PREPARED Doctors wearing protective gear with a possible flu patient in Mexico City.
Evolutionary biology may sometimes seem like an arcane academic pursuit, but just try telling that to Gavin Smith, a virologist at Hong Kong University. For the past week, Dr. Smith and six other experts on influenza in Hong Kong, Arizona, California and Britain have been furiously analyzing the new swine flu to figure out how and when it evolved.
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H1N1 An image of the newly identified swine flu virus. Partly because they can mutate very fast and can mix genes, viruses are turning out to be astonishingly diverse.
The first viruses from the outbreak were isolated late last month, but Dr. Smith and his colleagues report on their Web site that the most recent common ancestor of the new viruses existed 6 to 11 months ago. “It could just have been going under the radar,” Dr. Smith said.
The current outbreak shows how complex and mysterious the evolution of viruses is. That complexity and mystery are all the more remarkable because a virus is life reduced to its essentials. A human influenza virus, for example, is a protein shell measuring about five-millionths of an inch across, with 10 genes inside. (We have about 20,000.)
Some viruses use DNA, like we do, to encode their genes. Others, like the influenza virus, use single-strand RNA. But viruses all have one thing in common, said Roland Wolkowicz, a molecular virologist at San Diego State University: they all reproduce by disintegrating and then reforming.
A human flu virus, for example, latches onto a cell in the lining of the nose or throat. It manipulates a receptor on the cell so that the cell engulfs it, whereupon the virus’s genes are released from its protein shell. The host cell begins making genes and proteins that spontaneously assemble into new viruses. “No other entity out there is able to do that,” Dr. Wolkowicz said. “To me, this is what defines a virus.”
The sheer number of viruses on Earth is beyond our ability to imagine. “In a small drop of water there are a billion viruses,” Dr. Wolkowicz said. Virologists have estimated that there are a million trillion trillion viruses in the world’s oceans.
Viruses are also turning out to be astonishingly diverse. Shannon Williamson of the J. Craig Venter Institute in Rockville, Md., has been analyzing the genes of ocean viruses. A tank of 100 to 200 liters of sea water may hold 100,000 genetically distinct viruses. “We’re just scratching the surface of virus diversity,” Dr. Williamson said. “I think we’re going to be continually surprised.”
Viruses are diverse because they can mutate very fast and can mix genes. They sometimes pick up genes from their hosts, and they can swap genes with other viruses. Some viruses, including flu viruses, carry out a kind of mixing known as reassortment. If two different flu viruses infect the same cell, the new copies of their genes get jumbled up as new viruses are assembled.
Viruses were probably infecting the earliest primordial microbes. “I believe viruses have been around forever,” Dr. Wolkowicz said.
As new hosts have evolved, some viruses have adapted to them. Birds, for example, became the main host for influenza viruses. Many birds infected with flu viruses do not get sick. The viruses replicate in the gut and are shed with the birds’ droppings.
A quarter of birds typically carry two or more strains of flu at the same time, allowing the viruses to mix their genes into a genetic blur. “Birds are constantly mixing up the constellation of these viruses,” said David Spiro of the J. Craig Venter Institute.
From birds, flu viruses have moved to animals, including pigs, horses and humans. Other viruses, like H.I.V. and SARS, have also managed to jump into our species, but many others have failed. “It’s a very rare event when a virus creates a new epidemic in another species,” said Colin Parrish of Cornell University. In Southeast Asia, for example, a strain of bird flu has killed hundreds of people in recent years, but it cannot seem to move easily from human to human.
Only a few strains of influenza have managed to become true human viruses in the past century. To make the transition, the viruses have to adapt to their new host. Their gene-building enzymes have evolved to run at top speed at human body temperature, for example, which is a few degrees cooler than a bird’s.
Influenza viruses also moved from bird guts to human airways. That shift also required flu viruses to spread in a new way: in the droplets we release in our coughs and sneezes.
“If the virus settles down on the floor, then it’s gone,” said Peter Palese, chairman of microbiology at Mount Sinai School of Medicine. Winter is flu season in the United States, probably because dry air enables the virus-laden droplets to float longer.
Up to a fifth of all Americans become infected each flu season, and 36,000 die. During that time, the flu virus continues to evolve. The surface proteins change shape, allowing the viruses to evade the immune systems and resist antiflu drugs.
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