The age of the earth has intrigued people for a long time. Aristotle thought that our planet had existed forever, while a careful reading of biblical accounts suggested to theologians that it was created in 4004 BCE. In modern times,
radiometric dating makes it possible to determine the age of rocks and fossils, so we now know with confidence that Earth is 4.54 billion years old. Since a timespan so vast is difficult to contemplate, I find it useful to shrink it down to a single year, which starts with Earth’s formation on January 1. We can then look back on the events in this Earth Year from the vantage point of midnight on December 31—a stratagem that lets us view the history of our planet in its proper perspective.
For the first 200 million years of its existence, Earth remained a very inhospitable place. Only when the new planet had cooled down sufficiently could liquid water be retained on its surface. According to our Earth Year, oceans first appeared by the middle of January, and the robust fossil evidence tells us that by the middle of March, they were teeming with microbial life. Where and how these microorganisms originated is still uncertain, but it is conjectured that their earliest habitat was the vicinity of hydrothermal vents.
Springtime was also when an organism existed that is the ancestor of all forms of life that now thrive on Earth. That microbial creature, the Last Universal Common Ancestor, known as LUCA, possessed the 355 genes that all of today’s life forms have in common—from bacteria and dandelions to elephants and oak trees. Knowing the rate at which genes mutate, geneticists have estimated that LUCA was alive sometime in March. LUCA was probably just one of many primitive organisms, but over millions of years, this plucky ancestor proved better able to adapt to Earth’s changing environment than all the others.
Microbes were responsible for a dramatic change in Earth’s environment, beginning in August. Certain single-cell organisms managed to internalize a protein complex (now known as a photoreaction center, or PRC) that was capable of converting sunlight into usable chemical energy. Since this photoreaction generates oxygen as a byproduct, and the photosynthesizing organisms flourished in Earth’s sunlight, enough oxygen entered the atmosphere to alter its composition—with far-reaching consequences. Over millions of years, the air oxidized all the iron on the planet’s surface and caused the demise of most life forms that existed then. Oxygen was poison for them but a boon for the emergent oxygen-tolerant life forms—like us.
Around September, some of the single-cell microbes underwent another biological innovation that had prodigious effects. The genetic material (DNA, RNA) floating in the cytoplasm of these microbes became packaged in a membrane-enclosed nucleus, and such cells, known as eukaryotes, opened the door to multicellular life forms, in which all cells have the same genome. Eukaryotes incorporated several other organelles and, with their growing complexity, proved successful at adapting over millions of years, eventually evolving into the great variety of plants and animals that thrive on Earth today.
But all that was still far in the future. It was not until November 1 that land plants appeared, and by the middle of November, worms and primitive fishes existed. Early December saw the arrival of vertebrates and tetrapods, soon followed by reptiles, octopuses, and beetles. Disaster struck, however, on December 10 (250 million years ago), when an extinction event killed 90 percent of all sea animals. We do not know what caused this catastrophe, but life on Earth resumed in a most dramatic fashion, as an immense assortment of dinosaurs emerged and flourished for an astonishing 200 million years. Their dominance did not end until December 26 (65 million years ago), when another global disaster, possibly the impact of a meteor, killed 50 percent of all species, including all the dinosaurs—all except for the birds.
Now time seems to be speeding up. By December 29 many mammals, including bears, giraffes, and whales, were roaming Earth. Our humanoid ancestors did not appear until noon on December 31, when, geneticists tell us, they separated from our closest relatives, the chimpanzees. Our own species, optimistically named Homo sapiens, appeared in Africa a mere half hour before midnight, and by five minutes before 12, modern humans had populated all the continents.
Time now appears to pass at an increasingly dizzying rate. Earth’s last ice age ended a mere 90 seconds before midnight and ushered in the Holocene era, in which humans dominated and refurbished the planet. They invented agriculture, cities, empires, metallurgy, and writing during the last minute before midnight, while Caesar was assassinated just 15 seconds ago. Two seconds before midnight, in approximately the 17th century, modern science, arguably humanity’s greatest intellectual achievement, began to flourish and in time revealed to us the tortuous history of our planet and its inhabitants. But such enlightenment came at a price, for science also spawned the technological age, which hurtled forward at breakneck speed. The silicon transistor was invented just half-a-second (70 years) ago, and it quickly led to integrated circuits, computers, the Internet, mobile phones, and social media—with imponderable consequences for human society.
The clock has struck midnight, and I can catch my breath. I am left to face the future alone. It has not escaped my notice that photosynthetic organisms, close relatives of the phytoplankton that flourish in today’s oceans, succeeded in modifying our atmosphere only over many millions of years, whereas modern humans did so in just a few decades, by pumping enough carbon dioxide into the
atmosphere to affect Earth’s climate. Just as the newly oxygenated air had spelled doom for many species, so do greenhouse gases threaten many species of fauna and flora. Global warming played a role in the wildfires in Australia, which killed a billion animals and probably extinguished some species. The escalating temperature of the oceans is having similar effects, and rising sea levels are even now threatening human habitats in coastal areas.
Extinction events have punctuated the history of Earth many times in the past, but the current one differs from them in an important way: its severity could be limited by replacing fossil fuels, currently our principal energy source, with renewable and nuclear energy sources. Although this will require a degree of cooperation among nations that is notably lacking at present, it would largely prevent the economic and social disruptions that will surely follow inaction.
