Radiometric dating indicates the solar system formed approximately 4.6 billion years ago. However, long after a massive debris cloud gradually coalesced into the sun and planets, the compacting Earth was very far from being hospitable to life. As the sun grew in size and energy output, Earth was being pummeled by meteors that added to immense heat from internal friction. It’s said that molten lava covered the surface of young Earth for many millions of years in the forming stage.
Eventually the Hades-like surface of the evolving Earth cooled and water accumulated on the surface. Though wind and radiation may have been extreme, and the atmosphere may have lacked significant oxygen, conditions for life were improving. With abundant elements and water warmed by the sun, the stage was set for an amazing odyssey of chemical development, that would one day produce the only known meaning in our limited observance of the universe. The tremendous variety in the world today illustrates the myriad of possible atomic, molecular, compound, mixture and environmental combinations that shape elaborate and very different forms from the same basic building blocks.
A simplified view of atomic structure is represented by a nucleus of protons and neutrons that’s orbited by electrons. A negative charge of electrons and positive charge of protons is reportedly responsible for attraction and repulsion among atoms. Hydrogen, the lightest element, usually consists of one proton orbited by one electron. Occasionally the proton is joined in the hydrogen nucleus by one or two neutrons, and those isotopes of hydrogen are known as deuterium and tritium respectively.
And hydrogen, the simplest element is also believed to be the most abundant element in the universe, just as it’s very abundant on Earth, though mostly in the form of compounds with other elements. While people are familiar with the characteristics of gaseous hydrogen at room temperature and atmospheric pressure, it can also take the form of metal in low temperature and high pressure environments, like that believed to exist within the giant gas planet Jupiter, for example.
The formation of water molecules provide a simple example of how atoms, such as hydrogen and oxygen, combine to form molecules and release energy at the same time. Two molecules of hydrogen, consisting of two atoms each, combine with one molecule of oxygen, also containing two atoms, to form two molecules of water, with each molecule consisting of two hydrogen atoms attached to one oxygen atom. When hydrogen and oxygen atoms bond they fit together in less space than they occupied separately, making compact molecules that weigh more than oxygen or hydrogen, and releasing excess energy from smaller electron orbits as heat. The liberated energy is actually so significant that the combination of hydrogen and oxygen is used as rocket fuel.
Going back in history, some societies actually believed that fire was alive. They saw that fire did work and that it transformed matter; both the matter of its fuel source and substances affected by the heat energy and smoke it released. They also saw the birth and death of fire as it sparked to life and when it was extinguished.
At the time it wasn’t understood that what they were witnessing was a chemical reaction; the rapid oxidation of a fuel substance resulting in heat, light and chemical compounds of drastically altered property. Oxidation and reduction: common transformations that result in change of electric charge, affect many kinds of substances, and can occur with many substances, such as iron in the process is known as rust. And in providing our own internal energy and growth the processes are known as respiration and metabolism.
While humans are dependent on free oxygen in the air to combine with fuel molecules like glucose and amino acids to release energy, some life forms, such as bacteria and yeast utilize a less efficient metabolism that, although depending on oxygen containing compounds to break down, doesn’t require the input of free oxygen. Despite the amazing complexity of modern animals like humans, and actually owing to that complexity, animals are utterly dependent on a great number of environmental conditions.
As illustrated by the story of Joshua earlier in this book, people can’t survive for more than a few minutes without breathing free oxygen – though some exceptional individuals can hold their breath for considerably longer. Man’s persistent reliance on oxygen, a simple gas essential to some highly complex life functions, demonstrates both man’s basic chemical nature and the fantastic realities of material animation known as chemistry. Common elements like oxygen are found in astounding numbers of combinations that give form and substance to the world.
Carbon, like oxygen, is another element vitally important to life, and it’s believed to appear in more compounds than any other element on Earth, with nearly 10 million combinations. Carbon containing compounds are so prevalent, and carbon is so versatile, that it’s found in all known living things, giving rise to the expression “carbon-based life.”
Hydrogen, added to oxygen and carbon combine to form many types of biological compounds including sugar, cellulose, lignin, chitin, alcohol, fat and ester. Added to those elements, nitrogen allows formation of alkaloids; and sulfur, combined with hydrogen, oxygen, carbon and nitrogen can create amino acids and proteins. Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA); the complicated genetic codes for reproducing organisms; consist of only phosphorous, sulfur, nitrogen, carbon, oxygen and hydrogen. Together, these compounds are some of the essential substances of living organisms.
Carbon not only occurs in a huge variety of combinations with other elements, it also represents how environmental conditions affect substance formation, with pure carbon appearing in such diverse forms as graphite and diamonds. Graphite conducts electricity and is the soft gray/black substance used as “lead” in pencils. While diamond, in striking contrast, is brilliantly clear, doesn’t conduct electricity, and is the hardest known natural substance.
And that dramatic difference is due to the high temperatures and pressures deep underground that transform the loose mass structure of graphite into a dense cubic structure that provides both exceptional hardness and the fascinating ability to pass light. Though natural diamonds are rare and quite expensive relative to graphite and coal, today diamonds are made much more abundant through synthetic mass production; and some mature star cores are even thought by some to be solid diamond. In fact, the high quality and abundance of diamonds produced in modern facilities is causing the mystique of rarity so long prized by the public to slowly lose its luster; an environmentally beneficial side effect.
For all of the elemental potential it possessed, however, young Earth was vastly different than what we’re familiar with today. The planet looked barren, with a lonely quiet, save for the whistle of wind, splash of water and other elemental noises. For a long time rocks lay plain and dull, splashed by sterile waters. Though like its neighbors, barren Earth had yet to give birth to life, it did have amazing potential. There was tremendous potential for wondrous, new life. In the water and the soil and the sunlight there was unusual possibility.
Like jigsaw puzzles being pulled together by invisible forces, elements formed various combinations through internal attractions, with the aid of external stimuli; and as individual combinations produced or consumed energy at the molecular level they also made possible a growing range of alternative combinations. And like falling dominoes, particular events triggered further actions in chain reactions of growing duration. Action at the molecular level occurring constantly inside animate beings work together like cogs in extremely complex machines to enable the activity of life. But even when chemical combinations and reactions produce large amounts of energy or perform other substantial resource conversions, they may not produce repeatable or controlled results. Many action sequences “play out” like a dying fire when the necessary combinations of inputs and circumstances become unbalanced, but certain complex molecular sequences arrange to form circular systems that result in consistent sustainable actions or production based on persistent resource availability.
And billions of years ago, when Earth was still barren and young, with the passage of time, organic molecules came together in such specific combinations to form those self replicating processes. Eventually, amino acid chains capable of reproducing themselves evolved to form the foundation of genes: contributing enormously to the growth, development and diversification of life. It’s amazing to think the simple mixing and grouping of atoms that formed molecules as plain and abundant as water has led to the marvelously sophisticated process of precise replication found in animals as diverse as butterflies and whales. Even in this so-called information age the connection’s so easily missed between the astounding complexity of factors and conditions necessary to life, and the astounding magnitude of molecular combinations formed every second of every minute of every hour of every day of every one of the last four billion years.
That’s how long it’s taken for basic molecular combinations and chemical reactions to reach the level of intricate, interwoven, complementary relationships that make the complexity of life possible today. And much of the fascinating work of cellular replication is made possible by a special enzyme called DNA polymerase that splits the two helical strands of human DNA, found in the individual cells of the body, and proceeds to work down each strand, driven by simple molecular attractions and energy transfer; attaching a compliment to every one of the three billion nucleotides in each strand to produce two complete sets of DNA folded into tightly-packed chromosomes. After DNA replication, the cell divides in two, with each of the newly formed cells containing a full set of DNA.
And the amazing DNA polymerase enzyme is just one of thousands of different kinds of enzymes constantly performing tasks throughout the body. They’re the little assembly machines that keep us going. Complex protein structures, made from 100 to as many as 1,000 amino acid molecules joined together in such a way to allow them to break or combine specific bonds in other molecules, they perform such vital tasks as building cell walls, making ATP fuel molecules out of glucose, assembling hormones, and other critical production work.
Not surprisingly, enzymes themselves are produced by a complex enzyme called the ribosome; which reads messenger RNA strands that are copied from little segments of the DNA helix called genes by the RNA polymerase enzyme. Once the requisite amino acids are assembled, the new enzymes are released to drift in the cell’s cytoplasm, ready to perform their highly specialized tasks with the specific molecules and polymers that they interact with. And all of that intricate work is automatically guided in orderly fashion by the three billion character DNA sequence. However, those complex automatic cellular and molecular processes can be altered by even small chemical changes, and the sophistication of chemical processes is so great that what seems a simple cure for one ailment can fatally upset the balance of life in another way.
What is life anyway? Does life extract minerals from its environment and grow, like a crystal in a cave? Does it move and do work, like fire and wind? Is the combination of nitrogen with other elements, in the form of amines, life? How about the combination of amines and carbon, oxygen and hydrogen that form amino acids: fundamental building blocks of life? Is man making life forms by making amino acids for industry? And what about long chains of amino acids called proteins? Are they living?
That question becomes more intriguing when one considers that prions are disease agents believed to simply be forms of protein that cause transmissible spongiform encephalopathies, a group of incurable, fatal diseases that include scrapie in sheep and goats, Creutzfeldt-Jakob disease in humans, and bovine spongiform encephalopathy, commonly known as mad-cow disease. The fact that the contagious, causative agents of such deadly diseases are considered forms of protein, gives cause to ponder the definition of life.
It turns out that life is, rightfully, difficult to define. Some common definitions include criteria such as metabolism, growth, reproduction; and responses to stimuli or adaptation to the environment, that originates within the organism. Most of what people consider life meets those criteria. But some simple living organisms don’t. Since viruses, for example, are considered neither cellular nor possessing of internal metabolism, are they living? They attach to cells, inject their DNA, and replicate themselves with the host cell protein; often causing the host cell walls to burst; killing the cells and releasing the replicated viruses to repeat the cycle of parasitic reproduction and destruction. Viruses are deadly assassins: responsible for some of the more horrific historical afflictions of humanity that include smallpox, AIDS, SARS, influenza, polio, Ebola, meningitis, measles, hepatitis, rabies, yellow fever and others. Yet, they consist of little more than DNA or RNA inside protective coatings with projections capable of penetrating host cells.
And viroids are plant pathogens that are even simpler organisms; consisting of a short stretch of RNA without the protein coat typical of viruses. But clearly, as we would commonly want to kill those relatively simple entities, they are considered to be alive.
The difficulty people have had in assessing the origin of life results from misconceptions largely attributable to those old simplistic mythological perceptions we invented. People have traditionally looked for that WOW! moment; a specific place and time that life appeared. They were conditioned to expect one clear act of creation. But there is no such moment. There is no step in the long journey from atomic attraction to the complex life forms of today that can be defined as the beginning of life.
Regardless of the amount of complexity and evolutionary time invested, all life consists of repeating, controlled chemical reactions. Biological evolution is a long story of chemical evolution; the slow, random combination of more and more chemical processes resulting in ever more complex structures and activities. The development of life forms is gradual, with every advancement being just one small step in the four billion year journey. There simply is no origin of life to be found in that steady process of molecular arranging and system growth.
Steady as day passing into night, life spread around the world, adapting to different conditions and continuing to grow in complexity, ever building on past accomplishment. Tiny microbes too small to see, colonized the vast oceans as continents came together, broke apart, and migrated around the globe over time. From Earth’s youth until now, as long as there’s been life on Earth, microbial life like bacteria has been and will continue to be part of it. Because simple life has fewer requirements, those requirements are amply satisfied wherever complicated organisms flourish, even on and inside the more complicated organisms; such as the bacteria present in human digestive tracts. Of the trillions of cells in the human body, probably more than half are foreign cells like bacteria.
