Such a tranquil moment.
Imagine not only an empty caption balloon floating over the ginger tabby’s head (unless mice have learned to use scuba gear) but also a whole host of one-celled critters quietly developing in a Precambrian ocean much like this one.
Then, after billions of years, a Cambrian explosion of more complex marine life happens, eventually followed by a move of the animals we’re looking for — tetrapods (literally “four-foots,” although they were our physical ancestors, too) — out onto some tropical lagoon beach and spreading across the land.
Apart from Ginger up there and the improbable mice, that’s basically what happened.
But the few details known about Earth’s earliest history show a Precambrian world that often was much more tumultuous and risky than our own.
So, where were we again?
I don’t know the source for this computer simulation, but something big, perhaps along those lines, happened here 4.5 to 4.6 billion years ago.
Science offers wide-ranging ideas on every conceivable topic — especially anything involving Hadean Earth — but it would be confusing to cover them all. Let’s just go with a common hypothesis, explored in that video: a Mars-sized object hit Earth v1.0 soon after it formed.
After this impact, Physics and Chemistry decreed that the new Earth, Version 2.0, would have a rock-vapor atmosphere for a while, and our Moon would take shape about 19,000 miles away — just outside the new planet’s Roche limit. (Waltham; Zahnle et al.)
Had the Moon stayed that close, the Apollo program would have been much easier, but no, everything has to obey the law about conserving angular momentum and now there’s a 24-hour day and the Moon is hundreds of thousands of miles away.
At first, the planet spun around its axis much faster than it does today — a Hadean day/night might have only been 6 hours or so long! (Klatt et al.) — and centrifugal force, which still reduces your weight a little at the Equator (Strahler), produced quite a bulge of equatorial ocean water around Hadean Earth’s “waist.”
Torque resulting from the Moon’s attraction on this bulge transferred energy and angular momentum from Earth’s rotation to the Moon’s orbital motion. Simply put, Earth’s rotation slowed down and the Moon moved farther away. (Williams)
But we’re all alive, on a habitable planet. So there’s that.
Now let’s switch over to the Sports Cat analogy.
We’re almost ready for the kickoff that starts this game in which one-celled creatures evolve into cats and much more.
Before it begins, what are field conditions like down at the Hadean/Archean end of the stadium, around the 3- and 4-Ga (billion)-year lines, where players make their entrance?
Earth in the late Hadean/early Archean
After Earth stabilized, it cooled down a bit, developing a mafic (iron- and magnesium-rich) crust and protocontinents, oceans of salty water, and a gassy greenhouse atmosphere of some sort after the admittedly more hardcore vaporized-silicate one rained out. (Arndt and Nisbet; Catling and Zahnle; Hsia et al.; Morton; Sleep; Zahnle et al.)
On such a world, Physics and Chemistry would inevitably start processes that still shape life and evolution today.
Here’s some of what they had to work with in the Hadean:
- A more or less round planet, slightly tilted on its axis, with the equator getting more solar energy than the poles:
This gives you the general idea. It’s likely that centrifugal force made equatorial regions bulge out a lot more in the Hadean and Archean since Earth was spinning up to four times faster than it does now.
- Minerals on Earth’s surface, particularly mafic things like olivine — (Mg,Fe)2SiO4 — and hornblende — Ca2(Mg,Fe,Al)5(Al,Si)8O22(OH)2. (Source)
Okay — there’s no test. I just added those chemical formulas so you could see the ancient source of some vital human nutrients.
Take your vitamin pill today? If it’s one of the “complete” types, many of those elements will be on the ingredients list:
Is it a coincidence that these were present on our planet before and during the emergence of life?
Probably not. In fact, according to one theory, minerals and life evolved together!
If there had been, say, a lot of vanadium (V), cobalt (Co), molybdenum (Mo), or other elements in Earth’s first minerals, instead of mafic ones, the history of life might be very different — or perhaps life might never have started.
- A gas-containing atmosphere that behaved like our own, although its ingredients must have been different.
