The big book of how to build a human - your genome - is approximately 3 billion letters long.
The text is arranged in long molecules of DNA called chromosomes. Each letter of code in a chromosome is joined to its compliment - A to T and C to G - and the whole string is arranged in the iconic double-helix shape discovered by Watson and Crick in 1953.
Our chromosomes come in pairs, one from our father and one from our mother. So to be more accurate our instruction manual consists of 3 billion pairs of letters and since we have 2 copies of each "chapter" that makes a total of 6 billion pairs in every cell of our bodies.
Humans have 23 pairs of chromosomes. These are numbered 1-22 from the longest to the shortest plus another pair that determine our sex. X-X for females and X-Y for males. The number of chromosomes varies from species to species, and here we find a potential dilemma for evolution.
Evolution states that humans evolved from a common ancestor of other modern primates. However other primates have 24 pairs of chromosomes. This fact seems to make common ancestry impossible. If one of our ancestors lost an entire chromosome they would not have become the ancestor of anybody let alone the entire human race.
Geneticists were able to make a testable prediction. If evolution is true, somewhere in the human genome there should be a chromosome that was formed from the fusion of two chromosomes.
By comparing human and chimp genomes it was clear that human chromosome 2 had been formed in exactly this way. Geneticists have been able to read the two genomes side by side and match up the chromosomes that turned out to be more than 95% identical.
So somewhere in our ancestor's history, after they separated from the ancestor's of our closest relatives two of our chromosomes joined up end-to-end reducing our chromosome total to 46.
Interestingly a similar phenomena has been observed in populations of fruit flies. Some individuals who have a chromosome fusion breed freely with others who do not.
There is another feature of chromosomes that confirms the fusion of human chromosome 2 beyond all doubt. Every chromosome includes sections of code that are essential to the process of cell duplication known as mitosis. At each end there are chunks of repetitive code called telomeres. These suffer damage every time a cell duplicates - a sacrifice that protects the important code. Towards the middle of a chromosome is a section called the centromere that also plays a vital role in mitosis.
So a normal chromosome would have...
telomere --> code --> centromere --> code --> telomere
While a chromosome that has been formed by the fusion of two chromosomes would have ...
telomere --> code --> centromere --> code --> telomere --> code --> centromere --> code --> telomere
Guess what human chromosome 2 looks like.