Serial on cats

Volcanism: A Main Character?

That’s the caldera of Santorini Volcano behind this beautiful little poser.

Now put on your hard hats — it’s time for a little chemistry.

Very little.

In fact, just this: “What do you get when you remove the oxygen (chemical symbol O) from H2O?”

Okay, so there’s really no need for special gear (unless you want to follow a working geologist out into the field to collect and study some of the world’s oldest rocks).

You get H2 — hydrogen — of course.

Believe it or not, this answer is closely tied to the origin of life on Earth. So is volcanism: source of the heat energy powering that and other chemical reactions necessary for emerging life.

Rocks — solid as well as molten — are involved, too, in complex ways that range from providing nutrients to acting as templates and catalysts. (Hazen; Guttenberg et al., 2017, 2021; NASA, 2020b; Walker)

Ganymede looks very familiar, but it’s really not much like our Moon otherwise. (Image: Twitter)

In fact, rocks are so important that NASA is focusing its search for local ET to icy moons Europa (Jupiter) and Enceladus (Saturn), instead of on the photogenic Jovian orbiting snowballs Ganymede and Callisto.


Because subsurface oceans on Europa and Callisto probably are in contact with surface rock, not in between ice layers, as they appear to be on Ganymede and Callisto. Chances of finding life are a little better. (NASA, n.d.; 2020b, 2021c, 2021d)

Spacecraft passing close to Europa have even detected evidence for hydrothermal vents, like these on Earth (though no sign as yet of accompanying life):

You’ll need more than a hard hat down here!

The big puzzler is whether there’s a direct connection between geology and the evolution of life, and if so, what it is exactly.

That must go unanswered, but in this chapter we can get a general idea of evolutionary events on young, fiery Earth that must have occurred before LUCA (the Last Universal Common Ancestor of modern life) appeared.

Volcanism and continental shields

To bring things up to date:

The Earth [after the Moon-forming impact] began at rock-vapor temperatures and gradually cooled to habitability over 20–120 million years. By the time it was habitable, its mantle was mostly solid, but a few 100 K hotter than present. Volcanic rocks erupted to the surface and hydrothermal water circulated through hot rocks. At least some continental mass was exposed above sea level.

— Sleep (see reference list at end of post)

As we saw last time, the geology of Earth right after its magma ocean crusted over and cooled a bit was very different from today.

Using hard-hat-style geochemistry and geophysics, experts have figured out some of the details of how our planet took on its modern mode (plate tectonics).

Since we’re looking at evolution that ultimately led to cats, we don’t really need to know the minutiae of it all (some of which are controversial, anyway).

You can take my word for it that there was lots of volcanism going on while precellular life emerged, as well as the presence of lots of rocks above and below sea level.

Or, if you’re curious, here are two talking heads, describing some highlights in easily understandable terms, first in the classroom and then out in the wilds of Wyoming.

Enjoy! Or feel free to skip over this to the next section, where we get back to life’s early history again.

Note: At the end of this post, there is also a bonus video of what happens when a biologist and National Geographic get together to visit a continental shield in Guiana that is VERY different from the one discussed above, which is in Greenland and parts of Canada and the US.

Hydrothermal “hatcheries”

Earlier, we watched life thriving as seawater circulated through modern hydrothermal vents in a Caribbean deep sea trench.

Similar processes must have occurred in the oceans of Earth shortly after the planet’s formation 4.5 billion years ago. That’s just how geochemistry and geophysics work.

Evidence from ancient zircons strongly suggests that the planet did have oceans and running water that early. Perhaps, as the cooling mantle outgassed its nebular water, the resulting steam condensed into rain drops after Earth’s temperature went down a bit, and basins all over the world filled up. (Zahnle et al.)

The hydrological cycle had begun, but those alien-looking critters in the hydrothermal vent video weren’t around yet.

These shrimp and their companions may look primitive, but they’re actually highly evolved — yes, even little cellular things that are part of this ecosystem but don’t show up on camera.

To get technical for a moment, those tiny shrimp, etc., represent what many biologists call Eukarya (a domain that also includes every form of life you can see without magnification). The little cellular things belong to the other two Domains of Life — Archaea and Bacteria.

None of these thermophiles — “heat lovers” — physically resembles the first precellular life forms, which I imagine (probably incorrectly) as a sort of slimy goo on and/or in the rocks.


Because they aren’t direct descendants of the slimy goo.

Of course, something metaphysical can neither be proven nor ruled out. (Image: mayavase via Wikimedia)

Metaphysics aside, there seems to be overall scientific consensus that what I call slimy goo came first; after that, at some point, came LUCA, the Last Universal Common Ancestor, which already had well-developed cellular “stuff” (Walker et al.); and then from LUCA came the Three Domains, whose early forms left behind those famously ancient fossils. (Schopf; Walker; Walker et al.; Zahnle et al.)

The tricky part is identifying the huge evolutionary steps needed to transform slimy goo into LUCA in less than 500 million years (that is, the relatively short span of geological time, per Zahnle et al., between Earth’s cooling down to habitable temperatures and the moment when cyanobacteria, very similar to ones today, died 3.5 billion years ago, leaving their remains in an excellent fossilizing environment on what is now the western Australian craton).

It’s quite a problem for the boffins, especially because the fossil record suggests that major steps in evolution — like developing body structural components — often happen very slowly. (But this is very controversial, as we’ll discover later in the series.)

However, I do get an impression of general scientific agreement on one point, anyway: whatever its history, precellular life definitely wasn’t complex.

It probably lacked even a protective membrane at first! (Mulkidjanian et al.)

Hydrogen could satisfy its needs, and there was plenty of that available wherever water circulated through rock near magma. (Sleep; Sleep et al.)

