On the gross level (and I mean that in both senses of the word), there is the sheer number of cells in us that are not human. The adult human body has about 10 trillion human cells, and (depending on who you talk to) between 1 and 3 times more bacterial cells -- intestinal flora, bacteria hitching a ride on our skin, in our mouths, in our respiratory mucosa. Most of these are commensals at the very worst -- neither harmful nor helpful -- but a significant number are in a mutualistic arrangement with us, which is one of several reasons why the overuse of antibiotics is a bad idea.
Then there are the little invaders we can't live without -- namely the mitochondria, those tiny organelles that every high school biology student knows are the "powerhouses of the cell." What fewer people know is that they are actually separate organisms, descended from aerobic prokaryotes that colonized our cells 2.5 billion years ago (give or take a day or two). They have their own DNA, and reproduce inside our cells by binary fission the same way they did when they were free-living proto-bacteria.
Mitochondria [image courtesy of Louisa Howard and the Wikimedia Commons]
But that's not all. If you're a plant (I'm assuming you're not, but you never know), you have three separate ancestral lines -- your ordinary plant cells, the mitochondria, and the chloroplasts, which are also little single-celled invaders that now plants can't live without. But even that's not the most extreme example -- the microorganism Mixotricha paradoxa is a composite being made up of five completely separate ancestral genomes that have fused together into one organism.
But back to humans, if you're not already so skeeved out that you've stopped reading. Because it's even more complicated than what I've already told you -- geneticists Cedric Feschotte , Edward Chuong and Nels Elde of the University of Utah have just published a paper in which we find out that even our nuclear DNA isn't entirely human. 10% of our 30,000-odd genes and three-billion-odd base pairs...
... came from viruses.
We usually think of viruses as pesky little parasites that cause colds, flu, measles, mumps, and so on, but they're more than that. Some of them -- the retroviruses (HIV being the best-known example) -- are capable of inserting genetic material into the host's DNA, thus altering what the host does. Certainly, sometimes this is bad; both AIDS and feline leukemia are outcomes of this process. But now Feschotte, Chuong, and Elde have shown that some of our viral hangers-on have had their genes repurposed to work in our benefit.
These stowaway bits of DNA are called "endogenous retroviruses" (ERVs), and some of them seem to be associated with cancer. Others have been implicated in multiple sclerosis and schizophrenia. But what the researchers found is that not all of them are deleterious; the gene that allows us to digest starch, and (even more importantly) the gene that triggers the fusion of the developing embryo to the placenta, seem to have viral origins.
"We think we’ve only scratched the surface here on the regulatory potential of ERVs," Feschotte said.
All of which is pretty amazing. And it definitely gives one pause when you stop to think of how we define the word "organism." Am I a single organism? Well, not really. Besides my regular human cells, I've got trillions of mitochondria, each with their separate bacterially-derived genome; and 10% of what I think of as "my DNA" came from viruses, at least some of which has then been modified into genes that I depend on to survive. So humans -- and all living things -- are looking more and more like composite colonies of symbiotic life forms, representing a web of interrelationships that is so complex that it's mind-boggling.
So, to hell with the weird, exotic life forms from Star Trek. I'm too busy being blown away by how bizarre and cool the life here on Earth turns out to be.