We Are Animals Living in a Microbial World

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"Living in a material world,
And I am a material girl."

--"Material Girl" by Madonna

That song would have been much better if the word "material" was replaced by "microbial." Let me explain.

Animal evolution traditionally has been viewed as the result of interactions between animals or with the physical environment. For instance, bunny rabbits became fairly good at sprinting quickly because the slower ones got eaten by wolves. Likewise, because of environmental temperature fluctuations, mammals evolved hair to help keep their bodies warm. (Interestingly, elephants seem to use their sparse hairs to help cool off!)

However, this understanding of evolution overlooks a huge missing piece of the puzzle: bacteria. To put it bluntly, complex lifeforms probably would never have evolved on planet Earth if it weren't for bacteria. Besides oxygenating the early Earth, bacteria also play crucial roles in nutrient cycling. On a personal level, inside your body, friendly bacteria help metabolize food, block dangerous pathogens and even help develop the immune response. In fact, there are about 100 trillion of them and only 10 trillion cells of "you," meaning that your own body cells are outnumbered 10-to-1!

Because bacteria have always interacted with animals, it is very likely that bacteria helped shape animal evolution. And that is exactly the case being made by a new "Perspectives" article in PNAS titled, "Animals in a Bacterial World, a New Imperative for the Life Sciences."

The authors list several dozen examples to bolster their case, but a few in particular are worth mentioning.

About 3.8 billion years ago, a primordial cell formed, known as the last universal ancestor (LUA). From this organism, three domains of life evolved: Bacteria, Archaea (weird and bacteria-like) and Eukaryota (nucleated cells and multicellular organisms).
Since all life is related, most organisms have about 1/3 of their genes in common. Of the roughly 23,000 genes in the human genome, for instance, 37% are similar to genes in Bacteria and Archaea. Another 28% are similar to genes in unicellular eukaryotes. (Think: yeast and algae.) Thus, a full 65% of human genes show similarity to microbes. Only 6% are found uniquely in primates.

Not only do animals share evolutionary history with microbes, but they also continue to interact with them on a daily basis -- often in very profound ways. Bacteria living inside of animals can provide them with metabolic capabilities that the animal itself does not possess. Cows couldn't eat grass if it weren't for the resident microbes that ferment it in the cow's rumen. Surely, the evolution of the cow was heavily influenced by -- if not largely dependent on -- its microbial allies.

But cows aren't the only ones with helpful bacteria in their digestive system. As mentioned above, humans also come with a full complement of gut flora. In addition to aiding our digestive system, gut microorganisms also influence distant parts of the body -- including the brain. It is now thought that gut bacteria interact with the vagus nerve, allowing them to alter brain physiology.

The authors conclude that disturbing bacterial communities -- whether within our own bodies or in the environment -- could potentially have devastating impacts on all the rest of life. Considering that this is indeed a microbial world, they might very well be right.

Source: Margaret McFall-Ngai et al. "Animals in a bacterial world, a new imperative for the life sciences." PNAS. Published online before print: February 7, 2013. doi: 10.1073/pnas.1218525110

(Image: Phylogenetic Tree of Life via Wikimedia Commons)

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