Understanding Horizontal Gene Transfer In 'The Tangled Tree'
SCOTT SIMON, HOST:
Is our human view of life and lineage profoundly wrong? Darwin's tree of life, by which many of us see our ancestry - with genes and traits passed vertically, root to branch, from parent to child and so on for centuries - doesn't include what science has discovered over the past couple of generations. Horizontal gene transfer or HGT - genetic matter we don't inherit but acquire sideways, if you please, virally from other organisms, even other species. Roughly 8 percent of the human genome arrived that way. The tree of life is really a web.
David Quammen traces the story and implications of this discovery in his new book "The Tangled Tree: A Radical New History Of Life." He's an award-winning science, nature and travel writer. His work has appeared in National Geographic, Rolling Stone and The New York Times' Book Review. David Quammen joins us now from Bozeman, Mont. Thanks so much for being with us.
DAVID QUAMMEN: Thank you, Scott. It's great to talk with you.
SIMON: Has horizontal gene transfer been amazingly unheralded?
QUAMMEN: It has been. It's been known among some corners of microbiology since the 1950s. But it was really the explosion of genome sequencing that revealed it during the 1990s and into the 2000s.
SIMON: What does it mean that that we are, as you call us, composite creatures?
QUAMMEN: Well, we now know because of work in this field that, as you said, 8 percent of our human genome is viral DNA acquired by infection, by retroviruses, that got into not just our immune cells, the way the retrovirus HIV does, but into our genomes, into the reproductive cells and therefore became inherited. And there are also other parts of us that come by sideways transfer, in one form or another. Every one of our cells contains a little organelle called mitochondria. And we now know those are acquired bacteria - captured bacteria going back billions of years.
SIMON: One example you provide is the gene that produces a membrane between the placenta and the fetus.
QUAMMEN: That's right. This is a gene called syncytin-2. And it is acquired from a retrovirus. In the retrovirus, it forms an envelope - a sort of a membrane around the virus capsule. But it was acquired by animals and became the gene in mammals that creates a membrane between the placenta and the fetus. So without this acquired, adapted, repurposed viral gene, it would be impossible for a fetus to come to term in a human being.
SIMON: And to understand this, we are getting genetic matter from non-human - even non-primate sources.
QUAMMEN: That's correct.
SIMON: Boy, this really revises Darwinism, doesn't it?
QUAMMEN: It does revise Darwinism. The canonical view of evolution - the Darwinian view is that evolution occurs as genes descend from parents to offspring and are very gradually modified and branches diverge. The tree of life is the model used because it chose branches diverging. But now we understand that innovation in genomes doesn't always come gradually. Sometimes it comes suddenly, in an instant, by horizontal gene transfer. And that represents the convergence not the divergence of lineages.
SIMON: Is this the way, for example, so many anti-resistant bacteria have been able to protect themselves in recent years?
QUAMMEN: Absolutely. And resistance to one form of antibiotic in one bacterial strain arises gradually. But we now know that whole genes and packets of genes that confer that sort of resistance against multiple kinds of antibiotic can move sideways in an instant from one form of bacteria, from one species of bacteria into another - from salmonella into staphylococcus, from E. coli into streptococcus. And that's why this problem has spread around the world so quickly.
SIMON: This isn't a new danger that's been set off by any genetic tinkering? This is just nature, isn't it?
QUAMMEN: That's right. Movement of genes from one kind of bacterium to another, potentially making one more virulent, that's a natural phenomenon. It's been happening forever.
SIMON: What are some of the results of what, at one point, you refer to is this relentless bacterial togetherness? What are the implications we need to know about?
QUAMMEN: Just for understanding of who we are, we humans, what human identity means or does not mean, what the concept of a species means or does not mean and all of those things have been radically revised.
SIMON: Should it make us as human beings feel less individual?
QUAMMEN: Well, I think it should make us feel humble. Each human is an individual still in a very, very important sense. But it should remind us of that important Darwinian truth - and I think it's the deepest and the darkest of Darwinian truths - that we humans are not separate from nature. We're not above nature. We're part of nature. And now we know that not only is that true in the sense that we are animals evolved from other animals, but we also contain bacteria. We contain viruses. We are individuals, but we are individuals that are mosaics encompassing other individuals.
SIMON: I mean, the implication is we look out at the world and not just see that we have a place in it, see that we're all tied up in it.
QUAMMEN: That's right. What we might have thought of as our deepest identity, our genome, is a mosaic of pieces that have come from other branches on the tree of life. Now, I think we should celebrate that. We should be happy because we humans have figured that out. That's an enormously exhilarating accomplishment for the human species.
SIMON: David Quammen - his book, "The Tangled Tree." Thanks so much for being with us.
QUAMMEN: Thank you, Scott. Real pleasure to talk with you.
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