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.