Arrival of the Fittest: Solving Evolution's Greatest Puzzle
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Authors: Andreas Wagner
CURRENT
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First published by Current, a member of Penguin Group (USA) LLC, 2014
Copyright © 2014 by Andreas Wagner
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Illustrations by the author
LIBRARY OF CONGRESS CATALOGING-IN-PUBLICATION DATA
Wagner, Andreas, 1967 January 26–
Arrival of the fittest : solving evolution’s greatest puzzle / Andreas Wagner.
pages cm
Includes bibliographical references and index.
eBook ISBN 978-1-101-62816-4
1. Natural selection. 2. Evolutionary genetics. I. Title.
QH375.W327 2014
572.8'38—dc23 2014009774
While the author has made every effort to provide accurate telephone numbers and Internet addresses at the time of publication, neither the author nor the publisher is responsible for errors, or for changes that occur after publication. Further, the publisher does not have any control over and does not assume any responsibility for author or third-party websites or their content.
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CONTENTS
PROLOGUE
World Enough, and Time
I
n the spring of 1904, Ernest Rutherford, a thirty-two-year-old New Zealand–born physicist then working at McGill University in Canada, gave a lecture at the world’s oldest scientific organization, the Royal Society of London for Improving Natural Knowledge. His subject was radioactivity and the age of the earth.
At that time, scientists had long since forsworn the biblical accounts asserting that the earth was only six thousand years old. The most widely accepted dates had been calculated by another physicist—William Thomson, better known as Lord Kelvin—who had used the equations of thermodynamics and the earth’s heat conductivity to estimate that the planet was somewhere around twenty million years old.
In geology, that’s not a lot of time, and the implications were profound. The earth’s geological features could not have appeared within this duration if processes like volcanism and erosion proceeded at today’s rate.
1
But the real victim of Kelvin’s estimate was Charles Darwin’s theory of evolution by natural selection. Darwin had described himself as “greatly troubled at the short duration of the world according to Sir W. Thomson.”
2
He knew that organisms had not changed much since the last ice ages, and from such little change he inferred that the amount of time needed to create all organisms—alive today or preserved in fossils—must be truly enormous.
3
Twenty million years was not enough time to create life’s diversity.
But Rutherford, who had discovered the phenomenon of radioactive half-life only a few years before, knew that Kelvin was wrong, by at least several orders of magnitude. As he later recalled:
I came into the room, which was half dark, and presently spotted Lord Kelvin in the audience and realized that I was in for trouble at the last part of the speech dealing with the age of the earth, where my views conflicted with his. . . . The discovery of the radio-active elements, which in their disintegration liberate enormous amounts of energy, thus increases the possible limit of the duration of life on this planet, and
allows
the time claimed by the geologist and biologist for the process of evolution.
4
(emphasis added).
And that was that. Kelvin died in 1907. Rutherford won the Nobel Prize in 1908, and by the 1930s his radiometric methods had shown that the earth was around 4.5 billion years old. Darwin’s theory was saved, since the processes of random mutation and selection now had the time needed to create life’s enormous complexity and diversity.
Or did they?
Consider the peregrine falcon,
Falco peregrinus,
one of nature’s great predators and an organism of marvelous perfection. Its powerful musculature, matched with an extremely lightweight skeleton, makes it by far the fastest animal on earth, able to reach more than 200 miles per hour in one of its characteristic dives. All that speed translates into enormous kinetic energy when the falcon strikes its prey in midair with razor-sharp talons. If that impact alone does not deliver death, the falcon can sever the spinal column of its prey with a conveniently notched upper beak.
5
Before moving in for the kill,
F. peregrinus
needs to track down its prey. The targeting mechanism is a pair of eyes with full-color binocular vision, possessing resolving power more than five times greater than a human’s, which means that a peregrine can see a pigeon at distances of more than a mile.
6
Like many predators, the falcon has an eye with a nictitating membrane—a third eyelid—a bit like a windshield wiper that removes dirt while keeping the eye moist during a high-speed chase. The falcon’s eyes also harbor more photoreceptors, the rods that capture images in very low light, and the cones that provide color vision.
