The Origin of Life on Earth
By a few billion years ago, even the most distant planets in our solar system were fully formed. So, deep space was becoming relatively peaceful as the bombardment from galactic debris became less and less frequent on the surface of the Earth. However, the barrage was not entirely over. As the Sun traveled through the universe it bobbed up and down throughout the galaxy, taking the Earth with it as it went. Every thirty million years, the solar system then passed through the densest part of the Milky Way, composed of trillions of comets. So, anytime the Sun got too close, gravity disturbed the field of debris, catapulting huge chunks of ice in toward the planets.
As they traveled, for periods in excess of hundreds of thousands of years, the comets eventually began to warm up, causing immense clouds of gas and dust to trail through space behind them. Amidst this enormous orbit, incoming meteors, comets, and asteroids, the largest of which were the same size as the early continents, would occasionally vaporize themselves along with a portion of the surface that they impacted. Some of the largest impacts on the surface of the early Earth created enormous craters, set off gigantic waves, super-heated the atmosphere, and in some cases even lead to the evaporation of the upper region of the ocean. These blasts covered the entire planet with an incandescent rock vapor that broiled the surface of the Earth, over and over.
As a consequence of this relentless activity, the excessive heat sterilized everything down to a depth of about one thousand meters for quite some time. Nonetheless, since organic compounds were relatively common throughout the early universe, many comets, meteors, and asteroids already contained full-fledged amino acids such as glycine. These were composed of complex biological materials that had formed from simple carbon compounds that had been catalyzed by the intense radiation given off by the Sun. This was incredibly important because as these celestial objects traveled through the solar system, some of the massive chunks of ice and rock were pulled toward the Earth by its gravity, out from these fields of biochemical debris.
The heat and pressure of the resulting impacts then catalyzed a number of amino acid reactions, which fused combinations of carbon and other basic elements together to form more complex molecules. These global impacts evaporated the frozen comets, creating storm clouds over vast areas of the planet. This produced a deluge of hot, acidic rain that continued for millions of years as the relentless bombardment, along with frequent volcanic eruptions, sent searing hot gas and dust around the globe. At this point, the Earth possessed a reducing atmosphere which did not contain considerable amounts of methane or ammonia. Regardless of this, these gases were present since they were briefly emitted from volcanoes before being destroyed by the extreme ultraviolet radiation coming from the Sun.
At the greatest depths of the global ocean, carbon compounds were shielded from many of the ill effects of the surface world. As a result, some organic substances were able to survive the trip from deep space into the vast waters. These molecules then gave rise to a sort of primeval sludge as they combined by linking, losing molecules of water in heat catalyst reactions as dozens of different monomers became more and more resilient to the effects of entropy amidst the myriads of micro-environments which existed at the time. Each, and every, one of these gave rise to various different conditions of temperature, chemical composition, and available energy sources.
During the period of heavy bombardment, intense meteoroid and asteroid collisions produced heat and shockwaves that increased the chemical reaction rates amidst the primordial soup of organic molecules. Scores of carbonaceous chondrites also introduced a number of chemical compounds and various trace elements required for the formation of life, such as molybdenum. This seemingly catastrophic sequence of events both triggered and disrupted the processes of biopoesis during this panspermian seeding phase. This produced multiple origins of life on Earth, time, and time again.
As part of this, some of the more robust forms of the various prebiotic concoctions passed into the hydrothermal vents along mid-ocean ridges. These broiling hot water springs then precipitated out black plumes of mineral particles. Common among these clouds were sulfides of various different metals, like copper, lead, and zinc. As part of this, hot magma rose up where the water met the seafloor, bringing with it heat from the Earth’s interior. Cold seawater then percolated down the fractures to meet with the magma ten miles under the seafloor. The super-heated water then rose again, dissolving minerals from the rock and emerging at the ridge.
As the interacting organic molecules went through the vents, over and over again and again, the heat inside the vents provided the energy that drove the chemical reactions necessary to form increasingly stronger peptide linkages at very specific temperatures. Then, the cooler periods in the water cycle ensured that the newly synthesized molecules didn’t immediately break apart. In this way, simple substances like carbon dioxide and hydrogen sulfide were able to viably react with each other on iron-sulfur minerals such as pyrite and pyrrhotite.
Then, some of the first building blocks of life were converted into an array of nucleic acid molecules. Plus, nucleotides eventually even began to string themselves together to form ribonucleic acids. These complex polymerized molecules consisted of two strands of nucleotides, which were linked together to form a chain. The most resilient of these structures were made up of a sugar molecule called ribose, a phosphoric acid, and one of four different nitrogen-containing compounds. The latter were four bases that initially came in the form of adenine, guanine, uracil, and cytosine.
In general, the sugar ribose was produced through a multi-step process involving a condensation reaction consisting of molecules of formaldehyde while other molecules underwent wholly different processes of synthesis. As an example, adenine is a pentamer of hydrogen cyanide, while the nucleotides were produced through phosphate reactions.