What was Hadean/Archean air like? That’s a bigger question than you might think.
Air is a combination of gases, not just O2 (oxygen).
Each gas interacts in specific ways with sunlight, at the top of the atmosphere, and with everything at Earth’s surface, down at the bottom, as well as with all the other gases.
Gases can even leak down into the ocean and change its surface chemistry (fortunately for us all, this is a two-way street).
Knowing what the atmosphere was like during Hadean and Archean times would help the boffins to better understand terrestrial life’s first shaky steps.
Unfortunately, they don’t have any direct data from those times: air floats off, and it doesn’t fossilize.
Indirect evidence and computer modeling suggest that Earth’s early atmosphere contained carbon dioxide (CO2) and nitrogen (N); possibly also hydrogen sulfide (H2S), cyanide (HCN), carbon monoxide (CO), and methane (CH4), as well as water vapor (H2O). There certainly was no free-floating oxygen (O2) yet, let alone an ozone (O3) layer. Water and minerals bound up what little inorganic oxygen was available. (Arndt and Nisbet; Catling and Zahnle; Hsia et al.)
Author Douglas Adams offers some wardrobe tips for walking in that stuff:
.“Don’t step out!” said Dirk [to his friend Richard], putting up an arm, “The atmosphere is poisonous. I’m not sure what’s in it but it would certainly get your carpets nice and clean.”
Dirk was standing in the doorway watching the valley with deep mistrust.
… Reg…was busy fussing round Michael Wenton-Weakes, making sure that the scuba-diving suit he was wearing fitted snugly everywhere, that the mask was secure and that the regulator for the air supply was working properly.
…Reg finished smearing Vaseline on all the joins of the suit and the few pieces of exposed skin around the mask, and then announced that all was ready.
Dirk swung himself away from the door and stood aside with the utmost bad grace. “Well then,” he said, “be off with you…”
Again, you don’t need to memorize all that (although it will make Chemistry Cat very proud of you).
The chemical symbols are only there to show you something weird that I’ve noticed while reading up on this: only a few of the many elements on the periodic table are major players in the Hadean and Archean story, particularly carbon (C), nitrogen (N), iron (Fe), sulfur (S), hydrogen (H), and oxygen (O) (in water and minerals), combined in various ways (a few more, like phosphorus, will arrive when life enters the picture).
These keep showing up over and over again.
Why? For serious Science reasons, most of which are still being debated and, anyway, are over my head.
They all must have been very important for development during Earth’s toddlerhood.
Well, to jump ahead a bit and show you how useful these elements were, here’s what the three domains of life were probably up to during the mid-Archean (Arndt and Nisbet), around 3.5 Ga (billion years ago).
To them, “breathing” was eating and hydrogen (H2) was food (Sleep):
- Bacteria: CO2 + 4H2 → methane + 2H2O.
Yes, that’s Greek to me, too, but it worked out so well for bacteria that the methane they burped out might have shrouded Earth in a yellow haze like the one on Saturn’s moon Titan, which NASA has confirmed isn’t generated by Biology. (NASA 2020b)
- Archaea (bacteria-like extremophiles): 2CO2+ 4H2 → acetate + 2H2
- Our own group (Eukarya): Crickets chirping.
It’s not that eukaryotes were slow learners; they were way more advanced than bacteria (think Ferrari compared to a Ford Model A) and took more time to develop. (Knoll; Koonin; Reddy and Evans)
Anyway, there are those same ingredients again (methane and acetate are just other ways to combine C, H, and O).
And without sulfur (S) and iron (Fe), there might not be photosynthesis today!
Photosynthesis is at the base of our global food chain (not to mention fossil fuels) and probably started more than 3.7 billion years ago, i.e., shortly after the Archean eon began or even earlier. (Kitadai and Maruyama; Sleep; Zahnle et al.)
Bacteria came up with this way to turn solar energy into organic material (Hsia et al.; Reddy and Evans; Sleep), but they didn’t make free-floating oxygen with it at first, which is what we usually associate with the word “photosynthesis.”
