From the London Review of Books:
Meehan Crist She Has Her Mother’s Laugh: The Powers, Perversions and Potential of Heredity by Carl Zimmer
Picador, 656 pp, £25.00, August, ISBN 978 1 5098 1853 2
In contrast, I gave Carl’s big book a less credulous review in Taki’s Magazine.
… We tend to think of heredity as having something to do with traits that are passed from generation to generation, but in many ancient societies, the words for ‘kin’ and ‘kinship’ often denoted connections of mutual responsibility.
… So what do we mean when we say ‘heredity’ today? Zimmer, who writes a column for the New York Times and whose previous books include Soul Made Flesh and Parasite Rex, as well as a co-authored textbook on evolutionary biology, is a trustworthy guide in this inquiry. … ‘We use words like sister and aunt as if they describe rigid laws of biology,’ Zimmer writes, ‘but these laws are really only rules of thumb. Under the right conditions, they can be readily broken.’ This is clear if you widen the lens, as Zimmer so artfully does, to explore multiple channels of heredity, including the microbiome, epigenetics and culture. Along the way, he reveals that the way we talk about heredity – he got his height from his uncle; she has her mother’s laugh – isn’t linked to science at all. At every turn, Zimmer tries to complicate the concept of heredity and challenge received wisdom about why we are the way we are.
Just like in Turkey, the person who comes up with the most complicated theory wins.
You get half of your DNA from your mother and half from your father, but thanks to the specialised and ‘laughably baroque’ process of cell division known as meiosis, you and your sibling might get very different assortments of DNA from each parent. This explains why you may have more DNA from your maternal grandmother, say, than your paternal grandmother. Or why, if you have two siblings, you may be genetically more similar to one of them. Remarkably, researchers have found that a pair of siblings may share as much as 61.7 per cent of their DNA, or as little as 37.4 per cent. ‘Along the spectrum of inheritance, in other words,’ Zimmer writes, ‘some of our siblings are more like our identical twins, others more like cousins.’ She Has Her Mother’s Laugh is brimming with similarly surprising discoveries; and the cumulative effect is a radical destabilisation of the boundaries conventionally drawn around the individual, families, and even the human species.
A less politicized way to think of it instead of as “radical destabilisation” is that meiosis is random on average, but a little lumpy, like a child shuffling a deck of cards unadroitly.
So at the cellular level we are all what scientists call mosaics, ‘a rainbow of different genetic profiles’. Seen in this light, our intuitive tendency to equate genetic similarity with kinship looks a bit bizarre.
Alternatively, our intuitive tendency to equate genetic similarity with kinship looks extremely useful, just not perfect.
Zimmer cites research showing that out of any hundred pairs of third cousins, one pair wouldn’t share any identical segments of DNA.
Okay, so the kinship to genetic relationship glass at the third cousin level is 99% full.
Out of any hundred pairs of fourth cousins, 25 pairs wouldn’t share any identical segments. And yet, we would never say these cousins are not kin. When you look at heredity in terms of genes, using genes alone to define kinship (or even to draw strict boundaries round what it means to be human) starts to seem a little dubious.
Your family tree exists in a Platonic realm. It’s very real, but there are multiple ways to figure out what it is, none of them perfect. Your genes derive from your family tree, but imperfectly randomly.
The question of who we are related to also bucks intuition on much broader levels of human ancestry. Leaving DNA aside, if we think of our ancestors simply as people who procreated with each other, we soon run up against an inescapable paradox: We think of genealogy as a simple forking tree, our two parents the product of four grandparents, who are descended from eight great-grandparents, and so on. But such a tree eventually explodes into impossibility. By the time you get back to the time of, say, Charlemagne, you have to draw over a trillion forks. In other words, your ancestors from that generation alone far outnumber all the humans who ever lived. The only way out of that paradox is to join some of those forks back together. In other words, your ancestors must have all been related to each other, either closely or distantly.
Sure, but the real implication is that you are kind of inbred. You have to be. Not enough people were alive to fill in all the trillion slots in your family tree back 40 generations ago around the time of Charlemagne 1200 years ago. If you are of European descent, for example, you are likely descended from Charlemagne by millions of paths. On the other hand, you probably don’t have any bits of Charlemagne’s DNA copied in your DNA. But if you are European you probably have some bits copied from from subjects of Charlemagne.
… If you go back far enough in the history of a human population, you reach a point in time when all the individuals who have any descendants among living people are ancestors of all living people.
Sure, but you have to go back before modern humans, as we see no Neanderthal or Denisovan ancestors among most modern sub-Saharan Africans.
