Re: Cant fit it sorry!

Great question Anthony, I can see you've been thinking about this for quite some time. Since your question is so long, I'm going to try to address its various parts before I can address it as a whole.

So, lets take a look your first supposition, " If every human on the face of the earth has his own genes, or dna combinations...". OK, this is basically true. A gene is a segment of DNA which encodes information on how to make a particular protein from a small set of amino acids. Proteins are the bio-molecules that do 99% of the work in the cell. As members of the same species, all humans have the same number of genes, and it is estimated that it takes somewhere around 50,000 to 100,000 genes to make a human. In addition, we each have two sets of these 50,000 to 100,000 genes -- each set is inherited from one of our parents. So, even though we each have our own genes, those individual genes are not necessarily unique; we share half of them with each of our parents.

Ok, next we have, "and it has been this way since the beginning of time", which is basically not true. As far as we can tell, our species, Homo sapiens, has only existed for at most 100,000 years. Homo sapiens evolved from older species, like Homo erectus, and probably an Australopithecine species before that. Because we only know about these presumably ancestral species from fossils , we have no idea how many genes they had, or how similar their genes were to ours. However, because of genetic comparisons with the other Primates, we think that our Australopithecine ancestors evolved from a species of creatures that were also the ancient ancestors of Gorillas and two species of Chimpanzee, and that this ancient ancestral species lived about 5-7 million years ago. So, over the last six million years or so, the set of genes that our ancient ancestors had has changed quite a lot; it has changed in enough ways to give rise to four different species.

Now, we get to the meat of your question, "won't at some time in the future (assuming the human race survives more than say a million years) we run out of new amino acid combinations?" This question acknowledges the idea that genes give rise to functional proteins, made of amino acids, as well as the idea that changes in a protein's amino acid makeup (sequence) can give rise to changes in function, which is true in many cases. When we have two different versions of the same gene, we call each of these versions an allele of that gene. However, as I started to point out earlier, it is not necessarily the novelty of alleles or the uniqueness of the amino acid sequences of our proteins that makes us all distinct from one another. Half of the genes you have share identical amino acid sequences with half of the genes of each of your parents. What has been happening for the last hundred thousand years or so that Humans have been around is not simply the continual generation of new alleles via mutation. The genetic changes that make each of us unique have primarily been the generation of new combinations of alleles of various genes (accompanied by a slow process of random generation of new alleles) through sexual reproduction , as well as changes in the degree and manner in which extant genes are regulated. Sometimes, simply changing the area in which a particular gene is expressed in the organism, or the level at which that gene is expressed in its usual range, can have dramatic effects on an organism's phenotype. It is frequently the case that these sorts of changes do not involve any changes to the amino acids that are encoded by the gene, but instead involve changes to the DNA sequences between genes.

What this means is that instead of just considering the number of different amino acids that could change in 80,000 to 100,000 genes, we have to start considering the number of nucleotide (DNA level) changes that can be made throughout the entire genome (the organisms entire set of DNA). Another way to say that is to assert that even if we did run out of new amino acid combinations, people who have identical amino acid combinations might still be different because their genes are regulated differently. It is currently thought that our 80,000 to 100,000 genes constitute about 5% of our genome; the other 95% is thought to be comprised of spacers, regulatory domains, and other types of DNA that are collectively (and misleadingly) referred to as "Junk DNA". The DNA in the human genome is comprised by approximately 3,000,000,000 individual nucleotides, so our so-called Junk- DNA is comprised by about 2.85 billion nucleotides, and some subset of these can be changed to modify the expression of a given gene.

