Gould’s Belt

29.2 – 27.7 million years ago

Logarithmic History has had a lot of geology and biology lately, not so much astronomy. But all is not peaceful in the heavens.

Benjamin Gould is a nineteenth century astronomer who noted that a lot of bright stars in the sky — especially the bright blue stars that we know are very young — seem to fall along a ring tilted at a 20 degree angle to the Milky Way. This ring has come to be called Gould’s Belt (or the Gould Belt). The Belt is an ellipse about 2400 by 1500 light years across where there has been a recent wave of star formation. Our Sun lies within the belt, somewhat off center; the center lies in the direction of the Pleiades.

The Belt began forming maybe thirty million years ago. We’re not sure what happened. A supernova may have set off a wave of star formation, but it would have to have been a huge one. Or it may be that a gas cloud or a clump of dark matter passed at an angle through our part of the Milky Way, and started stars forming with its shock wave. There are features resembling Gould’s Belt in other galaxies. In any case, the Belt is one of the really striking features of our part of the Milky Way.

Whatever its cause, no one disputes its magnificence. Gould’s belt is the most prominent starry feature in the Sun’s neighborhood, contributing most of the bright young stars nearby. Nearly two thirds of the massive stars within 2,000 light-years of the Sun belong to Gould’s belt. If I were kidnapped by an alien spaceship and taken to some remote corner of the Galaxy, Gould’s belt is what I’d look for to find my way back home.

Ken Crosswell. Gould’s Belt.

If you’re in the Northern hemisphere you can look at the sky tonight and see the Milky Way in an arc in the Western sky, stretching from North to South. West of the Milky Way you’ll see some of Gould’s belt, an arc of bright stars running north to south from the Pleiades, through Taurus and the bright stars of Orion, and Canis Major. So tonight look at the stars, and drink a toast if you want, to your ape ancestors, who were just on the cusp of splitting off from monkeys thirty million years ago.

Dead baby monkeys

There’s a dark side to being a primate. A few years back a review article summarized data on rates of lethal aggression in non-human animals. The figure below shows some of the results. Several clusters of especially violent species stand out in the figure, including primates (redder is more violent). Bats are pretty nice, though (too bad about all the viruses).

Much of the lethal aggression in primates involves infanticide. Sarah Hrdy demonstrated back in the 1970s that infanticide occurs regularly in Hanuman langurs, monkeys in India. A male who takes over a group of females will systematically kill offspring sired by the previous male. If you think evolution is about the survival of the species, this is hard to explain. But it makes sense given the logic of the selfish gene. Females who lose an infant return more quickly to breeding again, and the father of the next infant is likely to be the killer of the previous one.

Primates may be particularly vulnerable to this grim logic, because they spend a long time as infants. Among primates, commonly,

           L/G>1

That is to say that the time, L, a female spends lactating for an infant (during which she is unlikely to conceive), is usually greater than the time, G, she spends gestating an infant. This puts particular pressure on males to hurry things along by eliminating nursing infants fathered by other males.

Death of Astyanax

As a result, infanticide is relatively common among primates, and females under particularly strong pressure to find ways to avoid it. Hanuman langurs live in one-male units, where a female has little choice about who she mates with. In other species, by contrast (most baboons, chimpanzees), multiple males reside with multiple females. In these species females are often sexually promiscuous, sometimes actively soliciting multiple males for sex. This is probably mostly a matter of confusing paternity sufficiently to suppress the threat of infanticide. There’s a general lesson here: females are not always monogamously inclined, but female promiscuity generally has different evolutionary roots than male promiscuity.

Ground up monkey brains

Short version: It looks like most mammals, at least most large mammals, have the brains they need, while primates, especially large primates, have the brains they can afford.

