Viruses reproduce rapidly and often with violent results, yet they are so rudimentary that many scientists don't even consider them to be alive. A virus is nothing more than a few strands of genetic material wrapped in a package of protein--a parasite, unable to function on its own. In order to survive, it must find a cell to infect. Only then can any virus make use of its single talent, wihc is to take control of a host's cellular machinery and use it to churn out thousands of copies of itself.
Thoses viruses were often highly infectious, yet their impact was limited by their ferocity: a virus may destroy an entire cutlure, but if we die it dies too. As a result, not even smallpox possessed the evolutionary power to influence humans as a species--to alter our genetic structure. That would require an organism to insinuate itself into the critical cells we need in order to reproduce: our germ cells. Only retroviruses, which reverse the usual flow of genetic code from DNA to RNA, are capable of that. A retrovirus stores its genetic information in a single-stranded molecule of RNA, instead of the more common double-stranded DNA. When it infects a cell, the virus deploys a special enzyme, called reverse transciptase, that enables it to copy itself and then paste its own genes into the new cell's DNA. It then bcomes part of that cell forever; when the cell divides, the virus goes with it...
The earliest mammals, ancestors of the spiny anteater and the duck-billed platypus, laid eggs. Then, at least a hundred million years ago, embryos, instead of growing in a shell, essentially became parasites. While only balls of cells, they began to implant themselves in the lining of the womb. The result was the placenta, which permits the embryos to take nourishment from the mother's blood, while preventing immune cells or bacteria from entering. The placenta is essentially a modified egg. In the early nineteenth-seventies, biologists who were scanning baboon placentas were surprised to see retroviruses on a layer of tissue known as the syncytium, which forms the principal barrier between mother and fetus. They were even more surprised to see that all the animals were healthy....
Luis P. Villarreal has posed that question many times, most notably in a 2004 essay, "Can Viruses Make Us Human?" ..."Viruses are molecular genetic parasites and are mostly recognized for their ability to introduce disease." Yet he goes on to argue that they also represent " a major creative force" in our revolution, driving each infected cell to acquire new and increasingly complex molecular identities....
For Villarreal and a growing number of like-minded scientists, the conclusion is clear: "Viruses may well be the unseen creator that most likely did contribute to making us human."
Saturday, December 15, 2007
New Yorker, Dec. 3, 2007
from "Darwin's Surprise" by Michael Specter: