother and 50% by my father. In turn, by applying the same reasoning at my parents' level. Each of my 4 grandparents have made, statistically speaking, a 25% contribution to my genetic makeup. Eight great-grandparents each contributed 12.5% and so on.
If we extend this backwards for 30 generations, or about 600 years, the potential number of people contributing to my genetic inheritance in that generation alone is 2 raised to the 30th power or roughly a billion. The population 600 years ago was probably only about 100 million, which just goes to show that we've all got plenty of inbreeding, although usually very 'distant' inbreeding, in our ancestry. Despite that, we should all recognise the vast number of people who have contributed to the heritable component of who we are.
If we take this to its likely conclusion, of course, then we need to recognise that all of us have descended from only one or a very few 'original' human couples. Just goes to show what sexual reproduction and a little mutation can do, given enough time. In fact, it is simply the application of plentiful and near-perfect replication over a great span of time that has brought us the vast wealth of life we see today from that special moment when the inert became vital.
Keep your, er, hand in
So get out there, pass it along. Have two kids. Tell them to each have two kids. Pass this on down the line. In 250 years you'll have over a thousand descendants, each with a bit less than a thousandth of your genetic input. Statistically speaking, the genetic part of you, will still be at 100%, even if widely distributed and greatly diluted!