Remember, we’re talking about a time when Biology had to tediously evolve everything from scratch.
These methods of using sunlight were the easiest:
2CO2 + S-2 + 2H2O + sunlight → organic matter + SO4-2.
Sulfur-based photosynthesis is still around, by the way (not green; it’s purple!):
CO2 + 4FeO + H2O + sunlight → organic matter + 2Fe2O3.
No known bacteria use iron this way today, but several groups still do have the cellular tools for it. (Hohmann-Marriott and Blankenship)
Those critters would “eat” either sulfur (S) from the atmosphere or iron (Fe) from mafic rocks and then excrete sulfate and ferric iron, respectively.
And all that H2O used in the formulas? It came from…
- Bacteria: CO2 + 4H2 → methane + 2H2O.
- Oceans, possibly greenish ones, thanks to the iron-eaters, with some sulfur-eaters also there to add a purple hue (Karecha).
Now imagine a yellow methane haze for the sky. Archean Earth was a colorful place!
Not surprisingly, with so much water available, some of those ancient little critters eventually worked out a way to cut out the iron/sulfur middleman (last chemical formula, I promise, but bear with me, please — it’s a world-changer!):
CO2+ H2O + sunlight → organic matter + O2
Or in words:
That’s the photosynthesis we know and love, although bacteria use cellular “stuff” instead of leaves, and 3 to 4 billion years ago, plants weren’t a thing yet.
Chemistry happens, whether or not life has found a way to exploit the chemical reaction or might be harmed by it.
But we were talking about Hadean field conditions, before Biology joined the game, and something else needs mentioning…
- Heat from both the Sun (fainter than today) and the planetary interior (hotter than today).
This was absolutely necessary, since chemistry operates very slowly in the cold — for instance, on Titan, where the rocks and boulders are made of water ice.
A little groundskeeping
Some of the major processes that Physics and Chemistry started up during the Hadean are too sciencey and specific for our purposes — ocean stratification, for instance.
Others are more general and very familiar to us all, including:
- Weather, seasons, and climate. Relevant to cats? You bet.
As we’ll see later in the series, ancestral cats apparently evolved in semi-arid deserts somewhere in Asia; the Family Canidae, in a somewhat more humid environment (North American grasslands).
Animal adaptations for basic functions run deep, so today we buy dry kitty litter for Fluffy but must take our domesticated wolf Fido outdoors on a regular basis, rain or shine.
Yet after the non-avian dinosaurs and many other groups went away 66 million years ago, the world was pretty much one big forest from the Equator up/down into the north and south polar circles. (Prothero)
Long after T. rex had disappeared, much of what is now the US West grew trees like this during the Miocene epoch (from 23 Ma to around 5 Ma). As the world cooled and the local climate dried out for various reasons, the great sequoias fell, one by one, without leaving seedlings; this happened everywhere except in a few humid locations like this one, along the California coast. (Lyle et al.) It’s a basic evolutionary fact of life: the future belongs to those who show up for it. Who knows? Maybe climate will change again and redwoods might conquer the continent! But it will happen veeeery sloooowly.
Many gradual but profound environmental changes like this, along with competition, evolutionary history, and so forth, have helped natural selection (Simpson) to eventually come up with cats and wolves.
And nothing is more changeable than the solar-powered dance of air and water!
Climate — weather plus physical geography and time — counts, too, but it changes more slowly.
Getting back to field conditions for a moment, one reason for seasonal and climate shifts is the influence of long-term astronomical cycles (hat tip to James Croll as well as to Milutin Milankovitch for spotting that).
Consequences of these Milankovitch-Croll cycles are significant but mild, at least in the short term.
That wouldn’t be the case if the Moon weren’t there to stabilize Earth’s axis!
Don’t worry. Our planet isn’t going to tip over (thanks, Moon!).
And no meteorology or astronomy degree is needed, either.
