Read She Has Her Mother's Laugh: The Powers, Perversions, and Potential of Heredity Online
Authors: Carl Zimmer
As you move further back in genealogical time, an even bigger paradox looms into view. 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.
The geometry of this heredity has long fascinated mathematicians, and in 1999 a Yale mathematician named
Joseph Chang created the first statistical model of it. He found that it has an astonishing property. 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.
To appreciate how weird this is, think again about Charlemagne. We know for a fact that Charlemagne has some living descendants, thanks to the genealogies proudly drawn by the Order of the Crown. But that fact, according to Chang's model, means that every European alive today is a descendant of Charlemagne. The order is hardly an exclusive club.
When Chang developed his model in 1999, geneticists couldn't compare it to reality. They didn't know enough about the human genome to even guess. By 2013, they had gained the technology they needed.
Coop and his colleague Peter Ralph, a statistician at the University of Southern California, set out to estimate how living Europeans are related to people who lived on the continent hundreds or thousands of years ago. They looked at a database of genetic variants collected across Europe from 2,257 living people. They were able to match identical stretches of DNA in different people's genomes, which they inherited from a common ancestor.
Ralph and Coop identified 1.9 million chunks shared by at least two of the 2,257 people. Some of the chunks were long, meaning they came from recent common ancestors. Others were short, coming from deeper in the past. By analyzing the chunks, Coop and Ralph confirmed Chang's study, but they also enriched it. They found, for example, that people in Turkey and England shared many fairly big chunks of DNA that they must have inherited from a common ancestor who lived less than a thousand years ago. It was statistically impossible for a single ancestor to have provided them all with all those chunks. Instead, living Europeans must have gotten them from many ancestors. In fact, the only way to account for all the shared chunks Coop and Ralph found was with Chang's model. Everyone
alive a thousand years ago who has any descendants today is an ancestor of every living person of European descent.
Even further back in time, Chang and his colleagues have found, the bigger the ancestral circle becomes.
Everyone who was alive five thousand years ago who has any living descendants is an ancestor of
everyone
alive today. The Order of the Crown may be big, but an early pharaoh of Egypt might be able to get a club seven billion strong.
I asked the scientists at the New York Genome Center to look beyond my cousins and use my genome to tell me something about my ancestry. They started with the simplest pieces of DNA to interpret: the mitochondrial DNA I inherited from my mother, and the Y chromosome I inherited from my father. By 2015, geneticists had built massive databases of both types of DNA, with sequences of hundreds of thousands of people. They organized the sequences in much the same way a taxonomist might classify insects, dividing them into classes, dividing those classes into orders, and so on. Large groups of men across the world have certain Y-chromosome mutations in commonâknown as haplogroups. I belong to haplogroup E, I learned. Its ranks are made up mainly of African men, but they also include some men from Europe and the Near East. Within that haplogroup, I belong to a smaller one known as E1, and within that, E1bâand so on all the way down to the haplogroup
E1b1b1c1.
That particular haplogroup includes some Jewish men. While that certainly jibed with my experiences with my father's side of the family, the snug fit began wiggling loose when I looked into the haplogroup further. Only a few percent of Jewish men carry E1b1b1c1. Many men who are not Jewish carry it as well; it's found across a range stretching from Portugal to the Horn of Africa to Armenia. When Napoleon died, one of his followers tucked a few hairs from his beard in a reliquary. In 2011, French researchers managed to extract some of his Y chromosome from them. They found that he belonged to the E1b1b1c1 haplogroup, too. The highest percentage of men with E1b1b1c1 yet found don't live in Israel. They live in the Jordanian city
of Amman. The second-highest percentage can be found among the Amhara, an ethnic group that lives in the highlands of Ethiopia.
The high percentage of men in Jordan with E1b1b1c1 suggests that it first emerged somewhere in the Near East, perhaps as long ago as ten thousand yearsâlong before the Jewish people existed. Thousands of years later, Arabs, Jews, and other peoples of the Near East spread into Africa and Europe, spreading the haplogroup with them. On its own, my E1b1b1c1 haplogroup cannot let me trace its path back through that ancestry (although I'm pretty sure Napoleon isn't my great-great-great-great-grandfather). All I can know is that there was probably an ordinary Near Eastern farmer some ten thousand years ago who acquired a harmless mutation in his Y chromosome that distinguished a new haplogroup, one that he unknowingly passed down to his son. But even among my male ancestors, that farmer holds no special place. He just happened to be the one from whom I inherited my Y chromosome.
On my mother's side, I discovered that I have a mitochondrial haplotype called H1ag1. It's found throughout much of western Europe, and has been found there for quite a while. When a genome sequencing center was built in Hinxton, England, the construction workers dug up across a 2,300-year-old skeleton. It turned out to have some bits of DNA in its bones. The Hinxton skeleton carried H1ag1, just like me. As for the original Ms. H1ag1, however, I can't say that she lived in Hinxton. I can't even say she lived in England.
People carrying the H1ag1 haplotype can be found today across northern Europe. I know I am their kin along the maternal line, but I can't know where our common ancestor lived. Scientists have drawn a tree of all of humanity's known mitochondrial DNA, and on it my H1ag1 branch sprouts next to other branches common in Europe. The European branches split off from branches common in Asia and the New World. The deepest branches on the tree are found in living Africans. By tracing the mutations along all the branches, scientists can estimate the age of the woman who carried the mitochondrial DNA that gave rise to all haplogroups today.
That woman lived in Africa about 157,000 years ago.
