Fossilized Intelligence

Traces of human civilization in the rock record

Over 60% of iron comes from one specific type of ore: the banded iron formation. These unique rocks concentrated in a brief moment of geologic time around 2.4 billion years ago during the Great Oxygenation Event, when photosynthetic, oxygen-producing bacteria first emerged.

You would not be able to breathe standing on the Earth of three billion years ago. Free oxygen is not naturally present in the atmosphere. It reacts with almost anything it touches; you know this as rusting or combustion (it’s even the reason you hear so many ads about why antioxidants are so important in our diets). So the oxygen that we and the other animals breathe must be put into the air by photosynthetic organisms like plants, and if we go back far enough in Earth history, these had not yet evolved. 

The first photosynthetic, oxygen-producing organisms were the cyanobacteria, and when they appeared, vast amounts of iron were dissolved in the oceans. The oxygen produced by these bacteria, which lived in these ancient seas, reacted with this iron. Iron minerals snowed onto the seafloor, forming the greatest concentration of iron deposits in the earth’s crust. It was only after this dissolved iron was depleted that oxygen accumulated in the oceans and the atmosphere. 

So even though we think of iron and the other metals as fundamentally inorganic, life played a critical role in forming the ores that have become the steel of our bridges and buildings. The appearance of cyanobacteria and their novel biochemistry changed the planet forever, not just by precipitating banded iron formations but in enabling animalian life. And today, we are living through another great shift, one that will also leave a tremendous mark on the geologic record: the emergence of intelligent civilization.

Mass extinction but more mammals?

Geologists often use the appearance and disappearance of fossilized organisms to define the boundaries between geologic ages. You may be already familiar with the most famous of these: the Cretaceous-Tertiary boundary, where non-avian dinosaurs went extinct thanks to a massive asteroid striking the Yucatan Peninsula. But the Cretaceous-Tertiary extinction (properly referred to as the Cretaceous-Paleogene or K-Pg) is only one of several mass extinction events. 

The Cretaceous-Paleogene boundary layer exposed along I-25 in Colorado. Source: NSF

The Earth’s biosphere is undergoing tremendous change due to human influence. Species that previously existed only on one continent will now be found around the world. Normally that is evidence of continental connections: when the isthmus of Panama formed three million years ago, armadillos and porcupines appeared in North America while the ancestors of modern bears, cats, and dogs walked to South America. But this will be the first time in earth history that large plants and animals homogenized planet-wide without the formation of a supercontinent. 

This is not simply the story of deliberate introduction of agricultural species like corn or livestock like cattle. Ships did not intend to carry rats along to islands where they have decimated local populations. Farmers did not think about the earthworms hiding in the roots of plants they brought to the new world; these have become invasive in many parts of North America, altering soil nutrient cycles. 

So what will future paleontologists find when studying our particular slice of earth history? Paradoxically, the current human-driven mass extinction has been accompanied with an increase in certain aspects of the biosphere. The total mammal biomass has increased fourfold from pre-human times, thanks to widespread cultivation of livestock fed by human agriculture. Of all mammal biomass, only 4% is wild, with humans accounting for just over one third and cows and pigs making up the remaining ~60%. Paleontologists will find a unique fossil succession around the world marking the beginning of intelligent civilization: mass extinctions of local species followed by the sudden appearance of humans, along with their domesticated livestock and crops with no local antecedents. 

Burrows and Middens

Bones are not the only mark life leaves in the rock record. There is a whole field of study, known as ichnology, dedicated to trace fossils. The most spectacular of these are dinosaur footprints, but the humble shrimp burrow has much to tell you about the environment in which it is found. How deep was the water? What was the water temperature and chemistry? By identifying the organism from its burrows, ichnologists shed light on the geologic conditions prevailing at that location and time. Organisms will also leave behind nests or other structures where they lived and ate.

The humble pack rat brings back all manner of items from its surroundings, such as seeds, leaves, and twigs, and it forms these into a midden. For paleoecologists investigating changes in plant or animal life through time, finding a pack rat midden is a gold mine, a snapshot of what was living in the local environment at its time. 

Compared to previous organisms, humans will leave behind vast traces. Future paleoanthropologists will be able to investigate human technology, behavioral patterns, and civilizational progression as recorded in the rock record. Imagine finding the treasure trove of a fossilized garbage dump or rail terminal. Just as with fossilized bones, these traces will only be found in locations of active subsidence and deposition. The places where New York and Chicago lie today will be scraped down to bedrock by glaciers during the next ice age; Houston and New Orleans are safer bets for preservation. 

Shrimp burrow in the loose sediment immediately below the sea floor. Tree roots may break into bedrock but they rarely extend deeper than a few dozen feet. Humans are unique in that our traces–mines, quarries, and wells–can go miles into the subsurface. If you fly over west Texas today, you will see tens of thousands of oil wells. At the surface, these consist of some processing equipment, some pipelines, and a network of small roads. But most of these wells extend beneath the surface for thousands of feet–steel pipes and cement that will be permanently sealed into the rock. If you were to erode away the surface in west Texas or any of the great oil provinces today, the landscape would be dotted with bizarre pillars of rusting steel and crumbling cement. 

New rocks and chemical traces

Evolution of novel life forms sometimes produces entirely new rock types. Much of the limestone around the world is made of shell fragments and other remnants of marine organisms. Coal did not exist before plants trapped the carbon of the atmosphere into their cell walls. Human civilization produces tremendous amounts of plastic which can take centuries to break down; already this has made its way into some very young rocks known as plastiglomerate.

But there will be easier ways to identify the start of human civilization in the rock record than finding the rare plastiglomerate. Human activity has left traces of microscopic particles and chemicals that can be found in sediments around the world. Industrial activity has produced large volumes of metals such as lead, aluminum, and titanium that can be found today trapped in muds. The burning of fossil fuels creates small, round, black particles known as fly ash that travel through the atmosphere and can be found even in remote bogs far away from civilization. And atmospheric testing of nuclear weapons during the 1950s & 1960s dusted the planet with new elements and rare isotopes such as plutonium and cesium-137. It is the appearance of these radionuclides that stratigraphers may use to mark the beginning of the Anthropocene, the new geologic period characterized by the influence of human civilization on the Earth and environment. 

Future Fossils

In writing this piece, I am not trying to argue against human activity, just as I would I never inveigh against the cyanobacteria for changing the atmosphere or the shrimp for disturbing the seafloor. Geologists bring the perspective of deep time; we understand the many cycles the earth has been through and the impact of life’s evolution on the planet. No one knows how long humans will be around or what the next phase of intelligence will even look like, but we can be sure that we have left dramatic marks on and in the Earth for future geologists to discover.