Planet of the horses

16.7 – 15.8 million years

Horses have probably been the single most important domesticated animal in human history. Also, more than with other livestock, people get attached to horses as individuals. I’m guessing that in history and literature there are more horses with individual names than any other animal. (Alexander the Great’s horse was Bucephalus, “Ox-head”; Muhammed’s was al-Buraq*; Charlemagne’s was Tencendur; Don Quixote’s was Rocinante; Gandalf’s was Shadowfax.) We’ll be hearing a lot more about horses and horse folk on Logarithmic History once we get to human history.

Being so charismatic, horses have featured in a big way in arguments over evolution. Thomas Henry Huxley (1825-1895), “Darwin’s bulldog,” knew he needed to find good evidence for evolution. When he visited the United States in 1876, he was ready to give a lecture based on horse fossils from Europe. But visiting Yale, he was so impressed with O. C. Marsh’s collection of horse fossils from the western United States, that he rewrote his lecture around it.

Henry Fairfield Osborn (1857-1935) was director of the American Museum of Natural History and a huge presence in American paleontology. He was active at a time when most scientists accepted evolution, but many weren’t so keen on Darwin’s theory of natural selection. He thought horses were a fine example of “orthogenesis,” the tendency of species to follow a fixed line of evolution, reflecting internal forces, maybe related to willpower. He thought that humans shared a migratory spirit with horses, so that anywhere horse fossils were found would be a good place to look for human fossils. This theory didn’t pan out too well. A massive AMNH expedition to Central Asia led by Ray Chapman Andrews found all sorts of wonders – dinosaur eggs, baluchitheres – but no fossil “pro-men.” Orthogenesis leant itself naturally to diagrams showing evolution from early to modern horses going in a straight line.

George Gaylord Simpson (1902-1984), paleontologist, was one of the great figures in the evolutionary Modern Synthesis that brought together Darwin’s theory of natural selection and Mendel’s genetics. There was no room for orthogenesis in the Modern Synthesis, and Simpson emphasized that the evolution of horses was a matter of adaptation to a changing environment – especially the spread of grasslands. Also that horse evolution looked more like a bush than a ladder.

Stephen Jay Gould (1941-2002) was the most widely recognized American evolutionary biologist of recent times. (For example had a spot on The Simpson’s — “Lisa The Skeptic,” Season 9.) Gould had his own take on the modern synthesis, taking the “bushes not ladders” theme for horses and other animals (including human ancestors), and pushing it a step further. According to the theory of “punctuated equilibrium” (formulated in collaboration with Niles Eldredge), species mostly change relatively little during the time they exist (evolutionary stasis). Most evolutionary change happens when a small population buds off to form a new species and reproductive isolation allows it to conserve any evolutionary novelties it has developed. This opens up the possibility of “species selection.” Applied to horses, for example, this could mean that horses were evolutionarily successful for some time not so much because individual horses were well-adapted, but because something about horses collectively (their harem mating system, maybe) made one horse species especially likely to generate new species. Both horses and primates seem to be especially prone to bud off new species:

Speciation and chromosomal evolution seem fastest in those genera with species organized into clans or harems (e.g., some primates and horses) or with limited adult vagility and juvenile dispersal, patchy distribution, and strong individual territoriality (e.g., some rodents). This is consistent with the … hypothesis … that population subdivision into small demes promotes both rapid speciation and evolutionary changes in gene arrangement by inbreeding and drift.

 * Richard Dawkins doesn’t believe that Muhammed’s horse, al-Buraq, carried him (i.e. Muhammed) to heaven and back.

Bunches of monkeys

Our descent, then, is the origin of our evil passions!! The devil under form of Baboon is our grandfather.

Charles Darwin, Noteboook M

Maimoun angushti shaitan ast.

(A monkey is the devil’s fingers.)

Tajik proverb

Monkeys and apes are not only exceptionally brainy, but also distinctively social. Most mammals are solitary (apart from mothers and their juvenile offspring of course). Among a minority of mammals, adult males and females set up pairbonds. And some mammals form larger groups. In most cases, however, these are relatively unstructured aggregations: a herd of buffalo is more like a crowd of people than a human community. A handful of mammals – elephants, cetaceans, and the majority of monkeys and apes – form more structured groups, enduring and internally differentiated.

