Hotblooded

Were dinosaurs warmblooded? More precisely, were they ectotherms, with low metabolic rates, like living reptiles, or endotherms, with high metabolic rates, like mammals and birds? (Yes, yes, you and I know that birds are dinosaurs, cladistically speaking, but you know what I mean.) And there are other possibilities: were the biggest dinosaurs, the sauropods, gigantotherms, keeping metabolic rates low, but staying warm through sheer size?

Endothermy is a big deal:

Elevated metabolic rates enable animals to remain active year-round at high latitude and altitude. They also enhance physiological performance, improve endurance, increase activity levels and facilitate rapid niche shift during environmental perturbations.

https://www.nature.com/articles/s41586-022-04770-6

Recently, it has become possible to address this question by looking at chemical signals of metabolic rates in fossil bones. The chart below (see link above) summarizes the results.

The upper branch of the tree are the diapsids – reptiles, dinosaurs, birds, and relatives. The lower branch is the synapsids ­– from dimetrodon way back in the day to mammals today. The chart shows that the earliest dinosaurs had high metabolic rates, as did the closely related early pterosaurs. And the sauropods were true endotherms. But some later dinosaurs actually gave up on endothermy: triceratops, stegosaurus, and the hadrosaurs (duck-billed dinosaurs) seem to be secondary ectotherms. Other dinosaurs went for more intense endothermy, like allosaurs and diplodocus and some close bird relatives.

In short, dinosaurs are diverse.

Coals to Newcastle

274 – 260 million years ago

It seems like Gaia really went on a bender in the late Carboniferous, getting drunk on oxygen. By some estimates, the atmosphere was over 30% oxygen back then, compared to 21% today. Living things took advantage of the opportunity. Insects apparently face an upper limit in size because they rely on diffusion through tracheas instead of forced respiration through lungs to get oxygen into their bodies. With more oxygen in the air, this limit was raised. The Carboniferous saw dragonflies with a wingspan up to 70 centimeters, and body lengths up to 30 centimeters, comparable to a seagull.

This happened because plants were turning carbon dioxide into organic matter and free oxygen, and the organic matter was accumulating. With carbon dioxide being removed from the atmosphere, the late Carboniferous and subsequent early Permian saw a reduced greenhouse effect, and global cooling. This was another Ice Age, with ice caps around the southern pole.

A lot of organic carbon ended up being buried. Much of the world’s coal, especially high quality anthracite, has its origin in Carboniferous tropical forests. Western Europe and eastern North America lay in the tropics at the time, and got a particularly generous allotment of coal. Three hundred million years later this bounty would fuel the early Industrial Revolution. (Thanks partly to some of my Welsh ancestors, who helped dig it up back in the day.) And here’s a song about coal mining, Dark as a Dungeon.

The boring billion

1.83-1.74 billion years ago

We’re now doing the history of the universe at the rate of 100 million years per day.

The Boring Billion* is a billion or so years, from maybe 1.85 to .85 billion years ago, in which Earth’s climate, ecology, and geography were relatively stable. For most of this period, life and Earth seem to have been locked into a very different set of chemical cycles than what we’re used to today.

Today we have a planetary fuel cell that keeps electron-hungry oxygen in the atmosphere and ocean separate from reduced carbon in minerals deep underground. Animals exploit this fuel cell by consuming and oxidizing organic matter. And a few centuries ago, human beings found another way of tapping Earth’s fuel cell: unearthing underground carbon and hydrocarbon stores, and oxidizing (burning) them to fuel the Industrial Revolution.

During the Boring Billion, however, a different, lower-energy, planetary fuel cell operated. There was some oxygen in the atmosphere – a few percent versus 21 percent today, not enough to make it breathable to us. But it’s likely that much of the ocean below a thin surface layer was anoxic, without free oxygen. Atmospheric oxygen still reached the ocean, but indirectly. On land, oxygen combined with sulfur compounds to yield sulfates. When these washed into the ocean, bacteria in the anoxic zone used them to produce hydrogen sulfide, the chemical that gives rotten eggs their bad smell.

