EARTH OF THE PAST

 On a planet called Earth, over 7 billion people were bustling about their daily lives. Each year, tens of millions were born, and tens of millions died, leaving traces of their existence in the air, water, soil, and even space. Yet, these signs would not last forever. In a few hundred years, our buildings would crumble, our stone monuments, plastic, Styrofoam, and even the evidence of our nuclear endeavors would fade away.

This realization led to an intriguing question: How could we be sure that we were the first advanced civilization on Earth? According to the Silurian Hypothesis, we couldn't. This hypothesis posed a fascinating premise that perhaps, long before humans, there might have been other advanced civilizations that left no trace for us to find.

The idea of the Silurian Hypothesis was inspired by an episode of Doctor Who, where intelligent reptilian creatures called Silurians awakened from 400 million years of hibernation due to nuclear testing. While this was a work of fiction, the hypothesis raised a profound possibility: What if there were once other advanced civilizations on Earth that have completely vanished?

Humans often think that their existence and their civilization are eternal, but history teaches us otherwise. Take ancient Egypt, for instance. For over 3,000 years and across 30 dynasties, Egyptians lived under the shadow of the pyramids, fished the Nile, and mingled with other cultures. To them, their civilization seemed everlasting, yet it too disappeared. Similar fates befell the Mesopotamians, the Indus Valley civilization, the Greeks, Nubians, Persians, Romans, Incas, and Aztecs. These great empires, once thriving with millions, left behind scant evidence of their grandeur.

Modern humans have been around for about 100,000 years, a mere blip in the hundreds of millions of years that complex life has existed on Earth. Given this vast expanse of time, it's conceivable that other intelligent species might have risen and fallen long before us. Would we even know they had been here?

Adam Frank and Gavin Schmidt explored this in their paper on the Silurian Hypothesis. They pointed out that our usual methods of studying ancient societies—through artifacts and ruins—only work for relatively recent history. When you want to look millions of years back, things get complicated. Earth's surface itself is dynamic, constantly reshaped by plate tectonics. Today's mountains were once ocean floors, and new lands form as old ones erode. The oldest surface land discovered, the Negev Desert, is only about 1.8 million years old.

Even fossils, our primary windows into ancient life, are rare. Specific conditions are needed for fossilization: hard body parts, rapid burial, high pressure, and low oxygen. Despite dinosaurs roaming the Earth for 180 million years, we have only a few thousand near-complete fossils. The odds of finding evidence of a short-lived species like Homo sapiens in the fossil record are slim.

Furthermore, less than one percent of Earth's surface is urbanized today. Artifacts like roads, cities, and machines would decay and disappear within a few thousand years. Even the aftermath of a nuclear war would eventually fade away. To detect ancient advanced civilizations, scientists must look for indirect evidence.

The Silurian Hypothesis suggests looking for markers of industrialization on a global scale. One key marker is changes in the isotopic composition of elements, which can be detected in sedimentary layers. For instance, human activities have altered the nitrogen cycle and increased the levels of certain metals like gold, lead, and platinum. Most notably, the burning of fossil fuels has changed the carbon isotope ratios in the atmosphere, known as the Suess effect, which is detectable in sediment cores.

Interestingly, a sudden global change in carbon and oxygen isotope levels was observed 56 million years ago during the Paleocene-Eocene Thermal Maximum (PETM). The PETM saw Earth's temperature rise by six degrees Celsius over 200,000 years, with fossil carbon levels spiking. Some scientists speculate that a massive volcanic eruption caused this, but the exact cause remains unknown. Could it have been evidence of an ancient civilization? Probably not, but it does show how such an event could leave a detectable mark.

The Silurian Hypothesis, while not proving the existence of ancient civilizations, provides a framework for searching for them, not just on Earth but also on other planets. The Drake Equation estimates the number of extraterrestrial civilizations in our galaxy, suggesting there could be anywhere from 150,000 to 1.5 billion. If intelligent life can arise multiple times on a single planet, as the Silurian Hypothesis proposes, it opens up exciting possibilities for finding civilizations throughout the galaxy.

Mars, once warmer and wetter, and Europa, one of Jupiter's moons with its saltwater ocean, are prime candidates for such investigations. While the authors of the Silurian Hypothesis don't believe ancient civilizations existed on Earth before humans, their ideas give us tools to explore and perhaps someday discover the rich tapestry of life that may have once existed, both on Earth and beyond.