If all animals vanished, most bacteria would still live on, but if all bacteria disappeared, we would die quickly.
Cells are the atoms of life, and life is what cells do.
— Franklin M. Harold, In Search of Cell History, p. 2
Above we were considering the evolution of our species, but now let’s go further back into the dressing rooms of life’s history. You have parents, you have grandparents. You have great-grandparents. Like a repeating template, every cell in your body has a similar heritage. Like a person, no cell is created out of the blue, it’s created from the cell that came before it. “Omnis cellula e cellula.” Every cell in your body represents an unbroken chain of cellular evolution going back four billion years. Each and every cell that makes your body claims an unbroken heritage dating back one-third of the age of the universe. And so does every cell in the grass of your lawn. And your favorite pet. Plants and animals share a same common ancestry; only unlike chimps — from whom we humans diverged about 6 million years ago — plants and animals (or what would much later become plants and animals) diverged about 1.6 BILLION years ago, while both existed only at the microbial stage. Really, any understanding of life on the scale of deep time starts with two basic facts, and let’s spend a moment or two to consider the first:
- The mind-blowing majority of the history of life on earth was microbial. Life on earth began some 4 billion years ago, and for 85% of those 4 billion years the most advanced life on the planet was single-celled bacteria, Prokaryotes, and later Eukaryotes — our ancient and primitive single-celled ancestors.
Life appeared on Earth within a few hundred million years, but for billions of years it was restricted to single celled organisms.
In the 1970s our current scientific classification of the Three Domains of Life was established: Bacteria and Archaea — the two Prokaryotic (and primary) domains, which have evolved very little in four billion years — and Eukarya, a minor offshoot whose cells seem more closely related to Archaea than Bacteria, though conversely the mitochondria within each cell seems more closely related to Bacteria than Archaea.
Eukarya is the domain that encompasses all complex life — all people, all plants, all animals, any and all life that is big enough for you to see (and some you can’t) comes from this small, modest and much-mutated branch. We’re an extended footnote in a thousand-page book. Eukarya represents a fraction of the life on earth, but it represents all the life that can be seen without a microscope. Eyes, ears, limbs, backbones, hair, scales and toes — BRAINS, for that matter — bacteria and single-celled life that dominates the biosphere have none of these things; these are tools and inventions only made possible with the joining of cells into complex structures. Eukaryotes have the freedom to link together cells like a kid tossing together Legos. With Eukarya, complexity built upon complexity… in contrast to bacteria, which has spent four billion years essentially in a flatline.
The consensus today is that Eukarya represents an unlikely merging of the two Prokaryotic types that occurred only once. Anyone of a certain age will remember those old Reeses Peanut Butter Cups commercials, speculating about the origins of the candy as two people walking on the street collide and exclaim, “Hey, you got chocolate in my peanut butter!” “Hey, you got peanut butter in my chocolate!” Pause. Taste. Both cry out, “YUM!” The analog here is that the mitochondria is that peanut butter that gave an unexpected zing to the chocolate.
According to Wikipedia, “the most accredited theory at present is endosymbiosis… The endosymbiotic hypothesis suggests that mitochondria were originally prokaryotic cells, capable of implementing oxidative mechanisms that were not possible for eukaryotic cells; they became endosymbionts living inside the eukaryote.” (https://en.wikipedia.org/wiki/Mitochondrion) In a symbiotic relationship, mitochondria ended up jettisoning much of its own genes, in the process becoming a power source for the eukaryotic cell. In the words of Nick Lane, with mitochondria — the “batteries” that power each eukaryotic cell — “Eukaryotes have 100,000 times more energy per gene than bacteria, which allows them to support far larger and more genomes and make far more proteins from each gene.” (https://www.youtube.com/watch?v=PhPrirmk8F4)
Given how early life on earth began, it seems a strong argument that simple life is an easy threshold to cross… considering the fact that Eukaryotic life seems to have emerged only ONCE in FOUR BILLION YEARS may suggest that this may be the truly “narrow needle to thread.” Achieving complexity, then, is akin to a lottery, that played endlessly and relentlessly over two and-a-half billion years, eventually resulted in winning numbers. The sheer immensity of time eventually gives rise to the unlikely. Once in four billion years is a very lucky stroke, but if time is on an immense enough scale all odds eventually fall away.