The term of art for this combination of no oxygen and lots of sulfides is euxinia, named after the Euxine, or Black Sea. When the Black Sea flooded 7500 years ago, the decay of organic matter used up all the oxygen below the top 150 meters or so, creating the world’s largest marine Dead Zone. But during the Boring Billion, it looks as if the whole ocean was largely euxinic, a Canfield Ocean.

And it may be that the dominant mode of photosynthesis was different back then too, with purple and green sulfur bacteria exploiting hydrogen sulfide and releasing relatively little oxygen in the process. There are bacteria today that can switch between aerobic and anaerobic photosynthesis depending on the supply of hydrogen sulfide. These photosynthetic bacteria are distinct from true algae, which are not bacteria but eukaryotes with chloroplasts. On the latest evidence true algae evolved at least 1.2 billion years ago, maybe 1.6 billion. They and their multi-cellular descendants – green plants – would eventually (after some extreme Ice Ages) make Earth a very different place.

* The correct, boring name for the Boring Billion is the Middle Proterozoic.

Compost

2.17 – 2.07 billion years ago

I’ve got a pre-capitalist exchange relationship with the cafe down the street – they give me coffee grounds for my compost pile, I bring them flowers later. 

For a long time, before I set things up with the coffee shop, I just let chopped up leaves sit in a pile in a corner of the yard. Nothing much happened. Eventually I wised up and did some research: proper composting requires the right balance of carbon and nitrogen. The nitrogen and some of the carbon go into building organic molecules. Most of the carbon, though, combines with oxygen to make carbon dioxide, releasing energy in the process for the bacteria that make the compost. The amount of energy is appreciable: on a mild winter day, with air temperature just above freezing, the inside of my compost heap was the temperature of a lukewarm bath.

But composting goes back way longer than my compost heap, and way longer than Homo sapiens has been around. In fact, composting may have gotten its start more than 2 billion years ago. Here’s the story:

Back when I was in college, a guy I knew was taking a course in organic chemistry. He despaired of mastering the material in time for the next exam. Another student who’d already taken the course advised him, “Just remember, electrons go where they ain’t. Everything else is details” This is a pretty good starting point for thinking about how life and Earth have coevolved. Carbon, sitting in the middle of the periodic table, with four electrons to share, and four empty slots available for other atoms’ electrons, is exceptionally versatile. It can form super-oxidized carbon dioxide, CO₂, where carbon’s extra electrons fill in oxygen’s empty slots. Or it can form super-reduced methane, CH₄, where carbon’s empty slots are filled by hydrogen’s extra electrons. Or it can form carbohydrates, basic formula CH₂O, neither super-oxidized nor super-reduced, where the oxygen atom gives the carbon atom some extra empty slots and the hydrogen atoms give the carbon atom some extra electrons.

Chemically, life is pretty much about reducing and oxidizing carbon compounds. Nowadays, this means, on the one hand, photosynthesis – using the sun’s energy to go from carbon dioxide to carbohydrates – and, on the other hand, aerobic respiration – running the same reaction in reverse to release energy. But this modern arrangement took eons to develop. It depended on geochemistry, specifically on the evolution of what has been called a planetary fuel cell, with an oxidizing atmosphere and ocean physically separated from buried reduced minerals. Given a breach in this separation, the oxygen and the reduced minerals will react and release energy. In future posts we’ll see how such short-circuits in the planetary fuel cell, a result of major lava flows, are probably the cause of most major mass extinctions.

The development of the planetary fuel cell was erratic. One puzzling episode is the Lomagundi Event, where oxygen levels went up 2.2 billion years ago and then back down 2.08 billion years ago. One theory is that the Event began with photosynthetic cyanobacteria pumping oxygen into the atmosphere. When these bacteria died, they just piled up, leaving a growing accumulation of organic matter in the world’s oceans. According to this theory, the end of the Event happened when other bacteria discovered they could oxidize the accumulated organic matter, and used up a good part of the atmosphere’s oxygen. In other words, the end of the Lomagundi Event marks the invention of composting.