Because the simplest organisms lack durable hard substances like bones, teeth and shells, they leave little lasting evidence of their presence. Most soft bodied creatures are easily recycled and don’t show up in the fossil record. So, little is known about the proliferation of the earliest microorganisms. But, whatever the sequence and circumstance of growth and development during Earth’s first billion years, fossils indicate simple colony forming, bacteria-like organisms were present by 3.5 billion years ago.
Even though they may not fossilize well, microbes are as tough as they come. Today many species of tiny organisms, like strains of bacteria, are so robust they’re called extremophiles. These microbes live in soil and water, under sheets of ice in Antarctica and Siberia, in pressure and heat hundreds and thousands of feet underground, under the astounding pressure at the bottom of the pacific ocean, in the corrosive salt environment of brine lakes, in hot springs and thermal vents where temperatures range as high as 250o F (121o C), and even in areas with super-high levels of radiation that would quickly kill a person.
And extreme tolerance isn’t the only characteristic that enables hearty life forms to flourish. When conditions are too harsh some organisms have the ability to go dormant until conditions improve. Many species of bacteria, for example, are known to turn into spores and enter a state of suspended animation when conditions are too inhospitable to sustain growth. Scientists have reported reviving bacteria that had been dormant for many millions of years.
Such reports dramatically illustrate the chemical nature of the animation called life. The molecular processes of life can come to a standstill, and as long as no processes of decay are able to act on the organism, the normal life functions can be restored when the organism is once again exposed to favorable environmental conditions. And the concept of suspended animation leads many to wonder how others might benefit from similar periods of dormancy.
But, suspended animation aside, there’s much that can be learned from the simplest creatures. They have been and continue to be instrumental in the proliferation and continuation of life on Earth. Some of the pioneer life forms such as photosynthesizing cyanobacteria are even believed responsible for causing dramatic environmental changes when Earth was one to two billion years old, including producing prodigious quantities of oxygen through the process of capturing the energy of sunlight known as photosynthesis. That major adaptation is believed by some to have significantly altered the life balance of the planet because many of the organisms that thrived up to that point in time aren’t believed to have been capable of tolerating free oxygen in the atmosphere and dissolved in the water.
After the appearance of the first cell-based organisms such as bacteria, simple single-cell organisms continued to proliferate and develop complex and diverse mechanisms of survival for about 2 billion years before some evolved into multicellular organisms, according to the fossil record as understood so far. The complexity and time investment in the intracellular foundation of complex organisms is so great that life may have grown no larger than a single cell for half the time it has existed on Earth. During that time tissues and structures were developed for growth, protection, feeding, locomotion, reproduction and other tasks necessary for even larger, more complex organisms.
Though tiny and simple by people’s customary perspective of reference, single-cell organisms are marvels of progress. Even tiny organisms such as modern bacteria have DNA consisting of millions and millions of base pairs of nucleotide molecules. And it’s that kind of complexity that allows gene sequences to guide the development of a thousand different enzymes, consisting of various long amino acid combinations, to carry out a single bacteria’s growth, feeding, division and other functions. Even the sophisticated ordering of the 10’s of trillions of cells in humans, is guided by the 3 billion pairs of nucleotides in DNA that reside in tightly packed chromosomes within the tiny individual cells.
Even tiny organisms have fascinating characteristics, such as the interesting variety found in single-cell organisms like euglenids, with similarities to both plants and animals. Some euglenids feed by phagocytosis, meaning they extend their cell wall around their food source, in a manner similar to bacteria, to form a food vacuole where the food is digested. And other euglenoids contain chloroplasts that capture energy directly from sunlight, as plants do.
