There is greater difference between the genetic code of the cholera bacterium and the tuberculosis bacterium than there is between the genetic code of the human being and the potato.
— Report of researchers who have recently sequenced all four genomes
Because the turtles [I was studying in Costa Rica] come out to nest after dark, much of my work was done at night. There was a great deal of waiting between turtles, plenty of time to sit on a driftwood log and think. In the first years of my research I was often the only one on the beach for miles. After ten or twenty minutes of sitting without using my flashlight, my eyes adapted to the dark and I could make out forms against the brown-black sand: the beach plum and coconut palm silhouettes in back, the flicker of the surf in front, sometimes even the shadowy outline of a trailing railroad vine or the scurry of a ghost crab at my feet. The air was heavy and damp with a distinctive primal smell that I can remember but not describe. The rhythmic roar of the surf a few feet away never ceased — my favourite sound. I hear it as I write in my landlocked office in New Jersey. And then, with ponderous, dramatic slowness, a giant turtle would emerge from the sea.
Usually I would see the track first, a vivid black line standing out against the lesser blackness, like the swath of a bulldozer. If I was closer, I could hear the animal’s deep hiss of breath and the sounds of her undershell scraping over logs. If there was a moon, I might see the light glistening off the parabolic curve of the still wet shell. Size at night is hard to determine: even the sprightly 180-pounders, probably nesting for the first time, looked big when nearby, but the 400-pound ancients, with shells nearly four feet long, were colossal in the darkness. Then when the excavations of the body pit and egg cavity were done, if I slowly parted the hind flippers of the now-oblivious turtle, I could watch the perfect white spheres falling and falling into the flask-shaped pit scooped into the soft sand.
Falling as they have fallen for a hundred million years, with the same slow cadence, always shielded from the rain or stars by the same massive bulk with the beaked head and the same large, myopic eyes rimmed with crusts of sand washed out by tears. Minutes and hours, days and months dissolve into eons. I am on an Oligocene beach, an Eocene beach, a Cretaceous beach — the scene is the same. It is night. The turtles are coming back, always back; I hear a deep hiss of breath and catch a glint of wet shell as the continents slide and crash, the oceans form and grow. The turtles were coming here before here was here.
— David Ehrenfeld, from Beginning Again
Imagine that a tiny creature, just 1/2 micron long, hitched a ride from outer space and landed on Earth just as it was formed 4.6 billion years ago. This indomitable creature, let’s call her Pri, decides to travel from where she lands, at a gaseous, ammonia-swirling prebiotic place that would one day be called Paris, toward the sun. Pri has been endowed with the ability to withstand all of Earth’s climates, but she moves very slowly, just 1/4 micron every hour, half her body length. At this pace she moves 5.5 microns a day, or 2mm (the width of a pinhead) every year. But she’s persistent. It doesn’t matter if the place she is traveling across is ocean, desert, or glacier, she keeps going, one pinhead-width every year.
This is what she sees.
For the first 3/4 of a billion years, it is a hostile and dramatic journey. There is no free oxygen in the air, no protective atmosphere, so temperature, winds, and geological activity are so wild that life for any creature (other than the alien Pri) is impossible.
Then, after 3/4 of a billion years, in a place not far from what would one day be called Ireland, she spots a few single-celled creatures called prokaryotes, floating in the warmer turbulent waters. They are about the same size as Pri, but they each contain these remarkable strands, a million pairs of molecules wound together in a tiny helix. What might they be for?
It takes another billion years of travel before Pri notices that some of these prokaryotes have evolved photosynthesis — they are making things from the sun! There is amazing diversity of these creatures — the ones called archaea and the ones called bacteria are utterly different, and even the bacteria are utterly different from each other. There is a huge amount of construction going on, even though, to a creature much larger than Pri, none of it would be visible. But something is definitely happening here. Pri is now a third the way through her great-circle Westward journey, in a place that would one day be called Greenland.
Another 400 million years pass, and suddenly it gets very cold, and the entire planet is covered miles deep in ice. She is now at the North end of what will one day be called Québec. Her trip is nearly half over, and she despairs that these strange creatures she has been seeing, working so hard, will not survive.
But they do. The ice melts and re-forms into another frozen ball, and melts again, but somehow these microscopic creatures endure, and they start to change the atmosphere in a way that calms the whole planet, makes it friendlier for all life on Earth. And after another 1.8 billion years, her trip now 90% done, Pri notices that some of these creatures have found a way to form together into multi-cellular life, into “organisms”. And an astonishing profusion of life begins. In the last 550 million years of her voyage, as she makes her way from what would one day be called Utah to her final rest in the Pacific Ocean, she witnesses the transformation of the planet by all this life, in a furious battle with the planet’s natural forces — alternately explosions of life and collapses into extinction. Here is her log for this last leg of her journey:
Only a bit over a kilometer from her ocean destination, 650 thousand years ago, Pri encounters the first of six ice ages that will freeze much of the planet, again, for much of the next 638 thousand years. When the last of these recedes, when Pri is only 24 meters from the ocean, the new monster species begins to extinguish all other life on the planet, organized now and using terrible new tools to destroy and to spread across the globe.
And when Pri is only one half meter, 250 years from the ocean, the monsters arrive at the ocean. And when she is only twenty centimeters, eight inches, 100 years from the ocean, the monsters block access to the ocean with huge buildings and walls, and foul the water until it is full of poisons, unable to support life.
And then, as she waits, patiently, as she’d done before for the other monsters and the ice and the storms to pass, the monsters suddenly vanish. As she waits and 100 years pass, most of the monsters die, and take with them most of the other life on the planet. Another Extinction!, she writes. So she waits a short thousand years more, the time it had taken her to crawl the last two meters of her astonishing halfway round-the-world journey. By then the buildings and walls have fallen, the water has cleansed itself, and Pri is able to make her final eight-inch hundred-year sprint, and her final log entry:
* Scientists now believe dolphins evolved from ungulate mammals, which some describe as the second most remarkable evolutionary invention in our planet’s history (next to the invention of heavier-than-air flight by birds).