One Freakish Event That Led to You and Ewe and Yew

Life arose on earth over three billion years ago, and for a long time, there were only one-celled organisms. These prokaryotes diverged and evolved in many ways, but making the leap from one cell to many cells (eukaryotes) was a paradigm shift that led to every living thing on earth that's big enough for us to see -including us. How did that happen? Before we could sequence genes, the prevailing theory was a gradual development as cells mutated, diverged, and evolved. However, recent genetic research has led credence to the idea that the first two-celled organism was a merger that only happened once.

The alternative—let’s call it the “sudden-origin” camp—is very different. It dispenses with slow, Darwinian progress and says that eukaryotes were born through the abrupt and dramatic union of two prokaryotes. One was a bacterium. The other was part of the other great lineage of prokaryotes: the archaea. (More about them later.) These two microbes look superficially alike, but they are as different in their biochemistry as PCs and Macs are in their operating systems. By merging, they created, in effect, the starting point for the first eukaryotes.

Bill Martin and Miklós Müller put forward one of the earliest versions of this idea in 1998. They called it the hydrogen hypothesis. It involved an ancient archaeon that, like many modern members, drew energy by bonding hydrogen and carbon dioxide to make methane. It partnered with a bacterium that produced hydrogen and carbon dioxide, which the archaeon could then use. Over time, they became inseparable, and the bacterium became a mitochondrion.

There are many variants of this hypothesis, which differ in the reasons for the merger and the exact identities of the archaeon and the bacterium that were involved. But they are all united by one critical feature setting them apart from the gradual-origin ideas: They all say that the host cell was still a bona fide prokaryote. It was an archaeon, through and through. It had not started to grow in size. It did not have a nucleus. It was not on the path to becoming a eukaryote; it set off down that path because it merged with a bacterium. As Martin puts it, “The inventions came later.”

The theory that an archaeon and a bacterium merged to make eukaryotes would give us a new tree of life that doesn't always fork. I couldn't help but picture this comic when reading about a singular event that changed everything. Ed Yong explains the developments that led to this idea at Nautilus.