Now, think about how long a sequence of DNA is needed to distinguish everyone on the earth. This sequence has to be just long enough so that everyone alive (6 billion people) could have a unique version of it. If the sequence were only one nucleotide long, then it would only be good for distinguishing four people, because there are only four possible nucleotide identities for a sequence one nucleotide long. These are A, G, T, or C. If the sequence was two nucleotides long, we would be able to distinguish sixteen people, because we could have one of four different nucleotides at the first nucleotide position, and one of four at the second position. These sixteen sequences would be AA, AT, AG, AC, TA, TT, TG, TC, GA, GT, GA, GC, CA, CT, CA, and CG. In fact, the easy way to calculate the number of unique sequences possible for a DNA sequence of a given length is, four (the number of nucleotides possible at each position) raised to the power n (where n is the length of the sequence), or 4^n. To cut to the chase, a sequence 17 nucleotides long could provide more than 17 billion unique sequences, and a sequence 20 nucleotides long could provide more than 1 trillion unique sequences -- enough for our population to grow 180 times larger!

Now, imagine that each of the 80,000 to 100,000 genes in our genome has three nucleotides in the so-called Junk DNA for which changes to those nucleotides can result in expression changes for that gene. That makes between 240,000 and 300,000 positions that could potentially serve to distinguish people regardless of the number of amino acid changes that are made to their genes. If a 20 position long sequence was sufficient to distinguish 1 trillion people, imagine how many people could be distinguished by changes at 300,000 positions (it is a ridiculously large number; much larger than a 4.5 followed by 307 zeroes, which is 4^511, as high as my computer would count).

OK, but as interesting as this all is, it is really just conjecture because (1) most of our genes (75%) only have one allele, (2) changes in the expression pattern of many of these genes is fatal, (3) things aren't really as randomly distributed as these scenarios suggest and (4) if you made all those dramatic expression changes in our genome, the resulting organism probably wouldn't be a "human" anwyay. So, lets get back to the rest of your question, "My theory is based on the fact that there should be a finite number of possible combinations and even though this number so great wouldn't we still at some point in the future run out of combinations producing a person with identical DNA?" If you reject the implication of the last section that 4.5x10^307 is vastly more humans than will ever have been alive, I will point out that people with identical DNA are born every day -- as the products of multiple births, mostly twins and sometimes triplets. In addition, if you are asking purely about identical DNA, i.e. identical genomes, then the chance of two unrelated people being born with identical DNA is about 1 in 4^3 billionth power (assuming everything was equal and random, which it is not).

In fact, because of the diversification (generation of new DNA sequences and new combinations of genes) that has gone on over the last 100,000 years, I suggest that it will become increasingly difficult to find people with identical DNA sequences as our species grows. This is because the process of mutation results in single nucleotide changes that occur at random places in the genome. So, the genomes of the descendants of any given individual gradually all become more and more distinct as the number of generations of descendants of that individual increases. So, as our species moves further into the future, our descendants will all be much more genetically different from one another than any of us are today. The degree of this difference is called 'average genetic diversity'. In fact, if you compare the average genetic diversity of our species to that of our closest primate relatives, the chimpanzees, you will find that our species of more than 6 billion individuals has the genetic diversity of a small population of chimpanzees. The entire chimpanzee species of100,000 - 200,000 individuals has a genetic diversity several times greater than our entire species. So it seems like there is still tremendous capacity for diversification in our species.

Finally, I want to point out that our genomes are not fixed in size or nature. They are "designed and intended" by evolution to adapt, to grow or shrink, and to change. In the long run, evolution is a process of innovation and diversification. Right now, we believe that all life on earth, every single organism alive today, is descended from one common ancestor that lived more than three billion years ago. In all of that time, that organism and its descendants have never 'run out' of 'new amino acid combinations' or even new DNA combinations. Each time evolution brings forth a new species, that species is different and fantastic in ways which have never been seen before, and once that species goes extinct, will never be seen again. Our species' history over the last six million years has been one of rapid change and evolution -- there is no reason to believe that our evolution has come to a halt, or that our genomes will remain frozen in their current state. Our ancestors of six million years ago evolved into many different species, four of which survive today. If our species survives another million years as you suggest, I expect that we will evolve into entirely new forms instead of revisiting the forms of people who are long gone.