Don’t panic, but eukaryotes — i.e., cats, frogs, people, plants, and any other living being whose cells have a nucleus — are way outnumbered by bacteria in this world (and maybe by extremophiles, too, although it’s hard to estimate the numbers of those rugged prokaryotes).
On the other hand, bacteria don’t do much; extremophiles, a/k/a archaeans, even less.
Sure, sometimes they work for us or against us, and there’s that one time, billions of years ago, when they changed the world by inventing photosynthesis (Blankenship; Hohmann-Marriott and Blankenship; Sleep), but bacteria mostly just wriggle around, minding their own business.
They’re not really equipped to do anything else.
That’s why I said not to panic.
Bacteria may outnumber us, but we eukaryotes are biological Formula One racers compared to those Model A’s, which is why we have been on top for several hundred million years now.
Yet no one knows for sure how this wonderful “racing machine” of ours evolved. (Knoll, 2014; Koonin)
Okay. “Evolved naturally.” Cats do NOT get metaphors!
Eukarya’s hidden strengths
First off, how can one biological domain include animals, plants, and fungi?
Obviously it isn’t based on appearance. You need a microscope to see how such different-looking forms of life are linked.
“Eukarya” means “true nucleus” — it’s that big round thing in the center, where the DNA is stored. Bacteria and extremophiles don’t have this (“Prokarya” = “before nucleus”); their DNA is free-floating in the cytoplasm. (Gouy et al.; Koonin)
Eukaryotes also have organelles — what I have called “stuff” in earlier posts. Following DNA-coded instructions from
the foreman the nucleus, these factory workers organelles do amazingly complicated things.
For instance, in humans living at high altitudes (and probably also in snow leopards), eukaryote “stuff” stimulates the body’s red blood cell production and increases hemoglobin concentration (Wang et al.)
Both of these handy adaptations deliver more oxygen to needy tissues when the air is thin.
Another example of why it’s good to be a eukaryote is how our organelles kick into overdrive when a cat (or a skateboarder) is injured. We may be complex, but we can also handle minor injuries in this contact sport called life.
Mitochondria — the red organelles in that cell image — power the whole operation by making ATP. (Koonin: University of Waikato)
Think of ATP as Fluffy’s fuel for those 3 a.m. wind sprints through the house.
It’s also the key ingredient in metabolic energy.
Just to mention it in passing, ATP biochemistry probably first appeared some 3 to 4 billion years ago or whenever it was that early forms of life first found uses for phosphorus and other elements that weathered out of rocks. (Burford et al.) Today’s plants still get nutrients this way.
There’s some horror in that video’s top-of-the-stairs imagery, so I want to briefly digress.
This is one way to find and maintain enough inner peace to have a functional, productive life despite the hard work it takes to keep those stairs moving 24/7/365.
It’s also far beyond the scope of this series.
As for cats, they have occasionally been worshiped or demonized during human history, but as far as anyone knows, cats have not yet developed their own metaphysics.
So let’s just bless them and all living things, according to my own tradition, and continue on.
In gladness and in safety,
May all beings be at ease.
Whatever living beings there may be;
Whether they are weak or strong, omitting none,
The great or the mighty, medium, short or small,
The seen and the unseen,
Those living near and far away,
Those born and to-be-born —
May all beings be at ease!
The Rise of Eukaryotes
Like gasoline in a car engine, ATP usually keeps our bodies going by burning oxygen.
But there also are ways for it to work without oxygen (anaerobically).
That’s how bacteria and extremophiles did it at first, back when they ruled the planet during its runaway-greenhouse days.
The byproducts of this photosynthesis were probably either methane or fermentation of some sort (Kharecha et al.; Sleep) — you could smell primordial Earth coming long before it appeared on your starship’s view screen!
Then, more than 2.5 billion years ago, cyanobacteria worked out a new way to do photosynthesis, and their waste product — oxygen — started building up. (Arndt and Nisbet; Goldblatt et al.; Klatt et al.)
It took some time for that pollution to have global effects, but when it did, the results were staggering!
From what I’ve understood of geochemistry papers on this, oxygen also reduces methane levels, and methane is an even more effective greenhouse gas than CO2. No wonder things got chilly!
Something new appeared after that: eukaryotes.
No form of “missing link” between them and prokaryotes has yet been identified in the Precambrian rocky record (Koonin), but in terms of molecular biology, these new critters were a blend of bacteria and extremophile, with some “stuff” of their own. (Eme et al.; Hsia et al.)
How did they evolve?
Scientists appear to be of two minds about their origin and about possible connections between eukaryote evolution and Earth’s newly available oxygen:
- Perhaps some sort of one-celled organism — there’s much debate about exactly what that might have been — ate an oxygen-producing proteobacterium that didn’t die; instead, it became an oxygen-using mitochondrion that provided its host with enough energy to develop into a “Formula One race car.” The eukaryote line that led to plants additionally took in a photosynthesizing cyanobacterium (Knoll, 2014; Martin et al. and sources therein).
I see this endosymbiosis — evolution through interactions between life forms — as an example of what some experts call the Red Queen hypothesis: the host gained a competitive edge, while symbionts outlasted their own competition (the free-floating “Gummy Bugs” in that image) because the host protected them from predators.
And maybe, just maybe, the planet got a new biological domain: Eukarya.
- In an alternative view of how eukaryotes might have evolved, oxygen wasn’t an enabler. It forced life to adapt or die.
