This drama of star birth and destruction continued until approximately 6-10 billion years ago when a cluster of stars formed at the outer edge of one of the spiral arms of the galaxy we call the Milky Way. Now spread over an arc tens of thousands of light years long, this cluster initially contained thousands of stars all within a diameter of only ten light years across. The larger of these exploded, seeding our general vicinity with an abundance of heavy elements, including a number of radioactive isotopes that would later play a crucial role in the advancement of life on earth.
Around 4.57 billion years ago, one particular accretion of gasses compacted to such a degree that it ignited the nuclear fusion reaction necessary to allow a medium sized star, our sun, to glow bright. The blast from this explosion, accompanied by the prevailing solar wind, swept the lighter elements to the outer reaches of the solar system, leaving only four rocky-cored survivors close in. The third member of this family, the Earth, was to eventually become our beautiful and life-sustaining home.
During its early history, numerous asteroids and comets regularly collided with Earth, keeping the surface a searing bath of molten rock. Around 4.5 billion years ago, it is believed that the Earth experienced a collision of unprecedented magnitude. A companion body roughly the size of Mars, named Theia, hit the Earth off center. It ejected a large portion of Earth’s mantle into space and most probably caused the planet’s axis to tilt to the 23° responsible for Earth’s yearly seasons. The combined ejected material would become our Moon. Other celestial bodies also visited the early Earth, bringing additional bulk material, complex carbon chains, and much-needed water to the planet. It took another one hundred million years or so for things to cool down and rocks to solidify. As the temperature continued to cool, starting around 4.2 billion years ago, the water vapor condensed as rain. This first storm must have continued for a considerable time, hundreds of thousands or possibly even a million years, covering the planet and forming a single great ocean. The turbulent waters of our ancient myths did, in fact, exist in Earth’s early history.
Between 4.1 and 3.7 million years ago, during a period known as the “Late Heavy Bombardment,” a barrage of asteroidal and/or cometary materials invaded the inner solar system–plowing through the Earth’s virgin ocean, vaporizing it, and re-melting much of its crust. One can readily see direct evidence of earth’s turbulent past by observing the numerous, uneroded craters pockmarking the moon.
After a time, the crust once again solidified and earth regained its global ocean. The only land was small basaltic islands, created through volcanism. The Earth appeared quite different in these early days; the sky shone pinkish-orange from abundant quantities of carbon compounds (carbon dioxide, carbon monoxide, and methane) and the seas reddish-brown due to its high iron content. The erosive seas made short work of these soft, volcanic basalt outcrops. The planet would probably look much the same today were it not for particular chemical reactions that were to follow.
In the early 1950s, chemists Stanley Miller and Harold Urey showed that by heating and then passing an electric arc through a mixture of methane, ammonia, and hydrogen, they were able to create a range of organic compounds. These included amino acids, the fundamental building blocks of all proteins and the basis for the rungs on the DNA ladder. Initially, it was believed that these conditions mirrored Earth’s early atmosphere, with photonic energy from the sun driving the process. However, Robert Ballard, the famed oceanographer and discoverer of the Titanic, discovered in the 1970’s complex life forms at depths of nearly 8000 feet below the surface–too far for sunlight to penetrate. Instead, these creatures lived off the nutrients and heat from undersea volcanic vents. Based on the iron sulfur world theory, we have now come to understand that the conditions for the initial generation of life on earth most likely occurred near these hydrothermal undersea vents–instead of in the atmosphere. Furthermore, it is now believed that the various minerals present at these vents acted a catalyst, helping jump-start the life process. More recent discoveries have also found amino acids in meteorites. This would indicate that these base building blocks of life need not have a terrestrial origin and may pervade the cosmos.
Next, through processes only partially understood, these amino acids formed more complex proteins, eventually becoming capable of self-replication. Miraculously, some of these managed to survive the numerous environmental stresses experienced by the early earth and went on to fill the seas. Around 3.5 billion years ago, the first “organized” life forms of bacteria and algae appeared.
Life continued in this limited fashion for another 2.5 billion years. During that time these simple life forms had a profound effect on the planet. Through the process of photosynthesis, early cyanobacteria began extracting carbon from the abundant supply of CO2 in the atmosphere and replacing it with oxygen. This oxygen reacted with the iron-rich oceans, precipitating out iron oxide (rust), and bonded with hydrogen to form additional water vapor. Eventually, the abundance of free oxygen would “poison” the prevalent anaerobic bacteria, causing a dramatic shift to those organisms that could coexist in this new world. Furthermore, super heating of volcanic basalt in the presence of water produces much harder and lighter crystalline rocks, such as granite. This material would both better resist the erosive forces of nature and essentially “float” on the earth’s mantle, forming the basis for our modern continents. Sometimes united into one, sometimes split apart as separate land masses, the land would now finally stand apart from the sea.
The next major breakthrough occurred approximately one billion years ago, when collections of cells began to cooperate and differentiate. This coincided with the formation of the first supercontinent, Rodinia. However, the promising future of planet Earth would next face its greatest challenge, for during the Cryogenian period (850-630 million years ago) the planet would experience a number of devastating ice ages. Some of these were so severe that it is believed glaciers covered much of the oceans and extended down to the equator.
Through a combination of effects the earth eventually warmed, once again returning to the predominantly blue planet we know. The now favorable environmental conditions–coupled with a substantial rise in sea level– provided a wide open niche for those organisms that survived the previous periods of glaciation to flourish. This dramatic proliferation of diverse life forms, named the Cambrian explosion, started between 540 and 500 million years ago. It populated the oceans, giving rise to shelled creatures and then the first vertebrates. One branch of vertebrates evolved into the fish that still fill our modern oceans. One such species, the Coelacanth, even survives to this day, 360 million years since it first appeared on the scene.
While the oceans teemed with creatures large and small, life had not yet ventured onto dry land. Then, between 445 and 365 million years ago, the first land plants appeared. Initially, these were simple algae, found near the water’s edge. Next came sporangia such as Rhynia, a simple plant with a few bifurcated stems each bearing a spherical spore pod. These would give rise to ferns, the first seed-bearing plants. It would take almost an additional 100 million years for the first trees, Archaeopteris, to appear. These looked not unlike Christmas trees and would eventually cover the earth in vast forests.
Vegetation on dry land presented an entirely new ecosystem to host an additional explosion of animal life. As these land-based plants continued to extract carbon from the atmosphere, the oxygen level correspondingly increased. In fact, it would reach levels that far exceeded that of the present day. These elevated levels allowed the first early animal colonists, arachnids and insects, to grow to gigantic proportions. Next, out of the sea came the amphibians, which in turn gave rise to the reptiles. One branch, the dinosaurs,