The late Dr. Stephen Jay Gould, a biologist and History of Science Professor at Harvard University, once noted that "The history of life is not necessarily progressive; it is certainly not predictable. The Earth's creatures have evolved through a series of contingent and fortuitous events."
Our Milky Way Galaxy is also the domicile of 100 billion stars in addition to our own golden, incandescent Sun. Our Star is situated very far from the heart of our Galaxy, where most of the other stars dwell, and where our own supermassive black hole--that weighs millions of times more than the Sun--resides in quiet, though restless, slumber, only to awaken now and then to hungrily devour bits of gas and star-stuff that wander too close to its maw. Our little luminous yellow Star is located halfway to the edge of the Milky Way along the Orion Spiral Arm.
Our Sun is circling around the center of the Milky Way at the speed of half a million miles per hour. Nevertheless, it takes about 200 million years for it to travel around once. Like other spiral galaxies, suspended like gigantic starlit pinwheels in Space, our Milky Way sports a bulge, a disk, and a dark matter halo. Dark matter is mysterious stuff, probably composed of some as-yet unidentified exotic particles that do not interact with light. This is why the dark matter is invisible, making its presence known only by its gravitational interactions with the luminous matter, that we can see.
Although the bulge, disk, and dark matter halo are all components of the same Galaxy, each contains different populations of objects. The halo and central bulge primarily host elderly stars, while the disk is filled with gas, dust, and much more bouncy, youthful stars. Our Sun is, currently, a member of the younger--or, at least, middle-aged--population, at less than 5 billion years of age. The Milky Way Galaxy itself is at least 5 billion years older than that. Our Galaxy is at a minimum 10 billion years old--and it is probably older. In fact, it might be one of the oldest galaxies in the Universe.
It is possible today for astrophysicists to determine the ages of certain stars. Specifically, this means that they can measure the amount of time that has elapsed since these stars were born as the outcome of condensation in dense knots studding huge, dark, frigid, interstellar molecular clouds composed of gas and dust. Some stars are quite youthful--for stars, that is--and are mere bouncing babies at "only" a few million years of age, or less. For example, such extremely young stars dance around happily in the "nearby" Orion Nebula. Our Sun and its family of planets, moons, and assorted other objects, formed about 4.56 billion years ago. However, many of the stars in our Galaxy were born much earlier!
Our tiny blue planet was born from an accretion disk revolving around the ancient Sun almost 5 billion years ago. About 4.53 billion years ago, the Earth and a Mars-sized object, sometimes called Theia, are thought to have crashed into one another, launching a multitude of moonlets into orbit around the ancient Earth. Eventually, these moonlets coalesced to form the Moon. The irresistible gravitational attraction of the newly-born Moon stabilized the Earth's fluctuating axis of rotation and set the stage for the conditions that were so necessary for the emergence of life. Long ago, the Moon was much closer to Earth than it is now. About 4.1 billion years ago, the surface of Earth finally cooled sufficiently for the crust to solidify--up until that time, both Earth and its newborn Moon were probably covered by global oceans of fiery magma. Also, at around this time, the atmosphere and the oceans of our planet formed.
The magic occurred sometime around 3.7 billion years ago. This was when the earliest and most primitive self-replicating nucleic acid tidbits emerged on our planet, probably derived from ribonucleic acid (RNA) molecules. The replication of these very primitive tidbits demanded energy; sufficient space to develop; and smaller, even more elementary, constituent building blocks. This necessary space was very rapidly used up by the flourishing brand new tidbits.. This resulted in bitter competition. Natural selection favored those molecules which were most efficient at propagating their own kind. Self-replicating material, in the form of deoxyribonucleic acid (DNA) molecules ultimately took over as the most efficient replicators. They quickly developed within enveloping membranes, which provided the stable and nurturing environment so necessary for them to continue replication.
About 3.9 billion years ago, the Late Heavy Bombardment occurred. This was when the greatest number of showering objects pelted the four inner planets: Mercury, Venus, our Earth, and Mars. The objects were probably comets, tossed out of their distant home that was situated beyond the outermost planet, Neptune, in what is called the Kuiper belt. These rudely evicted comets invaded the balmy inner Solar System, wreaking havoc as they rampaged through it. The invading comets of the Late Heavy Bombardment had likely been unceremoniously tossed out of their remote home as a result of the gymnastics engaged in by the giant outer planets--Jupiter, Saturn, Uranus and Neptune--which hurled them into the inner realm of our Solar System. This persistent shower of destructive, traveling chunks of alien ice and rock, possibly killed off any life that had already evolved, as the oceans boiled away. However, the theory of Panspermia suggests that life may have been transported to Earth by such rampaging meteorites. Of the two scenarios, however, the former is the most likely, and most (if not all) life existing on Earth prior to the Late Heavy Bombardment was extinguished.