So to conclude this first point, it is only in this last 15% of life’s history on earth (600 million years) that significant multicellular life has arisen. And if it seems sobering that 85% of life’s history was simple, unchanging and unicellular, here’s the second fact to consider…
- 90% (NINETY percent!) of the history of life on earth was confined to the oceans alone. For nine-tenths of the earth’s history, life was — by definition — a water event. Even today, there are life forms that can exist in the absence of oxygen… there are life forms that can exist without sunlight… but importantly, there are NO LIFE FORMS ON EARTH that can exist without water. WATER is the common solvent, water is the necessary cocktail-mixer through which all life on earth flows. Life began, thrived and perpetuated for more than four billion years in the protective alchemy of the oceans. The continents went unused for four billion years; barren and lifeless granite wastelands, untouched real estate for nine-tenths of earth’s existence.
And it’s not just that the land was simply without advantage, land was an impediment, a barrier… the environment out of the water was actively hostile to life. Unlike today, the early earth (the earth of the Hadean era and Archean era) did not possess an ozone layer to permit life outside of the water. Ultraviolet radiation would have cooked anything outside of the protective haven of the water. What created the ozone layer? An abundance of oxygen. It was oxygen that allowed life to flourish on land, just as it was oxygen that allowed for the development of complexity. Oxygen was the catalyst that allowed life to overcome both of the two hurdles listed above. And where did this abundance of oxygen come from? Well, in a carefully choreographed dance it was created by… life. Life — and the conditions for life — are basically a positive feedback loop.
We live in an oxygen-rich atmosphere today, the earth’s cocktail is a 78/21 mix of nitrogen and oxygen, but the earth of the Archean Eon — which began about 4 billion years ago, and lasted until 2.5 billion years ago — was not an earth we would recognize, even if we could survive our first breath of methane and carbon-dioxide. There was no free oxygen in the atmosphere to begin with. Oxygen may be the third most abundant element in the universe, but it is rarely found by itself — oxygen is a sticky atom and is usually found bonded with other types of atoms. The free oxygen in our atmosphere is a byproduct of life, and our ozone layer is a construction of biology. Those countless sci-fi movies that feature people breathing air on uninhabited planets are overlooking simple physics: any oxygen-rich atmosphere would not be likely without life. Free oxygen is a bio-signature, and as we look out into the universe today any atmosphere containing it would be a strong indication of an inhabited world.
The presence of oxygen in a planetary atmosphere is the litmus test of life: water signals the potential for life, but oxygen is the sign of its fulfillment — only life can produce free oxygen in the air in any abundance.
— Nick Lane, Oxygen: the Molecule That Made the World, p. 2
The initial rise of oxygen in the atmosphere is referred to today as the Great Oxygen Catastrophe, and anyone who would challenge the notion that any one species can have an impact on the earth’s climate need only look back to what simple bacteria did two-and-a-half billion years ago. The history of the earth is written in its geology; rocks tell the story of life on earth and can tell us a lot about changes in the atmosphere. There have been several Extinction Events in the history of life on earth, but this first one was the only one caused by LIFE itself…
When blue-green algae ruled the world…
The main source of oxygen in the atmosphere is oxygenic photosynthesis, and its first appearance is sometimes referred to as the oxygen catastrophe.
