The Dawn and Dusk of the Age of Mammals

In Beasts Before Us, paleontologist, Elsa Panciroli, traces the evolutionary history of the mammal class of the animal kingdom, of which humans are members, to its origins about 300 million years ago.  It’s a tedious recitation of multitudes of now extinct species from their earliest ancestors up to the dawn of the age of mammals that began 66 million years ago after the abrupt end of the age of dinosaurs. But it’s also a rewarding read because it reminds us of our close relationships with other animals as well as the ways in which we are different.  Those differences predict which mammals will survive the forthcoming sixth great extinction that humans have inflicted on life on Earth.

Mammals living today have in common only one characteristic that distinguishes them from other classes of close relatives.  The subdivisions of mammals alive today have mammary glands that produce milk to feed their young.  The three subdivisions of mammals are monotremes, marsupials, and placentals.  Monotreme species alive today are platypus and echidna whose young are hatched from eggs, but are milk fed by their mothers.  Marsupials are born at an undeveloped stage and carried to term in their mothers’ pouch.  By far the largest group, placentals carry their developing offspring inside the mothers’ abdomen until birth. 

The earliest ancestors of mammals were four-legged vertebrates called amniotes. Amniotes were named for the membrane that lined the hard shells of their eggs, protecting the embryo.  The development of the amniotic membrane provided protection needed to lay and hatch eggs on land rather than the ocean where earlier forms of life lived.  This evolutionary development was associated with the transition of life from the ocean to the land.  The earliest amniotes diverged to take two different evolutionary paths, one as reptiles and dinosaurs (sauropsids) and the other as mammals (synapsids).  Pause here briefly to contemplate our close relationships with other animals. 

The Science of Paleontology

Beasts Before Us is also interesting as a history of paleontology, the branch of science that studies fossils of plants and animals to determine the evolutionary history of life.  Beasts focuses on advances in modern paleontology, but this article takes readers further back in time to appreciate how recently we learned about the scale of past extinctions that predict future extinctions.

Prior to the 19^th century, an understanding of extinction was inconsistent with prevailing Western belief that the world was created by God as complete, perfect, and unchangeable.  In the late 17^th century fossils of extinct animals were discovered that appeared to be unlike any living species.  Inquiring minds began the search for an explanation for what happened to these unknown species. 

George Cuvier is credited with establishing the modern concept of extinction in a lecture to the French Institute in 1796.  Cuvier is sometimes called the “founding father of paleontology.”  He rejected the theories of evolution, believing instead that extinctions could be explained by “cyclical creations” and catastrophic natural events such as floods. 

The fossil record is limited in what it can tell us about life in deep time because it does not preserve the remains of extinct species with equal reliability.  Bones survive to tell the tale with greater accuracy than soft tissues and plants.  Paleontology is developing techniques to compensate for gaps in the fossil record, drawing from other scientific disciplines, such as botany, biochemistry, mathematics, and engineering. 

Since more than 99% of all species that ever lived on Earth—more than five billion species–are now extinct, we can only imagine the difficulty of the task of piecing together the complete phylogenetic tree of life.  Beasts Before Us gives us a current view of what has been accomplished to date.  Clearly it is not the end of the story and much of the story is still speculative. 

Divergent Evolution

The 300-million year journey from the first ancestors of mammals to modern mammals of today is a story of divergent evolution, the accumulation of differences between closely related populations within species that lead to new species. Tracing that long process was until recently dependent upon the fossil record and was therefore focused on changes in bone structure, particularly teeth, jaws, and skulls for which the fossil record is more intact. 

These bone structures are important clues about the diet of animals. The teeth of herbivores, insectivores, and carnivores are different.  “Mammal fossils can be distinguished and named based on their teeth alone.” (1) Nearly half of all mammal species are rodents, a name that comes from the Latin word for gnaw.  Their long front teeth grow continuously as they are ground down by gnawing on tough plant material such as tree bark in the case of beavers or the wooden shingles on my home in the case of squirrels.

The digestive systems of mammals also diverged to accommodate their different diets (or vice versa).  Carnivores typically have a short intestinal track where digestion is accomplished with enzymes and resident microbial communities.  Herbivores have a longer digestive system in which plant material is fermented in a series of separate chambers in the case of ruminants (cows, sheep, deer, etc.). 

Divergent evolution creates diverse species with diverse abilities to exploit different ecological niches while reducing competition between species.  Shortly after the divergence of mammal and reptile lineages, the characteristic most consequential to the fate of those lineages was endothermy (warm-bloodedness) in mammals and ectothermy (cold-bloodedness) in reptiles. The divergence of this characteristic occurred about 250 million years ago, shortly after the divergence of mammal and reptile lineages. 