The first clues that living humans get their mitochondria from a single woman in Africa first emerged in 1987, thanks to research in the lab of Allan Wilson, a geneticist at the University of California at Berkeley. Reporters swiftly nicknamed this unknown woman Mitochondrial Eve. The name stuck like superglue.
Newsweek
ran a cover story about the research, illustrating their cover with a brown-skinned Adam and Eve.
It would take years for scientists to trace back the Y chromosome of all living men. According to the latest research,
he lived in Africa 190,000 years ago, at the dawn of our entire species. Soon enough that man was christened Y-chromosome Adam. He now enjoys a Wikipedia page of his own. It's easy to imagine Mitochondrial Eve and Y-chromosome Adam as the parents of all humanity, dropped down into a Pleistocene Garden of Eden. The fact that Eve didn't show up in the garden until thirty thousand years after Adam died is one of those minor scientific details that cannot undermine a seductive metaphor.
It took a couple of weeks for Mark Gerstein to work over my genome. He and his students wanted to analyze the short fragments of DNA with their own software and create their own map. Once they had pinned down the location of the vast majority of Illumina's fragments, they could then determine which variants I carried. And they could try to figure out what those variants meant to me. When I paid my second visit to Gerstein, I was surprised that he wasn't leading me back to his office. Instead, he led me to a conference room down the hall.
Eight of Gerstein's graduate students and postdoctoral researchers were waiting for us, flanking two sides of a long table, all with laptops and wireless keyboards at the ready. They had me sit down at the head of the table so that they could show me slides on a giant monitor on the wall in front of me.
The first slide was labeled “Individual Z Overview.”
It was only then that I realized why so many of the scientists I contacted for my little project were proving to be so strangely helpful. To them, I am Individual Z. It was as if I was a frog that had hopped into an anatomy
class with my own dissecting scalpel, asking the students to take a look inside.
For the next two hours, Gerstein's team picked over my genome, showing me broken genes and duplicated genes and genes with mutations that altered how my proteins worked. But what struck me most of all was what they found when they compared my genome to two other people'sâa pair of anonymous volunteers who agreed years beforehand to have their DNA sequenced and made publicly available. One of them was from Nigeria and the other from China.
Gerstein's team identified a total of 3,559,137 bases in my genome that were different from the human reference genome. These variants are known as single-nucleotide polymorphisms, or SNPs for short. They include the variants that make me a carrier for things like familial Mediterranean fever, as well as ones that influence traits that have nothing to do with disease, like my skin color, and ones that have no effect on my biology at all.
The Nigerian and the Chinese had a similar number of single-nucleotide polymorphisms. But those variants did not distinguish the three of us in any clear way. Sushant Kumar, a postdoctoral researcher in Gerstein's lab, made me a Venn diagram to drive the point home. All three of us have 1.4 million single-nucleotide polymorphisms in common. There were another 530,000 that I shared only with the Chinese person but not with the Nigerian. And there were 440,000 single-nucleotide polymorphisms that I shared with the Nigerian alone. All told, 83 percent of my variants were present in at least one of their genomes.
We were three people of African, Asian, and European descent, from three corners of the world. Three races, some might say. And yet we shared far more than what set us apart.
The concept of race is not like the moon or hydrogen. It is not a feature of the natural world beyond our social experience. Up until the Middle Ages, writers never used the word
race
in the sense that it would later take onâreferring to a sharply defined biological group of people whose
members were bound together by heredity. Ancient writers certainly recognized differences among peoples from different parts of the world. But they didn't explain them with taxonomy.
The word
race
seems to have first taken on a modern complexion during the Habsburg rule of Spain. The country was filled with people of different ancestriesâChristian Celts, Romans, Jews, Africans. When the persecution of the Jews began, other Spanish people began to think of themselves as belonging to a particular groupâOld Christians. To prove they were Old Christians, noble Spanish families had to demonstrate that they had no Jewish ancestry. In other words, that they didn't have a single drop of Jewish blood.
Noble families struggled to prove their ancestry had been pure since time immemorial.
When Spain established an empire in the New World, it now had another group of people to distinguish itself from. The Spanish conquistadors, the conquered Indians, and the imported African slaves now shared the same countries. The governments came up with a legal hierarchy with the Spanish on top, Africans in the middle, and Indians at the bottom.
But the people of the New World would not respect those boundaries. Through marriage or rape, people from different races had children together. The colonial governments needed to invent new categories, with new names. In Mexico, the viceroy sliced his subjects into
fine distinctions:
Spaniard and Indian beget
mestizo
Mestizo
and Spanish woman beget
castizo
Castizo
woman and Spaniard beget Spaniard
Spanish woman and Negro beget
mulato
Spaniard and
mulato
woman beget
morisco
Morisco
woman and Spaniard beget
albino
Spaniard and
albino
woman beget
torno atrás
Indian and
torno atrás
woman beget
lobo
Lobo
and Indian woman beget
zambaigo
Zambaigo
and Indian woman beget
cambujo
Cambujo
and
mulato
woman beget
albarazado
Albarazado
and
mulato
woman beget
barcino
Barcino
and
mulato
woman beget
coyote
Coyote
woman and Indian beget
chamiso
Chamiso
woman and
mestizo
beget
coyote mestizo
Coyote mestizo
and
mulato
woman beget
ahà te estás
To the north, England brought Africans to their own colonies in the 1600s to work their fields. The Africans worked at first alongside European servants, subject to the same laws, but over the course of decades the colonial governments
gradually singled out the people from Africa for harsh treatment. By the early 1700s, free Negroes had lost the right to vote or bear arms, while those still enslaved were recognized by the law as slaves for life, and their children inherited their bondage.