Longer version: One reason for being interested in monkeys is that they’re brainy mammals. Here’s the conventional graph illustrating that:

Larger mammals tend to have larger brains, but the relationship is non-linear. Multiplying body mass by x doesn’t multiply brain mass by x. Instead it multiplies brain mass by about x^.75. In other words, Brain Mass is proportional to (Body Mass)^.75. Equivalently (taking the logarithm of both sides) Log[Brain Mass] is equal to .75 times Log[Body Mass], plus a constant. So Log[Brain Mass] plotted against Log[Body Mass] gives a straight line with a slope of .75. That means that if one mammal has 16 times the body mass of another, it’s expected to have 8 times the brain mass, 10,000 times the body mass means 1000 times the brain mass, and so on. The thing to note is that primates defy expectations. They have larger brains than would be expected based on their body sizes.

But we’ve recently learned that primates – especially big ones – are even more special than this graph suggests. Susan Herculano-Houzel has pioneered a technique that involves chopping up brains (or parts of brains), dissolving their cells to make a kind of brain soup, and counting cell nuclei. This allows her to estimate how many neurons there are in different brains.

Major findings: Among most mammals, the number of neurons increases more slowly than brain size. Increase brain size by x, and you increase number of neurons by about x^.67. (H-H shows this flipped around. Increase number of neurons by x and you increase brain mass by x^1.5.) But primates are exceptional; the relationship is nearly linear. An x-fold increase in primate brain size corresponds to about an x-fold increase in number of neurons. Humans follow the primate rule here. We have about the same density of neurons as other primates. When you combine the exceptionally large brain sizes of humans (exceptional even relative to our brainy primate relations) with a standard high primate neuron density, you get an animal with an enormous number of neurons. By contrast, a rodent with a human sized brain, if it followed rodent rules for how neuron numbers increase with brain size, would have only 1/7 as many neurons.

Neurons are expensive. Most large animals economize by cutting back on neuron density. A cubic centimeter of cow brain has fewer neurons, and consumes energy at a lower rate, than a cubic centimeter of mouse brain (although of course the cow’s brain is bigger). By contrast, large primates are extravagant, devoting exceptionally large energy budgets to running their brains. And human brains are exceptionally costly. An important question for the study of human evolution is how we paid the bill for such costly brains. That’s a story for later. But another part of the story starts back in the early Cenozoic, when monkeys committed to a different set of rules for building brains.

And here is a chart giving absolute numbers of  cortical neurons (cneurons) for a bunch of species. Scott Alexander has some thoughts about the moral implications. Short version: skip the pork for dinner (and skip the elephant, chimp, and manflesh. But you knew that). Beef might be okay. Better is lobster.

And Werner Herzog is probably okay with you eating chicken.

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Bird life

So we rode all around the park until quite late talking and philosophizing quite a lot and I finally told him that I thought, after all, that bird life was the highest form of civilization. So Gerry calls me his little thinker and I really would not be surprised if all of my thoughts will give him quite a few ideas for his novels. Because Gerry says he has never seen a girl of my personal appearance with so many brains. 

“Gentlemen Prefer Blondes” Anita Loos

Maybe bird life is not the highest form of civilization. But a recent book, The Parrot in the Mirror: How Evolving to be Like Birds Makes Us Human, makes the case that primates in general, and humans in particular, are special in ways that show convergent evolution with birds. To wit:

Vision: Mammals in the Mesozoic Era were largely nocturnal. With the great dinosaur extinction, some mammals moved into diurnal niches. Primates take this further than most. Like birds, primates are highly visually oriented. Birds have exceptional color vision, with four types of color-sensitive cone cells in their retinas. Most mammals have just two. But Old World monkeys and apes (and you, unless you are colorblind) have three types of cones. On the other hand, birds and primates are less attuned to smells than most mammals.

Longevity: Birds are long-lived relative to mammals. At any given size, a bird is likely to live maybe twice as long as a typical mammal. (Something to consider when choosing a pet.) Primates are also longer lived than most mammals, and human beings long-lived even among primates. Birds can afford to slow down their life histories and senesce more slowly, because being able to fly puts them at lower risk from predators. Primates too live life in the slow lane, relying on brains and sociality to cut down on the predation and other extrinsic mortality that push many mammals to live fast and die young.