Hydrothermal vents must have been almost everywhere, given the abundance of water, rocks, and ongoing volcanism at this point in the planet’s history.

As I understand it, planet-wide hydrothermal circulation disturbs the delicate, sterile balance of Earth’s rock, water, and carbon cycles. In short, it causes a chemical gradient of hydrogen that the first living organisms could have used as energy. (Sleep; Sleep et al.)

Still at Santorini, but the cat might be a different individual. (Image: Kathryn Burrington, CC BY-NC 2.0)

Of course, early Life didn’t put on a hard hat and reason out things like “CO2 + 4H2 –> CH4 + 2H2O,” producing methane (which soon turned Earth’s runaway greenhouse atmosphere into a yellow haze, per NASA 2020b), or “2CO2 + 4H2 –> 2CH2O + 2H2O,” producing acetate, which could provide the hungry little things with organic molecules as well as energy. (Both of these equations are from Sleep and Sleep et al.)

No, Life simply found a good “diner” — early hydrothermal springs on the seafloor, perhaps, or a geothermal area on land — and started chowing down on the daily special: H2 and other goodies, like boron (warning: this links leads to a hard-hat zone, but hey, it describes possible chemical ingredients for ancient life on Mars!). (Mulkidjanian et al.; Sleep; Sleep et al.)

Wait. On land?

Everyone knows life originated in the sea! Look at the hydrothermal vents!

True, but there are important problems with the notion that life on Earth only evolved beneath the waves.

  • For one thing, some sort of Late Heavy Bombardment was still happening at the time, with huge space rocks coming in; the biggest impacts could boil oceans. After that, incredibly hot rain would then fall, and surface life would only become possible again after a few thousand years. The likeliest long-term survivors of such catastrophes would be microbes able to withstand high temperatures — thermophiles — and also wearing “hard hats,” i.e., living in Earth’s crust just as some unrelated microbes do today. (Sleep)
  • For another, those hydrothermal vents are very deep. Their residents live off chemical energy, which works but isn’t very productive. (Sleep; Sleep et al.) If all life began down there, how could some of those primordial microbes work out a way to convert sunlight into energy — photosynthesis — so they could take over the world? (Corsetti et al.; Schopf)

    The first microbe that could dispense altogether with H2 proliferated with its productivity increasing by a factor of thousands above the previous total primary productivity of the Earth. It occupied all suitable environments on the scale of years to thousand of years. Its descendents held most of the tickets in the subsequent evolutionary lottery.

    — Sleep

  • Too, here’s something you might not know: The popular idea that the chemistry of our cells is like that of seawater is a misconception.

    Mulkidjanian et al. get into this, but you’d want a hard hat to read the whole paper, although it’s pretty interesting.

    A plain-language interpretation of the relevant nontechnical part by this layperson is that, although there are similarities, there are also important differences between cell biochemistry in all living things (including us) and seawater.

    This undercuts the idea that life’s very first appearance was intimately connected with the ocean.

    The researchers also describe some really weird chemistry in parts of all modern cells.

    This is identical to a geochemical environment that ancient life would have encountered in certain parts of land-based volcanic hydrothermal systems, like those known today at Yellowstone National Park and at two geothermal power-generating sites: The Geysers, in California, and Larderello, in Italy.

    Warned you about that hard hat! But remember — the first precellular life was a thermophile survivalist, and this sort of thing was its battlefield. According to these researchers, that “vapor dominated zone” is where the slimy goo could evolve. “Meteoric water” is just rain and ground water; “exhalations” are surface fumaroles; and “thermal waters” are hot springs. (Image: Figure 1, Mulkidjanian et al., PNAS Open Access)

    In fact, Mulkidjanian’s team note that the world’s largest known vapor-dominated zones are underneath The Geysers and Larderello.

    Relax. You can visit all of these places today without fear of a new living horror of some type oozing out at you from a crack in the ground (dibs on the movie idea, though!).

    For seriously hard-hat-chemistry reasons, the researchers report that, under current conditions on Earth, this vapor zone is much too acidic to support Life; nothing is evolving down there now.

    It was only possible back in the day when Earth’s atmosphere was a runaway greenhouse, loaded with carbon dioxide.

    I have no idea how many scientists accept this hypothesis, since it can’t be tested in the field now and, per Sleep, such early forms of life would leave little evidence in the geologic record.

    Still, the paper has hundreds of citations and is published in a reputable journal. Take this interesting idea for what it’s worth.

Because the geothermal-origin option requires a smoggy Earth atmosphere, life must have evolved before plate tectonics began, if it’s correct.

Plate tectonics locked up all that excess atmospheric carbon dioxide in our planet’s mantle, leaving the balanced carbon cycle we know today. (Sleep)

The other alternative — life originating at hydrothermal vents — is much less time limited, despite its problematical parts. And Time usually is on Life’s side.

Perhaps both ideas are right.

Having more than one type of deeply buried “hatchery” certainly would improve the chances for precellular life surviving Earth’s very hostile environment long enough to get established.

We probably will never know for sure what happened.

As Dr. Sleep points out in his paper:

Nascent life competes with nonlife (Nowak and Ohtsuki 2008). There is selection both for efficient gathering of resources and for faithful reproduction. Once the fidelity of reproduction crosses a threshold, life wins. The population explosion colonizes all connected environments on a time scale of years to thousands of years. It is highly unlikely that
the Earth’s meager geological record preserves this event.

So. Once life got started, what happened next?

Somehow, slimy goo evolved into LUCA, and the Three Domains of Life eventually appeared. Members of one of those domains won the evolution lottery by inventing photosynthesis and then poisoned the world with their excessive waste: oxygen.

“We did it, and we’re proud!” — Stromatolites. (Image: Pat Scullion, CC BY-ND-NC 2.0)

This, and more, is in the next chapter.


