Recently my spouse & I reviewed Jennifer Raff’s Origin: A Genetic History of the Americas for the American Biology Teacher magazine (in brief: Raff’s book is lovely, you should read it! I’ll include a link to our review once it’s published!), which deftly balances twin goals of disseminating scientific findings and honoring traditional knowledge.

By the time European immigrants reached the Americas, many of the people living here told stories suggesting that their ancestors had always inhabited these lands. This is not literally true. We have very good evidence that all human species – including Homo sapiens, Homo neaderthalensis, and Homo denisovans among possible others – first lived in Africa. Their descendants then migrated around the globe over a period of a few hundred thousand years.

As best we know, no lasting population of humans reached the Americas until about twenty thousand years ago (by which time most human species had gone extinct – only Homo sapiens remained).

During the most recent ice age, a few thousand humans lived in an isolated, Texas-sized grassland called Beringia for perhaps a few thousand years. They were cut off from other humans to the west and an entire continent to the east by glacial ice sheets. By about twenty thousand years ago, though, some members of this group ventured south by boat and established new homes along the shoreline.

By about ten thousand years ago, and perhaps earlier, descendants of these travelers reached the southern tip of South America, the eastern seaboard of North America, and everywhere between. This spread was likely quite rapid (from the perspective of an evolutionary biologist) based on the diversity of local languages that had developed by the time Europeans arrived, about five hundred years ago.

So, by the time Europeans arrived, some groups of people had probably been living in place for nearly 10,000 years. This is not “always” from a scientific perspective, which judges our planet to be over 4,000,000,000 years old. But this is “always” when in conversation with an immigrant who believes the planet to be about 4,000 years old. Compared with Isaac Newton’s interpretation of Genesis, the First People had been living here long before God created Adam and Eve.

If “In the beginning …” marks the beginning of time, then, yes, their people had always lived here.

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I found myself reflecting on the balance between scientific & traditional knowledge while reading Gabriel Andrade’s essay, “How ‘Indigenous Ways of Knowing’ Works in Venezuela.” Andrade describes his interactions with students who hold the traditional belief in partible paternity: that semen is the stuff of life from which human babies are formed, and so every cis-man who ejaculates during penetrative sex with a pregnant person becomes a father to the child.

Such beliefs might have been common among ancient humans – from their behavior, it appears that contemporary chimpanzees might also hold similar beliefs – and were almost certainly widespread among the First Peoples of South America.

I appreciate partible paternity because, although this belief is often framed in misogynistic language – inaccurately grandiose claims about the role of semen in fetal development, often while ignoring the huge contribution of a pregnant person’s body – the belief makes the world better. People who are or might become pregnant are given more freedom. Other parents, typically men, are encouraged to help many children.

Replacing belief in partible paternity with a scientifically “correct” understanding of reproduction would probably make the world worse – people who might become pregnant would be permitted less freedom, and potential parents might cease to aid children whom they didn’t know to be their own genetic offspring.

Also, the traditional knowledge – belief in partible paternity – might be correct.

Obviously, there’s a question of relationships – what makes someone a parent? But I also mean something more biological — a human child actually can have three or more genetic contributors among their parents.

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Presumably you know the scientific version of human reproduction. To wit: a single sperm cell merges with a single egg cell. This egg rapidly changes to exclude all the other sperm cells surrounding it, then implants in the uterine lining. Over the next nine months, this pluripotent cell divides repeatedly to form the entire body of a child. The resulting child has exactly two parents. Every cell in the child’s body has the same 3 billion base pair long genome.

No scientist believes in this simplified version. For instance, every time a cell divides, the entire genome must be copied – each time, this process will create a few mistakes. By the time a human child is ready to be born, their cells will have divided so many times that the genome of a cell in the hand is different from the genome of a cell in the liver or in the brain.

In Unique, David Linden writes that:

Until recently, reading someone’s DNA required a goodly amount of it: you’d take a blood draw or a cheek swab and pool the DNA from many cells before loading it into the sequencing machine.

However, in recent years it has become possible to read the complete sequence of DNA, all three billion or so nucleotides, from individual cells, such as a single skin cell or neuron. With this technique in hand, Christopher Walsh and his coworkers at Boston Children’s Hopsital and Harvard Medical School isolated thirty-six individual neurons from three healthy postmortem human brains and then determined the complete genetic sequence for each of them.