This is why, as has been repeatedly pointed out in recent years, every European alive today is a descendant of Charlemagne. Such ancestral tree-twisting is hard to keep up with, but it reveals that the obsession with being a ‘direct descendant’ of a celebrated historical figure has more to do with the way certain relationships are culturally valued – for example ‘legitimate’ v. ‘illegitimate’ children – than with science. In a sense, we are all royals, even if we don’t all have royal DNA in our genomes. And yet, we are obsessed with genealogies. ‘By one estimate,’ Zimmer writes, ‘genealogy has now become the second most popular search topic on the internet. It is outranked only by porn.’
Like I said, your family tree exists Platonically, like perfect circles and triangles. This doesn’t mean you can perfectly determine it from a DNA scan, but that’s another tool that has become useful in knowing more about your family tree in recent years.
In 2002, a geneticist called Jonathan Pritchard and his team at Oxford, collaborating with Noah Rosenberg at Stanford, found they could use a program they’d designed called Structure to identify clusters of people based only on their DNA. The program scanned genetic variation and assigned each individual’s DNA to one or more groups of ancestors who shared similar variations. When the researchers set the parameters of the program to sort people into five groups, they found clusters that matched the continents the people lived on, which meant the program roughly grouped Africans, Eurasians, East Asians, Pacific Islanders and people in the Americas. Crucially, these ancestral groups didn’t have sharp boundaries. ‘Where two clusters met on a map of the world,’ Zimmer writes, ‘the researchers found people who had some DNA that linked them to one group, and some that linked them to the other.’
In 1491, the Atlantic Ocean was a pretty sharp boundary among “people in the Americas” and Eurasians and Africans. There may or may not have been some genetic exchange in the prior few thousands years, such as the historically recorded Vikings in North America or the recent bizarre finding that, apparently, some Amazonian tribes have a small percentage of genes that look like those of Andaman Islanders in the Indian Ocean, but, in general, as of 1491 the Atlantic was an awfully sharp boundary. And, anyway, how is a sharp boundary crucial? What’s the sharp boundary between young and old or hot and cold? People could walk from, say, China to France in 1491, but how many did? In 1491, could you tell people in China apart from people in France, even if in Turkestan there were people who were kind of in-between?
In fact, he and his collaborators found that ‘the overwhelming amount of genetic diversity was between individuals. The genetic differences between major groups accounted for only 3-5 per cent.’ Rather than defining biological boundaries between racial groups, cutting-edge genetic studies like Pritchard’s suggest a dissolution of these boundaries.
As the late anthropologist Henry Harpending pointed out, racial differences in genes are on the same scale as we see in our extended families. For example, he noted, that if he were informed that he had a grandson he’d never met and drove over to meet him and saw ten boys playing on the front lawn, would he be able to distinguish his grandson from the other boys who aren’t his grandsons? … He’d have a better than random guess, but it wouldn’t be easy to single out one out of ten. On the other hand, could he distinguish one white boy from nine non-white boys? Sure. Race and grandchild relationships turn out to be pretty similar in genetic magnitude.
… ‘But any resemblance between genetic clusters of people and racial categories concocted before genetics existed can have no deep meaning.’
But, oddly enough, the genetic clusters of people and racial categories concocted before genetics by looking at people and testing their biochemistry and asking them where their ancestors came from and so forth and so on give highly similar answers. Not perfectly similar. For example, Carleton Coon in 1965 theorized that just as the Laplanders of Scandinavia appeared to have some East Asian ancestry, the heavily bearded Ainu of Northern Japan were likely to have some Caucasian ancestry. Well, the first turned out to be sort of true, but the Ainu don’t appear to have any particular relation to Europeans.
Just because your genome has variants statistically more similar to variants in the genomes of other people on the same continent, that doesn’t mean you are all members of some shared biological ‘race’, or that you share a similar skin colour, that ubiquitous cultural marker for race.
Just because your ancestors and mine come from the same continent and you and I share the gene markers of people from that continent doesn’t mean that you and I share some biological “race” because, obviously, … well … because Carl Zimmer says you don’t.
… More recently, David Reich’s Who We Are and How We Got Here, as well as his March op-ed in the New York Times, ‘How Genetics Is Changing Our Understanding of “Race”’, prompted an impassioned response.＊ In an open letter published on Buzzfeed 67 scientists flatly stated that Reich ‘misrepresents the many scientists and scholars who have demonstrated the scientific flaws of considering “race” a biological category’.
Because what could David Reich know that 67 Buzzfeed scientists plus Carl Zimmer don’t know?
… To drive this point home, Zimmer shows how the emerging field of paleogenetics uses the DNA extracted from ancient skeletons to offer an even longer view of human history, revealing that we are all genetic mongrels and any notion of biological racial purity is just a fantasy. Among the outmoded biological concepts that science suggests we need to abandon is that of a ‘white’ race.
White people don’t exist, but white guilt is forever.