Before we get into that, let’s see how a biologist views some of the oldest rocks on Earth.

As it turns out, he doesn’t even notice them.

He’s more interested in frogs. (There are also five wild cats in this ecoregion, including jaguars and pumas.)

But those views of the famous Venezuelan table lands are breathtaking!

Feel free to get your own “Lost World on, too.

Sir Arthur Conan Doyle was wrong, though, about the tepuis being volcanic and only going back to dinosaur days.

They are actually Precambrian in age, about 1.7 billion years old, and part of the Guiana Shield. This craton is one óf about thirty-five ancient fragments of continental crust, and one of the youngest, believe it or not.

Craton sections in parts of Canada and Greenland go back some 4 billion years or more and are considered the oldest by many, though not all of the writers whose papers I’ve read. (Carlson et al.; Gradstein et al.; Palin and Santosh)

Featured image: mikjáll, CC BY-ND 2.0.


Carlson, R. W.; Garçon, M.; O’neil, J.; Reimink, J.; and Rizo, H. 2019. The nature of Earth’s first crust. Chemical Geology, 530: 119321.

Corsetti, F. A.; Olcott, A. N.; and Bakermans, C. 2006. The biotic response to Neoproterozoic snowball Earth. Palaeogeography, Palaeoclimatology, Palaeoecology, 232(2-4): 114-130.

Doolittle, W. F., and Brown, J. R. 1994. Tempo, mode, the progenote, and the universal root. Proceedings of the National Academy of Sciences, 91(15), 6721-6728.

Falkowski, P.; Scholes, R. J.; Boyle, E.; Canadell, J.; and others. 2000. The global carbon cycle: a test of our knowledge of Earth as a system. Science. 290: 291–296.

Fitch, W. M., and Ayala, F. J. 1995. Preface. Tempo and Mode in Evolution: Genetics and Paleontology 50 Years After Simpson. Washington: National Academy Press.

Gradstein, F. M.; Ogg, J. G.; and Hilgen, F. G. 2012. On the geologic time scale. Newsletters on Stratigraphy. 45(2):171-188.

Guttenberg, N.; Virgo, N.; Chandru, K.; Scharf, C.; and Mamajanov, I. 2017. Bulk measurements of messy chemistries are needed for a theory of the origins of life. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 375(2109), 20160347.

Guttenberg, N.; Chen, H.; Mochizuki, T.; and Cleaves, H. J. 2021. Classification of the Biogenicity of Complex Organic Mixtures for the Detection of Extraterrestrial Life. Life, 11(3): 234.

Hazen, R. M. 2017. Chance, necessity and the origins of life: a physical sciences perspective. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 375(2109): 20160353.

Maizels, N., and Weiner, A. M. 1994. Phylogeny from function: evidence from the molecular fossil record that tRNA originated in replication, not translation. Proceedings of the National Academy of Sciences, 91(15), 6729-6734.

Morton, M. C. 2017. When and how did plate tectonics begin on Earth?

Mulkidjanian, A. Y.; Bychkov, A. Y.; Dibrova, D. V.; Galperin, M. Y.; and Koonin, E. V. 2012. Origin of first cells at terrestrial, anoxic geothermal fields. Proceedings of the National Academy of Sciences, 109(14): E821-E830.

NASA. 2020a. Can we find life? Last accessed July 12, 2021.

___. 2020b. Life in our Solar System? Meet the neighbors. Last accessed July 12, 2021.

___. 2021. NASA selects 2 missions to study “lost habitable” world of Venus. Last accessed July 12, 2021.

___. 2021a. Then there were 3: NASA to collaborate on ESA’s new Venus mission. Last accessed July 12, 2021.

___. 2021b. Venus overview. Last accessed July 12, 2021.

___. 2021c. The searchers: How will NASA look for signs of life beyond Earth? Last accessed July 12, 2021.

__. 2021d. Life in the universe: What are the odds? Last accessed July 12, 2021.

___. 2021f. What’s out there? The exoplanet sky so far? Last accessed July 12, 2021.

___. 2021e. Mars 2020 Perseverance rover. Last accessed July 12, 2021.

___. n.d. Europa Clipper: Ingredients for life. Last accessed July 12, 2021

Palin, R. M., and Santosh, M. 2020. Plate tectonics: What, where, why, and when?. Gondwana Research.

Prothero, D. R. 2006. After the Dinosaurs: The Age of Mammals. Bloomington and Indianapolis: Indiana University Press. Retrieved from

Schopf, J. W. 1994. Disparate rates, differing fates: tempo and mode of evolution changed from the Precambrian to the Phanerozoic. Proceedings of the National Academy of Sciences, 91(15), 6735-6742.

Simpson, G. G. 1944. Tempo and Mode in Evolution. New York: Columbia University Press.

Sleep, N. H. 2010. The Hadean-Archaean environment. Cold Spring Harbor Perspectives in Biology, 2(6): a002527.

Sleep, N. H., Bird, D. K., & Pope, E. C. (2011). Serpentinite and the dawn of life. Philosophical Transactions of the Royal Society B: Biological Sciences, 366(1580), 2857-2869.

Taylor, S. R., and McLennan, S. M. 1995. The geochemical evolution of the continental crust. Reviews of Geophysics, 33(2): 241-265.

Walker, S. I. 2017. Origins of life: a problem for physics, a key issues review. Reports on Progress in Physics, 80(9): 092601.

Walker, S. I.; Packard, N.; and Cody, G. D. 2017. Re-conceptualizing the origins of life. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences,375: 20160337.

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Zahnle, K.; Schaefer, L.; and Fegley, B. 2010. Earth’s earliest atmospheres. Cold Spring Harbor Perspectives in Biology, 2(10): a004895.

