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Joshua Lederberg: In Tune With The Enemy

March 7, 2006

Jun 7, 2003     Dov, in biologicalEvolution forum.

Two quoted SUPERB paras (The Scientist,Vol 17,Issue 11,20. Jun 3,2003)  from "Getting in Tune With the Enemy", by J Lederberg.

(A) " Today, we are carrying around 500 different integrated retroviruses in our own genome. After millions of years of evolution,  the ancient viruses now perform indispensable defense functions for the host. The microbes that co-inhabit our bodies show considerable self-restraint by moderating the virulence of disease, especially in well-established relationships with animal hosts. Systemic pathogens such as staphylococci and streptococci, that long ago invaded us and now live within our bodies, rarely secrete extreme toxins. In consequence, probably a third of us are walking around as healthy carriers of these bugs."

(B) " Multitudes of bacteria and viruses occupy our skin, our mucous membranes and our intestinal tracts, and we must learn to live with them in a "truce" rather than victory. Understanding this cohabitation of genomes within the human body-what I call the microbiome-is central to understanding the dynamics of health and disease.

THE ENEMY SHOULD HAVE WON:… From an evolutionary point of view, microbes are extremely successful. They can grow and evolve in cycles of 20 minutes or less. A community of a billion cells can be replaced overnight from a single seed. Tens of billions of cells can be cultured in a single small test tube.

By contrast, the human species has a total population of less than 10 billion, quite modest on the microbial scale. Each human is multicellular and large, with a costly, long developmental cycle. Unlike people, germs readily exchange genes within and between various species. They don't speciate or differentiate into genetically isolated organisms as we do. In fact, these bugs engage in promiscuous lateral gene transfer, making the microbial world a kind of DNA-based worldwide Web that shares genetic information that can move from one bug to another.

When, for example, antibiotics get into our sewage system and kill some bugs, it is the occasional resistant mutant that survives. These survivors can then transfer their newfound immunity to the genes of other microbes, including pathogenic species that foment human disease.

These rapidly evolving bugs can gang up on humans through synergies of organisms that provoke mild disease, which, when joined with others, become virulent. This may prove to be the case with SARS, which appears to be a variation of the common cold virus. Humans, by contrast, are not only genetically isolated from other species (we get no biological benefit from evolutionary innovations in mice or monkeys), but the cells of the human germ line are sealed off in our gonads, insulated from most of the vicissitudes of the body. Whatever that body might learn by way of generating immunity, let's say against a new virus, cannot be passed on to one sperm or egg to the next generation. New generations have to learn it all over again.

In short, the competitive evolutionary odds seem cast very much in favor of the bugs. We see this mismatch when great plagues and epidemics sweep the world. By the raw evidence, the capability of evolving bugs should have trounced us eons ago.

So why haven't they? Why are we still here, sharing the planet with the bugs? They haven't extinguished us simply because microbes have a shared interest in the survival of the host, humans and other multicellular creatures. The bug that kills its host is at a dead end.

BUT THEY NEED US: Biologically speaking, the reason we are still here is because microbes need live hosts for their own survival. This reality allows us to establish some of the ground rules of evolutionary success in the microbial world. It is as if they have read the Bible and know Genesis: They go forth and disseminate as their first rule. They multiply. Next, according to Malthusian and Darwinian doctrine, they have to be the fittest in order to survive so that they can produce the largest number of offspring they can. "

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