Brains: Birds have relatively small, i.e. light-weight, brains, as part of being lightly built for flight. But their small brains pack in lots of smaller neurons compared a similar size mammal brain.

[C]ompared with mammals, very small birds, with similarly small brains, will have many more neurons than similarly sized mammals. Small songbirds can weigh in at barely a tenth the weight of a common mouse, but sport more than double the number of neurons. Meanwhile, some of the heaviest bird brains, which are found in the macaws, the big, colourful South American parrots, weigh in at perhaps 20–25 grammes. This is a bit bigger than the brain of a common European rabbit. Yet while the rabbit has about half a billion neurons in its whole body, the macaw can have over three billion in its brain alone, a number more in line with much larger giraffes and baboons There are really only four types of animal that get into the billions of neurons: whales, large mammals such as elephants and seals, primates, and brainy birds (parrots and crows).

https://www.amazon.com/Parrot-Mirror-evolving-birds-makes/dp/019884610X/

Pair bonds. Many birds, especially passerines (perching birds, including songbirds = most bird species) pair up, with males and females cooperating to care for offspring. There’s only so much nutrition you can pack into an egg, so baby birds are often pretty helpless, and need two parents to take care of them. Human infants too are pretty helpless; there’s only so big a fetus can get before birth. And children, growing slowly, take a long time to become independent. So human mothers too commonly have to enlist helpers in providing for their offspring.

Vocal communication. Among birds, parrots, hummingbirds, and songbirds show exceptional vocal learning abilities. (Among mammals, the closest nonhuman exemplars are cetaceans.) In birds, vocal learning may happen during juvenile sensitive periods (in humans: think about learning the local accent in childhood) or may be more open-ended (in humans: think about adding to your vocabulary throughout life). Bird vocalizations can be socially transmitted, and different communities can develop distinctive dialects.

And even among birds, parrots are really exceptional. The Parrot in the Mirror gives them a whole chapter to themselves. Parrots are a pinnacle in the evolution of intelligence. 

And they’ve got rhythm.

Age of mammals

Linnaeus chose one trait – mammary glands / lactation – to define the order Mammalia. This was not a purely scientific decision. Like many authorities in eighteenth century Europe, he was concerned that the common practice of wet-nursing was unnatural and dangerous, and he wrote a pamphlet urging the advantages of women nursing their own infants.

But mammals do not owe their Cenozoic success to any one trait. True, there is a central theme in mammalian evolution.

[T]he over-arching attribute manifested by the origin of the mammals is increasing homeostatic ability: the maintenance of a constant internal environment in the face of a fluctuating external environment, by means of high-energy regulatory processes (Kemp p. 18)

But this homeostatic ability is supported by a whole series of interrelated traits that evolved in tandem. Here’s a summary diagram. 

Evolving a whole set of coordinated traits like this is a much slower business than optimizing a single trait. It is a matter of correlated evolution, in which small changes in one character allow for small changes in other characters, along an “adaptive ridge.”

It took several hundred million years, from synapsids, to therapsids, to cynodonts, to mammals, to put the mammalian package together. And even after mammals had appeared and begun to diversify, it would take an extraordinary catastrophe at the end of the Cretaceous before the Age of Mammals would really begin.

For an excellent popular introduction, try I Mammal: The Story of What Makes Us Mammals

Life at sea: whales and sailors

48.3 – 45.8 million years ago

The end-Cretaceous mass extinction knocked off not only the dinosaurs (except for birds), but also air-breathing marine predators like mososaurs and plesiosaurs. Birds and mammals started moving into the empty niche: penguins from early on, and eventually whales.

(Cartoon by Sam Gross. Not scientifically accurate.)