7
Its photoreceptors render even long-wavelength ultraviolet light visible.
A marvel indeed. But even more marvelous is knowing that every one of those brilliant adaptations is the sum of innumerable tiny steps, each one preserved by natural selection, each one a change in a single molecule. The deadly beak and talons of
F. peregrinus
are built from the same raw material as its feathers, the protein molecules known as keratin, the human versions of which make up your hair and nails.
8
For color vision, those extraordinary eyes depend on opsins, protein molecules in the eyes’ rods and cones. Crucial for their remarkable acuity are their lenses, composed of transparent proteins known as crystallins.
9
The first vertebrates to use crystallins in lenses did so more than five hundred million years ago, and the opsins that enable the falcon’s vision are some seven hundred million years old.
10
They originated some three billion years after life first appeared on earth. That sounds like a helpfully long amount of time to come up with these molecular innovations. But each one of those opsin and crystallin proteins is a chain of hundreds of amino acids, highly specific sequences of molecules written in an alphabet of twenty amino acid letters. If only one such sequence could sense light or help form a transparent cameralike lens, how many different hundred-amino-acid-long protein strings would we have to sift through? The first amino acid of such a string could be any one of the twenty kinds of amino acids, and the same holds for the second amino acid. Because 20 × 20 = 400, there are there are 400 possible strings of two amino acids. Consider also the third amino acid, and you have arrived at 20 × 20 × 20, or 8,000, possibilities. At four amino acids we already have 160,000 possibilities. For a protein with a hundred amino acids (crystallins and opsins are much longer), the numbers multiply to a 1 with more than 130 trailing zeroes, or more than 10
130
possible amino acid strings. To get a sense of this number’s magnitude, consider that most atoms in the universe are hydrogen atoms, and physicists have estimated the number of these atoms as 10
90
, or 1,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000. This is “only” a 1 with 90 zeroes. The number of potential proteins is not merely astronomical, it is hyperastronomical, much greater than the number of hydrogen atoms in the universe.
11
To find a specific sequence like that is not just less likely than winning the jackpot in the lottery, it is less likely than winning a jackpot every year since the Big Bang.
12
In fact, it’s countless billions of times less likely. If a trillion different organisms had tried an amino acid string every second since life began, they might have tried a tiny fraction of the 10
130
potential ones. They would never have found the one opsin string. There are a lot of different ways to arrange molecules. And not nearly enough time.
When the seventeenth-century lyric poet Andrew Marvell bemoaned, “Had we but world enough, and time” to avoid the “deserts of vast eternity” that lay before him, he was attempting to unlock his mistress’s bedchamber, not the secrets of nature. But he was on to something. Common wisdom holds that natural selection, combined with the magic wand of random change, will produce the falcon’s eye in good time. This is the mainstream perspective on Darwinian evolution: A tiny fraction of small and random heritable changes confers a reproductive advantage to the organisms that win this genetic lottery and, accumulating over time, such changes explain the falcon’s eye—and, by extension, everything from the falcon itself to all of life’s diversity.
The power of natural selection is beyond dispute, but this power has limits. Natural selection can
preserve
innovations, but it cannot
create
them. And calling the change that creates them random is just another way of admitting our ignorance about it. Nature’s many innovations—some uncannily perfect—call for natural principles that accelerate life’s ability to innovate, its
innovability
.
For the d last fifteen years, I have been privileged to help uncover these principles, first in the United States and later, joined by a group of highly talented researchers, in my laboratory at the University of Zürich in Switzerland. Using experimental and computational technologies unimagined by Darwin or Rutherford, our goal is not to discover individual innovations, but to find the wellsprings of
all
biological innovation. What we have found so far already tells us that there is much more to evolution than meets the eye. It tells us that the principles of innovability are concealed, even beyond the molecular architecture of DNA, in a hidden architecture of life with an otherworldly beauty.
These principles are the subject of this book.