In fact, many scientists believe that green plants resulted from a symbiotic relationship between chloroplast containing organisms and phagocytes that enveloped them but didn’t digest or otherwise fatally harm them. And there’s much more to those unusual euglenids that contain plant-like chloroplasts; they also have animal-like eyespots that detect light, and primitive muscles for movement; resembling as such, tiny plants that can wiggle toward sunlight.
By 1,200 million years ago multicellular algae was present. Though algae would later give rise to seaweed; leaves, roots, flowers, seeds and other organ structures characteristic of vascular plants were a long time from being realized. And within a few hundred million years later, soft-bodied, worm-like animals began to develop primitive central nervous systems. Sponges too, developed near the same time period; apparently descended from colony forming choanoflagellate protists.
Sponges are considered animals even though they lack many of the traits commonly associated with animals. For example, they don’t have a nervous system or internal organs like most animals. Also lacking are circulatory and digestive systems, as fuel and waste products diffuse directly through the walls of individual cells from and to circulating ocean water. Although sponges reproduce sexually, combining genes of two parents, they also reproduce by budding and can regenerate missing parts. And their lifestyle also differs significantly from most animals in that young sponges are mobile in the larval stage, but soon attach to objects where they remain sedentary for the rest of their lives.
Another evolving marine animal, the jellyfish, had more of the complicated systems people are accustomed to seeing in animals, such as muscles, a digestive system, and a neural net. However, unlike many modern animals, the jellies’ digestive systems had only one opening for both food intake and waste expulsion, and the neural nets weren’t connected to a central brain. Sponges, primitive worms, and jellyfish were joined by ancient arthropods, the ancestors of crustaceans, insects and arachnids, that also appear in the fossil record of the period.
Beginning about 542 million years ago species evolution and differentiation accelerated in what is known as the Cambrian explosion, with a widespread and diverse population documented in the fossil record. Because the trilobite was relatively large, populous and its shell fossilized well it’s probably the most well known animal from the time of the Cambrian explosion. Life was still relatively small and primitive, and Earth still looked barren by today’s standards but by the end of the Cambrian explosion all of the major animal body types seen today had formed.
Worm-like creatures continued to progress, developing hearts and gill-like structures to compliment their growing central nervous systems. Some of that line sired little fish-like animals with notochords that would eventually develop into backbones, and those little worm-fish gave rise to later generations of fish and other vertebrate animals including amphibians, reptiles, mammals and even birds. Every animal with a backbone alive today, and others like the woolly mammoth and dinosaurs of years past may owe their existence to those little prehistoric worm-fish.
Generations of fish continued to diversify, as plants, descended from green algae, moved onto land, along with fungi. And finally, after billions of years of watery evolution, the first animals to move onto land appear to have been arthropods like millipedes. Today the vast majority of animal species, about 90% of the total, including crustaceans, arachnids and insects, are members of the arthropod family, those possessors of jointed exoskeletons. And so, with the expansion of plants and then arthropods, colonization of the great land frontier, so bleak and barren, so sun-baked and windswept to that point, was underway en masse less than 500 million years ago. The nichiation of land settlement had begun.
Subsequently, under the shimmering surface of seas and inland waters, generations of fish and other marine animals continued to spread out, seeking their own place in the world. While the ancestors of squids and swordfish prowled the open oceans, man’s ancestors were living on the fringe, exploring the boundary between water and land. Pushing along the shallows with paired fins and gulping air to supplement oxygen-poor waters, the tetrapod ancestors of humanity and other non-arthropod land animals prepared for a move out of the water, away from predators and competition, but into a world of harsh extremes.
As tetrapods evolved into amphibians, their stout fins were slowly transformed through many, many generations into legs; their air sacs developed into lungs, and they moved onto the land. But they weren’t yet ready to move away from water. It would be up to amphibians’ reptilian descendants to develop amniotic eggs and skin tough enough to tolerate dry conditions away from water; enabling them to follow plants and arthropods further inland.