People around the world seem to be naturally inclined to distinguish major animal life forms according to whether they walk, fly, swim, slither, or creep, so evolutionary shifts in modes of travel – the origin of flight, the return to the sea – really catch people’s imagination – and provoke Creationists. The whale story is particularly dramatic. When Darwin was tried to account for the evolution of whales from a land-dwelling ancestor, he cited accounts of bears swimming and feeding in water, and wrote “I can see no difficulty in a race of bears being rendered, by natural selection, more and more aquatic in their structure and habits, with larger and larger mouths, till a creature was produced as monstrous as a whale.” This statement attracted so much ridicule that Darwin took it out of later editions of The Origin of Species. But he turns out to have been very much on target. We now have a great sequence of whale ancestors. The sequence runs from today’s Pakicetus — a wolf size meat-and-fish eater that splashed along the shores of the ancient Tethys sea separating Africa from Eurasia — to the “walking whale,” Ambulocetus, and on to true whales. We have even begun to detail some of the genetic changes that went with the return to the sea. Darwin was sort of on the right track thinking of bears, but anatomy and genetics put the ancestors of whales firmly among artiodactyls – hooved animals including hippos, pigs, and cows.

Whales are famously large. Marine mammals in general tend toward bigness: one theory is that large body size (low ratio of surface area to volume), and an insulating layer of blubber, are adaptations to reduce heat loss. Whales, particularly baleen whales, take it further with dietary adaptations that let them get huge.

Remarkably there may be a parallel in human evolution. Polynesians have the largest body sizes of any living people, and this too may be an adaptation to conserve heat in a maritime environment.

The Polynesian people who settled a wide area of the tropical Pacific have a large and muscular body phenotype that appears to contradict the classical biological rules of Bergmann and Allen. However, a scrutiny of the conditions actually experienced by these canoe voyagers and small-island dwellers suggests that in reality the oceanic environment is labile and frequently very cold, and from it tribal technology offered little protection. The Polynesian phenotype is considered to be appropriate to, and have undergone selection for, this oceanic environment.

Strange relations and island continents

54.0 – 51.1 million years ago

We’ve seen a great many catastrophes in the history of life, and been reminded of the role of sheer chance in evolution. But the Cenozoic also sees a dramatic adaptive radiation and the steady progress of arms races among survivors of the great dinosaur die-off. Four large scale groupings of placental mammals have already appeared: Afrotheres (aardvarks, hyraxes, elephants, and sea cows), Xenarthrans (anteaters, armadillos, and sloths), Laurasiatheres (shrews, hedgehogs, pangolins, bats, whales, hoofed animals, and carnivores), and Supraprimates (aka Euarchontoglires, including rodents, tree shrews, and primates). This grouping of mammals is anything but obvious – it’s only with DNA sequencing that it has emerged. What’s noticeable is the association with different continents: Afrotheres with Africa, Xenarthrans with South America, and the others with the monster content of Laurasia (Eurasia and North America). Looking beyond placental mammals we see other continental associations: marsupials flourish in South America and Australia, and giant flightless “terror birds” carry on rather like predatory dinosaurs in South America.

There is a pattern here. Evolutionary arms races are most intense in the supercontinent of Laurasia (eventually joined by India and Africa). The island continents of South America and Australia stand apart, and they fare poorly when they start exchanging fauna with the rest of the world. We’ll see a similar pattern – large areas stimulate more competition, and more intense evolution, isolated areas are at a disadvantage – when we look at modern history, with ocean voyages effectively reuniting Pangaea. (This is a major theme of Alfred Crosby’s Ecological Imperialism and Jared Diamond’s Guns, Germs, and Steel.)

The worst day in the history of the Earth

66.5 million years ago

This April 5 on Logarithmic History marks the most famous mass extinction ever, the one that did in the dinosaurs (okay, okay, the non-avian dinosaurs).