“Oxygenic photosynthesis seems to have been invented but once in all of earth’s long history, by precursors of the bacterial clade that we call the cyanobacteria.” (Franklin M. Harold, In Search of Cell History, p. 158) When, exactly, single-celled cyanobacteria first emerged is a little unclear in the fossil record, but what is clear is that by three billion years ago its colonies were prospering in shallow oceans. Sure, bacteria, big deal… but this was bacteria that photosynthesized oxygen. Photosynthesis. In the words of Professor Brian Cox, “the task is so complex, that unlike flight or vision, which have evolved separately many times during our history, oxygenic photosynthesis has only evolved once.” (Wonders of Life, Ep.5, BBC, 2013) What made cyanobacteria such a game changer was that in finding a way to utilize photosynthesis as a metabolic process it unleashed as a byproduct… oxygen. In school one infers (somewhat erroneously) that plants create oxygen, but really, the only place oxygen atoms are created is within the fusion processes of stars — what plants do is release oxygen. Plants take in carbon dioxide. What is carbon dioxide? It’s CO2 — one carbon atom bonded with two oxygen atoms. Plants utilize the carbon atom but simply discard the oxygen atoms. Plants free oxygen. Multiply this effect by the trillions over millennia, and you have the makings of a world-changing atmospheric event.
Simply, cyanobacteria seems to have overwhelmed the biosphere. Over a span of hundreds of millions of years, countless colonies and countless generations of this blue-green algae released into the atmosphere then-toxic (to other life forms) amounts of free oxygen. A 2007 NASA study suggests the tipping point to have been about 2.5 billion years ago. Simply, competing anaerobic organisms (methanogenic Archaea) that metabolized methane would have found oxygen toxic, seen their numbers and habitats dwindle and eventually faced extinction. It was basically a “metabolic format war,” as unfathomable generations of a cyanobacteria overpopulation polluted the biosphere of the earth over trillions of generations. What resulted from this process was an earth atmosphere rich in oxygen… and ultimately rich enough that an ozone layer formed, making it possible, for the first time, for life to leave the water.
“The rise in atmospheric oxygen was far from uniform [over time],” as Franklin Harold observed (In Search of Cell History, p. 158). “There seem to have been at least two discrete steps, the first around 2.45 billion years ago and a second right at the beginning of the Phanerozoic, 540 million years ago.” In short: “Atmospheric oxygen attained its present level just in time to support animal evolution.” So, while complex life may have taken a long time in the record to appear, there’s a good argument to be made that it may have occurred at the earliest possible opportunity.
Multiple lines of evidence from evolutionary biology, geochemistry, and systems biology build a compelling case for a central role of O2 in the evolution of complex multicellular life on earth.
— Victor J. Thannickal, “Oxygen in the Evolution of Complex Life and the Price We Pay” (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2720141/)
It is somewhat startling (and humbling) to realize that it is only in the past 10% of the history of the earth that life colonized land. And in essence, we never fully left the water; we are water-based life, and as such, we are still tied to the water… we simply evolved to carry it with us. That’s why your body is made mostly of water, that’s why you’d die faster from dehydration than from hunger. We are essentially walking, talking water balloons, and water balloons that require refilling every day… that’s what you do every time you go to the refrigerator and grab a Zima. Hey, do they still make Zimas? What about Snapple? Well, anytime you feel the need to grab a soda or a juice, you’re just paying the price for the evolutionary bargain that our distant ancestors struck 400 million years ago as they left life’s native habitat.
As to our friends and relatives, the cyanobacteria, they still exist today, in smaller numbers. Stromatolites, the rock-like physical products their colonies leave behind, dot shorelines in remote locations of South Africa and Australia. Cyanobacteria — single-celled blue-green algae — once dominated the biosphere and changed the world with its waste. It unintentionally exterminated much of its metabolic competition and fathered the dynasty of life that still stands today. In the words of Neil DeGrasse Tyson, “you can argue that they [cyanobacteria] were the most disruptive creature ever to live on the face of the earth.” (StarTalk, ep. 2.10, National Geographic, 2015) Simply, our entire domain of life today — every plant and animal on earth right now — owes its existence to cyanobacteria and the Oxygenation Event.
All of this, [our biosphere today exists] because of a waste product pumped out by microscopic bacterial slime, operating on an industrial scale in those ancient seas. The empire of the stromatolites was, without doubt, the greatest in the history of the earth. Forget the Romans, the Persians, even the dinosaurs. These humble bacterial mounds dominated the planet for over 2,000-million years, and they engineered its greatest transformation.