The return to higher oxygen levels in the atmosphere would take some time, and deposition of reduced sediments.

Matchmaking life

Oxygen’s got a brand new pair of roller skates, hydrogen’s got a brand new key.

You can find romance if you want in the periodic table, with elements on the right side of the table, just an electron or two short of a full shell, cruising for partners over on the left side, with an electron or two to share. The folk-singing sisters Anne and Kate Macgarrigle made a song out of this, NaCl, about a love affair between a chlorine atom and a sodium atom. “Think of all the love you eat when you salt your meat.”

And living things are matchmakers, middlemen making a living by arranging liaisons between the two sides of the periodic table, harvesting the energy released by moving electrons around. The basic mechanism is shown below.

Take a reducing compound willing to give up an electron and an oxidant looking for an electron. Electrons go where they ain’t: starting on the left, electrons (e-) from the first compound pass through a series of protein complexes (grayish circles) embedded in a membrane (horizontal lines). The passage of electrons from one complex to the next in the membrane powers the movement of protons (H⁺) from one side of the membrane to the other. For each pair of electrons passing down the chain, ten protons are moved across the membrane. Finally the electrons are united with the oxidant (shown as O₂ generating H₂O). Electrons keep being pulled through the chain as long as reducing and oxidizing agents are available.

This is the first part the job. The result is a lot of extra protons on one side of the membrane and a large difference in electrical potential between the two sides. For the second part, another protein complex (ATP synthase) uses this potential difference to turn ADP (adenosine diphosphate) into ATP (adenosine triphosphate). ATP is the fundamental energy carrier of life, powering the cell’s chemical reactions when it turns back into ADP. (Creatine supplements work for athletes and bodybuilders because creatine helps with ATP synthesis.)

This is the basic mechanism of respiration (and most of photosynthesis), just as DNA replication is the basic mechanism of heredity. Understanding the origin of this mechanism is a major challenge in understanding the origin of life. Nick Lane walks through some of the theories.

Up this point in Earth’s history, living things are tiny and not very interesting structurally. But they are hugely diverse biochemically, making use of a great array of different reducing and oxidizing compounds. For example, methanogens use hydrogen seeping from undersea vents to reduce carbon dioxide, producing methane. This may have been of some importance to the planet as a whole. Methane is a greenhouse gas, trapping infrared radiation even more effectively than carbon dioxide. The Sun three billion years ago was fainter than it is today; it may be methanogens that kept a young Earth from freezing over.

The curve of binding energy

9.86 – 9.33 billion years ago

More on stardust and us.

Looking at the abundance of different elements in the universe, we get the following:

Note that the vertical scale is exponential. Each tick marks a hundred-fold increase in abundance over the tick below, so there is vastly more hydrogen and helium in the universe than any other element. As noted in the last post, all the elements except hydrogen and helium were formed after the Big Bang, spewed out by supernovas and the collisions of neutron stars. In general, heavy elements are less abundant because it takes more steps to produce heavy elements than light ones. But the curve is not smooth. The lightest elements after hydrogen and helium (lithium, beryllium, boron) are relatively rare, because they get used up in the nucleosynthesis of heavier elements. And there is a saw tooth pattern in the chart, because nucleosynthesis favors atoms with even numbers of protons. So we get lots of oxygen, magnesium, silicon, and iron, the main constituents of our planet. Lots of carbon too. Finally, iron (Fe) is more than 1000 times more abundant than might be expected based on a smooth curve. Iron nuclei are especially stable because binding energy, the energy that would be required to take the nucleus apart into its constituent protons and neutrons, reaches a maximum with iron. Here’s the famous curve of curve of binding energy (nucleons are protons and neutrons):

An implication of this curve is that if you can split a really heavy nucleus, of Uranium-235 say, into smaller nuclei (but still heavier than iron), you will release energy equal to the vertical difference between U-235 and its lighter fission products (not shown) on the vertical scale. This is lots of energy, way more than you get from breaking or forming molecular bonds in ordinary chemical reactions. And if you can fuse two light nuclei, of hydrogen say, into a larger nucleus, you can get even more energy. When we split uranium, we are recovering some of the energy that colliding neutron stars put into synthesizing the heaviest elements. When we fuse hydrogen, we are extracting energy left over from the Big Bang that no star got around to releasing. (This doesn’t violate the Law of Conservation of Energy, because the negative gravitational potential energy of the universe cancels the positive energy represented by the matter. So the total energy of the universe is zero.)

