Arrival of the Fittest: Solving Evolution's Greatest Puzzle by Andreas Wagner Page A

down, at a steady clip, unceasingly, no matter how long the exploration continued, a hundred, a thousand, or ten thousand steps, hours, days, weeks, until we ran out of time and needed to do other work. We realized that the innovability of an evolving metabolism would not exhaust itself in our lifetime. 47
    Innovability in the metabolic library is near limitless, and for that both genotype networks
and
diverse neighborhoods are required. They are the two keys to innovability. Genotype networks guarantee that evolving populations can explore the library. Without them the lethal punishment of losing viability would be inevitable. But without diverse neighborhoods in this library, exploring a genotype network would be pointless: The exploration would not turn up many texts with new meanings.
    Any librarian who wanted to organize a human library in this way would be locked away. Even if a thousand books told the same story in different ways, no sane librarian would create sections that placed books with all manner of different meanings next to one another. And he would certainly not pack
different
neighborhoods around synonymous texts with books in
different
subject categories.
    But a closer look reveals that the metabolic library’s catalog is far from a madman’s febrile fantasy. Human libraries are useful only because we
have
librarians who make catalogs suitable for us, where books on photovoltaics stand on one shelf, those on French literature on another, and so on. For a library whose readers have no catalog and can only take random steps, and where missteps are punishable by death, it would be disastrous, because they would be stuck on whatever shelf they started. They would be idiots savants, world experts in one area but completely ignorant in all others, and could never learn anything new—not a smart strategy for surviving in an ever-changing world. For such readers, the metabolic library is perfect, uncannily well set up for innovation. It guarantees eternal learning and innovability.
    Even more uncanny: Life’s other libraries are organized the same way.

CHAPTER FIVE
Command and Control
    I t’s hard to beat milk as a metaphor for goodness. Lady Macbeth’s husband is too full of the “milk of human kindness” to commit regicide, the third chapter of Exodus promises the Hebrews a “land flowing with milk and honey,” and to this day we call harmless things “safe as mother’s milk.” But for more than half of the world’s population, a healthy glass of milk is decidedly not good. It means bloating, gas, and diarrhea. The reason is a lack of lactase, the enzyme that prechews the milk-sweetening sugar lactose for our bodies. Without it, bodies cannot break down lactose, leaving it to gut bacteria that happily scavenge this unused fuel and leave waste products with nasty side effects.
    When those lactose-intolerant adults were babies, they could digest the sugar in their mother’s milk just fine. Their lactase gene was turned on—in technical terms, it was
expressed
—which means that their bodies transcribed the DNA instructions for lactase into RNA and translated this RNA into the needed enzyme. The bodies of lactose-intolerant adults have switched the lactase gene off permanently and no longer express it. Genes like the lactase gene, which our bodies can turn on or off, are
regulated
genes
.
    For most of human history, the “off” position in adults was the norm. If you are lucky enough to tolerate lactose, you have a mutation in the lactase control region, a stretch of DNA near the lactase gene that leaves the lactase gene turned on well into adulthood. Chances are that your distant ancestors were milk-drinking cattle farmers, because mutations that cause lactose tolerance first spread through pastoral populations, like those of East Africa and Scandinavia. And they spread blazingly fast, from zero to more than 90 percent of some populations, in a blip of time, the eight thousand or so years since