Main Character: Plate Tectonics

That tiger — and the little avian dinosaur in the foreground, keeping a respectful distance away from the cat — are walking along one of many river beds that cross the Terai, a flat grassy wetland that runs along the feet of the Himalayas in Bhutan, India, and Nepal.

This particular river is in Nepal, according to the photographer.

As you can see, Terai soil is deep and fertile, but mountain floods can slash through it easily. They also bring down the nearby towering range piece by piece as rounded boulders and cobblestones.

Thanks to plate tectonics, though, the Himalayas continue to rise despite this constant assault by rain and ice.

Down in the flatlands, a young Ganges River flows through the Terai, gathering in lesser streams like the one shown above and growing in size and volume as it travels more than a thousand miles eastward and then south to the distant Bay of Bengal.

India and Nepal established important nature preserves here in the early 1970s. Bengal tigers are also protected elsewhere in the region, including the Sunderbans: a vast mangrove forest that covers the Ganges Delta of India and Bangladesh.

What does all that have to do with plate tectonics?

Well, this:

A few caveats to this excellent video: Per sources that I have read, other factors were also at work during the great greenhouse-icehouse transition, but let’s save that for Chapter 18. As I understand it, there is consensus on the evolution of whales, but otherwise the India-Asia collision and its effects on plant and animal life were very complex, as this abstract shows. Not all experts agree with Dr. Hughes. In a later chapter, though, we’ll look into another, even more controversial hypothesis: that big cats might have evolved in Tibet!

Cats and Plate Tectonics

Take the part in this video where they mention cooling, for instance.

Based on how cats behave now and the ways that behavior has shaped their anatomy and that of their fossil relatives down through time, it’s likely that Family Felidae evolved to fill a predator niche in an ecosystem that existed in between the forest’s edge and an open plain. (Martin).

That was ideal! There was sufficient cover to sneak up on prey (and trees to scoot up into when danger threatened), as well as just enough open space for a short sprint and deadly pounce. (Werdelin)

Now try to imagine a place like that in Late Cretaceous times.

Despite what you’ve seen in the “Jurassic Park” movies (Akhmetiev and Beniamovski; Prothero; Vajda and Bercovici), it might not have existed:

  • The non-avian dinosaurs’ world was warmer and muggier
  • Much of what is now land was covered in water back then, with the central plains of North America and Eurasia hidden at times underneath warm, shallow seas
  • Vast swamps were turning into thick coal beds
  • Per the best-known fossil record of those times (North America’s Hell Creek Formation), a rainforest grew in what’s now North Dakota

A surprising and, in some ways, delightful world, but it was no place for a cat.

The difference between that world and our own is distinctive. (Image: Bureau of Land Management, CC BY 2.0)

Any clearings that opened up from fire, storms, or “veggie-saurus” overgrazing probably filled in quickly with a jungle-like tangle of plants competing for light.

Of course, the K/T (K/Pg) extinction 66 million years ago changed everything for animals and plants, but it didn’t turn Earth into an icehouse.

Plate tectonics, as described in the video about India, as well as other factors (Lyle et al.) did that.

Plate motion, however, is slow, so global climate took millions of years to cool off and dry out.

A decent habitat for stalk-and-pounce predators only developed at around the 35-Ma line in our cat/sports field analogy.

And the first cat-like mammal predators — nimravids — then showed up.

We are jumping way ahead of the story here, but it’s also interesting to see how plate tectonics helps us answer other questions about cats.

For instance:

  • Since Earth is obviously an ocean world (Taylor and McLennan), why did we (and cats, among many other forms of life) evolve on dry land?

    Answer: Because there has been continental crust on this planet, as well as seafloor, for almost as long as Earth has been around. (Morton; Sleep) Marine life of some sort came first, but Life started exploiting the vast resources available on land just as soon as it could.

  • Why did cats first show up in the Northern Hemisphere? (Werdelin et al.)

    Answer: Because once they came on the scene in Eurasia roughly 30 million years ago (Werdelin et al.), cats couldn’t swim across oceans to get into the Southern Hemisphere. Plate movements first had to form land bridges with Africa (about 19 million years ago) and South America (some 3 million years ago). Australia is still isolated; cats arrived there just a few hundred years ago by ship along with Europeans. (Agusti and Antón; Prothero: Werdelin et al.)

  • Why are there so many hybrids among small wild Latin American spotted cats?

    Answer: Okay, that question might not occur to you unless you’re into wildlife conservation or have read my ocelot lineage eBook.

    They really are adorable. (Image: Márcio Motta, CC BY-NC-ND 2.0)

    Trust me: Even experts have a difficult time sorting out pampas cats, tiger cats, and some other species.

    These little cats reached South America so recently that some of them haven’t completely settled into stable species yet.

    Sure, 3 million years is a long time to you and me, but evolution, like plate motion, takes time. These beautiful kitties are still undergoing what biologists call an adaptive radiation.

    Genome research shows that similar hybridization occasionally happened in the distant past with other feline lines, like 10-million-year-old Panthera (the big cats). (Figueiró et al.; Werdelin et al.)

    For that matter, mammals — particularly our group, the Eutherians — had a huge adaptive radiation after the K/T (K/Pg) extinction. (Prothero)

    This is one of the ways that evolution works.

Plate tectonics and climate

We just saw how plate tectonics built land bridges and changed world climate in ways that helped cats to evolve and migrate.

So plate tectonics is a player in this series about Family Felidae.

To explore it, though, we need to strip away vegetation and other forms of life that we usually think of as “the natural world” and look at the underlying raw power of Earth as shown by things it has wrought.

That’s not easy, since the planet is much bigger and older than us.

