Since the late 1970s, there have come three big surprises about what we humans are and about how life on our planet has evolved.
The first of those three surprises involves a whole category of life, previously unsuspected and now known as the archaea. (They look like bacteria through a microscope, but their DNA reveals they are shockingly different.) Another is a mode of hereditary change that was also unsuspected, now called horizontal gene transfer. (Heredity was supposed to move only vertically, from parents to offspring.) The third is a revelation, or anyway a strong likelihood, about our own deepest ancestry. (It seems now that our lineage traces to the archaea.) So we ourselves probably come from creatures that, as recently as forty years ago, were unknown to exist.
One of the most disorienting results of these developments is a new challenge to the concept of “species.” Biologists have long recognized that the boundaries of one species may blur into another—by the process of hybridism, for instance. And the notion of species is especially insecure in the realm of bacteria and archaea. But the discovery that horizontal gene transfer (HGT) has occurred naturally, many times, even in the lineages of animals and plants, has brought the categorical reality of a species into greater question than ever. That’s even true for us humans—we are composite individuals, mosaics.
It’s not just that—as you may have read in magazine articles—your human body contains at least as many bacterial cells as it does human cells. (This doesn’t even count all the nonbacterial microbes—the virus particles, fungal cells, archaea, and other teeny passengers inhabiting our guts, mouths, nostrils, and other bodily surfaces.) That’s the microbiome. Each of us is an ecosystem.
I’m talking about something else, a bigger and more shocking discovery that has come from the revolution in a field called molecular phylogenetics. (That phrase sounds fancy and technical, but it means merely the use of molecular information, such as DNA or RNA sequences, in discerning how one creature is related to another.) The discovery was that sizeable chunks of the genomes of all kinds of animals, including us, have been acquired by horizontal transfer from bacteria or other alien species.
How could that be possible? How could genes move sideways, between species, not just vertically along ancestral lineages? The mechanisms are complex, but one label that fits most of them is “infective heredity.” DNA can be carried across boundaries, from one genome to another, by infective agents such as bacteria and viruses. Such horizontal gene transfer, like sex, has been a source of freshening innovation in otherwise discrete lineages, including ours—and it is still occurring.
This is an aspect of evolution that was unimagined by Charles Darwin. Evolution is trickier, far more intricate, than we had realized. The tree of life is more tangled.
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These discoveries should not merely complicate our magisterial human self-image, but also help lead us toward a wiser and humbler understanding of our place—collectively and as “individuals” within the “species” Homo sapiens—in the story of life on Earth.
It’s a story in which we humans are important protagonists but not the ultimate and predestined heroes. It’s a story in which heredity has moved sideways as well as vertically and all the conventional hierarchies and boundaries have proven more imperfect, transgressible, and leaky than we had supposed. But these revelations don’t diminish our responsibility, as humans, to respect and preserve the diversity of living creatures, with all their own mosaic genomes and tangled lineages, who cohabit the planet with us. On the contrary, I think. All this should make us only more amazed, respectful, and careful. Life on Earth is wondrous precisely because it’s so complicated.
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