Just a few years back, we had news of one of the most extraordinary fossil discoveries ever, in North Dakota: a graveyard of fish piled on one another by a tsunami-like wave, and mixed with burned trees, and the remains of mammals, mososaurs, ammonites, and insects, and a partial triceratops, formed within hours of the asteroid impact that wiped out most life on Earth. Here is a news release, 66 million-year-old deathbed linked to dinosaur-killing meteor, and here is an article from the New Yorker, The Day the Dinosaurs Died.

The end-Cretaceous extinction isn’t the biggest ever, but it’s the one everybody knows about. The Disney movie Fantasia (1940) did a version of the event, set to Stravinsky (and mixing up Jurassic and Cretaceous dinosaurs). In Terence Malik’s movie The Tree of Life, a predatory dinosaur discovers compassion in an encounter with a hadrosaur just before all their kind are wiped out by an asteroid: mass extinction meets the Book of Job.

The discovery that dinosaurs (and about 70% of all species in total) probably went extinct as a result of an extraterrestrial impact did more than anything else to bolster catastrophism. For most of the history of modern geology, geologists have mostly argued instead for uniformitarianism: the same slow processes we see today caused past geological and evolutionary changes. When evidence for an impact was first discovered – a thin layer of iridium, presumably extraterrestrial — paleontologists were pretty uniformly hostile: no physicist was going to tell them how to do science. But by now the evidence is overwhelming that the asteroid impact that left the Chixculub crater, in what is now the Yucatan, was largely responsible for the end-Cretaceous extinctions (although the volcanic eruptions that created the Deccan traps in India may also have played a role).

But at the same time that evidence has increasingly vindicated the catastrophist position, new discoveries in paleontology have increasingly brought home that one group of dinosaurs survived the extinction. Most people think of birds and dinosaurs as two quite distinct kinds of animal. But birds are just as much dinosaurs as bats are mammals. Many dinosaurs had many of the distinctive features of birds – warm-bloodedness and high metabolic rates (probably), wishbones, an advanced respiratory system, feathers (sometimes brightly colored, sometimes used for courtship), and parental care for nests of eggs and juveniles. It’s even possible that some flightless dinosaurs, like the turkey-sized Caudipteryx, were secondarily flightless, descended from flying ancestors like Archeopteryx. We don’t have to hope for The Lost World or Jurassic Parkto come true to see living dinosaurs; a trip to the park, with The Sibley Guide to Birds (or appropriate guidebook for your region) in hand, will do it.

Leaves of grass

71.5 – 67.7 million years ago

Not as dramatic as the evolution of Triceratops or T. rex, but of more lasting consequence, is the evolution of grasses (Poaceae). We know from coprolites – fossil feces — that grass was around by the Late Cretaceous, so the coevolution of grass and grazers had already begun with dinosaurs. These early grasses were not widespread. It would take climate shifts and more evolution (toward using carbon dioxide more efficiently) to create the sort of grasslands we are familiar with.

Grasses have played a central role in human evolution and human history. Human beings evolved in tropical grasslands, and some evolutionary psychologists think we still have an instinctive affinity for this environment. The domestication of grasses (wheat, barley, oats, millet, rice, corn) was one of the great revolutions in human prehistory, and grasses provided most of the calories people ate for most of recorded history. Contact along the frontier between grasslands supporting pastoralists and grain growing lands supporting peasants is one of the great engines of historical dynamics.

Grasses grow from the base of the leaf, not the tip of the stem, which is what allows them to recover from being grazed. This makes them a recurring symbol both of the transitoriness of life (“All flesh is grass, and all the goodliness thereof is like the flower of the field,” Isaiah 40:6) and its resilience.

Brahms used another verse about grass in the second movement of his German Requiem “For all flesh is as grass, and all the glory of man as the flower of grass. The grass withereth, and the flower thereof falleth away,” 1 Peter1:24. (Here is the German text and English translation.)

