It’s often said that we humans share 50
percent of our DNA with bananas, 80 percent with dogs, and 99 percent with chimpanzees.
Taken literally, those numbers make it sound like 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 don’t 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 huge sections 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 up 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 percent of their
genome and 25 percent of ours. And there’s another problem: just as 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 doesn’t really tell us that much about how similar or different two creatures are.
But that doesn’t mean we can’t learn anything by comparing their genomes. DNA contains a
record of the evolutionary relationships between all organisms. It’s a garbled record – but
by reading closely, we’ve 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 percent chimp, but we are 100 percent great ape…and at least a little bananas.
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