The Key to Complex Life Might Lie Miles Below Our Feet
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Eons ago, long before T. rex or any other large multicellular life roamed the planet, life on Earth got stuck. After inventing single-celled organisms and teaching them biochemical tricks such as the energy-creating miracle of fermentation, evolution cranked out nothing but simple communities of microbes. It was the evolutionary equivalent of visiting every car dealership in the world and finding only Kia sedans, but in different colors. Scientists call this stagnant period, which spanned from about 1.8 billion years ago to 600 million years ago, the “boring billion.”
Then, suddenly, everything changed. The long snooze gave way to the Cambrian explosion, the most rapid, creative period of evolution in the history of our planet. In the blink of a geologic eye (hundreds of millions of years), all the basic biology needed to sustain complex organisms was worked out, and the paths to all modern life, ranging from periwinkles to people, branched off. Mega sharks hunted in the oceans, pterodactyls took to the skies, and velociraptors terrorized our mouselike mammalian ancestors on land.
What drove this instantaneous, epic change in evolution has been one of the great unsolved problems of evolutionary theory for decades. In their attempts to solve the riddle, some researchers have recently turned not to the Cambrian explosion but back to the boring billion, and in so doing, they might finally have found the answer. The origins of the Cambrian explosion, these scientists say, may lie not in life itself but deep in Earth’s interior. If they’re right—if the evolution of organisms really is woven together with the evolution of planets—their hypothesis will have profound consequences in our search for life beyond Earth.
Earth’s crust—the thin upper layer on which all life resides—is broken into 15 or so major and minor plates, more than 100 miles thick in some places. Below it, stretching thousands of miles down toward the center of the planet, lies the mantle, a thick layer of rocky stuff that’s so hot, it’s more goo than solid. The mantle has been slowly cooling since the planet’s formation but remains locked into a constant, slow-motion boil: Heat from the planet’s core sends the goo from the depths up toward the crust and back down again. The circular motion at the top of the mantle drags Earth’s tectonic plates along at a few centimeters a year, the same rate your fingernails grow. This is the continental drift, which generates the occasional earthquake and sometimes births volcanoes. In some places, plates get pulled apart, creating new crust. More important for the story of life are the zones where plates crash into each other, creating mountain ranges.
Plate tectonics is a fundamental feature of Earth. As far as astronomers know, other planets may have broken-up crusts, yet ours is the only one with continually shifting plates. But what if continental drift got lazy? This is exactly what some scientists think may have happened during the boring billion. Perhaps, as the Earth cooled, the heat flow from its core was disrupted; perhaps chemical changes in the mantle itself altered how it responded to that heat. In either of those cases, the conveyor belt carrying the plates around the Earth’s surface could shut down for hundreds of millions of years or more.
[Read: Why Earth’s history appears so miraculous]
The boring billion occurred during the reign of the supercontinent Rodinia, a vast landmass that covered a significant portion of the Earth’s surface. Earth’s land had been swept into supercontinents before, but evidence from billion-year-old mineral deposits suggests that Rodinia may have formed just as continental drift effectively shut down (or at least significantly slowed). A Rodinia without major tectonic shifts would have offered early microbes a planet-girdling geologic stability that lasted for hundreds of millions of years. With no towering mountain ranges being born from apocalyptic collisions between continental plates, the supercontinent would have been exactly the kind of place that could put evolution to sleep.
About 900 million years ago, the planet-size engine within the Earth may have restarted. Evidence from mineral studies that track the creation and destruction of crustal material seems to indicate that plate tectonics might have not only begun again but also entered a vigorous phase unlike anything Earth had seen before. The new continental conveyor belt pulled Rodinia apart and slammed the newly separated landmasses into one another. Earth’s first sky-puncturing, continent-spanning mountain chains were born.
Such a rapid and destructive breakup of Rodinia would have created new environments around the planet that pushed life to adapt quickly and dramatically. Perhaps that’s why the first multicellular organisms—some of which were branching, frond-like creatures made of interconnected tubes—appear in the fossil record within 100 million years of Rodinia’s destruction. This dramatic increase in evolutionary creativity was accompanied by an equally dramatic increase in the abundance of life. Geochemical data that track carbon cycling suggest that, after the end of Rodinia’s reign approximately 700 million years ago, the Earth’s biological productivity skyrocketed. Across 100 million years, biological activity increased almost 100-fold. As rain washed over those new Himalayan-style mountains, the rocks would have been weathered into their elemental components, which could then flow downstream, filling the seas with a burst of nutrients that fueled a burst of life. Each new generation offered a chance for an innovative mutation that might eventually lead to eyes, wings, or a fancy nervous system.
[Read: A possible break in one of evolution’s biggest mysteries]
Much work remains to be done to confirm this story. For example, before scientists can be sure whether plate tectonics stopped or slowed down before the boring billion—or just became more vigorous afterward—they need better ways to rebuild timelines from geochemical evidence. They need to not only determine which plate was where and when, but also tie that evidence to a more detailed understanding of what was happening in the mantle and the rest of the planet’s deep interior. And even if researchers can prove that the Cambrian explosion happened just (geologically speaking) after plate tectonics restarted with a bang, the timing might still have been mere coincidence.
What is clear from the emerging research, however, is that the evolution of life and the evolution of Earth must be considered as one inseparable process. The links between the boring billion and plate tectonics are only one preliminary data point. Scientists are, for example, sure that about 2.5 billion years ago, the evolution of a new kind of photosynthesis flooded Earth’s atmosphere and oceans with oxygen. That “great oxygenation event” profoundly rewired the future evolution of the Earth as atmospheric oxygen led to the solar-radiation-shielding ozone layer, which led to life colonizing the continents, which led to big-brained creatures like us emerging.
These lessons in the “coevolution” of life and planets matter for how humans understand Earth now, as we push our planet’s evolution in new and dangerous directions. And they will matter even more as astronomers continue to scour the universe for life on other planets. Understanding how life reshapes its planets, and vice versa, can help astronomers zero in on which planets to search, including larger worlds, which are more likely to retain their heat and have longer periods of active plate tectonics.
[Read: Humans are ready to find alien life]
We humans finally have the technology and the scientific understanding to begin in earnest the search for life beyond Earth—the chance to encounter beings that come closer to matching our strange capacity to sense and make sense of the world. But to find that complexity, we must take the lessons of the boring billion to heart. Planets are not just a stage on which the drama of life’s evolution takes place. They are main characters too.