It’s enough that we get a feel for how long Nature takes to turn rainforest into something resembling the Kalahari Desert (a modern semi-arid region that’s home to African wildcats and black-footed cats, among others) and for slight changes in Earth’s rotation and orbit to help bring on or end an ice age. (Agustí and Antón; Herbst; Strahler)
Tens of thousands to millions of years!
Evolution, which is insanely complex, responds to those changes, but this, too, happens over geological spans of time. (It also happens between generations, by the way (Simpson) — outside of FX work, you will never see a living being evolve before your very eyes.)
Artificial selection by humans is quicker, but getting back to our example, can Fido learn to ignore its evolutionary past and use the potty?
It looks like we’re the ones in training!
What if this walk in the snow was, for some reason, a matter of Pleistocene life or death? The presence of a companion would make it easier to struggle through. It’s very likely that pets helped our own evolution.
- Tectonics. Weather and climate surround us constantly. Tectonics underlies geography — the way most of us experience Earth (please let me hide in your luggage if you have a space tourism ticket).
You know, things like mountains (which support everything from snow leopards to bobcats); rift valleys (providing cheetahs with plenty of running room, lions with territory for their prides); and the high plains:
This Mongolian Pallas cat got the poor bird, but it might not get far. Hawks and other raptors are big enough to carry off this furry little kitty. Everyone wears camouflage out here! Insulated as we are by the successes of our “…drive … to get as far away from Nature as possible” (Gaiman and Pratchett), we forget how hard it is for hungry life to get enough energy to keep going until it can reproduce: another basic evolutionary requirement.
Tectonics gives the broader picture behind these familiar features of our world.
Take continents, for instance.
In Hadean/Archean times, they probably were flat and mostly underwater. Nevertheless, they rose high above the seafloor, just like today. (Taylor and McLennan)
Thanks to vertical tectonics like that, cats (and people) don’t have gills. There has always been land for us to evolve on.
Plate tectonics — pieces of Earth’s crust moving horizontally — didn’t go on during the Hadean (and was quite iffy over the next 2 billion years), but much later in Earth’s history it enabled cats to spread across the Northern Hemisphere after they evolved.
Because they couldn’t swim the ocean. No land connections existed yet with Africa, South America, and other parts of what had been the southern half of Earth’s most recent supercontinent, Pangea.
Here’s one expert’s reconstruction of Pangea:
Now here’s something to think about: plate tectonics has not been found anywhere else in the Solar System yet, although some planets and moons do seem to have active tectonics.
Plate tectonics might be both rare and vitally necessary to life on Earth.
And it didn’t have to happen.
But let’s wait until the next couple of posts to watch Earth somehow avoid the totally solid crust fate of the Moon, Mars, and many other Solar System neighbors.
This is still very much under debate, but here are two videos I found showing results from one of the many models of how that might have gone:
“Let me out!” — Mantle plume. “No.” — Crusty old Mars.
And brand-new Earth (wait for it, at around 1:40)
Important note: These are modelled superplumes, like those assumed by some researchers to have happened (spoiler) in the late Archean, not typical modern hot spots that burn through a tectonic plate.
Takeaway point: Tectonics seems very boring and academic to us laypeople, unless there’s an earthquake or eruption, but it affects every living being on the planet all the time.
- Weathering. Last one I’ll include, though there are many others at work.
Weathering is a crucial Earth process that we never notice, although it happens around us all the time.
It’s also impossible to describe in detail without sounding like a chemistry or physics major, but let’s try to sum it up in plain English.
Your house and car finish weather chemically, and Fluffy uses claws to mechanically weather the scratching post.
Houses, cars, and Fluffy didn’t exist on Hadean Earth, which was basically a great big chemistry set, with the Sun and its own core heat serving as Bunsen burners. Physics, Chemistry, and eventually Biology (next post) ran around in white coats.
Chemical weathering happened: 4.4-Ga-old zircon crystals show that.
How did it happen?
That’s also a big question because no one can be sure what the atmosphere was like back then.
But there was a hydrologic cycle: Water evaporated off the sea surface, condensed into clouds, dropped down, and flowed back toward the ocean to take this ride again.