Social evolution is path dependent: primate social organization is affected by ecology, but also has a strong phylogenetic component.  This makes it possible to offer a tentative reconstruction of the stepwise evolution of stable sociality in primates. Here’s an evolutionary tree, showing inferred transitions between solitary living, and multimale/multifemale, unimale/multifemale, and pairbonded groups:

A diagram of the possible evolutionary dynamics looks like this:

And the accompanying story goes like this: about 52 million years ago, the solitary nocturnal ancestor of monkeys and apes switched to being diurnal. This allowed for the exploitation of a whole range of new foods, but it also exposed the ancestor to new forms of predation. The first step in the evolution of monkey and ape sociality, then, was aggregation in multimale/multifemale groups to cut predation risk. At first these groups would have been loosely structured and unstable, but eventually they would have evolved into something like what we see today among most Old World monkeys: stable (sometimes lifelong) networks of relatives and friends, dominants and subordinates, nested within enduring communities.

A later development, going back to 20 million years ago or less, was a shift, among some of these social primates, to unimale/multifemale groups, or pair-living family groups.

The inferred development of structured social groups in primates bears a remote similarity to the evolution of eusociality among social insects. According to current theories, the starting point for eusociality is the development of a defended nest site where females lay eggs and raise offspring. Sometimes an established nest site is so valuable that it’s adaptive for the next generation of offspring to stay on when they mature rather trying to found new nests. The eventual result may be the evolution of a highly structured society, with strong reproductive skew: some nest members specialize in reproduction, others in foraging or defending the nest. The latter may evolve into an obligately sterile caste.

Primates too have developed an intensified, structured sociality as a response to obligate group living. But the parallels with eusocial insects go only so far. The great majority of primates give birth to one offspring at a time. There are no queen bee baboons whelping one vast litter after another and pushing subaltern kin into caring for them. This goes for humans as well. Our species matches the social insects in the scale of cooperation, but we manage this through a complicated dance of coalition-building and reputation management. For a primate, building a honey-bee style superorganism has to be an aspiration rather than a reality.

We were never chimps

It’s natural to turn to our closest living relatives, the great apes (chimpanzees and bonobos, gorillas, orangutans), for insights into what our remote ancestors were like. But the fossil evidence suggests that current great apes aren’t a good guide to our past. Below is a figure from a recent article reconstructing diet and habitat for Morotopithecus and some relatives, from just over 20 million years ago, and later. It looks like all these guys inhabited relatively open woodland – trees interspersed with grass – rather than the closed canopy tropical forest that is the modal habitat for all the living great apes. Also, they may have specialized in consuming more young leaves, and less fruit than, say, chimpanzees. On current evidence, then, our closest living relatives have all evolved away from our common ancestors, to become (not terribly successful) tropical forest specialists.

Also, more on this later: gorilla and chimpanzee knuckle walking may be a poor model for locomotion in our common ancestor. Asking how we evolved from a chimp-like ancestor is probably asking the wrong question. 

Pace Jared Diamond: a good book and a snappy title, but we are not The Third Chimpanzee.

Planet of the apes

17.6 – 16.7 million years ago

We are now covering history at the rate of one million years a day

The Miocene (23 – 5 million years ago) is a period of extraordinary success for our closest relatives, the apes. Overall there may have been as many as a hundred ape species during the epoch. Proconsul (actually several species) is one of the earliest. We will meet just a few of the others over the course of the Miocene, as some leave Africa for Asia, and some (we think) migrate back.

Sometimes evolution is a story of progress – not necessarily moral progress, but at least progress in the sense of more effective animals replacing less effective. For example, monkeys and apes largely replace other primates (prosimians, relatives of lemurs and lorises) over most of the world after the Eocene, with lemurs flourishing only on isolated Madagascar. This replacement is probably a story of more effective forms outcompeting less effective. And the expansion of brain size that we see among many mammalian lineages throughout the Cenozoic may be another example of progress resulting from evolutionary arms races.

But measured by the yardstick of evolutionary success, (non-human) apes — some of the brainiest animals on the planet — will turn out not to be all that effective after the Miocene. In our day, we’re down to just about four species of great ape (chimpanzees, bonobos, gorillas, and orangutans), none of them very successful. Monkeys, with smaller body sizes and more rapid reproductive rates, are doing better. For that matter, the closest living relatives of primates (apart from colugos and tree shrews) are rodents, who are doing better still, mostly by reproducing faster than predators can eat them.

So big brains aren’t quite the ticket to evolutionary success that, say, flight has been for birds. One issue for apes may be that with primate rules for brain growth – double the brain size means double the neurons means double the energy cost – a large-bodied, large brained primate (i.e. an ape) is going to face a serious challenge finding enough food to keep its brain running. It’s not until a later evolutionary period that one lineage of apes really overcomes this problem, with a combination of better physical technology (stone tools, fire) and better social technology (enlisting others to provision mothers and their dependent offspring).

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.