David Quammen – The Tangled Tree: A Radical New History of Life

David Quammen – The Tangled Tree: A Radical New History of Life

A book on evolution that is causing some waves. In standard Darwinian evolution, genetic traits and mutations are passed on to the next generation only if they affect gametes—sperm and eggs. This is called vertical evolution. A mutation in someone’s skin cells, for example, is non-heritable. Lamarckian evolution, long since disproved, posited that such things could, in fact, be passed on. Some Lamarckians even posited that things like memories or learned aversions could be genetically passed on from one organism to another.

This turned out not to be true. But as scientists are now discovering, there actually is a mechanism for genetic change during the same generation, and a way to pass genetic information horizontally from one cell or organism to another during the same generation, rather than vertically through the generations. This is not Lamarckian evolution in the old sense, but it is conceptually related.

The key to this horizontal evolution is in the large swathes of junk DNA in every organism’s genomes. These lengthy patches don’t activate any traits or seem to do anything. A few do, but most don’t. The new thinking, since 1980 or so and still being tested, is that much of our junk DNA, though not all of it, does not come from mutations. It comes from retroviruses that invade cells and merge with local DNA.

This happens all the time throughout the body. Such mergers are usually genetic gibberish and do nothing. But occasionally the additional code can accidentally cause new characteristics to emerge. But these aren’t passed on to descendants unless they happen to hit the lottery by merging not just with a gamete, but the rare gamete that ends up being fertilized. Despite odds of less than one-in-a-trillion-trillion, these lightning strikes have happened often enough that retroviral junk DNA makes up a sizable portion of every plant and every animal’s genetic code, though the process has taken about two billion years. It’s a good the odds of this happening are so small, otherwise our DNA would be almost endless by this point!

This revelation, especially as concerns non-gamete cells, may someday have significant medical applications, from HIV treatment to cancer. The line between viral diseases and genetic diseases may be a blurry one. But it is too soon to tell, and Quammen could go a bit further in tamping down speculation. Lamarck isn’t vindicated, but he wasn’t entirely wrong, either.

Quammen explains, far better than I can, that this discovery has profound implications for our place in the tree of life, and even the very shape that tree takes. All life is even more deeply interconnected than we already thought. Quammen also tells the story of how this theory of horizontal evolution was thought to be quackery just a few years ago, but is rapidly becoming mainstream thinking among evolutionary biologists. Much of the research happened in Wisconsin, where I was born, and in Illinois, where I now live, which is a nice little coincidence.

Note, however, that horizontal evolution does not displace traditional natural selection over generations. It adds to it.


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