The Deep Past is Really, Really Deep
“Millions of years” is a difficult thing for our minds to imagine. The oldest humans usually only live about 100 years. So the span of ten thousand lengthy lifetimes is just one million years.
Take that into consideration with the following: the Mesozoic Era lasted from about 255 million years ago to 66 million years ago. It is bookended by two mass extinctions – the Permo-Triassic Extinction (also known as The Great Dying because it is estimated that over 90% of life on Earth died out at that time) and the Cretaceous-Paleogene Extinction (the one that killed most of the dinosaurs).
To expand that even further, Earth is estimated to be about 4.5 billion years old. That’s forty-five million human lifetimes.
Rocks Tell the Story
You may be wondering how those times have been determined and how scientists know. The answer lies in rocks. Not individual rocks, but formations of rock that were deposited, buried, melted, uplifted, folded, cut through, split apart, bombarded by asteroids and meteorites, frozen and thawed, changed by chemical reactions, and sculpted by wind and water. The rocks tell the story of Earth's history.
From these rocks, scientists divide Earth's history into Eons, Eras, Periods, and Epochs. These different chunks of time are organized into what we call the Geologic Time Scale.
We find changes in the rocks and changes in the fossils within those rocks that help us puzzle out where one chapter of the story ends and another begins. Often, vast spans of time are missing. And sometimes we’re able to see how rocks and fossils are the same in two parts of the world that were once together. Scientists also find minerals that contain radioactive elements that change predictably over time. When the mineral forms, the “clock” is set. This is why rocks and fossils are the key to understanding how our planet has changed since its beginning.
Chapter One: A World with No Life
From what scientists can tell, for about the first billion years, this planet was lifeless. Earth was a hot chunk of molten rock teeming with... nothing. Early on, it got a glancing hit by what was likely a Mars-sized planet that shot a molten glob of our world into orbit around us. That glob cooled and is what we know as the Moon.
Other rocky planets and moons in our solar system and beyond would have their own geological stories to tell. And while we don’t know with certainty if life exists out there in space, here on Earth, written in stone, the fossil and geologic records provide glimpses into the deep past.
The Secret Life of the Universe
The Mystery of Life's Beginnings
Eventually, life on Earth began. How it happened is a mystery, though there are several hypotheses with evidence supporting and challenging each. Hot vents in the ocean, impacts bringing different elements to Earth, “primordial soup”... All are possibilities, but none are certain. One thing we do know, however, is that all life that exists on Earth now comes from a single ancestry, as we share common DNA with every living thing on Earth. And despite billions of years with conditions that support life, scientists have never discovered evidence that life itself arose more than just once.
The Earliest Chapter of Life
The earliest evidence of life in the fossil record is from about 3.5 billion years ago, found in rock formations called stromatolites. Stromatolites are formed when groups of microbes grow into a living mat that gets covered in sediment; then another group forms into a mat on top of that and gets covered; then another, layer by layer. This happens over considerable stretches of time. Not all stromatolites are that ancient; they are still built by similar microbial and sedimentary processes to this day. However, the oldest are the earliest examples of the history of life being written in stone. And stromatolites get even more interesting than that, as I'll explain in my next blog post.
To Be Continued...
Paleontology is about so much more than dinosaurs. (Though dinosaurs are really cool and there is a lot to learn from them!) Throughout the summer, I’ll talk more about some of the most remarkable paleontological stories as we travel through time from the ancient past to the current day. To steal a phrase from the film version of Jurassic Park: Hold onto your butts.
Join Harris County Public Library for our Summer Reading Program as we explore the theme Unearth a Story. We’ll have exciting events and activities at all our HCPL branches! Sign up on Beanstack to track your reading, gain prizes, and be part of the fun!
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