The short chains of preliminary ribonucleic acid, which were only about a hundred and sixty bases long, could then act as the hereditary instructions and as a catalyst for the various reactions involved in replication, so it was possible for this particular structure to make copies of itself without the need for other kinds of molecules. This process began with the separation of two nucleotide chains, each of which then acted as a template for the assembly of a new complementary chain. As the old chains separated, each nucleotide in the two chains attracted a complementary nucleotide.
Following this, nucleotides were then joined to one another by hydrogen bonds to form the rungs of a new ribonucleic acid molecule. As the complementary nucleotides were fitted into place, the phosphate group of one nucleotide bonded to the sugar molecule of the adjacent nucleotide, forming the side rail of the new ribonucleic acid molecule. This process continued until a new nucleotide chain formed alongside the old one, using the oldest form of replication on Earth.
Once this crucial step was achieved, variations in the sequences occurred which could compete with each other, leading to more complex arrangements. These ribonucleic acid molecules could also make copies of their own sequences, so they could replicate both ribonucleic acid strands, and the cycle could be repeated indefinitely. So, eventually, the natural selection of specific ribonucleic acid molecules made a primitive form of protein synthesis possible. That is to say, once ribonucleic acid was available to catalyze peptide ligation, proteins large enough to self-fold were able to emerge.
Through transcription, a section of one strand acted as a pattern to produce a new strand, called messenger ribonucleic acid. Through translation, amino acids were linked together in a particular sequence, dictated by the messenger ribonucleic acid, to form proteins. As a result, the subsequent generations of nucleic acid molecules took a selective advantage because the proteins that they were making favored their replication through mutual evolution. Thus, proteins became very abundant in a relatively short period of time. At this point, viruses also evolved from complex self-replicating molecules which consisted of nucleic acids and proteins.
Then, more and more prebiotic structures began to form because organic chemicals tended not to remain uniformly dispersed, but separated out into layers amidst the churning waves of the worldwide ocean. This allowed them to be surrounded by tight skins of molecules that formed when fatty acid vesicles grew into filaments that shook loose in the raging tides of the hostile planet. These, often spherical, aggregations of lipid molecules then provided locally segregated environments that allowed for the selective absorption of simple organic solutions from the surrounding medium. This process of coacervation ultimately established the basis of metabolism, which would eventually serve as the precursor to more complex biological reactions such as respiration, digestion, and circulation.
As part of this preliminary process, certain spheroidal, heterotrophic systems encountered the advantage of building anatomical scaffolding with tubular components. Thus, microtubules first formed when a globular protein with the property of longitudinal self-assembly underwent an evolutionary change thereby altering its faculty of lateral association. In this way, single filaments tended to form cylindrical sheets to better resist shearing and compression. This brought forth a significant degree of stability in these otherwise delicate structures which had been prone to disintegration.
After advancing in this way, new generations of viruses coevolved with the most primitive life-forms in response to these developments. This happened just before organisms diverged into the taxonomic domains. Right after that, bacteria, archaea, and eukaryotes began to diversify, as proto-cells parasitized the larger common ancestor cells. In this way, viruses played a central role in the earliest phases of organic evolution, serving as the all-important means of effectively transferring genes between different species. This inevitably increased genetic diversity by driving the processes of development, however, as a consequence of this, the genes that were not required by parasitism were gradually lost through regressive degeneracy.
Nonetheless, the point is that the universe finally produced the most basic resources necessary for the organic processes of living, so the genetic code began to replicate as life began to procreate. In this way, populations of individuals began to undergo an increase in the number of organisms through a process of copying that resulted in the creation of additional entities, as reproduction quickly replaced replication. As a result, this typically resulted in two nearly identical manifestations of the original organism, although certain cells had a relatively high rate of mutation.
These kinds of changes were most often the result of changes in the hereditary material within an organism. Such transformations were most often caused by a substitution of one, or more, of the nucleotide pairs in a molecule, thereby producing a change in the resulting protein. As a consequence of this phenomenon, mutations quickly became the primary source of variability among the evolving populations of primitive creatures on the early Earth.
In general, the life-forms that resulted from this particular set of events consisted of self-contained and self-maintaining entities. As the first animate objects within this biosphere, these preliminary organisms could perform a number of sophisticated functions. For instance, they could undergo growth through metabolism by converting environmental materials into body mass. In addition to this, organic systems are able to respond to stimuli by way of a range of predetermined reflex actions. Living beings are also capable of systematically maintaining their high level of organized complexity amidst an entropic space-time continuum.
Most importantly, life-forms could finally reproduce which would enable them to evolve between generations to remain within the set parameters of a specific ecological niche, through the power of adaptability to the environment. However, these creatures were still so primitive that they didn’t even exchange genetic material when they reproduced. This was tremendously important because it meant that one surviving mutation could give rise to an entirely new species in a single generation. Furthermore, every viable organism was also able to store its own set of instructions for carrying out each of these novel activities.
