The Loom : Carrying Ancient History In The GutAlas, the story is more complex than this, so you do need to and read Carl Zimmer's essay. Fascinating stuff.
... Giardia, many researchers suspected, was one of those early-branching eukaryotes. This suspicion was generated at first by simply eyeballing the creatures. They are quite weird. Their teardrop-shaped bodies have eight tails for swimming and a suction pad to clamp onto the wall of the intestines. They also carry two nuclei, each with its own DNA. How Giardia manages to keep all those genes coordinated--and why it even has two nuclei--remain mysteries. Bizarre single-celled eukaryotes are pretty easy to find. What set Giardia apart from most other eukaryotes was what it lacked. Scientists could not find a lot of those compartments in which the business of most eukaryote cells takes place.
Most significantly, it was missing mitochondria. Lots of things take place inside these sausage-shaped structures, most importantly the generation of ATP, the energy-bearing molecule found in all living things. Mitochondria started out as free-living bacteria and later evolved into permanent symbionts inside the eukaryote cell. (Mitochondria still carry some DNA of their own, which bears a strong resemblance to one group of free-living bacteria.) The fact that Giardia seemed to be missing mitochondria hinted that it was a transitional eukaryote...
I probably invited this bug onboard during a particularly rough college train trip through Mexico. I didn't realize I was carrying a possible echo of biology's "big bang".
Incidentally, speaking of parallels to cosmology, there's apparently a suspicion among some biologists that there's a kind of "dark matter" component to biology. They're looking for terrestrial life forms so bizarre that we don't even recognize them as "living" ...
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