When I travel into the tidelands, the moon is often on my mind. Without the moon, there would be no sea slugs attacking hydroids, no sculpins ambushing amphipods. Nor would there be a sentient mammal at the ocean’s edge to witness these spectacles.
Recently, from a little boy obsessed with space, I learned that scientists have identified a second moon orbiting Earth. The boy shouted about this discovery while racing away from an ocean wave. Then he turned and sprinted toward receding water, challenging the tidal surge. Back and forth he ran, yelling above the shouting surf about two moons in the sky. When I left the tidepools and returned to my computer, I found out he was right.
Though so tiny and distant it cannot be seen with the unaided eye, an asteroid that serves as a second companion to Earth has been discovered. This "second moon" that loops around our planet is not massive enough to impact life on Earth. But without Earth’s main satellite—the singular Moon I grew up with—large tides would not exist, nor would the boy who shouted about space as he ran with the waves. If the cosmic accident that created the Moon hadn't occurred, the Earth might be bereft of life—a dead sphere of stilled oceans without a single twitching microbe in all the world’s seas.
Shortly after Earth formed amid the chaos of our young solar system, a rock the size of Mars careened into Earth with a glancing blow, splashing fiery debris into space. This molten mess coalesced into our Moon. The massive, nearby satellite became the main agent that moves tides on Earth. (The more massive but more distant sun influences the tides, and a complicated interplay of other factors also contributes, but the Moon is the star of the tidal show.)
The Moon in its infancy was much closer to Earth than it is now. Had creatures been around to observe it, they would have seen a shining celestial body occupying a vast swath of sky—the fantastic imagery of a science fiction story. There was other weirdness, as well. Because Earth spun faster after being hit by the object that formed the Moon, days were half as long as they are now. Earth’s spin has been decreasing ever since: a day has stretched from a hyperactive 12 hours to a more leisurely 24 hours as the planet’s rotation has slowed. The flux of the ocean tides has also been diminishing. As the Moon moves away from Earth, the gravitational influence of our planet's satellite decreases, rendering the rise and fall of oceans less dramatic.
As I wander the ocean’s edge, I imagine a primordial Earth with a nearby Moon that fills the horizon with its cratered face. I imagine towering high tides every six hours that rush for miles inland, drowning vast amounts of terrain. I imagine an ebbing ocean flow exposing tidelands so wide they stretch beyond sight. And I imagine life on Earth without a Moon.
Earth’s satellite alternately exposes and covers the rocky edge of the ocean, forcing organisms to adapt to dynamic conditions. Much of the life that now exists at the ocean’s edge would be absent if not for our large Moon. From the shore crab that seals water in its gill chambers as it ambles onto land, to the sculpin that breathes air to survive stranding on dry sand, to the anemone that covers itself with bits of stone and shell to screen its soft flesh from the sun's rays, so many creatures of the intertidal zone would not have evolved on a moonless Earth.
On a static planet of still seas, there would be no pressure for creatures to adapt to a constantly cycling intertidal zone of ocean and land, of water and air. Evolution would unfold at a much slower pace, and with far fewer species to its credit. Consider all the organisms that are tuned to lunar cycles, all the creatures that evolved to hunt by the light of the moon, all the animals that depend on ocean tides.
Evolution is also driven by plate tectonics, a process that would not exist without the Moon. The impact that created our lunar companion scrambled the crust of our planet, allowing granite to rise above the ocean. Great shields of granite formed continents that interrupted the monotony of a water world. Earth's water-land interface (landmasses lapped by oceans) has been a driving force in the evolution of life. Creatures crawled from the sea onto dry rock, developing into forms so numerous and so varied we are still discovering them, even as we plan missions to search the cosmos for life in alien seas.
Plate tectonics not only created the landmasses where complex life flourished, it also accelerated evolution. On this active Earth, terrain is continuously reshaped. Magma rises and cools, forming solid rock; crustal plates sink and melt back into magma. The changing planetary surface challenges organisms to adapt to shifting land and sea. Mountains rise, rifts open. And in the endless scramble for survival, the slow collide of continents exacts costs for some species while offering opportunities for others.
While dynamic geology and daily tides serve as engines of evolution, dramatic shifts in climate pose an existential threat to life. The Moon’s gravitational interplay with our planet keeps Earth's tilt—and thus its climate—stable. Mars, in contrast, lacks a large moon. Throughout its history, the Red Planet has wobbled on its tipping axis, trading searing heat for deadening freeze. The severe climactic shifts that would have been caused by a moonless Earth's axis swinging back and forth could have posed an insurmountable challenge to life as it moved slowly from the simplicity of microbes to the complexity of intelligent life. For this evolutionary process to play out across millennia, life required vast spans of time with a relatively stable climate. The vicious climate changes of a moonless Earth might have stalled life’s march at a microbial crawl.
Plate tectonics also creates an atmosphere conducive to complex life, recycles carbon to stabilize the climate, and generates the magnetic field that shields life on Earth from damaging solar winds. Had the collision that made the Moon not occurred, life’s complexity and diversity on this planet would be radically diminished. Without the Moon, Earth might have microbes, but it wouldn’t be home to big-brained apes.
Some scientists take the Moon’s connection to life even further—all the way to life's origin. They propose that the self-replicating molecules from which all known life on Earth evolved might have been born in lunar tides. The building blocks of nucleic acids could have been stranded on drying shores; the receding water of ebb tides might have concentrated molecules in the salty environment necessary for DNA and RNA to form. Our most distant ancestor might have risen from the saline chemistry of a tidepool. This thought stirs my imagination as I stare into pellucid pools in search of strange creatures—or, like that little boy who shouted about a second satellite, I run along the shore, racing waves and chasing receding water while the Moon works its tidal magic.
Some scientists argue that life got started not in the intertidal zone but around thermal vents on the seafloor. Regardless of life’s origin, whether in the ocean’s deep basement or along its shifting edge, lunar tides have profoundly shaped evolution on Earth. A large moon might be as elemental to complex life as carbon, oxygen, hydrogen, nitrogen, phosphorus and sulfur.
As we wade into the cosmic ocean, we would do well to search for planets that not only harbor liquid water but also have massive moons. The probability of discovering life on another world increases substantially if that world is orbited by a satellite as large as the one that lifts and lowers our planet’s waters, driving life’s diversity at the ocean’s edge.