Without these seemingly eternal mountains, there would no tiger and bird standing in a dry Nepalese river bed, no Ganges, no Sundarban mangroves, probably far fewer people in India and parts of Asia, and a VERY different global climate. Today, humanity provides an exclamation point to Himalayan grandeur, but how did Life ever get started when the whole world was so stark and silent?

Although it operates on geological time scales, plate tectonics is the force behind earthquakes and eruptions and other changes that disrupt our daily lives.

It also underlies the whole field of physical geography, which covers natural features that shape evolution and human society in many ways.

An ever-changing “blue-and-white marble” — complicated almost beyond human comprehension? Who would have expected that! (Image: NASA)

Plate tectonics is also part of an interwoven geologic/atmospheric/hydrological planetary systems network that affects us all personally.

We need to take that seriously.

Like me, you’ve seen and perhaps also have been turned off by the news coverage of “climate change.”

It isn’t pretty: political fighting over policy never is, because no one is ever objective and everybody’s ugly side comes out.

Just for the record, I think it demeans Science whenever any of its members use labeling like “denier” on those who disagree with them, rather than undertaking the difficult task of calmly reasoning with them to open their minds.

Unfortunately, there is a lot of that going on, and it’s just as unhelpful as the other side’s unrealistic view, basically summed up as “so what?”.

Cutting past all the mean stuff, scientists do know something important that the rest of us don’t: our dynamic Earth goes through some intense climatic “mood swings” over geologic time.

Unfortunately, because this involves multiple factors and feedbacks, including but not limited to plate tectonics, no one can be very certain about the why and how of it.

Venus, top, by NASA/JPL-CalTech: Saturn’s ice moon Enceladus, bottom, by NASA.

One of the more extreme of these episodes, when the planet was more or less completely iced over, was the Cryogenian.

It happened about 1 billion years ago. You might have heard it called “Snowball Earth.” (There were actually several such events, but only the most recent one — the Cryogenian — has left much evidence; we’ll get into that a little later in this post.)

Life on Earth survived the Cryogenian and actually went into the famous “Cambrian explosion” after the last traces of its globe-covering ice melted away.

But the hellish conditions on our “twin,” Venus, clue us in that other, more lethal extremes than a deep freeze are possible on the planetary climate spectrum.

Earth might have been like that in early Hadean times, soon after its formation (Morton), but not since then.

As we’ll soon see, plate tectonics stopped that runaway greenhouse by cycling carbon dioxide into Earth’s mantle. (Sleep)

The third planet from the Sun has had other, milder “mood swings,” like the global greenhouse conditions that non-avian dinosaurs enjoyed or the icehouse that we have today.

Changes from one “mood” to another can happen fairly quickly.

The geologic record shows that conditions cross a threshold of some sort and — just as one example — soon, global CO2 somehow nosedives and you’ve now got ice on Antarctica (Lyle et al.) — a continent that often was habitable during its long history.

This fossil leaf grew on a bush- to tree-sized fern in Antarctica during the Permian. (Image: James St. John, CC BY 2.0)

It’s only a little ice at first, but the delicately balanced climate mechanisms and feedbacks have shifted; one thing leads to another, and eventually Antarctica is deeply frozen, while continental ice sheets are moving back and forth over the world’s other polar land masses every hundred thousand years or so. (Lyle et al.; Prothero; Zachos et al.)

No one yet knows much about the world thermostat and its thresholds, other than that when Earth crosses one of those, the environment changes in complex ways and there is nothing we can do but get ready for the consequences.

While few of us would be distraught if there never was another Pleistocene-style ice age, sea level rise and climate shifts during the transition out of an icehouse “mood” would play expensive and often deadly havoc with our carefully arranged artificial world.

We might be seeing the start of that now, although the sociopolitical static around this hot-button issue makes it impossible to be truly objective. Time will tell.

And there’s this to think about, too: Although greenhouse gas levels are still quite low compared to those at some points in Earth’s history, we are the Pleistocene’s children.

How well will we do at another another planetary “setting”?

Quite well, I suspect, given our adaptability and technology, but there is also the question of how Earth’s regulatory systems will react to the extreme climate forcing that humanity is unloading on them today.

What climate thresholds might we unwittingly drive the planet across, with potentially dreadful consequences?

We just don’t know. Earth’s experiment in the evolutionary viability of advanced intelligence has only been going on for a few hundred thousand years — it’s too early yet to tell whether that will be successful or a dead end.

So let’s think about cats instead — well, at least about the point where we left off in this series.

There weren’t any cats yet, but young Earth, a little more than 4 billion years ago, had cooled enough to have a fairly solid mantle, as well as an outer crust covered with oceans that were kept filled by a water cycle that operated in a smoggy, methane/CO2 atmosphere that would kill us with a single breath.

Earth compared to other planets

When we last visited Hadean Earth, a few hundred million years after its formation, the magma ocean that had covered this planet’s surface had cooled off and crusted over, while the mantle underneath it was “freezing” into rock from the bottom up.

US Geological Survey

That’s not to say Earth’s mantle was ever totally solid. It’s not rock-solid today, when temperatures down there are much cooler than they were in Hadean times: gigantic currents of hot rock still convect through the mantle just as if it was a simmering pot of soup on the stove.

Chemical and physical differences aside, simmering soup actually is a good analogy for the mantle — and not just because solids (crystals), liquids (magma), and gas (water vapor and other volatiles) are combined down there (Oppenheimer) like meat/vegetables, broth, and savory steam in a stew.

After all, the mantle’s base is in contact with an extremely hot stove burner planetary core.

And this is what some of that “soup,” a/k/a the upper mantle or asthenosphere, looks like on those rare occasions when it bubbles over. This eruption is in Iceland — a special case.

And the pot lid? Today it’s Earth’s lithosphere — continental and oceanic crust.

