Scientists have been making some remarkable discoveries about viruses recently that may change the way we think about life. One place to start understanding what it all means is by looking at this picture.
You can’t help put see a bright triangle with its three corners sitting on top of the black circles. But the triangle exists only in your mind. The illusion is known as a Kanisza triangle, and psychologists have argued that it plays on your brain’s short-cuts for recognizing objects. Your brain does not bother to interpret every point of light that hits your retina in order to tell what you’re looking at. Instead, it pulls out some simple features quickly and makes a hypothesis about what sorts of objects they belong to. It’s fast and pretty reliable, allowing you to make quick decisions. For getting us through our ordinary lives, it’s good enough. But as a guide to objective reality, it is far from perfect. What’s really weird about the Kanisza trinagle is that even when you accept that it doesn’t exist (cover up the circles and watch it disappear) you can still can’t stop yourself from seeing it. You just have to accept that your brain’s short-cuts are fooling you.
Scientists have documented lots of illusions that may expose many other mental short-cuts. And it’s possible that one of them may interfere with the way we think about life. For most of the history of Western thought, natural philosophers tried to divide up living things into species and other groups on the belief that each group shared an underlying nature–an essence. Birds all have feathers, setting them off from other animals. People always give birth to people, rather than rabbits or trout. But recent psychological research suggests that essentialism is not something we come to after years of careful thought. We are essentialists from childhood. (For a nice summary of this research, see this recent article by University of Michigan psychologist Susan Gelman.) Children seem to put things into categories and come to believe that there are deep, non-obvious differences between the categories, even if they don’t know what those differences are. The essence of these things is stable, children believe, and intrinsic–particularly when those things are species.
Why do we have this essence-perceiving faculty in our brains? One possibility–an adaptationist explanation–is that it helps us to predict how things will act, and allows us to come up with a reliable response. If you meet a lion, you don’t need to sit down and get to know that individual lion to figure out how it will act. A lion is a lion, and you run. Of course, that particular lion might be blind or tame or a guy in a lion suit. But you’re probably better off just letting the essence of lions be your guide.
Essences can act as a rough guide to organizing the world. A bird guide distinguishes different species by their unique colors and shapes. But our essentialist brains can also get us into trouble. In the 1700s naturalists could not draw clear lines between species of plants that could clearly hybridize. The discovery of the platypus in the early 1800s–an animal that nursed its young like mammals but laid eggs unlike any other mammal–posed an enormous headache. When Darwin and other scientists began arguing that humans shared a common ancestry with chimpanzees and gorillas, anatomists such as Richard Owen desperately tried to find traits in the human brain that would firmly set us apart–signs, as it were, of our unique essence. Owen failed, and today’s research on the human genome helps to show what a futile effort he was making. Humans are different, just like each species is, but they are also linked to other species by common descent. They have no more of a special essence than the branches on a tree.
Which brings us to viruses. Viruses have traditionally been considered fundamentally different than "true" organisms, such as bacteria, animals, and plants. That’s because all viruses that scientists studied were just simple bags of genes, made up of tiny bits of genetic material encased in protein shells. They were not truly alive, because their few genes could only be copied and turned into proteins with the help of a cell’s biochemical machinery. Outside a cell, they were inert, lifeless packages drifting through the world, waiting to bump into a new host.
Last year this essence of viruses began to blur. Scientists discovered a gigantic virus capable of making 150 proteins, including enzymes for repairing DNA and for translating a gene’s code into protein. Its entire genome is 1.2 million base pairs long–about twice as long as the smallest genomes of parasitic bacteria. These viruses are not rare flukes. Just a few days ago, scientists reported on how they plumbed a database of DNA gathered by Craig Venter from the Sargasso Sea and found signs that there are a lot of these giant viruses floating out in the oceans.
Today, viruses from another part of the world blurred their essence even more. Scientists reported in Nature the discovery of strange viruses from hot springs in Italy. The viruses reproduce inside microbes, and when they burst out of their host, they do not remain inert. Instead, they continue developing, growing tails made out of filament-shaped proteins that are encoded by their own genes. It’s not clear from the report whether the viruses can make the proteins themselves, or if their hosts make them and then squirt them out into the surrounding water. But whichever the case, the scientists conclude that viruses "may be even more biologically sophsticated than previously recognized."
The discoverers of the "living" virus compared some of its genes to those of other organisms and argued that it has an ancient history, descending from organisms that lived four billion years ago, before the major branches of life had emerged. Some critics have argued that these viruses actually stole the genes from their hosts and incorporated them into their own genome, but the original team has rebutted them in a paper submitted to Virus Research. It is still possible that these viruses stole some of their genes from their hosts, because the evidence of viral gene theft is now overwhelming. On the other hand, viruses seem to have sometimes donated their genes to their hosts. Some researchers have even argued that many of the key components of our own cells, from DNA-copying enzymes to DNA itself–began as viruses.
So try to ignore that urge to see viruses as a separate kind from us, just as you try to ignore the triangle that isn’t there. Despite what we may think, life is a wonderful blur.