cells, and they passed those instructions down to their descendants.
These hidden viruses were not limited to just one oddball breed of chickens. Weiss and other
scientists found avian leukosis virus embedded in many breeds, raising the possibility that the virus
was an ancient component of chicken DNA. To see just how long ago avian leukosis viruses infected
the ancestors of today’s chickens, Weiss and his colleagues travelled to the jungles of Malaysia.
There they trapped red jungle fowl, the closest wild relatives of chickens. The red jungle fowl
carried the same avian leukosis virus, Weiss found. On later expeditions, he found that other species
of jungle fowl lacked the virus.
Out of the research on avian leukosis virus emerged a hypothesis for how it had merged with
chickens. Thousands of years ago, the virus plagued the common ancestor of domesticated chickens
and red jungle fowl. It invaded cells, made new copies of itself, and infected new birds, leaving
tumors in its wake. But in at least one bird, something else happened. Instead of giving the bird
cancer, the virus was kept in check by the bird’s immune system. As it spread harmlessly through the
bird’s body, it infected the chicken’s sexual organs. When an infected bird mated, its fertilized egg
also contained the virus’s DNA in its own genes.
As the infected embryo grew and divided, all of its cells also inherited the virus DNA. When the
chick emerged from its shell, it was part chicken and part virus. And with the avian leukosis virus
now part of its genome, it passed down the virus’s DNA to its own offspring. The virus remained a
silent passenger from generation to generation for thousands of years. But under certain conditions,
the virus could reactivate, create tumors, and spread to other birds.
Scientists recognized that this new virus was in a class of its own. They called it an endogenous
retrovirus—endogenous meaning generated within. They soon found endogenous retroviruses in
other animals. In fact, the viruses lurk in the genomes of just about every major group of vertebrates,
from fish to reptiles to mammals. Some of the new endogenous retroviruses turned out to cause cancer
like avian leukosis virus, but many did not. Some seemed to be effectively muzzled by their host.
Certain endogenous retroviruses carried by mice cannot infect mice cells, for example, but they can
readily spread among rat cells.
Other endogenous retroviruses turned out to be crippled, carrying mutations that robbed them of the
ability to make full-fledged viruses. They could still make new copies of their genes, however, which
were then reinserted back into their host’s genome. And scientists also discovered some endogenous
retroviruses that were so riddled with mutations that they could no longer do anything at all. They had
become nothing more than baggage in their host’s genome.
Endogenous retroviruses can linger in their hosts for millions of years. In 2009, Aris Katzourakis,
an evolutionary biologist at the University of Oxford, discovered hundreds of copies of endogenous
retroviruses in the genome of the three-toed sloth. Their genes closely matched those of foamy
viruses, free-living pathogens that infect primates and other mammals. Katzourakis concluded that
foamy viruses infected the common ancestor of three-toed sloths and primates, which lived a hundred
million years ago. In primates, they’ve remained free-living. In the sloth lineage, however, they
became trapped in their host’s DNA and have remained there ever since.