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 monkey’s voyage

35.9 – 34.0 million years ago

The Oligocene (starting today on Logarithmic History) sees a major diversification of anthropoid primates (monkeys, apes, and humans). Among the anthropoids, the major evolutionary split is a geographic one, between platyrrhines (New World monkeys) and catarrhines (Old World monkeys, apes, and humans). Aegyptopithecus is one of the earliest primates that clearly falls on the catarrhine side of that split (although the split must go back earlier).

At Logarithmic History we traffic in Big Questions, and one of the biggest questions of all is the balance of natural law and accident in making our world. Thus physicists have long hoped to find that the laws governing our universe reduce to just a few fundamental equations, but we saw at the beginning of this blog that they are now confronting the possibility that our universe is just one among many, and that the laws of physics in our universe may incorporate a large dose of historical accident. With the discovery of extra-solar planets, we’re just beginning to get an idea of how typical or atypical our solar system is. And we’ll have a lot of opportunities to ask whether there are Laws of History (an old idea now undergoing a revival in the new field of cliodynamics*) when we move into the historical period later in the year.

The field of biogeography – the study of the geographic distribution of species – has seen some major pendulum swings in this regard. Darwin was intensely interested in questions of biogeography mainly because they could provide support for the theory of evolution. His approach could fairly be called eclectic. From sometime in the second half of the twentieth century however, a lot of biologists thought they could do better than just answering particularistic questions about how species A got to island Z (sometimes disparaged as “stamp collecting”). They wanted to find scientific laws.

Edward O. Wilson was an early pioneer in this area. Along with Robert MacArthur, he developed a theory of island biogeography which was supposed to get the field out of its natural history phase, and turn it into a predictive science. According to MacArthur and Wilson, the number of species on an island is set by a predictable equilibrium between extinction (smaller islands have higher extinction rates) and colonization (remote islands have lower colonization rates). Being a good scientist Wilson actually put this theory to the test by getting an exterminator to “defaunate” (it means what you think it means) some little mangrove islets, and showing that they returned to very close to their predicted equilibrium numbers of animal species after a while.

For the biogeography of continents (and larger islands once part of continents) the quest for scientific laws took a different turn. The discovery of continental drift and plate tectonics encouraged a school of “vicariance biogeography.” Vicariance biogeographers liked to trace current biogeographic distributions to the wanderings of continents. They were highly allergic to explanations involving accidental long-distance dispersal over big stretches of ocean.

Alan de Queiroz, in The Monkey’s Voyage: How Improbable Journeys Shaped the History of Life, provides a highly readable overview of the decline (if not quite the extinction) of the vicariance school in the face of mounting evidence for flukish dispersals as a major factor in biogeography. The dispersal of monkeys to the New World is a dramatic case in point. (Guinea pigs and their relatives are another.) About the only scenario that makes sense involves a raft of trees washing out to sea (most likely from the Congo basin) and eventually delivering a few parched, scared monkeys to the island continent of South America, where they eventually spawned the whole range of species – spider monkeys, squirrel monkeys, howler monkeys, tamarins, marmosets, capuchins – we know today. Sheer accident: change the weather a little, leave the monkeys stranded at sea a little longer, and the whole history of primates in the New World is erased.

* so new my spellchecker doesn’t recognize it.

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

Strange relations and island continents

53.1 – 50.3 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.)

Gould’s Belt

30.3 – 28.8 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.

The monkey’s voyage

35.9 – 34.0 million years ago

The Oligocene (starting today on Logarithmic History) sees a major diversification of anthropoid primates (monkeys, apes, and humans). Among the anthropoids, the major evolutionary split is a geographic one, between platyrrhines (New World monkeys) and catarrhines (Old World monkeys, apes, and humans). Aegyptopithecus is one of the earliest primates that clearly falls on the catarrhine side of that split (although the split must go back earlier).

At Logarithmic History we traffic in Big Questions, and one of the biggest questions of all is the balance of natural law and accident in making our world. Thus physicists have long hoped to find that the laws governing our universe reduce to just a few fundamental equations, but we saw at the beginning of this blog that they are now confronting the possibility that our universe is just one among many, and that the laws of physics in our universe may incorporate a large dose of historical accident. With the discovery of extra-solar planets, we’re just beginning to get an idea of how typical or atypical our solar system is. And we’ll have a lot of opportunities to ask whether there are Laws of History (an old idea now undergoing a revival in the new field of cliodynamics*) when we move into the historical period later in the year.

The field of biogeography – the study of the geographic distribution of species – has seen some major pendulum swings in this regard. Darwin was intensely interested in questions of biogeography mainly because they could provide support for the theory of evolution. His approach could fairly be called eclectic. From sometime in the second half of the twentieth century however, a lot of biologists thought they could do better than just answering particularistic questions about how species A got to island Z (sometimes disparaged as “stamp collecting”). They wanted to find scientific laws.

Edward O. Wilson was an early pioneer in this area. Along with Robert MacArthur, he developed a theory of island biogeography which was supposed to get the field out of its natural history phase, and turn it into a predictive science. According to MacArthur and Wilson, the number of species on an island is set by a predictable equilibrium between extinction (smaller islands have higher extinction rates) and colonization (remote islands have lower colonization rates). Being a good scientist Wilson actually put this theory to the test by getting an exterminator to “defaunate” (it means what you think it means) some little mangrove islets, and showing that they returned to very close to their predicted equilibrium numbers of animal species after a while.

For the biogeography of continents (and larger islands once part of continents) the quest for scientific laws took a different turn. The discovery of continental drift and plate tectonics encouraged a school of “vicariance biogeography.” Vicariance biogeographers liked to trace current biogeographic distributions to the wanderings of continents. They were highly allergic to explanations involving accidental long-distance dispersal over big stretches of ocean.

Alan de Queiroz, in The Monkey’s Voyage: How Improbable Journeys Shaped the History of Life, provides a highly readable overview of the decline (if not quite the extinction) of the vicariance school in the face of mounting evidence for flukish dispersals as a major factor in biogeography. The dispersal of monkeys to the New World is a dramatic case in point. (Guinea pigs and their relatives are another.) About the only scenario that makes sense involves a raft of trees washing out to sea (most likely from the Congo basin) and eventually delivering a few parched, scared monkeys to the island continent of South America, where they eventually spawned the whole range of species – spider monkeys, squirrel monkeys, howler monkeys, tamarins, marmosets, capuchins – we know today. Sheer accident: change the weather a little, leave the monkeys stranded at sea a little longer, and the whole history of primates in the New World is erased.

* so new my spellchecker doesn’t recognize it.

Life at sea: whales and sailors

50.2 -47.6 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.

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