Deeper ecology

Carl Zimmer writes:

We are, fundamentally, a fusion. As I wrote in my essay [subscription req'd - I don't have one] for Science on the origin of eukaryotes, there's now a wealth of evidence that our cells evolved from the combination of two different microbes. The mitochondria that generate fuel for our cells started out as free-living bacteria. Today, they still retain traces of their origin in the bacterial DNA they carry, as well as their bacterial structure, including the membrane within a membrane that envelops them.
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In today's issue of Nature, James Lake [of the University of California, Los Angeles, a veteran researcher on the early history of life] questions whether we may be too quick to assume that only eukaryotes are the result of fusion. He observes that aphids depend on a species of bacterium called Buchnera to digest their food, and Buchnera in turn contains other bacteria on which its own survival depends. These two bacteria are still distinct enough from each other that we can tell them apart. But what if two bacteria joined together billions of years ago and their identities blurred together? How would we tell them apart?
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When scientists dredge up muck from the ocean floor, for example, they often find different species of microbes living together in tight clumps. They have to live close to other species to survive because each species takes care of chemical reactions that their partners can't carry out on their own. That intimacy makes it easier for individual genes to move from host to host, as viruses infect different microbes or as microbes die and other microbes slurp up their genes.
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Our cells, in other words, are not just microbes within microbes; they are microbes within microbes within microbes: a true Russian doll of evolution.

Microbes Within Microbes Within Microbes

Slightly related post: Matryoshka selves, strange loops and inner voices