"Tree of Life" is Tangled: the Web

In the last blog, we saw how “endosymbiosis” complicated the idea of the tree of life. David Quammen’s book, The Tangled Tree, continues.

 

Why have bacteria gotten resistant to antibiotics? The usual culprits are indiscriminate over-use, and the evolution of bacteria themselves. But here’s a less known point. The genes that evolved in one bacterial species to confer resistance to the antibiotics seemed to become prevalent in almost all bacterial species. How could this be? Sure, it could happen by chance in one species, but how could the same exact solution popup in other species?

 

Wait, it gets weirder. Did you know that some of these antibiotics resistant genes existed even before the first antibiotics were discovered by humans? Huh? This sounds like a reversal of cause-and-effect! What was going on?

 

But before we answer those questions, let’s go to the 1920’s, when Fred Griffith stumbled onto something when testing two variants of a bacteria. Type I was virulent (dangerous), Type II was harmless. He killed Type I (the deadly one) and mixed it with the living, harmless variant. What happened next was stunning:

“Dead virulent I, plus living mild II, becomes… living virulent I. Something weird had happened. It sounded like zombie bacteria.”

Either the dead bacteria had come back to life. Or the dead ones had somehow converted the living ones into something, er, deadly:

“This wasn’t a sci-fi movie, and neither of those options was supposed to be possible.”

 

Does it all sound like that riddle, wrapped in a mystery, inside an enigma?

 

Now let’s see the answers. First up, the antibiotic resistant genes that existed before antibiotics. Turns out many antibiotics are derived from compounds. But near those very compounds, bacteria have been living for ages. Ergo, they needed to develop resistance to the compound… long before humans discovered antibiotics. And thus, those genes had evolved already… in certain bacteria.

 

Next up, how did those genes move across species of bacteria? Turns out our conventional view that genes can only be inherited vertically (parent to child) is wrong. It turns out genes can move horizontally, from one individual to another, and even across species. The popular term for this is Horizontal Gene Transfer (HGT). And HGT is how those antibiotic resistant genes moved, and continue to move across bacterial species.

 

Last up, remember that dead virulent I + living mild II = living virulent I case? It was again a case of HGT, the transfer of virulent genes across variants. HGT can happen between living and dead entities as well.

 

Soon, scientists could find long gene sequences that were common across totally unrelated species, across sizes, across kingdoms, across levels of complexity, including humans. Those HGT transfers were the cause, and it has been going on for billions of years.

 

HGT has major implications on how life works. It blurs the line between species since genes are exchanged across species. It also shows that recombination of genes can happen without sex or random mutations. Were such transfers always useful to the recipient? Sometimes yes, sometimes they were harmful, and often they didn’t do any good or bad. The last scenario (neither good nor bad) was yet another example of the famous selfish gene: it only cares to make copies of itself.

 

The tree of life view, with its clean forking-only idea, had moved from Darwin’s time to include the occasional convergence (last blog) and now included something that looks like a web of interconnections. As you might imagine, the details of how much HGT changed the picture was hotly debated. It was tongue-in-cheek called the war between “tree huggers” and “tree cutters”, the tree in this case being the tree of life! Leading Quammen to remark:

“Some scientists did appear to be more religious about science than about religion.”

 

Quammen’s’ book title of the tree being tangled is so apt, isn’t it?