Back in the Hadean, it was crust that formed when the magma ocean cooled down, right? Sort of like the dark scum that eventually covered Kilauea Volcano’s cooling 2021 lava lake?


Well, no doubt that old magma ocean crusted over like that at first.

However, newborn planets and other large rocky worlds apparently have alternatives as their thick outer layer hardens up.

And two of the three types of crust that Taylor and McLennan describe apply to Earth at different stages in its development.

According to these experts, a planet or moon’s outer crust can be:

  1. Primary: Material from the Solar System’s formation. As it cools, some minerals crystallize early and float to the top: for example, this is how Taylor and McLennan say that the light-colored lunar highlands formed.

    The rock/ice outer layers of some Jupiter and Saturn moons might be primordial, too.

    There was recent news from Greenland about the discovery of 3.6-billion-year-old crust from the Hadean magma ocean. I don’t know how that fits in with widespread consensus that 4-plus-billion-year-old zircon crystals from Australia’s Jack Hills show evidence of continental weathering — something unlikely to happen when there was a global magma ocean. (Palin and Santosh; Sleep; Taylor and McLennan)

  2. An eruption on Io, while the Galileo spacecraft was passing by.
    (Image: NASA)

  3. Secondary: Internal heat from radioactive decay melts part of the mantle, causing basalt eruptions on the primordial crust: think the dark parts on the Moon — the lunar maria.

    Another example is Jupiter’s moon Io; here, the heat also comes from interactions with Jupiter’s immense gravity field.

    Palin and Santosh see this Jovian moon as a possible analog for very early Earth, although Io is much smaller. If that’s the case, then such lava flows would have buried the original crust that hardened over our world’s Hadean magma ocean.

    By the way, Taylor and McLennan include Mars and Venus in this category.

    So do Palin and Santosh, but they point out some unusual details, like relative young volcanism on Mars (less than 40 million years old) and an unknown catastrophe on Venus that resurfaced the entire planet some 300 million years ago, as well as a few peculiar surface features on both planets (see the discussion in their paper).

  4. Details aside, the above examples are both forms of what geologists call single-plate or stagnant-lid tectonics.

    There is only one known example of Taylor and McLellan’s third type of planetary crust, and we’re living on it right now:

  5. Tertiary: Another name for this is mobile-lid tectonics, or as most of us call it, plate tectonics.

    Taylor and McLennan note that, as the planet’s surface layers recycle down into the mantle, “continuous distillation” produces magmas that are more like granite than basalt — in other words, the continental crust of today.

    Note that this isn’t as dense as basalt seafloor, so it doesn’t go down into the subduction zone.

    Instead, depending on relative plate motion, this crust either piles up into something like the Himalayas or else slips sideways, as those parts of California west of the San Andreas Fault are doing right now.

  6. Exactly how and when plate tectonics began is an unsettled scientific question, but it must have required many favorable factors, most notably, the long-term presence of water on the surface. (Palin and Santosh)


    Get your license first.

    You try subducting a dry rock plate some time!

    It can’t be done. Friction is too strong, and the plate also won’t be bendy enough for the move. (Palin and Santosh)

    Earth’s mantle outgassed water vapor as it “froze” during the first 10 million years or so of the new planet’s existence, so there was plenty of water available in streams and oceans to get a stable hydrologic cycle going. (Zahnle et al.)

    Something similar probably happened as Mars and Venus also cooled down. Venus might even have been the Solar System’s first habitable world! (NASA, 2021)

    But those worlds didn’t have water for long. And the Moon lost all its volatiles, including water, during its formation during a gigantic impact. (Sleep)

    So, until space scientists prove otherwise, it seems that Earth’s being a long-term ocean world has also made it the only Solar System body to have plate tectonics.

    Is that also why it hosts life?

    That question is much bigger than it appears to us laypeople, judging by the number of research papers found on Google Scholar that address it and related topics.

    From the little I’ve read, some models reportedly suggest that single-plate worlds could hold on to water for a long time. This implies that simple life might possibly exist elsewhere.

    But this did not happen on Mars, and space scientists still wonder about Venus.

    There might be a broader consensus on this point: complex life is possible only on a planet that has plate tectonics. (Palin and Santosh)

    One reason for this is that plate tectonics, besides keeping greenhouses gases down, also recycles essential nutrients for life. (NASA, 2020b)

    The rock cycle

    At first, on Hadean Earth, only water and carbon cycled through various stages:

    • Mantle outgassing formed oceans, whose surface waters evaporated into vapor that eventually condensed into rain and made its way back to the sea — the hydrologic cycle.
    • That rain water chemically interacted with CO2 in the air to form weak carbonic acid that leached minerals out of the rocky ground when it fell and brought everything into the sea, where further reactions turned it into carbonate rocks — the carbon cycle.

    But apparently rocks were cycling, too.

    The oldest minerals on Earth — those 4-plus-billion-year-old Australian zircon crystals — are from metamorphosed clay-like sediments. (Sleep)

    This sort of thing was NOT supposed to be going just a couple of hundred million years after the Solar System formed, while the planet was so hot and primitive:

    Yet this, or some similar process, obviously was happening together with the water and carbon cycles.

    While its origins aren’t clear, whatever was going on with Hadean rocks eventually became the plate tectonics we know today.

    But metamorphism and other geologic changes have ruined the few stony archives that have survived from those times, leaving only chemical clues about that Hadean process, like isotopes.

    Ruby, by Stranger Than Kindness via Wikimedia, CC BY-SA 3.O; Uncut diamond, by USGS via Wikimedia,public domain; Mayan jadeite amulet, by Metropolitan Museum of Art via Wikimedia, public domain.

    There are sometimes clues about plate tectonics in precious stones, too. Jadeite and ruby, for example, are only found in subduction-zone settings and can be billions of years old.