With basic cell and body structures well developed, competition in the waters, and vast lands to populate; rapid and vigorous evolutionary change ensued. Animals adapted to the proliferating plant life by consuming more vegetation and growing ever larger. The arthropods, being first to land and also first to take to the air, took advantage of the wide open space and abundant resources. Change was happening fast, and by 280 million years ago insects were living large, as exemplified by the mighty Meganisoptera dragonfly, with an astonishing 2 foot wingspan. However, even they pale in comparison to the impressive reptiles that came to dominate the land, patrolling vast forests of club moss and tree ferns.
By 150 million years ago giant dinosaurs were common. They would grow into the lumbering, towering, fearsome giants that live on in modern imagination. The ground shook under the strain of hundred-ton giants that stood above the trees and pushed them aside like so much tall grass. Hearts filled with terror and creatures great and small fled the scene of mighty tyrannosaurs gashing beasts as big as elephants with teeth the size of daggers in bites so powerful they would split today's mighty alligators in half.
In a diverse and contradictory world, while dinosaurs as big as tractor-trailers were gorging on lush greenery, plants produced the first fragile flowers, ancestors of those brilliantly vibrant gems of today’s dazzling landscapes. How exquisite it would be to witness the marvelous ecology of the first slight flowers and the greatest behemoths to ever walk the planet. Of course, people weren’t there to witness it, but mankind’s reptilian ancestors were, along with other lesser-known dinosaurs. They may have been minor players at the time, but in the distant future some of the prodigy of those small dinosaurs would have their turn.
Before small, feathered dinosaurs evolved into birds that would touch the sky, descendants of other small dinosaurs were giving live birth and producing milk to nourish their young. In the shadow of their Earth-shaking relatives, the first mammals were putting on fur and regulating their body temperatures. Mankind’s roots run deep on mother Earth, just as all life does. Humanity’s small ancestors ran with the giants, and witnessed the flowering of a planet.
Man’s humble ancestors diverged from the ancestors of mice about 75 million years ago. Shortly thereafter in geologic time, about 65 million years ago, the Cretaceous-Tertiary extinction occurred. That most famous extinction event, that wiped out the large dinosaurs and changed the face of life on Earth, was one of about seven major extinction events. Though numerous factors can contribute to mass extinction, many scholars believe the Cretaceous-Tertiary extinction event was caused by weather changes triggered by a massive asteroid impact.
It was likely fortuitous for man’s ancestors that survived the great dying off that they were small and could regulate their body temperature, because the impact dust cloud is believed to have resulted in wintry conditions that would have caused widespread hypothermia, disease and starvation. Such a sudden, dramatic event, though devastating, would have been similar to, but more traumatic than, other weather extremes like ice ages that have periodically put a big freeze on the planet. Over Earth’s long life, due to continental drift, variations in solar output, and changes in Earth’s inclination and distance from the sun, all the continents have experienced both tropical sunshine and polar ice coverage at some point in time.
Whatever the cause of the Cretaceous-Tertiary extinction, the sudden collapse of the reptile reign created opportunity for mammalian expansion and ascension. Though the evolutionary process is slow, diversity of size and lifestyle blossomed, and mammals grew to fill niches formerly occupied by the dinosaurs. While man’s little ancestors were searching trees for insects and fruit, the ancestor of horses was the size of modern foxes and elephants were smaller than modern cattle. Even the ancestors of whales and dolphins were still spending time on land. But what may be even more strange to modern senses is the idea that grasses hadn’t yet evolved; according to fossil records they didn’t appear until about 35 million years ago. Still later, as the slow march of time continued, about 30 million years ago, New World monkeys and Old World primates diverged, perhaps separated by an expanding Atlantic ocean as South America slowly drifted away from Africa.
The parade of life has continued to give rise to exceptional and interesting specimens around the world. Today’s variety of plants and animals is as diverse and fascinating as any time in Earth’s history; ranging from tiny colonies of microbes inhabiting deep ocean vents and the digestive tracts of animals, to the giant redwoods of California blocking the sun as they tower to heights greater than 370 ft. Regardless of sun or shade, some animals including bats and dol