Starting to figure this all out was part of a scientific revolution that made physics in 1950 look very different from physics in 1900. The new physics resolved a paradox in the study of prehistory. Geologists were pretty confident, based on rates of sedimentation, that the Earth had supported complex life for hundreds of millions of years. But physicists couldn’t see how the sun could have kept shining for so long. The geologists were right about deep time; it took new physics to understand that the sun got its energy from fusing hydrogen to helium (via some intermediate steps).

As the scientific revolution in atomic physics was picking up steam, it was natural to assume that it would be followed by a revolution in technology. After all, earlier scientific revolutions in the understanding of masses and gases, atoms and molecules, and electrons and electromagnetism, had been followed by momentous innovations in technology: the steam engine, artificial fertilizers, electrification, radio, to name just a few. But in some ways, the Atomic Age hasn’t lived up to early expectations. The atom bomb brought an earlier end to the Second World War, but didn’t change winners and losers. The bomb was never used again in war, and it’s a matter for debate how much the atom bomb and the hydrogen bomb changed the course of the Cold War. Nuclear energy now generates a modest 11% of the world’s electricity (although this number had better go way up in the future if we’re serious about curbing carbon dioxide emissions). And a lot of ambitious early proposals for harnessing the atom never got anywhere. Project Plowshare envisioned using nuclear explosions for enormous civil engineering projects, digging new caves, canals, and harbors. Even more audacious was Project Orion, which developed plans for a rocket propelled by nuclear explosions. Some versions of Orion could have carried scores of people and enormous payloads throughout the solar system. Freeman Dyson, a physicist who worked on the project, said “Our motto was ‘Mars by 1965, Saturn by 1970.’”

On the purely technical side these plans were feasible. There were concerns about fallout, but the problems were not insurmountable. Nevertheless both Plowshare and Orion were cancelled. Regarding Orion, Dyson said “… this is the first time in modern history that a major expansion of human technology has been suppressed for political reasons.” The history of the Atomic Age  and its missed opportunities is one more refutation of pure technological determinism. How or even whether a new technology is exploited depends on social institutions, politics, and cultural values.

Speaking of which, Where Is My Flying Car?

Enjoy it while it lasts

105 – 209 CE

If a man were called to fix the period in the history of the world, during which the condition of the human race was most happy and prosperous, he would, without hesitation, name that which elapsed from the death of Domitian [96 CE] to the accession of Commodus [180 CE]. The vast extent of the Roman empire was governed by absolute power, under the guidance of virtue and wisdom. The armies were restrained by the firm but gentle hand of four successive emperors, whose characters and authority commanded involuntary respect.

Edward Gibbon, The Decline and Fall of the Roman Empire, Chapter 3

Gibbon doesn’t include China in this assessment of the state of the world, but for that country too, under the Eastern Han dynasty, there was a period of stability and prosperity, lasting from the death of the usurper Wang Mang in 24 CE to the outbreak of the Yellow Turban peasant uprising in 184 CE. During this time, the Roman and Han empires so completely dominated their respective portions of Eurasia that they enjoyed relative peace. Toward the end of the second century CE, both empires had populations around 50-60 million; world population was perhaps 190 million. In the succeeding centuries both empires would experience major population declines and political collapse. As a result, the world’s total population may have declined as well.

Ian Morris’ attempt to quantify historical progress, set forth in his book The Measure of Civilization, tends to corroborate Gibbon. Roman diets and biological well-being may or may not have been very good, but their standard of living, measured by energy capture – the energy embodied in livestock, household goods, dwellings, public buildings, infrastructure, and so on – may have reached heights unmatched in the West for a millennium.

Of course Gibbon’s view is a retrospective one, and didn’t anticipate the vast rise in standards of living that eventually followed the industrial revolution.

