"Tree of Life" is Tangled: Fusion

When we think of evolution, most of us have the “tree of life” view. Life started. Mutations occurred. Most were harmful, but a few proffered some advantages, and became more widespread. Over time, the extent of change due to cumulative mutations was so large that they couldn’t mate, and that’s when we say a new species has arrived. Or to use the tree of life analogy: the trunk of the “tree” has split into two branches. And the process repeats itself. The branches fork further and so we eventually see so many species.

 

This is also the view Darwin himself had. But over time, science and technology is showing that the tree of life view isn’t entirely right. Since the “tree” view is so widespread and easy to understand, David Quammen calls his book on the topic as The Tangled Tree.

 

Evolution moves slowly. Or so we are taught. Yet, the difference between bacteria and pretty much all complex life forms is colossal. How did that happen? Lynn Margulis noticed a key difference between bacteria and complex life forms is the existence of mitochondria within cells. Therefore, she ventured, at one point one organism (mitochondria) took symbiosis (mutual benefits) to an extreme when it moved into the cell of the other organism. She termed it “endosymbiosis”:

“It was the biggest leap in the history of life… reflected in the differences between bacteria and more complex organisms.”

The theory wasn’t accepted initially for obvious reasons: it seems too improbable. But then the DNA of the mitochondria was shown to be different from the DNA of the rest of the body, and that certainly aligns with her fused organism theory. The full implication isn’t obvious, so let’s spell it out:

“One organism could exist permanently inside the cells of another, replicating itself as the cells replicate themselves, and becoming part of a new, composite and heritable entity.”

 

Notice how this doesn’t fit into our tree analogy? Branches don’t merge, they fork. But endosymbiosis was telling that branches can merge. Not often, but it happens. The tree was starting to get tangled…