Summary
After ancient stars exploded, their remnants conglomerated through gravity to one day form a place called Earth. Primordial Earth was a giant, red ball of magma and smoldering rocks; it was hellish and ablaze with erupting volcanoes and fiery skies. But over time hails of comets and asteroids bombarded the Earth to form the oceans causing Earth's outer layer to cool and turn grey; those heavenly bodies also seeded the oceans with rich organic chemistry which, somehow, eventually turned into the first microbe. Nearly one billion years later, photosynthesis was invented—this oxygenated the world and set the stage for the emergence of large, complex, multi-cellular organisms.
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Earth's formation
As the extraordinary physicist Richard Feynman once said, all physical phenomena in the universe is the result of a myriad of different atoms interacting with one another in an infinite variety of combinations. Scientists have known for at least the past century that life is the result of chemistry—a science which catalogues all of the countless ways in which electrons of some atom can effect, in some way, another atom. Despite this apparent and seeming complexity, remarkably, all life on Earth—and its infinite number of permutations—is made of just four fundamental building blocks: hydrogen, oxygen, carbon, and nitrogen. Fundamentally, all of the complexities and permutations of life boils down to something very simple: four kinds of atoms which chemically interact with one another. It is astonishing that all of this complexity is the result of something quite simple going on.
These elements are star stuff—the ashes of ancient stellar alchemy—and are the most abundant elements in the universe. Since star stuff eventually condenses into planets—like Earth—the fundamental ingredients of life were already abundant on the Earth after its formation. But, as Carl Sagan once said in Cosmos on the discussion of the origin of life: what is special about life are not the atoms which go into it, but rather the way in which those atoms are put together. This why carbon is special: it allows atoms to be put together in many different ways. (This Khan Academy video gives an excellent introductory explanation on the flexibility of carbon.) For many scientists, it is difficult to imagine any form of life besides carbon-based life since carbon's flexibility is necessary (at least for Earth-based life) for so many important biological functions—well, at least for all life which we know of. But who knows? Perhaps somewhere else in the galaxy there are exotic beings which are not carbon-based. Some scientists have argued that perhaps silicon-based life could be possible since this element is also very flexible. But since silicon, unlike carbon, is a very rare element in the universe, it is far less likely for silicon-based life to evolve than for carbon-based life to evolve. Well, that is a very profound thing to ponder; but that tangent aside, we have to address at least one part of our original question. How could simple molecules have formed from these four basic ingredients?
Scientists are fairly certain that the answer to this question, as Darwin long ago speculated in a letter to a friend, is that the Earth's primordial sea was a soup of chemicals undergoing complicated chemical interactions. What exactly these chemical interactions were is not fully known, but what we do know is that they, somehow, transformed a soup of atoms and chemicals into the first microbe.
Origin of the sea
The earliest eon in Earth's history is known as the Hadean eon which began during the formation of the Earth 4.5 billion years ago and ended roughly 4 billion years ago. During this time, Earth was a hellish place. It was a giant ball of magma with ferocious lava eruptions; it was continuously bombarded with meteoroids and micrometeorites; it was also covered by ultra-energetic UV light emitted by the Sun. Although invisible to us, this UV light was so energetic that it cooked the surface of Earth to extraordinarily hot temperatures of over 2,000 degrees Fahrenheit. Any rocks or life which might have formed during this time would have been annihilated—absolutely eviscerated—leaving no "detectable trace" whatsoever. Geologists and paleontologists need at least some rocks or fossils of long-dead creatures to make reliable predictions of what kinds of rocks and lifeforms were present back then—in other words we need something to work with to be able to know what things were like back then. For this reason, due to this lack of any preserved evidence, the Hadean eon is not very interesting—or more precisely not very useful—for studying the origin of life.
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We'll begin our study of life's origin in the earliest eon in which rocks and fossils were preserved—the Archean eon—a vast sweep of time starting when the Earth was only 4 billion years old and ending when the Earth was 2.5 billion years old. It was during this time period when the Earth's oceans formed. Let's explain how this happened. About 3.9 billion years ago in a time period known as the period of Late Heavy Bombardment, hails of meteorites—each containing very minute traces of water—collided with Earth and deposited water into its surface. Despite each meteorite not containing much water, after 20 million years of continuously bombarding Earth, all of that water contributed by each meteorite started to add up and immense puddles of water eventually formed the oceans. Astonishingly, every last drop of water on Earth can be traced back to a meteorite during the time period of Earth's formation and Late Heavy Bombardment. Over time, the water cooled the surface of the Earth's crust and the Earth—once resembling a red, glowing ball in space—turned grey. As a result of the formation of the oceans, the Earth cooled to about 170 degrees Fahrenheit. Not only did meteorites form the oceans, but they seeded the ocean bottoms with minerals and proteins made of amino acids. The oceans were turned into a soup of chemicals and proteins which, somehow, would eventually go from chemistry to biology and create the first microbe.
