Like spring, clouds are both young and very old. Their existence is fleeting, but the basic natural laws underlying cloud-forming movements of air and water haven’t changed for billions of years.
Mountains seem eternal, but they are in constant motion: forced upward by plate tectonics; weathering down into pebbles and dirt under the influence of wind, rain, and gravity, filling up the valleys; and then rising up again as a new range.
Our planet’s hydrologic, atmospheric, and rock cycles are closed systems that have been interacting in complex ways for more than 4 billion years (giga-annos, or Ga).
How did the very distant ancestors of cats, plants, and all other life ever manage to break through and set up a fourth Earth system — the biosphere?
The answer, in a word, is volcanism.
Life is a survivor
Remember this, from last time?
It’s one of several different ways to model how plate tectonics began, a billion or more years after Earth formed.
The most important point for us doesn’t appear in that video. It’s this — whenever and however the first subduction zones, continental collisions, and spreading centers got going, the process stirred Earth’s mantle around and also broke apart the lid-like outer crust that probably had been present until then. (Palin and Santosh; Reddy and Evans)
That means major trouble when it happens outside a movie, in the real world.
The planetary effects of the transition to plate tectonics must have been cataclysmic (Stern and Miller), yet life was present and rode it out.
Although the whole thing likely played out over hundreds of thousands to millions of years, mass extinctions that dwarf anything known in the fossil record were inevitable.
But life itself survived.
No one knows how. Little direct evidence from Precambrian times has been preserved in the rocky record, so geoscientists can only make informed, computer-assisted best guesses about those days.
Maybe it was a case of “what doesn’t kill you makes you stronger.”
There was lots of volcanism going on before plate tectonics shook up the world. (Palin and Santosh)
And life and volcanoes do have something in common: both are out of equilibrium with their surroundings. (Barge et al.)
Earth’s core is as hot as the Sun, while the temperature of space is close to absolute zero.
That is way out of thermodynamic balance, so our planet constantly vents its inner heat out into the surrounding cold, mostly through volcanism. (Oppenheimer)
This heat flow powers a planet-sized chemistry/physics “lab” where almost anything can and does happen, driving environmental changes that living beings must adapt to or else perish. (<Ernst et al.; Strahler)
Forms of life that were best adapted to conditions similar to the changes wrought by the transition to plate tectonics survived that change and kept evolving. We’ll probably never know what the losers were like.
Life needs energy
How is life out of balance?
A living being busily consumes and produces at the same time, constantly moving toward or away from equilibrium but never quite reaching that perfectly balanced stasis point (until it dies).
See? It’s not just you — all forms of life experience this struggle!
Everything from pachyderms to protists burns energy to stay alive and therefore needs to regularly fuel up along the way. Life is always hungry. (Brown et al.; Hohmann-Marriott and Blankenship)
As I understand it, scientists suspect that the first life on Earth harvested energy off the ever-present chemical and/or pH imbalances in areas where water was heated by nearby molten rock. (Barge et al.; Kitadai and Maruyama; Mulkidjanian et al.; Root-Bernstein; Saladino et al.; Sleep; Sleep et al.)
Three to four billion years later, such places still provide a sort of 24-hour, all-you-can-eat buffet:
If you’re curious about the earliest forms of life, have seventy minutes to spare, and don’t mind geoscience jargon, this SETI video explores one common hypothesis about primitive life.
Since there was plenty of time lying around (the Archean eon covers about a third of Earth’s total history), some of those extremely primitive first life forms — slimy goo, perhaps, huddled around hydrothermal vents or in geothermal pools — eventually were able to evolve a cell membrane.
And then they didn’t need to get their meals from the Hot Rock Cafe any more. (Durzyńska and Goździcka-Józefiak)
Wherever they were, one-celled critters could now collect energy just by moving elemental ions like H+ through their newfangled protective membrane — a biological process called chemiosmosis. (Hsia et al.; Lane et al.)
Hydrogen was plentiful in Hadean and Archean seas. Too, Earth’s early atmosphere was loaded with carbon dioxide, which helped along the biochemistry of life. (Catling and Zahnle; Hsia et al.; Sleep)
As a result:
- Some unicellular forms — perhaps ancestral extremophiles — happily produced methane this way. That’s an even stronger greenhouse gas than CO2, so these methane-makers would have helped to keep Earth’s surface temperatures above freezing despite the dim sunlight of those times. (NASA, 2020b: Reddy and Evans; Sleep).
- Other microbes — the group that led to bacteria — combined the same ingredients (hydrogen and carbon dioxide) a little differently to make complex organic (carbon-based) matter for themselves, as well as energy. (Sleep; Sleep et al.)
Technically, each of these approaches is respiration (Brown et al.), not photosynthesis, since it doesn’t require sunlight.
Still, the second method could have developed into photosynthesis — a much better way to get life-sustaining energy. (Sleep)
Photosynthesis, but not oxygen yet
Without photosynthesis happening at the base of the food web, there would be no cats, no trees, indeed, no complex life at all.
You might think that plants own the copyright on it, but they’re newbies. Bacteria invented photosynthesis a long time ago. (Blankenship; Sleep)
Presumably this happened once the Sun was closer to its present brightness, though Earth was still spinning fast and days may have been only a third as long as they are now. (Klatt et al.; Sleep)
While its exact origins are a mystery, evidence found in Greenland rocks suggests that photosynthesis was going on as far back as 3.8 Ga. (Sleep) Certainly it was common by the middle Archean. (Arndt and Nisbet)
But this wasn’t the chlorophyll-green photosynthesis that we know and love today. Plants were not yet a thing.
Bacteria first used either sulfur that filtered out of the smoggy early atmosphere into upper ocean waters or else iron from volcanic rocks on land or in the sea. (Hohmann-Marriott and Blankenship; Kharecha et al.)
Granted, the iron-based photosynthesis might have looked greenish, but that was for very different chemical reasons.
Sulfur-based little critters were probably purplish-gray. Now imagine those multicolored Archean waters under a yellow methane sky!
Young Earth was strange in many ways, but it always has been our world.
By the way, sulfur-based photosynthesis is still around, including in this small section of one of the US Great Lakes:
All Precambrian life in the sea, and perhaps on land, too, existed in two-dimensional mats like these, on rocks or in a stromatolite. Three-dimensional living didn’t really take off until the Cambrian period. (Finke et al.; Peterson et al.; Sleep)
The type of photosynthesis that these purple bacteria are doing doesn’t produce oxygen.
The research team in that video, in order to investigate their idea that day length and the Great Oxidation Event might have been connected, uses the sinkhole as a proxy for Archean Earth. There was almost no free-floating O2 in the air back then. (Goldblatt et al.; Hsia et al.; Klatt et al.)
However, that changed when some cyanobacteria worked out a way to split water with sunlight. They get their H2 energy this way and we get the beautiful, life-giving, world-changing O:
More about those chloroplasts, and the appearance of Eukarya — the group that includes animals, plants, and much more — next time.
Featured image: Victoria Virozhko/Shutterstock
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