You can understand a great deal about people, places, and things by examining their past. This is even more true of the ocean, which has at least a 3.8 billion-year history, give or take a few hundred million years. Now that’s a lot of birthdays! Over the course of its existence, it went quickly from mostly fresh water to salt water, has seen entire populations of plants and animals evolve and go extinct, and has been divided into oceans (plural) as huge land masses drifted apart. It is even thought to have frozen over at least twice and possibly as many as four times in its long history. (Don’t worry, the last freeze was about 600 million years ago, and the bigger problem now is global warming, not cooling.)
In this chapter, we transport you back in time to the birth of the ocean and trace its long history of supporting the evolution of various forms of marine life, a few of which you’ll meet up close and personal. We then fast-forward to the present to describe the current condition of the ocean and its inhabitants, along with the impact it has on Earth overall. We wrap things up by taking a peek into the possible future of the ocean to see where it may be heading. (This chapter gets wet, messy, and maybe a little hard to follow at times, so if you want to skip ahead, we would understand. But it’s also really cool, so we hope you stick with us.)
Discovering How the Ocean Got Its Start
Our ocean covers 71 percent of our planet and accounts for nearly 97 percent of its water. That may represent a mere drop in a bucket on a cosmic scale, but it’s respectable on a planetary scale.
If you had that much water in your basement, you’d want to know where it came from, and scientists have been trying to answer that question since, well, about the time they started asking questions. The two big questions are: Was that water always here? and if not, How the heck did it get here? These aren’t exactly “Which came first, the chicken or the egg?” questions, but they’re sort of along the same lines. Either water came from the space debris that formed Earth (so the makings of the ocean were here already), or water arrived via comets or asteroids crashing into Earth after it was already formed. (Note: Comets are made of dust, rock, and ice, and they tend to fly farther from the sun than asteroids do. Asteroids are made mostly of metal and rock and varying amounts of water and tend to hang out closer to the sun. Meteors are comets that enter Earth’s atmosphere.)
In this section, we present the two leading theories of how all that water got here as we explore the ocean’s formation.
The wet planet theory
The prevailing theory is that the water and/or the chemicals needed to make water were already here when Earth was formed. In other words, Earth formed as a “wet planet.” How planets are formed is also a subject of debate, but generally speaking, they form when particles of dust and gas clump together. In its early days, our solar system was a cloud of dust and gas (or clumps of dust and gas). Gravity caused the matter to collapse in on itself as it began to spin, forming the Sun at the center and the planets around it.
According to this theory, the ocean was formed when water (in the form of vapor) slowly escaped from Earth’s hot molten interior into the atmosphere surrounding the cooling planet. This degassing, as it’s called, occurred over millions of years. As the planet cooled to below the boiling point of water, the vapor slowly condensed into clouds and rain began to fall for centuries or even millennia. At some point, estimated at between 4.4 to 3.8 billion years ago, enough water had been wrung from the sky to create the primeval ocean.
The water delivery truck theory
The competing theory is that water came after Earth was fully formed. This theory asserts that thousands of watery/icy comets and asteroids containing hydroxide (a water precursor) delivered water to Earth. Back then, Jupiter was slightly closer to the sun, and its presence and gravity could have shifted the orbit of these comets and asteroids, putting them on a collision course with Earth. Heat from Earth and the sun melted the ice and formed the ocean.
To test whether Earth’s water came from comets or asteroids, scientists looked at the composition of the water itself. They compared the ratios of hydrogen isotopes (hydrogen atoms with slightly different nuclear masses) among water samples from Earth, asteroids, and comets. None of them matched exactly, but Earth’s water was more similar to that of water contained in asteroids.
Tracing the Evolution of Ocean Life
Although scientists may not be able to pin down where all the ocean water came from, one thing we do know for sure is that life on Earth started in the ocean. In fact, it started not too long after the ocean was formed and has continued evolving ever since. Although the ocean probably existed for at least a few hundred million years before signs of life appeared, that’s not very long from a geological perspective. Because life has existed in the ocean for most of the time that the ocean itself has been around, most of the ocean’s history is commonly presented as a timeline designating key stages in Earth’s geological progression coupled with the corresponding evolution of life. Kind of convenient, isn’t it? In this section, we trace the fascinating progression of that evolution, but first, we need to define a few key terms:
Life is the condition that distinguishes plants and animals (organic) from inorganic matter, such as rocks, minerals, metals, and other substances not derived from living organisms.
Evolution is the process by which species of organisms arise from earlier life-forms and change over time through natural selection.
Natural selection is the process by which organisms better adapted to their environment tend to survive and produce more offspring, while those less adapted tend to die out.
Getting the evolutionary ball rolling
So how did life begin? Well, honestly we still don’t know for sure. One theory is abiogenesis — that life spontaneously arose from non-living material. Another theory, called panspermia, suggests that life came from space on a comet or asteroid. But the leading theory is the RNA World Hypothesis. RNA is similar to DNA but is structured as a single strand as opposed to a double strand and is made up of different nucleobases (the molecular building blocks of RNA and DNA). According to the RNA World Hypothesis, early life on Earth originated with simple RNA molecules that were able to self-replicate (create copies of themselves) and create protein molecules — organic compounds that are an essential part of all living organisms.
Another quality of RNA that makes many scientists believe that RNA drove early evolution is that it has the capacity to evolve through interactions with its environment. The RNA World theory has it that diverse RNA molecules formed (how this happened is still not known) and began to evolve and compete for survival. As they evolved, some RNA molecules began to cooperate with one another to develop genetic code, form proteins, and build cells. Eventually, RNA gave rise to DNA, which has the capacity to store more complex blueprints for living things.
Going