(After this I’ll give dates as numbers without the “CE”.)

Quest for fire, or, Eisenhower steak

833 – 789 thousand years ago

This post, and some subsequent tweets, are a few days late. I was touring wineries in the Sunnyslope region of Idaho, a little ways outside Boise. Recommended, if you get a chance; you could pick up some wines that would pair well with Eisenhower steak (recipe below).

On June 3 on Logarithmic History, our ancestors had gotten as far as steak tartare. Now it’s time for an Eisenhower steak (cooked directly on the coals; see below).

What really distinguishes humans from other animals? We’ve covered some of the answers already, and will cover more in posts to come. But certainly one of the great human distinctions is that we alone use fire. Fire is recognized as something special not just by scientists, but in the many myths about how humans acquired fire. (It ain’t just Prometheus.) Claude Lévi-Strauss got a whole book out of analyzing South American Indian myths of how the distinction between raw and cooked separates nature from culture. (I admit this is where I get bogged down on Lévi-Strauss.)

Until recently the story about fire was that it came late, toward the latter days of Homo erectus. But Richard Wrangham, a primatologist at Harvard, turned this around with his book Catching Fire (which is not the same as this book), arguing that the taming of fire goes back much earlier, to the origin of Homo erectus. Wrangham argues that it was cooking in particular that set us on the road to humanity. Cooking allows human beings to extract much more of energy from foods (in addition to killing parasites). Homo erectus had smaller teeth and jaw than earlier hominins and probably a smaller gut, and it may have been fire that made this possible. Cooking is also likely to have affected social life, by focusing eating and socializing around a central place. (E O Wilson thinks that home sites favored intense sociality in both social insects and humans.)

Surviving on raw food is difficult for people in a modern high-tech environment and probably impossible for people in traditional settings. Anthropologists are always looking for human universals, and almost always finding exceptions (e.g. the vast majority of societies avoid regular brother-sister marriage, but there are a few exceptions, including Roman Egypt and Zoroastrian Iran). But cooking seems to be a real, true universal. No society is known where people got by without cooking. Tasmanians, isolated from the rest of the world for 10,000 years, with the simplest technology of any people in recent history, had supposedly lost the art of making fire, but still kept fires going and still cooked.

Recent archeological finds have pushed the date for controlled use of fire back to 1 million years ago, but not all the way back to the origin of Homo erectus. This doesn’t mean Wrangham is wrong. Fire sites don’t always preserve very well: we have virtually no archeological evidence of the first Americans controlling fire, but nobody doubts they were doing it. It could be that it will be the geneticists who will settle this one. The Maillard (or browning) reaction that gives cooked meat much of its flavor generates compounds that are toxic to many mammals but not (or not so much) to us. At some point we may learn just how far back genetic adaptations to eating cooked food go.

An alternative to an early date for fire, there is the recent theory that processing food, by chopping it up and mashing it with stone tools, was the crucial early adaptation.

Whenever it is exactly that humans started cooking, the date falls in (Northern hemisphere) grilling season on Logarithmic History, so you can celebrate the taming of fire accordingly. It doesn’t have to be meat you grill. Some anthropologists think cooking veggies was even more important. I recommend sliced eggplant particularly, brushed with olive oil to keep it from sticking, and with salt, pepper, and any other spices.

On the other hand, Homo erectus probably appreciated a good Eisenhower steak, cooked directly on the coals. (Yes, this actually works pretty well.)

Eisenhower Coal-Fired Steak

Named for the 34th president of the United States, who liked to cook his steaks directly on the coals, this preparation will create a crunchy, charred exterior with rosy, medium-rare meat inside.

Lump hardwood coals work better than briquettes for this recipe because they burn hotter. Be sure you use long-handled tongs. (Sorry, this method is for charcoal or wood grilling only.)

You might find an uneven exterior crust, especially when using lump charcoal, because it is irregularly shaped (unlike the uniform briquette pillows). If that happens, try to position the steak so that it is more directly on the coals and gets an even char. Clasp the steak in the tongs and rap the tongs against the edge of the grill to knock off the occasional clinging ember. If you have some ash, flick it off with a pastry brush.

