It is often said that we humans share 50% of our DNA with bananas, 80% with dogs, and 99% with chimpanzees. Taken literally, those numbers make it sound that we could pluck one cell from a chimp and one from a human, pull out the tangled bundles of DNA known as chromosomes, unroll each one like a scroll, and read off two nearly identical strings of letters. But in reality, the human and chimp scrolls do not sync up so easily. In the six to eight million years since we split from our last common ancestor, chance mutations and natural selection have changed each of our genomes in radical and unique ways. Two human scrolls fused, leaving us with 23 pairs of chromosomes to chimps’ 24. Other large mutations revised a huge section of text – duplicating a chunk of a human DNA here, erasing a chunk of chimp DNA there; while, throughout the scrolls, tiny mutations swapped one letter for another.
When researchers sat down to compare the chimp and human genomes, those single-letter differences were easy to tally, but the big mismatched sections weren’t. For example, if a genetic paragraph, thousands of letters long, appears twice in a human scroll, but only once in its chimp counterpart, should the second copy count as thousands of changes, or just one? And what about identical paragraphs that appear in both genomes, but in different places, or in reverse order, or broken into pieces? Rather than monkey around with these difficult questions, the researchers simply excluded all the large mismatched sections – a whopping 1.3 billion letters of DNA, and performed a letter by letter comparison on the remaining 2.4 billion, which turned out to be 98.77% identical. So, yes, we share 99% of our DNA with chimps, if we ignore 18% of their genome and 25% of ours. And there is another problem: just a small tweak to a sentence can alter its meaning entirely or not at all, a few mutations in DNA sometimes produce big changes in a creature’s looks or behavior, whereas other times lots of mutations make very little difference. So just counting up the number of genetic changes does not really tell us that much about how similar or different two creatures are. But that does not mean that we cannot learn anything by comparing their genomes.
DNA contains a record of the evolutionary relationships between all organisms. It is a garbled record, but by reading closely, we have been able to glean enough information to refine the evolutionary trees we started drawing long before genome sequencing was around. We may not actually be 99% chimp, but we are 100% great ape and at least a little bananas.