According to Charles Darwin, you are the product of your biological mother and father, just as they were the product of their biological mother and father, and so on back to the earliest days of humans and to all the species that came before.
In this view of evolution, inheritance always flows vertically, with genes — and the traits that they define — passing from ancestor to descendent. This is often depicted as a tree, with species branching from the trunks and limbs, always diverging from what came before, but never converging.
While this is a nice story, and an easy way to depict the history of life, it turns out that it’s only partially correct. The truth is much messier and raises many questions about your identity and the nature of the human species.
“We are not precisely who we thought we were. We are composite creatures, and our ancestry seems to arise from a dark zone of the living world, a group of creatures about which science, until recent decades, was ignorant,” writes science writer David Quammen in The Tangled Tree: A Radical New History of Life .
What he’s referring to is the recent discovery that all animals, plants and fungi may have descended from the Archaea, the bacteria-like third domain of life (alongside the bacterial Prokarya and the Eukarya, which make up everything with a cell nucleus, including humans).
The archaea are odd, ancient microbes, often living in extreme environments like hot springs, sewage and salty lakes. Some even produce energy without needing oxygen and make methane.
These creatures don’t seem very humanlike. But recent research, writes Quammen in a New York Times article, suggests that human cells resemble one group of Archaea known as the Lokiarcheota, which live near a hydrothermal vent on the bottom of the Atlantic Ocean.
But there’s more to our tangled microbial past. As science writer Ed Yong describes in his book, I Contain Multitudes: The Microbes Within Us and a Grander View of Life, our bodies are teeming with microorganisms. They live on our skin and in our mouth, throat, stomach and gut. Some researchers estimate that our bodies have 1.3 times as many microbial cells as human cells.
These aren’t just lazy hitchhikers. They play important roles in our health — helping us digest our food, making vitamins in our gut and affecting our mental health. But our microbial connections go much, much deeper than our intestines — all the way to the nucleus of our cells (which, as Eukaryotes, we have).
Quammen writes in his book: “Roughly 8 percent of the human genome consists of the remnants of retroviruses that have invaded our lineage — invaded the DNA, not just the bodies, of our ancestors — and stayed. We are at least one-twelfth viral.” We are like an internet meme that’s gone viral.
It’s not clear, though, at what point humans — or pre-human species — picked up this viral DNA. Also, while some of the viral DNA is nonsense (meaning it has no apparent function), other bits play important roles in the functioning of our bodies.
Quammen writes in the New York Times that French scientist Thierry Heidmann and his colleagues found that one bit of viral DNA is responsible for “creating an essential layer between the placenta and the fetus during pregnancy.” In the virus, this gene creates an envelope around the virus — in people, it makes a different kind of wrapper, in this case, around the fetus.
The placenta is essential because it allows nutrients to be delivered to the fetus and waste products removed, but it also keep’s the mother’s immune system from attacking the fetus as a foreign body. The placenta is also a defining characteristic of mammals — it’s part of what makes us who we are.
Humans are not unique in carrying DNA from microorganisms — an opossum from South America, a frog from West Africa and a tenrec from Madagascar have all picked up sections of DNA from somewhere other than their direct ancestors.
This type of transfer is sometimes referred to as “infective heredity” — what Quammen calls “an infection that transforms identity,” or the genetic equivalent of a blood transfusion.
In bacteria, it’s known as horizontal gene transfer, which differs from the vertical gene transfer of an organism passing its genes onto its offspring. Horizontal gene transfer is common among bacteria. This is how antibiotic resistance passes from one species of bacteria to another.
In the earliest days of life on earth, this process was even more common. Bacteria were floating around in the primordial soup with very permeable membranes, allowing them to easily transfer genetic traits between individuals. At some point in the history of life, bacteria — and later organisms — became more protective of their DNA. But not entirely.
With more powerful genetic sequencing, scientists have been able to read the genomes of humans and many other species in detail. As this genetic knowledge has grown, the separateness between seemingly distant organisms has started to break down.
We now know that inheritance doesn’t just pass from parent to offspring, but also sends branches across the canopy to other species. Some researchers have tried to capture this genetic overlap by redrawing the tree of life. Quammen described one of these attempts as “a tangle of pipes in someone’s basement, set in place by a manic plumber.”
Even beyond how to draw the family tree, these discoveries raise questions of what it means to be human, or to be you as an individual. If you take away all the bacteria living inside you, would you be the same person (or even able to survive)? What about the viral DNA lurking inside your cells?
As Quammen puts it on an episode of RadioLab: “The categories that we apply to the world — categories like individual and species — now appear more blurry. The edges are fuzzy.”