What made this alchemy possible? The answer, in part, is viruses.
Viruses are constantly swarming into our bodies. Sometimes they make us sick; sometimes our immune systems vanquish them; and sometimes they become a part of ourselves. A type of virus called a retrovirus makes copies of itself by inserting its genes into the DNA of a cell. The cell then uses those instructions to make the parts for new viruses. HIV makes a living this way, as do a number of viruses that can trigger cancer.
On rare occasion, a retrovirus may infect an egg. Now something odd may happen. If the egg becomes fertilized and gives rise to a whole adult individual, all the cells in its body will carry that virus. And if that individual has offspring, the virus gets carried down to the next generation.
At first, these so-called endogenous retroviruses lead a double life. They can still break free of their host and infect new ones. Koalas are suffering from one such epidemic. But over thousands of years, the viruses become imprisoned. Their DNA mutates, robbing them of the ability to infect new hosts. Instead, they can only make copies of their genes that are then inserted back into their host cell. Copy after copy build up the genome. To limit the disruption these viruses can cause, mammals produce proteins that can keep most of them locked down. Eventually, most endogenous retroviruses mutate so much they are reduced to genetic baggage, unable to do anything at all. Yet they still bear all the hallmarks of viruses, and are thus recognizable to scientists who sequence genomes. It turns out that the human genome contains about 100,000 fragments of endogenous retroviruses, making up about eight percent of all our DNA.
Evolution is an endlessly creative process, and it can turn what seems utterly useless into something valuable. All the viral debris scattered in our genomes turns out to be just so much raw material for new adaptations. From time to time, our ancestors harnessed virus DNA and used it for our own purposes. In a new paper in the journal Nature, a scientist named Samuel Pfaff and a group of fellow scientists report that one of those purposes to help transform eggs into adults.
In their study, Pfaff and his colleagues at the Salk Institute for Biological Sciences examined fertilized mouse eggs. As an egg starts to divide, it produces new cells that are capable of becoming any part of the embryo–or even the membrane that surrounds the embryo or the placenta that pipes in nutrients from the animal’s mother. In fact, at this early stage, you can pluck a single cell from the clump and use it to grow an entire organism. These earliest cells are called totipoent.
After a few days, the clump becomes a hollowed out ball. The cells that make the ball up are still quite versatile. Depending on the signals a cell gets at this point, it can become any cell type in the body. But once the embryo reaches this stage, its cells have lost the ability to give rise to an entirely new organism on their own, because they can’t produce all the extra tissue required to keep an embryo alive. Now the cells are called pluripotent. The descendants of pluripotent cells gradually lose their versatility and get locked into being certain types of cells. Some become hematopoetic cells, which can turn into lots of different kinds of blood cells but can no longer become, say, skin cells.