And the most famous poem about grass, by Walt Whitman, perhaps strikes the right elegiac note for the dinosaurs, who meet their doom tomorrow:

A child said, What is the grass? fetching it to me
with full hands;
How could I answer the child?. . . .I do not know what it
is any more than he.

I guess it must be the flag of my disposition, out of hopeful
green stuff woven.
Or I guess it is the handkerchief of the Lord,
A scented gift and remembrancer designedly dropped,
Bearing the owner’s name someway in the corners, that we
may see and remark, and say Whose?

Or I guess the grass is itself a child. . . .the produced babe
of the vegetation.

Or I guess it is a uniform hieroglyphic,
And it means, Sprouting alike in broad zones and narrow
zones,
Growing among black folks as among white,
Kanuck, Tuckahoe, Congressman, Cuff, I give them the
same, I receive them the same.

And now it seems to me the beautiful uncut hair of graves.
Tenderly will I use you curling grass,
It may be you transpire from the breasts of young men,
It may be if I had known them I would have loved them;
It may be you are from old people and from women, and
from offspring taken soon out of their mother’s laps,
And here you are the mother’s laps.

This grass is very dark to be from the white heads of old
mothers,
Darker than the colorless beards of old men,
Dark to come from under the faint red roofs of mouths.

O I perceive after all so many uttering tongues!
And I perceive they do not come from the roofs of mouths
for nothing.

I wish I could translate the hints about the dead young men
and women,
And the hints about old men and mothers, and the offspring
taken soon out of their laps.

What do you think has become of the young and old men?
What do you think has become of the women and
children?

They are alive and well somewhere;
The smallest sprouts show there is really no death,
And if ever there was it led forward life, and does not wait
at the end to arrest it,
And ceased the moment life appeared.

All goes onward and outward. . . .and nothing collapses,
And to die is different from what any one supposed, and
luckier.

Dino habilis

84.5 – 80 million years ago

When Jane Goodall reported in 1960 that chimpanzees at her field site in Gombe, Tanzania, were making tools, she made headlines. The discovery toppled a supposed pillar of human uniqueness, enshrined in the catchphrase “Man the Toolmaker.” Louis Leakey, upon getting the news from a telegram sent by Goodall, wrote “Now we must redefine tool, redefine Man, or accept chimpanzees as humans.”

We’ve grown more blasé since then, and the gradually accumulating evidence for dinosaur tool use has attracted a lot less attention. The evidence is similar to what we find for early human ancestors, Homo habilis and earlier. It takes the form of manuports, stones that have been transported a long way out of their geological context. Although the inference was resisted at first, it now looks almost certain that they were picked up and carried, probably by dinosaurs who used them as hammers to smash bones (or, less plausibly, to attract mates). Most suspected manuports weigh just a few pounds (a kilogram or two), and the most likely tool users in this case are the various raptors. A small fraction of manuports are much larger – up to fifty pounds (over twenty kilograms) – and may have been carried by larger dinosaurs like Tyrannosaurus rex. T. rex was clearly a carnivore, but people have long wondered how she got her meat, whether by hunting or scavenging. The latest finds raise the possibility that she was a specialized predator, dropping rocks from above to smash armored prey like tortoises and ankylosaurs.

Tool use is sometimes taken as evidence of sophisticated cognition. With dinosaurs, however, we may be seeing something different, the evolution of complex tool-using instincts over many tens of millions of years, no more indicative of high intelligence than honeybee dances or spider webs. If there is a parallel with human evolution it is not that dinosaurs were especially smart, but that the process of evolution, given as a starting point a pair of forelimbs not being used for locomotion, is likely to find (or exapt) some new function for them.

The suggested evolutionary scenario for instinctive tool use in dinosaurs is supported by findings regarding another group of organisms, still abundant today: ethologists discovered decades ago that complex tool using abilities without a high level of intelligence are present among some varieties of gull. More recently, these gull abilities have turned out to be widespread.