    Diamonds — crystallized carbon — are the most famous ancient gemstones; and since they form at depth, perhaps from subducted carbonate seafloor rocks, diamonds also record changes happening in the mantle underneath continents. (Palin and Santosh; Sleep; Stern and Miller, 2018, 2021)

    These clues can be, and often are, interpreted in very different ways.

    When did plate tectonics start?

    Carbonate rock formation in Hadean oceans removed some CO2 from Earth’s runaway-greenhouse atmosphere (Zahnle et al.), but not enough to shut that down or even to cool things off very much.

    Scrubbing that much carbon dioxide out of the air was a job for global plate tectonics, which can store vast amounts of carbon deep underground. (Sleep)

    But for a few hundred million years, at least, early Earth was too hot to make sturdy enough tectonic plates for subduction.

    Yes, there were continents, almost from the get-go, though probably not like the ones we know today. (Morton; Sleep)

    Even so, all you could get back then with such a thin, weak crust was overturn like this:

    As seen on Kilauea’s 2021 lava lake, when it was fresh.

    View through Kilauea summit webcam B1, August 14, 2021.

    I like to think of the islands of basalt on that lava lake as similar to early Hadean continents, but don’t quote me on it.

    The experts that I’m basing this section on — Morton; Palin and Santosh; Sleep; Taylor and McLennan — only report that there was a change in the old continental material, around 2.5 to 3 billion years ago, from basalt-like mafic rock to more granitic rock.

    At that point, the planet’s crust would have been cooler and thick enough to maintain active subduction zones.

    Since granite is the sort of material produced by the “continuous distllation” process of modern plate tectonics, most experts believe that is when it all started, 2.5 to 3 billion years ago.

    Palin and Santosh explain earlier evidence of an active rock cycle, like the Jack Hill zircons and certain multi-billion-year-old gemstones, as the product of localized tectonic movements that hadn’t yet gone global.

    (A note for continuity: Don’t forget that, as we saw in earlier chapters, the oldest fossils known are 3.5-billion-year-old cyanobacteria — Life somehow was doing its thing, at least in simple ways, while Earth’s surface matured.

    This might not have been coincidental, per Hazen, who writes “Minerals and life coevolved, with most mineral species mediated by life…”)

    There’s an even wilder idea about early plate tectonics out there.

    A small but vocal minority argues that the start of mobile-lid plate tectonics would have had much more significant effects than anything found to have occurred 3 billion years ago. (Morton: Stern and Miller, 2018, 2021)

    While everyone seems to agree that early plate tectonics subducted away most of the carbon dioxide left over from planetary formation — leaving what Sleep calls “a modest concentration of CO2 in the air and the ocean” — Stern and Miller think that the start of plate tectonics did much more than that.

    They also claim that this happened much later in the planet’s development, when the geological record indeed does show a major upheaval.

    In brief, Stern and Miller suggest that the development of plate tectonics happened around 1 billion years ago and caused such massive changes in young Earth’s previously stable climate and oceanographic systems that our planet went into what I referred to earlier as a “mood swing.”

    That is, about a billion years ago — when plate tectonics began, according to Stern and Miller — Earth froze up, with ice all the way down to the Equator.

    This has definitely happened, and more than once over geologic time.

    The only disagreement about the most clearly documented event, 1 billion years ago, is whether it was a “hard” Snowball Earth — a solid ice shell — or a “Slushball” Earth with some patches of open water, perhaps in the tropics and/or around volcanoes. (Corsetti et al.; Hoffman et al.; Moczydlowska)

    (Continuity note again: Life soldiered on through this, too.)

    Why Earth froze over is another matter. There are more than twenty hypotheses floating around besides the one proposed by Stern and Miller, most of them unrelated to the question of when plate tectonics started.

    Getting back to that issue (and leaving for later chapters the “Cambrian explosion” of life that occurred when the giant snowball finally melted), I like the way Morton resolves this controversy.

    She writes:

    Consensus may be a long way off, but in some ways, everybody might be right: Perhaps plate tectonics itself has gradually evolved to operate how it does at present over billions of years, such that it’s looked different at different times in Earth’s past.

    Whatever form it has taken as the planet slowly cooled, plate tectonics certainly has kept Earth habitable, beautiful, and occasionally terrifying for billions of years.

    “Thanks a lot, Plate Tectonics.” — Everybody living near an erupting fire mountain. (Note: This video is from January 2020.
    At the time of writing, Taal is still restless but the alert level has been lowered to 2 on a four-point scale.)

    Volcanoes mess up that otherwise perfect symmetry of water, carbon, and rock cycles in so many ways.

    But if they didn’t, would the Earth’s balanced energy budget have anything to spare for Life?

    Let’s find out more about that in the next chapter.

    Featured image: Paco Como/Shutterstock


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    Falkowski, P.; Scholes, R. J.; Boyle, E.; Canadell, J.; and others. 2000. The global carbon cycle: a test of our knowledge of Earth as a system. Science. 290: 291–296.

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Main Character: Earth

Great news!

A spacecraft has found definite signs of life on a habitable world!

Well, it was Earth and the craft was a probe named Galileo that flew past its home in 1990 for an equipment check before sailing on to explore the Solar System as far as Jupiter. (Here’s how that turned out.)

This isn’t from Galileo, but it is definitely cool. In 2020 NASA turned some of its data on Earth into music and released the video. Check out which instrument is “playing” atmosphere, water, etc., at the YouTube page.

Still, congratulations to the rocket scientists!

And even though our focus is on how cats evolved, we do need to look at Earth and ask the basic questions: where did it come from? What makes our planet such a good stage for, and cast member in, that ensemble play we call Life?