Today, falling rain dissolves some atmospheric CO2 along the way, forming a weak carbonic acid solution and leaching the more easily dissolvable elements, like calcium, out of rocks as the raindrops splash down.
That elemental ion solution, a/k/a groundwater, then returns to the sea, where Chemistry and (nowadays) Biology bind up carbon and other ions into solid things like seashells and limestone.
Those just sit around until plate tectonics eventually disposes of them one way or another, thus locking up a surprisingly large amount of what was once the greenhouse gas CO2 for a very long time (and therefore keeping Earth cool).
When you encounter the term “carbon sink” in news stories or online, this is one of the processes they mean.
Back in the day mechanical weathering also happened from pounding ocean waves, earthquakes, impacts during the Heavy Bombardment, and any other Hadean/Archean forces strong enough to shatter rocks.
“Aha!” said Chemical Weathering, gazing at all those stony fragments, “more surface area for me to work on!”
“Ho ho!” Physics, Chemistry, and Metaphysics might have said, “more minerals and surface area to help organize the materials for life!”
The Hadean rock cycle was a little different from our own, since plate tectonics hadn’t begun yet. But volcanism always produced surface rock with mafic minerals for wind and water to busily weather back down into the sea.
These processes are actually very complicated, and there also must have been associated less obvious processes going on during the Hadean and Archean, just as there are today.
No one really knows what it was like.
Over millions of years Physics and Chemistry, left to themselves, would have set up the real-world equivalent of a complex, well-oiled machine, once Earth had stabilized from its fiery, violent birth.
There’s no room for life in such a carefully balanced/counterbalanced global system. Yet life found a way.
Probably by exploiting the fact that Earth’s core, even today, is as hot as the Sun, while the temperature of space is close to absolute zero.
That and more, in the next post.
Featured image: Murat Art/Shutterstock
Agustí, J., and Antón, M. 2002. Mammoths, sabertooths, and hominids: 65 million years of mammalian evolution in Europe. New York and Chichester: Columbia University Press.
Arndt, N. T., and Nisbet, E. G. 2012. Processes on the young Earth and the habitats of early life. Annual Review of Earth and Planetary Sciences, 40: 521-549.
Bell, E. A.; Boehnke, P.; Harrison, T. M.; and Mao, W. L. 2015. Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon. Proceedings of the National Academy of Sciences, 112(47): 14518-14521.
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.
Catling, D. C., and Zahnle, K. J. 2020. The Archean atmosphere. Science Advances, 6(9): eaax1420. https://www.science.org/doi/10.1126/sciadv.aax1420
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.
Durzyńska, J., and Goździcka-Józefiak, A. (2015). Viruses and cells intertwined since the dawn of evolution. Virology journal, 12(1), 1-10.
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.
Gaiman, N., and Pratchett, T. 2012. Good Omens, retrieved from https://play.google.com/store/books/details?id=-o-2KpQlFNsC .
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. https://royalsocietypublishing.org/doi/full/10.1098/rsta.2016.0353
Herbst, M. 2009. Behavioural ecology and population genetics of the African wild cat, Felis silvestris Forster 1870, in the southern Kalahari. PhD thesis, University of Pretoria.
Hohmann-Marriott, M. F., and Blankenship, R. E. 2011. Evolution of photosynthesis. Annual Review of Plant Biology, 62: 515-548.
Hsia, C. C.; Schmitz, A.; Lambertz, M.; Perry, S. F.; and Maina, J. N. 2013. Evolution of air breathing: oxygen homeostasis and the transitions from water to land and sky. Comprehensive Physiology, 3(2): 849.
Kharecha, P.; Kasting, J.; and Siefert, J. 2005. A coupled atmosphere–ecosystem model of the early Archean Earth. Geobiology, 3(2): 53-76.
Kitadai, N., and Maruyama, S. 2018. Origins of building blocks of life: A review. Geoscience Frontiers, 9(4): 1117-1153.
