A lot of people ask, 'Do you think humans are parasites?' It's an interesting idea and one worth thinking about. People casually refer to humanity as a virus spreading across the earth. In fact, we do look like some strange kind of bio-film spreading across the landscape. A good metaphor? If the biosphere is our host, we do use it up for our own benefit. We do manipulate it. We alter the flows and fluxes of elements like carbon and nitrogen to benefit ourselves—often at the expense of the biosphere as a whole. If you look at how coral reefs or tropical forests are faring these days, you'll notice that our host is not doing that well right now. Parasites are very sophisticated; parasites are highly evolved; parasites are very successful, as reflected in their diversity. Humans are not very good parasites. Successful parasites do a very good job of balancing—using up their hosts and keeping them alive. It's all a question of tuning the adaptation to your particular host. In our case, we have only one host, so we have to be particularly careful.

Parasites are not only incredibly diverse; they are also incredibly successful. There are parasitic stretches of DNA in your own genes, some of which are called retrotransposons. Many of the parasitic stretches were originally viruses that entered our DNA. Most of them don't do us any harm. They just copy and insert themselves in other parts of our DNA, basically replicating themselves. Sometimes they hop into other species and replicate themselves in a new host. According to one estimate, roughly one-third to one-half of all human DNA is basically parasitic.

Researchers keep identifying new species, but they have no idea about the life cycle of a given species or its other hosts. They cut open an animal and find a new species. Where did it come from? What effect does it have on its host? What is its next host? They don't know and they don't have time to find out, because there are too many other species waiting to be discovered and described.