Make Ahead: The steaks can be seasoned and refrigerated up to 4 hours in advance. Bring them to room temperature before they go on the fire.

INGREDIENTS

• 1 teaspoon olive oil
• Two 1 1/2-inch-thick boneless rib-eye steaks (about 28 ounces total)
• 2 teaspoons coarse sea salt
• 2 teaspoons freshly cracked black pepper

---

DIRECTIONS

Prepare the grill for direct heat. Light the charcoal; when the coals are just covered in gray ash, distribute them evenly over the cooking area. For a hot fire (450 to 500 degrees), you should be able to hold your hand about 6 inches above the coals for 2 or 3 seconds. Have a spray water bottle at hand for taming any flames. But use it lightly; you don’t want to dampen the heat too much, and some flames here are fine.

Meanwhile, brush the oil on the both sides of the steaks, then season both sides liberally with salt and pepper.

Once the coals are ready, place the steaks directly on the coals (see headnote). Cook, uncovered, for 6 minutes on one side, then use tongs to turn them over. Cook for about 5 minutes on the second side.

Transfer the steaks to a platter to rest for 10 minutes. Serve as is, or cut them into 1/2-inch-thick slices.

Calories and curves

1.31 -1.24 million years ago

This figure is from a neat recent paper comparing energy expenditure (TEE or Total Energy Expended) and fat among humans and our closest relations: chimpanzees (genus Pan), gorillas (Gorilla), and orangutans (Pongo). (The numbers are adjusted for differences in overall body mass.)

What stands out here is that humans are a high energy species. Also we carry a lot more body fat than the other great apes. This applies particularly to women, who need a lot of extra fat to meet the high energy demands of human infants. But it even applies to men. For both sexes, a high energy life style means you want to carry around an extra reserve of fat in case of emergencies.

We don’t know how long ago our ancestors decided to crank up their energy consumption. Maybe back with the rise of Homo erectus (just a few days ago on Logarithmic History). Or maybe later, when the typical modern human pattern of slow maturation was more firmly in place. At some point in the near future, we’ll actually nail down the specific genetic changes leading humans to accumulate more fat, and be able to put a date on the change. It may be that the distinctively human mating system also arose back then, with human females concealing ovulation (no chimp-style monthly sexual swellings) but advertising nubility (with conspicuous fat deposits appearing at puberty).

A high energy life-style also goes with extensive food sharing and changes in human kinship. (Here’s me, on beating Hamilton’s rule through socially enforced nepotism.)

Hotblooded

Were dinosaurs warmblooded? More precisely, were they ectotherms, with low metabolic rates, like living reptiles, or endotherms, with high metabolic rates, like mammals and birds? (Yes, yes, we both know that birds are dinosaurs, cladistically speaking, but you know what I mean.) And there are other possibilities: were the biggest dinosaurs, the sauropods, gigantotherms, keeping metabolic rates low, but staying warm through sheer size?

Endothermy is a big deal:

Elevated metabolic rates enable animals to remain active year-round at high latitude and altitude. They also enhance physiological performance, improve endurance, increase activity levels and facilitate rapid niche shift during environmental perturbations

https://www.nature.com/articles/s41586-022-04770-6

Recently, it has become possible to address this question by looking at chemical signals of metabolic rates in fossil bones. The chart below (see link above) summarizes the results.

The upper branch of the tree are the diapsids – reptiles, dinosaurs, birds, and relatives. The lower branch is the synapsids ­– from dimetrodon way back in the day to mammals today. The chart shows that the earliest dinosaurs had high metabolic rates, as did closely related early pterosaurs. And the sauropods were true endotherms. But some later dinosaurs actually gave up on endothermy: triceratops, stegosaurus, and the hadrosaurs (duck-billed dinosaurs) seem to be secondary ectotherms. Other dinosaurs went for more intense endothermy, like allosaurs and diplodocus and some close bird relatives.

In short, dinosaurs are diverse.