At this point in the great chain of being, within one hundred and fifty million years after peptides first emerged on Earth, translation became accurate enough to unambiguously link the sequences of individual proteins with relatively short sequences of individual ribonucleic acid genes. So, as a result of mutation, protein enzymes enabled the evolving cells to perform new chemical reactions, or at least refine less efficient processes. As a result, enzyme-dependant metabolism gradually replaced its primitive predecessors.
Following this, the emergence of deoxyribonucleic acid molecules brought forth a crucial refinement in the information processing systems of animate entities. So, the replication of genetic material was now able to take place just before cells divided. This process was identical to that of ribonucleic acid, except for the fact that thymine had now replaced uracil. Aside from the development of this new nitrogen base, everything else remained the same. During cell division, the macromolecular spiral would unwind and split in half and each side would pair with a new structure, thereby forming two sets of chromosomes both having the same sequence of nucleotides.
During this process, a purine would match up with pyrimidine and these pairs would form a code in such a way that three of the pairs together would then determine the position of amino acids in a protein. In general, the type of protein that was made depended on the kind and order of amino acids in a given sequence. This typically occurred in such a way that any change in the nucleotide sequence of a deoxyribonucleic acid molecule would produce a change in the amino acid sequence so that a different protein would be generated.
As the genetic components in biological systems became increasingly more complex, there were greater advantages to storing their codes in a separate molecule. As such, deoxyribonucleic acid soon copied all of the instructions contained in the ribonucleic acid genes. Having then been displaced by deoxyribonucleic acid and the more effective protein enzymes, ribonucleic acid was relegated to the intermediate role that it now plays.
Genes were then able to serve as the specific nucleotide segments in a chromosome that work to encode the inherited patterns which are expressed throughout an ancestral lineage by way of the similarities and differences that are produced in the traits most often expressed in that population. As part of this, genes became the smallest most successful unit of recombination, being the areas in chromosomes that consist of subunits that are closely related in their action and that must be present in an unbroken set to give their characteristic effect.
Ultimately, chromosomes became the primary structures that carried the instructions which affected the various individual characteristics among a specific population, thus forming the general basis of genetic inheritability and variability. In this way, particular molecular structures were able to transmit vital information from one cell to another and hence one generation to the next. As such, chromosomes began to control organisms in such a way as to maximize the overall survival of their genetic material.
At this time, the weather on the surface of the Earth was similar to the meteorological conditions of modern times. Torrential rains would occasionally poor down as electrical storms raged through the sky above. In addition to this, pools of boiling mud bubbled, geysers spewed incredibly hot water into the air, and active volcanoes hurled super-heated ash into the atmosphere. All in all, these kinds of ecological factors served to create vast wastelands on the inland regions of the continents.
Regardless of these facts, the first modern cellular organisms were undeterred by all of this, emerging as they did from the ocean depths. As such, the original populations of prokaryotes were now able to refine many of the simpler systems that primitive cells had been using up to this point. Such tremendous efficiency then made these creatures incredibly successful, in a relatively short period of time. As a result, genetic mutations soon led to several different kinds of bacteria.
Eventually, some of these ubiquitous single-celled creatures even became responsible for a number of crucial biological activities such as putrefaction and fermentation. Then, as the bacteria propagated, eukaryotes began as one cell but, gradually, developed a membrane-bound nucleus, in which deoxyribonucleic acid was stored as chromosomes. These nuclei dictated the structure of a cell’s proteins and by this means controlled several of the activities of the cell itself. At this time, eukaryotes also began to contain other organelles that evolved through the incorporation of prokaryotes into the cell.
At first, these bacteria invaded the cells, but instead of being digested, the invaders formed a symbiotic relationship with their hosts. So, as time went on, the bacteria came to depend on the host cells for survival, and in return, they provided their hosts with energy. More to the point, at this stage of organic evolution, single-celled organisms began to possess the cytological equivalents of a digestive system, excretory system, respiratory system, skeletal system, immune system, reproductive system, and cardiovascular system.
Next, the first opportunity for sexual reproduction arose, through the emergence of different kingdoms of life. This allowed certain organisms to combine half of their genes with half of another individual’s genes. As such, new combinations of genes were subsequently produced every generation. This increase in their variation also increased the opportunity and likelihood for change over successive generations. Occasionally, these offspring inherited new characteristics that gave them a survival and reproductive advantage in their local environments, and these characteristics tended to increase in frequency in the population, while those that were disadvantageous decreased in frequency through the process of natural selection.
As a result of this, the shores began to harbor generation after generation of strange new life. Algae were one of the most prolific of the more refined organisms that emerged. While cabbage-shaped laminated silicate structures grew in the shallows, another industrious type of algae was hard at work on one of the most important steps toward modern existence. At this stage, cyanobacteria became the first organism to use photosynthesis as a life-sustaining process, thereby replacing the chemosynthesis used by earlier forms of life. This started massive oxygenation of the planet’s atmosphere, which served as a crucial turning point for life, including that of our ancestors, going all the way back to the very first lifeform, and beyond.