We don’t think of good old O2 this way, but oxygen actually is poisonous because it’s so reactive. (Auten and Davis; Gladyshev)
That’s why mammals need to take in antioxidants. As well, they have evolved biological processes that sequester oxygen while allowing its life-support uses. (Castillo and Hernández; Hsia et al.)
Perhaps one of the adaptations that early life came up with to handle increasing oxygen levels was this newfangled eukaryote cell. (Fedonkin)
To me, that sounds more like the environment shaping evolution — the Court Jester effect rather than the Red Queen’s.
Of course, these two ideas aren’t mutually exclusive. Evolution is very complex, and there was enough time lying around for all sorts of things to happen.
In any event, eukaryotes evolved slowly at first. Free-floating oxygen was now present but nowhere near the level (about 20%) that it is today.
Bacteria also continued to dominate, building either reef-like stromatolites or flat mats mostly in the sea but possibly also onshore and along nearby waterways. (Beraldi-Campesi; Hohmann-Marriott and Blankenship; Finke et al.; Kenrick and Crane; Knoll, 2003; Reddy and Evans; Sleep)
Here’s what many Precambrian coasts and rivers probably looked like then, if you take out all the green (no land plants were present then to hold the soil together as a river bank, per Reddy and Evans).
Red fits in because red algae (some of the oldest known eukaryotes) were around then.
You would also need to level the mountains in the background.
After all, this was during Earth’s Boring Billion phase.
Everything — air, sea, and land in the form of a supercontinent or two — was more or less on hold for a while. (Klatt et al.; Mukherjee et al.; Palin and Santosh)
There was little to no mountain building going on to raise rocks back up again after erosion had worn them down.
It’s impossible to see cats evolving on that delta, isn’t it? And yet this is where cats come from.
That very early stage, with a billion years’ worth of shaping from the “Red Queen” (eukaryote v. eukaryote, eukaryote v. prokaryote) and “Court Jester” (volcanism, for instance, changes in ocean chemistry, or biological stress when nutrient runoff slowed after the mountains eroded away) — was absolutely necessary to eukaryote evolution for some reason. (Mukherjee et al.)
Whatever happened, it worked. The eukaryote line diversified twice during the Boring Billion: once when the line that would develop plants split off on its own and again when the remaining eukaryotes split in two, with one line developing as fungi and the other — the one we’re interested in — eventually turning into animals. (Fedonkin; Mukherjee et al.)
Well, bacteria still ruled (Hoffman et al.; Knoll, 2014) and the lineage that brought about today’s animals hadn’t yet arrived.
But eukaryotes had lots of energy and some surprising tricks up their sleeves, thanks to that very innovative “factory” in their cells.
To make a long story short, the plant division of Eukarya, Inc., got so good at producing oxygen with photosynthesis, on land, as well as in the sea, that they probably helped to freeze the Earth at least twice, for tens to hundreds of millions of years at a stretch. (Hohmann-Marriott and Blankenship)
Remember that video up above about how bacteria likely contributed to a global glaciation when they took greenhouse gasses out of the air and added in oxygen?
The same thing happened with the much more productive eukaryotes, only bigger and harder. Let’s close with a video about that, after pointing out that:
- There probably were geological and astronomical factors involved, as well as the eukaryotes. However, Earth’s climate was warm and eukaryote microfossils do show a surge in diversity just before the big chill hit. (Knoll, 2014: Stern and Miller, 2021)
- Some researchers think the whole Earth was ice-capped, and only its closeness to the Sun kept it from going into permanent deep freeze a la Jupiter’s moons Europa and Ganymede.
Others argue for some open seawater, at least seasonally, and perhaps some ice-free patches of land. After all, some thirty known species of unicellular eukaryotes did survive. But an estimated 50% to 60% of all known microbial species apparently went extinct. (Hoffman et al.; Moczydłowska)
- After enough geological time, experts suspect, greenhouse gasses released by volcanoes built up to the point where the global ice cap melted, and quickly, too, in about two thousand years.
Because of density differences, most of that meltwater sat on top of the briny sea for a while, where the Sun heated it to estimated temperatures of 104° F at the poles and around 140° F at the equator. (Hoffman et al.) Remember, most life inhabited the sea at this point!
Why does all that matter?
Because eukaryotes became major players in the game soon after the last Snowball Earth, and also somewhere around that time, metazoans — the first animals — appeared.
There is no proof that those dramatic climate changes were responsible for those evolutionary developments (Corsetti et al.; Hoffman et al.; van Maldegem et al.), but one can’t help but wonder.
Stress certainly can affect evolution, and icy cold followed in a geological instant by scalding hot tropical seas is about as extreme a stress as you can get without wiping out life.
Wel, a little more on that when we get to the “Cats as Animals” section. Here’s the Snowball Earth video.
Another one of those long ones, with well over a million views.
Evolution is so very complex.
One-off biological and geological events like endosymbiosis and the occasional natural catastrophe, and also many ordinary events, must have gone into the making of eukaryotes and eventually Family Felidae.
But Precambrian fossils are few and far between, and those ancient rocks are often difficult to interpret.
Scientists can only speculate on how life and Earth have shaped each other down through time.
In any case, once eukaryotes appeared, they still had a long journey ahead of them before they could start purring and tom-catting around.
Speaking of tomcats…
Next time: Finishing Up the Precambrian, Part 3 — Cats and Sex
Featured image: Tatiana Davidova/Shutterstock
Sources: Full list will come at end of “Finishing Up the Precambrian.” Specific references in this section:
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University of Waikato
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