Only mammals and birds are generally capable of generating their body heat internally.  Over millions of years they also evolved insulation that conserves body heat with fur, feathers, and blubber in the case of marine mammals. A diet high in sugar and fat also helps to maintain body heat. Cold blooded animals depend on external heat sources such as sunlight to be active.  These crucial differences in mammals and reptiles relegate them to different ecological niches to which they are suited, for example:

• Mammals and birds can survive in colder climates than reptiles.
• Mammals and birds can be more active at night when it is cooler.
• Mammals and birds can be more active for longer periods of time than reptiles.
• Mammals and birds can live below ground where it is colder in summer and warmer in winter than above-ground temperatures.
• On the other hand, mammals and birds must eat more and more frequently than reptiles. 

These significant differences are partly responsible for the sudden transition from the age of dinosaurs to the age of mammals 66 million years ago.  During the age of dinosaurs, mammals were small, lived below ground, and ate primarily insects.  This lifestyle avoided competition with huge dinosaurs that dominated the land. 

When the asteroid hit the Earth 66 million years ago, the climate was suddenly and drastically transformed from a tropical climate to a cool, partly sunless climate.  Vegetation adapted to a tropical climate quickly died, depriving dinosaurs of their food if they weren’t killed outright by the impact. 

Beasts paints a vivid and dire picture of the cataclysmic event that ended the age of dinosaurs.  The asteroid created a crater almost 100 miles in diameter and 12 miles deep.  “An earthquake larger than any recorded in human history would have made the Earth reverberate like a bell.  The thermal shockwave would have flash-fried all life for hundreds of miles.  The blast of air probably flattened forests as much as 1,000 kilometers away…[the impact] created a mega-tsunami at least 330 feet in height…[that] mounted the coasts of North American and barreled inland like a liquid steam-roller…The dust in the atmosphere swirled its way around the planet until it enclosed all life in its smothering grip.  The sun rose, but as little as half of its light could penetrate the dust in the atmosphere.  The sulphur in the dust combined with water droplets to rain sulphuric acid on the land, burning away the green vegetation…Few animals bigger than a Labrador dog survived the extinction event.”  (1; not verbatim)

The fifth extinction predicts the consequences of the sixth extinction

Small mammals were safely below ground and they didn’t require the great quantities of plant food required by dinosaurs.  Mammals inherited the Earth and over millions of years they evolved into some 5,500 mammal species today of which 90% are still small bodied, most of them rodents.

The final chapter of Beasts uses the consequences of the fifth extinction that ended the age of dinosaurs to predict the consequences of the anticipated sixth extinction because “Humans are replicating many of the conditions of previous mass extinctions.” (1)

• Animals are likely to become more active at night, when temperatures are cooler.
• Animals are likely to find some respite by living below ground where temperatures are more moderate in winter and summer. 
• Animals will move to more temperate regions if they can.

The animals that are most likely to survive will be small generalists, who need less food, are not fussy about what they eat, and are more capable of tolerating heat. Think rats. Beasts advises, “If I were you, I’d say goodbye to any wild animal bigger than a pig—zoos are likely to be the only refuges for them in the future we are creating.”

Birds were the only descendants of dinosaurs to survive the fifth great extinction.  They are expected to fare better in the sixth extinction for much the same reason:  they can be active at night; they eat insects as well as plants; they are more mobile than most classes of animals.  We often hear dire predictions of the fate of birds, but in fact they are less threatened than other classes of animals.  A recent study reported that 21% of reptiles are threatened with extinction, a higher risk than birds (of which about 13 percent of species are threatened with extinction) and slightly less than mammals (25 percent). Amphibian species are at highest risk with about 40 percent of species in danger of extinction.  We hear more about birds because their popularity motivates greater media coverage about them.

I will also presume to give my readers some advice. 

• Quibbling about whether native plants are superior to non-native plants is like arguing about the color of the lifeboat. It really doesn’t matter.  Soon enough we will be glad to have ANY vegetation that is capable of living in the climate we have created. The universe is indifferent to the survival of any specific species of life.
• You can do more for the environment and the animals that live in it by stopping the use of pesticides than by planting native plants. 
• Be humble about what you think you know.  Many important scientific concepts such as evolution and extinction are less than 200 years old and the cause of the extinction of dinosaurs was discovered less than 50 years ago.  What you learned 50 years ago may need to be reconsidered and revised.  A rapidly changing situation requires that we keep an open mind to new information.
• Set meaningful priorities.  Climate change is an existential threat to all life on Earth.  Ask yourself how we can justify the destruction of healthy trees that sequester the carbon that contributes to climate change? 

---

1. Elsa Panciroli, Beasts Before Us: The Untold Story of Mammal Origins and Evolution, Bloomsbury Sigma, 2021