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Origin of plate tectonics and land masses
Around the time period of Late Heavy Bombardment, despite the Earth, with its newly formed oceans, being more congenial for the emergence of life, the Earth back then was still a very chaotic place. The Earth was spinning so fast that an entire day lasted only six hours. And the Moon—looming large in the sky—was only a little more than a dozen thousand kilometers away. The Moon exerted tremendous tidal forces on the oceans stirring tumultuous currents and waves in the sea; and the Earth spun so rapidly, that ferocious winds of up to 300 mph—dwarfing the mightiest hurricanes of today—enveloped the Earth's surface. But very gradually, over millions of years, the Moon moved away from the Earth and the Earth's rotation slowed. Over the course of the next 100 million years, magma erupted from beneath the sea floor and this magma—cooled by the oceans—emerged above the ocean surface as black, rocky, volcanic islands.
But how did larger land masses first form? The Earth's crust and everything on it—such as the oceans, volcanic islands, and, later on in the future, mountains and other landmasses—sit on top of hot, flowing magma. Sometime in the early Archean, Earth's crust got torn apart into pieces—these pieces are called plate tectonics. To us, the land masses on the Earth seem fixed and unchanging. But that's the case because we can only at best imagine a timescale about the length of a human lifetime. But on the scale of billions of years, the Earth's surface is constantly undergoing change and getting reshuffled. The writer John McPhee very concisely captured the restlessness of Earth's surface best in the aphorism: the summit of Mount Everest was once—a very long time ago—beneath the surface of the ocean. Over time periods of billions of years, all the plate tectonics—pieces of Earth crust—are constantly moving because they get carried along with the flowing, circulating magma that they float on. The entire geological history of Earth is characterized by the drifting movements of these plate tectonics. Throughout Earth's history, the plate tectonics are continually colliding with one another and getting torn apart. Whenever these things happen, sometimes land masses on them collide. Those volcanic islands which emerged in Earth's early history eventually smashed into one another to form the first continents. And after billions of more years, these continents continued to drift due to the movement of the plate tectonics and eventually collided to form the first supercontinents.
After Late Heavy Bombardment
Modern stromatolites in Shark Bay, Western Australia.
It is generally accepted that the first microbe evolved from chemistry in the Earth's oceans about 3.5 billion years ago after the period of Late Heavy Bombardment ended.11 We'll explain in detail how during the Archean eon, trillions upon trillions of these microbes dramatically changed the Earth's chemistry and ecology. To put it very briefly as to how they did this: they oxygenated the Earth's atmosphere which precipitated a great extinction event, despite being a prelude for the emergence of terrestrial life; they rusted the Earth leaving billions of tons of rusty, orange rocks behind; and they even caused an ice age. But how could such minuscule creatures so profoundly alter the planet? Let's now delve into the details—the story told by science—as to how during this interval of time, microbes did all of this. The story begins with an an expedition to Australia, where a team of geologists collected rock samples from a rock formation which is roughly 3.5 billion years old. They uncovered the fossilized remnants of ancient stromatolites. These remnants look like black, rocky, alien-like, mushrooms as shown in the image above. But, there was a problem: it is impossible to figure out how these stromatolites first formed merely by studying their 3.5 billion year old fossils. The geologist David Flannery found a way around this problem by taking a trip to Shark Bay where he studied still-living stromatolites. He discovered that stromatolites were being built by a colony of tiny little creatures: a one millimeter thick layer of single-celled microbes. But these microbes Dr. Flannery was looking at were different from those living 3.5 billion years ago; they were a special kind of microbe known as cyanobacteria. Cyanobacteria is able to use the Sun's energy to convert water and carbon dioxide—which was teeming the primordial oceans—into glucose (a simple form of food) and oxygen. Although the first microbes appeared around 3.5 billion years ago, the first kind of microbes capable of undergoing photosynthesis—cyanobacteria—did not evolve until much later around 2.45 and 2.32 billion years ago. Around this window of time, food and photosynthesis were invented. And in the latter process, they left their signature in layers of Earth rock. How? Well, the oxygen they produced oxidized iron in the Earth's rocks. This "rusted" billions of tons of Earth's rocks turning them orange. As this was happening, all of the oxygen was getting absorbed by the iron; but eventually, after all of the iron became oxidized, for the first time oxygen filled the oceans and atmosphere. This event is known as The Great Oxidization Event.
This event visibly changed the color of the Earth. Around that window of time (2.45-2.32 billion years ago) when cyanobacteria began to fill the Earth with oxygen, the Earth's sky—once hazy orange—filled up with so much oxygen, that it turned blue. But still—even though we're three billion years in time since the Earth's formation—life forms more complex than single-celled organisms had not yet evolved. There were no animals, reptiles, or plants; only simple, single-celled microbes. We'll soon see that these cyanobacteria caused the greatest series of ice ages in Earth's history; these ice ages were the key to the origin of more complex lifeforms.
This article is licensed under a CC BY-NC-SA 4.0 license.
References
1. SciShow. "A Brief History of Life: Survival Is Hard". Online video clip. YouTube. YouTube, 13 July 2016. Web. 09 June 2017.
2. Wikipedia contributors. "User:Triangulum." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 3 Apr. 2017. Web. 9 Jun. 2017