Only then will details in later chapters make sense, for instance, why cats have four legs and a long tail (mammal predators do have other options); the whole cat-dog thing and whether T. Rex ate any of their direct ancestors; why cats have pretty fur but scary claws and teeth (compared to our own flat fingernails and chompers), and so forth.

Continue reading “Main Character: Earth”

Does Life Exist Elsewhere?

Whatever happened with that news last year about finding life on Venus? Something about phosphine, whatever that is.

And what about the cigar-shaped space rock with the unusual name (Oumuamua) they found in 2017 and called an “interstellar visitor”? Aliens, right? Where’s the follow-up on that?

The shortest, simplest answer to both these versions of the Big Question is that scientists are working on it.

Their scientific method is a very useful tool for getting to the root of things, but it takes time. Too, jargon and the technical details involved do not make for reader-friendly stories.

That’s why journalists usually wait for results to be announced in simple language.

Many years can pass in between press conferences. And sometimes other research teams come up with different results in the meantime, which the journalists also must report.

“…the launching of this ‘bottle’ into the cosmic ‘ocean’ says something very hopeful about life on this planet.” — Carl Sagan via Wikipedia.

This extended, open-ended process generally leaves us laypeople feeling confused and a little put off by Science — except when the topic is “Life Out There.”

THAT always gets our attention.

It appeals to our gut feeling that, if humanity keeps searching long enough, we’ll find ET someday, looking back at us and glad to discover that it’s not alone in this huge universe.

Is that a valid hope or are we just projecting our social selves onto the cosmos?

Alien life isn’t impossible

I’ve found out something cool while reading through the sources for this chapter of the series on how cats evolved.

Continue reading “Does Life Exist Elsewhere?”

What Is Life?

Trigger warning for people who have been traumatized by or become anxious around animals.

Adalbert Dragon/Shutterstock

Life is incredibly precious, you realize seconds after meeting a cat like this and knowing that you might come out on the losing end here.

The usual way we see our lives — as something to get through each day — evaporates under this jaguar’s frank stare.

Welcome to the food chain, pal!

Like it or not, you and I are part of the great web of life.

Individually and as groups, we try to avoid the scary and unpleasant parts by insulating ourselves from Nature as much as possible. This often works, too.

However, Nature is bigger than us. Bottom line: like any other species, H. sapiens eats and can be eaten.

Now for the good news.

“Cave lions suck!” “Bears, too! — Prehistoric people. (Image:
EOL, CC 2.0)

Human beings have been dealing successfully with predators like this jaguar and even worse for hundreds of millennia.

The survivors of such encounters have passed along to us a built-in emergency mode that gives our famously big brains a chance to think their way through a crisis.

Continue reading “What Is Life?”

Time: Human vs. Geological

Life on Earth is strange, and I don’t just mean physically. It’s odd how life goes on here.

In terms of time, we are so out of sync with our planet!

About 75% of the Earth’s outer crust, where we live, is composed of rock similar to this. (Image: James St. John, CC BY 2.0)

First, look at our natural surroundings — steady as a rock (most of the time, anyway).

And usually very, very old.

Then look at humans, or at cats — each born helpless; struggling to reach maturity; struggling more to survive and reproduce; and then aging and passing away.

It all happens quickly, too (at least to an outside observer: parts of our own lives seem to take forever).

In the wild, cats don’t live long, maybe five to ten years, or a little more if they’re tough and lucky.

More beautiful than any rock. (Image: SantiPhotoSS/Shutterstock)

Exceptionally elderly people might live for a hundred years, but even this is short compared to the social fabric that they are wrapped in. While often resembling a patchwork quilt, its history goes back many centuries.

The current British monarch, for example, is in her mid-90s. That isn’t very old, considering how long her royal house has been around, and it’s positively youthful compared to the age of her kingdom.

Still, what do centuries and millennia mean to a multimillion-year-old rock?

Nothing, of course. It’s inert, although there may be something living underneath it or even inside. The rock’s components — silica, oxygen, and various other elements — are just chemistry, facts for nerds to ponder.

Biology is where it’s at, and we’re at the top of the heap!

This delusion is so powerful that most of us need a strong reason to ask the really interesting question — what does that multimillion-year-old rock mean to us?

Continue reading “Time: Human vs. Geological”

To Make A Cat

Look! A cat!

No, seriously. Have you ever really looked at one of these before?

The kitty doesn’t have to be Fluffy, although house cats are a lot easier to study at home than, say, mountain lions or tigers.

Believe it or not, apart from size and a few lifestyle-related anatomical details, you do have a little mountain lion/tiger there!

The essence of Cat is not so easy to describe. (Image: Olas, CC BY-SA 2.0)

That’s because all members of family Felidae are built alike. (Turner and Anton)

What is a cat?

This information is from Kitchener et al., Wright and Walters, and some fun hours spent watching house cats — my own and friends’ cats.

The long feline body is much more supple than that of a gray wolf (Fido’s closest relative; I use wolves for comparison because dogs have been domesticated longer and in many cases don’t look much like their forebear now; outside the show ring, Fluffy still resembles its African wildcat ancestor in many ways).

Continue reading “To Make A Cat”


Have you ever wondered why the spring season in both hemispheres feels as old as forever and as young as a newborn baby — at the same time?

Evolution goes back billions of years, but new possibilities open up whenever life awakens and reproduces itself.

On a related note, ever wonder about how cats evolved? Or how closely related house cats — the “lions in our living room” — really are to the big cats?

Sabercats cannot be ignored. (Image: Wim Hoppenbrouwers, CC BY-NC-ND 2.0)

And what about those saber-toothed cats?

Earth’s apex predators have gone from T. Rex & Company to today’s lions, tigers, and other carnivorous mammals (including domestic cats, which are apex small predators in most human-dominated habitats).

Why did evolution take this route?

Continue reading “Introduction”