Sometime between the Late Heavy Bombardment and 2.5 billion years ago, the very first precious and delicate tiny tidbits of life appeared. These fragile cells used carbon dioxide as a carbon source, and they were also adept at oxidizing inorganic materials to extract the necessary energy. Eventually, these fragile cells developed the ability to perform glycolysis. Glycolysis is a chemical process that liberates the energy of organic molecules such as glucose, and it generates Adenosine-5'-triphosphate (ATP) molecules as short-term energy sources. ATP continues to be used by almost all organisms on Earth, virtually unchanged, to this day.
About 3.5 billion years ago the very last universal ancestor of all the species now living on Earth existed. At this time, a split occurred between Bacteria and Archaea. Bacteria went on to generate ATP.
Photosynthesizing cynanobacteria evolved about 3 billion years ago. These minute bits of life used water as a reducing agent thereby producing oxygen as a waste product! The oxygen that was first oxidized dissolved iron in the oceans, manufacturing iron ore. The oxygen concentration in Earth's atmosphere subsequently rose, and poisoned many of the existing forms of bacteria. The Moon, at this time, was still very close to Earth, appearing as an enormous companion world in the sky--and it caused tides that were 1,000 feet high. The Earth was also continually subjected to extremely strong, hurricane force winds. Both of these occurrences resulted in a great deal of mixing and shaking that is believed to have enhanced evolutionary processes.
The first known footprints on land date to about 530 million years ago. This suggests that animal migrations onto the land from the oceans might have predated the evolution of terrestrial plants. About 475 million years ago, the first plants moved onto land. These primitive plants evolved from green algae that accumulated around the edges of ancient lakes. The first primitive plants were joined in their migration by fungi, which may have aided the invasion of land through symbiosis.
About 363 million years ago, our Earth began to look like home. Insects skittered over the land, and would soon evolve wings and flutter through the air. Sharks roamed the ancient seas with predatory intent. Vegetation covered what was once barren and desolate land with lovely seed-bearing plants. Beautiful, dense forests began to flourish. Four-limbed tetrapods started to adapt to terrestrial life. This era marks the triumphant Great Crawl, when our ancestors that still dwelled in ancient seas, heroically took their first tentative steps out from the water on to the land. Millions of years later, some of the heroic descendants of those brave little creatures would leave their footprints in the dust of the now much more distant Moon.
About 65.5 million years ago, the catastrophic Cretacious-Tertiary Extinction occurred. The disaster killed off about 50% of the animal species on Earth, including all non-avian dinosaurs. This event is believed to have been caused by the impact of a very large asteroid or comet that blasted out a huge crater in the Yucatan. The birds of the modern world are the descendants of those remnant species of avian dinosaurs that were able to survive this event.
About 14,000 years ago, during the so-called Anthropocene period, human beings evolved to attain their present dominant role over other species.
In August 2012, astronomers around the world celebrated the 35th anniversary of the launch of the Voyager 2 spacecraft. This hearty spacecraft would be the first--and so far only--craft to make the long journey to the two dazzling blue and green, remote ice-giant planets, Uranus and Neptune. Voyager 2, and its sister spacecraft, Voyager 1 (launched 16 days after Voyager 2), are still out there exploring, streaking away from our Sun, zipping out to the edge of our Solar System--and beyond. Scientists are breathlessly awaiting the moment when the sister crafts zoom out to the other side--tearing into interstellar space!
"Even 35 years on, our rugged Voyager spacecraft are poised to make new discoveries as we eagerly await the signs that we've entered interstellar space," Dr. Ed Stone told the press on August 21 2012. Dr. Stone is Voyager project scientist at the California Institute of Technology in Pasadena. He added that "Voyager results turned Jupiter and Saturn into full, tumultuous worlds, their moons from faint dots into distinctive places, and gave us our first glimpses of Uranus and Neptune up-close. We can't wait for Voyager to turn our models of the space beyond our Sun into the first observations from interstellar space."
On February 14, 1990, NASA commanded the Voyager 1 spacecraft to photograph the planets of our Solar System. One image that Voyager 1 returned was that of our Earth, taken from a record distance of "more than 4 billion miles". The fuzzy photograph shows a "pale blue dot" dangling like a tiny spherical bead against an almost incomprehensibly expansive and barren blackness.
In a commencement address delivered on May 11, 1996, the late Dr. Carl Sagan of Cornell University, told of his own thoughts on the deeper meaning of the historic photograph: "Look again at that dot... On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives... It has been said that astronomy is a humbling and character-building experience. There is perhaps no better demonstration of the folly of human conceits than this distant image of our tiny world. To me, it underscores our responsibility to deal more kindly with one another, and to preserve and cherish the pale blue dot, the only home we've ever known."