This revealed that no two neurons had exactly the same DNA sequence. In fact, each neuron harbored, on average, about 1,500 single-nucleotide mutations. That’s 1,500 nucleotides out of a total of three billion in the entire genome – a very low rate, but those mutations can have important consequences. For example, one was in a gene that instructs the production of an ion channel protein that’s crucial for electrical signaling in neurons. If this mutation were present in a group of neurons, instead of just one, it could cause epilepsy.

No human has a genome: we are composite creatures.

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Most scientists do believe that all these unique individual genomes inside your cells were composed by combining genetic information from your two parents and then layering on novel mutations. But we don’t know how often this is false.

Pluripotent (“able to form many things”) cells from a developing human embryo / fetus / baby can travel throughout a pregnant person’s body. This is quite common – most people with XX chromosomes who have given birth to people with XY chromosomes will have cells with Y chromosomes in their brains. During the gestation of twins, the twins often swap cells (and therefore genomes).

At the time of birth, most humans aren’t twins, but many of us do start that way. There’s only a one in fifty chance of twin birth following a dizygotic pregnancy (the fertilization of two or more eggs cells released during a single ovulation). Usually what happens next is a merger or absorption of one set of these cells by another, resulting in a single child. When this occurs, different regions of a person’s body end up with distinct genetic lineages, but it’s difficult to identify. Before the advent of genetic sequencing, you might notice only if there was a difference in eye, skin, or hair color from one part of a person’s body to the next. Even now, you’ll only notice if you sequence full genomes from several regions of a person’s body and find that they’re distinct.

For a person to have more than two genetic contributors, there would have to be a dizygotic pregnancy in which sperm cells from unique individuals merged with the two eggs.

In the United States, where the dominant culture is such that people who are trying to get pregnant are exhorted not to mate with multiple individuals, studies conducted in the 1990s found that at least one set of every few hundred twins had separate fathers (termed “heteropaternal superfecundication”). In these cases, the children almost certainly had genomes derived from the genetic contributions of three separate people (although each individual cell in the children’s bodies would have a genome derived from only two genetic contributors).

So, we actually know that partible paternity is real. Because it’s so difficult to notice, our current estimates are probably lower bounds. If 1:400 were the rate among live twins, probably that many dizygotic pregnancies in the United States also result from three or more genetic contributors. Probably this frequency is higher in cultures that celebrate rather than castigate this practice.

Honestly, I could be persuaded that estimates ranging anywhere from 1:20 to 1:4,000 were reasonable for the frequency that individuals from these cultures have three or more genetic contributors.** We just don’t know.

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I agree with Gabriel Andrade that we’d like for medical students who grew up believing in partible paternity to benefit from our scientific understanding of genetics and inheritance – this scientific knowledge will help them help their patients. But I also believe that, even in this extreme case, the traditional knowledge should be respected. It’s not as inaccurate as we might reflexively believe!

The scientific uncertainty I’ve described above doesn’t quite match the traditional knowledge, though. A person can only receive genetic inheritance from, ahem, mating events that happen during ovulation, whereas partible paternity belief systems also treat everyone who has sex with the pregnant person over the next few months as a parent, too.

But there’s a big difference between contributing genes and being a parent. In Our Transgenic Future: Spider Goats, Genetic Modification, and the Will to Change Nature, Lisa Jean Moore discusses the many parents who have helped raise the three children she conceived through artificial insemination. Even after Moore’s romantic relationships with some of these people ended, they remained parents to her children. The parental bond, like all human relationships, is created by the relationship itself.

This should go without saying, but: foster families are families. Adopted families are families. Families are families.

Partible paternity is a belief that makes itself real.

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** A note on the math: Dizygotic fertilization appears to account for 1:10 human births, and in each of these cases there is probably at least some degree of chimerism in the resulting child. My upper estimate for the frequency that individuals have three or more genetic contributors, 1:20, would be if sperm from multiple individuals had exactly equal probabilities of fertilizing each of the two egg cells. My lower estimate of 1:4,000 would be if dizygotic fertilization from multiple individuals had the same odds as the 1:400 that fraternal twin pairs in the U.S. have distinct primary genetic contributors. Presumably a culture that actively pursues partible paternity would have a higher rate than this, but we don’t know for sure. And in any case, these are large numbers! Up to 5% of people from these cultures might actually have three or more genetic contributors, which is both biologically relevant and something that we’d be likely to overlook if we ignored the traditional Indigenous knowledge about partible paternity.

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header image from Zappy’s Technology Solution on flickr