The Origin, and the Sickening, of Our Species

If one of your hominoid ancestors hadn't gotten a viral infection millions of years ago, you might look really, really different today.

Illustration by Christoph Neimann

BEFORE TRANSLATION
"On the basis of estimates of integration time from species distribution and LTR divergence, we calculate that the 35 proviruses examined here represent in total over 490 million years of evolution. If all 6 of the proviruses are the result of inter-element recombination, then one such event is fixed, per 10Kb of repeat sequence, every 82 million years ... The true rate of recombination may be much higher."
Illustration by Christoph Neimann

The Paper: Evidence for genomic rearrangements mediated by human endogenous retroviruses during primate evolution. Published in Nature Genetics, December 2001

The Authors: Jennifer F. Hughes and John M. Coffin

The Gist: If one of your hominoid ancestors hadn't gotten a viral infection millions of years ago, you might look really, really different today.

In the beginning, before the first landlubbers crawled out of the murky ocean depths, viruses were everywhere. These parasites infected our earliest ancestors, and today, many millions of years later, bits of their genes live on in our genes.

For three decades, scientists have wondered whether the viral DNA within us is more than merely a vestige of our lowly origins. Could these infectious organisms have assisted in driving our evolution--by helping, say, to turn a foraging ape into a more cerebral toolmaker?

"We probably have more viruses in our genes than we have genes in our genes," says John Coffin, a molecular biologist at Tufts University School of Medicine. The latest analysis of the human gene sequence reveals somewhere around 40,000 "provirus DNA sequences." Proviruses were once infectious and, at some point between 10 and 50 million years ago, infected our hominoid ancestors' egg or sperm cells, becoming a permanent part of their DNA. Those sperm or eggs became people who went forth and were fruitful.

Coffin wanted to know if a virus could affect an animal's evolution through genetic recombination--a process whereby chromosomes in sperm and egg cells exchange genes. Genetic recombination is the reason why you look different from your siblings (and, for that matter, from everyone else). Inside the cells that make up sperm and eggs, chromosomes from each of your parents pair up. While they are pressed together, the chromosomes often break, and each chromosome swaps a portion of its genetic material with the other. Then the chromosomes glue themselves back together and separate, as they go on to become sperm and eggs. Since each has picked up new genetic material, the constellation of physical characteristics they carry has been substantially altered.

Normally such recombinations take place at identical positions on the two chromosomes. But proviruses provide additional sites for exchange because their front ends look exactly like their back ends. Could two proviruses at different chromosomal positions in a sperm or egg cell have swapped genetic material in a similar fashion? And in the process, could they have swapped not just their own genes but rearranged some of the animal's chromosomes as well?

Yes. Jennifer Hughes, a graduate student in Coffin's lab, examined the DNA sequences of some 35 proviruses that are part of the human genome and found evidence of recombination in six cases. Not only that, but the events could have changed our ancestors' genome enough to jump-start the divergence of humans from apes and other species. Perhaps the genetic recombination made an ape's sex cells incompatible with those of all the other apes in its troupe--except for a sibling. It is possible that two siblings paired up and created their own troupe of baby apes who in turn multiplied and became humans.

Since people are infected with viruses all the time, it is also possible that such a genetic recombination could happen again. But don't expect it to happen in your lifetime. Coffin guesstimates such recombinations become part of our permanent genetic makeup about once every 82 million years.