Klatt, J. M.; Chennu, A.; Arbic, B. K.; Biddanda, B. A.; and Dick, G. J. 2021. Possible link between Earth’s rotation rate and oxygenation. Nature Geoscience, 14(8): 564-570.
Knoll, A. H. 2014. Paleobiological perspectives on early eukaryotic evolution. Cold Spring Harbor Perspectives in Biology, 6(1): a016121.
Koonin, E. V. 2010. The origin and early evolution of eukaryotes in the light of phylogenomics. Genome Biology, 11(5): 1-12.
Lyle, M.; Barron, J.; Bralower, T. J.; Huber, M.; and others. 2008. Pacific Ocean and Cenozoic evolution of climate. Reviews of Geophysics. 46: RG2002.
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? https://www.earthmagazine.org/article/when-and-how-did-plate-tectonics-begin-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. https://www.pnas.org/content/109/14/E821.long
NASA. 2020a. Can we find life? https://exoplanets.nasa.gov/search-for-life/can-we-find-life/ Last accessed July 12, 2021.
___. 2020b. Life in our Solar System? Meet the neighbors. https://exoplanets.nasa.gov/news/1665/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. https://www.nasa.gov/press-release/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. https://www.nasa.gov/feature/then-there-were-3-nasa-to-collaborate-on-esa-s-new-venus-mission Last accessed July 12, 2021.
___. 2021b. Venus overview. https://solarsystem.nasa.gov/planets/venus/overview/ Last accessed July 12, 2021.
___. 2021c. The searchers: How will NASA look for signs of life beyond Earth? https://exoplanets.nasa.gov/news/1681/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? https://exoplanets.nasa.gov/news/1675/life-in-the-universe-what-are-the-odds/ Last accessed July 12, 2021.
___. 2021f. What’s out there? The exoplanet sky so far? https://exoplanets.nasa.gov/news/1673/whats-out-there-the-exoplanet-sky-so-far/ Last accessed July 12, 2021.
___. 2021e. Mars 2020 Perseverance rover. https://mars.nasa.gov/mars-exploration/missions/mars2020/ Last accessed July 12, 2021.
___. n.d. Europa Clipper: Ingredients for life. https://europa.nasa.gov/why-europa/ingr.edients-for-life/ Last accessed July 12, 2021
Oppenheimer, C. 2011. Eruptions That Shook the World. Cambridge: Cambridge University Press. Retrieved from https://play.google.com/store/books/details?id=qW1UNwhuhnUC
Palin, R. M., and Santosh, M. 2020. Plate tectonics: What, where, why, and when?. Gondwana Research. (PDF)
Prokoph, A.; Ernst, R. E.; and Buchan, K. L. 2004. Time-series analysis of large igneous provinces: 3500 Ma to present. The Journal of Geology, 112(1): 1-22.
Prothero, D. R. 2006. After the Dinosaurs: The Age of Mammals. Bloomington and Indianapolis: Indiana University Press. Retrieved from https://play.google.com/store/books/details?id=Qh82IW-HHWAC
Reddy, S. M., and Evans, D. A. D. 2009. Palaeoproterozoic supercontinents and global evolution: correlations from core to atmosphere. Geological Society, London, Special Publications, 323(1), 1-26.
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. http://m.cshperspectives.cshlp.org/content/2/6/a002527.long
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. https://royalsocietypublishing.org/doi/full/10.1098/rstb.2011.0129
Strahler, A. N. 1970. Introduction to Physical Geology, second edition. John Wiley & Sons.
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.
Waltham, D. 2015. Milankovitch period uncertainties and their impact on cyclostratigraphy. Journal of Sedimentary Research, 85(8): 990-998.
Wessner, D. R. 2010. The origins of viruses. Nature Education, 3(9), 37. https://www.nature.com/scitable/topicpage/the-origins-of-viruses-14398218/
Zahnle, K.; Schaefer, L.; and Fegley, B. 2010. Earth’s earliest atmospheres. Cold Spring Harbor Perspectives in Biology, 2(10): a004895. http://m.cshperspectives.cshlp.org/content/2/10/a004895.long