Something in common
According to a survey from the Pew Research Center, 33% of Americans believe that God created humans in their present form within the last 10,000 years and another 24% believe that God guided evolution over millions of years to create humans as they exist today. While scientists should respect people’s deeply held religious beliefs, it is disconcerting that so many of our fellow citizens are not convinced by the evidence—nearly universally accepted by biologists—that organisms (including humans) evolve through natural selection acting on random variation. I’m working on an explanation of evolution that some evolution doubters may be willing to consider.
Does belief in evolution even matter? Even in a world so dependent on science and technology, is it an issue that much of our populace does not accept the scientific arguments for so fundamental a process?
I think it does matter and it is an issue. The reluctance to accept the reality of evolution erodes support for scientific evidence as a whole, leads to a failure to understand phenomena like antibiotic resistance, cancer progression, and vaccination, and results in a personal sense of blame for diseases or genetic syndromes that arise from the occasional mistakes of DNA replication. The unwillingness of a large fraction of our society to acknowledge well-established scientific truths belies our standing as a technologically advanced nation and endangers our future progress.
Regardless of their belief in evolution, though, many of our fellow citizens readily provide DNA samples to commercial services that genotype a large number of their genetic markers. From the resulting data, a DNA donor may learn that she is, say, 1/2 Irish; 1/4 German; and 1/16 Norwegian. How do we know this?
To geneticists, the reasoning is simple. Two siblings share 1/2 their DNA because they have the same common—and recent—ancestors, their mom and pop. Each parent supplies a child with one chromosome from each of a set of 23 matching pairs. Because recombination has scrambled the DNA of the chromosomes in the eggs and sperm of their parents, siblings don’t get the exact same fragments of their parents’ chromosomes, and so on average they share only 1/2 their DNA.
First cousins share about 1/8 of their DNA because they, too, have some of the same common—and recent—ancestors, but in this case it’s only one of their two sets of grandparents. So while you share 1/4 of your DNA with each of your grandparents, with your first cousins it’s half of that.
Second cousins share only 1/32 of their DNA because their common ancestor is a set of great-grandparents. And so on. The child of unrelated partners brings two distinct genetic lineages together. As you go back and trace the DNA of more and more distant cousins, you’ll see that they share fewer common ancestors, and recombination each generation continued to scramble up chromosomes.
DNA testing services can identify fourth cousins who share about 1/500 of their DNA and perhaps fifth cousins who share about 1/2000. You’re happy when these distant relatives show up, because while they may not look a whole lot like you, they represent other descendants of one of your long-ago ancestors. If that ancestor happened to live in Norway—which we know because modern day Norwegians have many of the same DNA markers as those who lived there long ago—you have an explanation for why you are said to be a small fraction Norwegian.
Now imagine you provide a DNA sample to one of these testing companies, but imagine, too, that the detection of tiny haplotype blocks that indicate identity by descent has gotten really really good. The company genotypes your markers and finds that you have a match with someone who is your thousandth cousin, because she shares 1/22001 of your DNA. The common ancestor of you and this distant cousin lived about 20,000 years ago.
The testing company pushes the technology even further and comes up with someone who is your ten thousandth cousin; your common ancestor with him lived about 200,000 years ago, just as anatomically modern humans first came on the scene.
But you’re still not satisfied and demand even more distant relatives. So you get back a match with Individual O, who is your 300,000th cousin; you share a common ancestor but that ancestor lived about 6 million years ago. You’re excited to meet this new relative, so the company takes you to a local zoo and introduces you to Individual O, who happens to be an orangutan.
Soon you get an email message that yet another relative has been located through a match with a DNA donor. This Individual D, who is your 4,500,000th cousin, shared an ancestor with you about 90 million years ago. Amazingly, D lives with you and your family and is 100% German, as in German shepherd: you took him for a walk just a few hours ago.