12E The Age of Mammals

The End Cretaceous extinction was the transition point from small protomammal insectivores to the orders of placental mammals that inhabit our world. The extinction event wiped out the large dinosaurs, classified as Aves due to their bird affinity, and in general only allowed the smaller, less dangerous birds, to pass through the extinction event. This event was like the door that opened up the world to mammals and ultimately to humans.

God made the beasts of the earth after their kind, the cattle after their kind, and creeping things after its kind. In the ancient Hebrew animal classification scheme, “beasts of the earth” refers to carnivorous placental mammals, “cattle” refers to grazing animals, and creeping things includes small mammals and reptiles. The word translated as made, asah, implies that God made a modification, not a new creation. The earth had already brought forth what were probably protomammals (v. 24). The likely implication of the definite article in v. 25 (the beasts, the cattle) vs. no definite article in v. 24 (beasts, cattle) is that the result of the intervention was the modern mammals that inhabit the present earth.

Genesis 1:25. And God made the beasts of the earth after their kind, and the cattle after their kind, and everything that creeps on the ground after its kind; and God saw that it was good.

Although God might have intervened in mammal evolution during the entire Cenozoic Era, there is one problem in mammal evolution that might point more directly toward divine intervention. The origin of placental mammals is the second biggest problem in paleontology, after the unexplained origin of vertebrates. Paleontologists observe no placental mammal fossils in the Cretaceous (before 66 Ma), including the three insectivore ancestors of the modern groups of placental mammals (Section 4-8). Then there are three different groups of placental mammals directly after extinction. The placental mammal condition is a relatively complex change from the nonplacental condition (Section 4-8). It isn’t like a fin became shorter due to a few DNA mutations in the noncoding region of a Hox gene. In efforts to explain the fact that there were no placental mammals in the Cretaceous, and the fact that their ancestors were three different species of nonplacental mammals, paleontologists have proposed three different models of placental mammal evolution (Section 4-8), none of which account for all the data. The analysis is complicated by the fact that there are different types of placentae in cattle and sheep (cotyledonary), horses and pigs (diffuse), dogs and cats (zonary), and rodents and primates (discoid). Other differences in placentae include invasiveness and villous conditions of placentae.

Unlike v. 24, Moses listed beasts of the earth as a kind, cattle as a kind, and creeping things as a kind, in v. 25. The order and distinction between kinds is important because it is different than v. 24, which lists cattle, creeping things, and beasts of the earth of one single kind. The Hebrew categories do not align perfectly with modern scientific mammal classifications.

Carnivores evolved almost immediately after the End Cretaceous extinction, as did some herbivores. Primitive stem carnivorans appeared in the early Paleocene. The Carnivoramorpha began 66.043 Ma. The Periptychidae were herbivores that lived only 500,000 years after the K-Pg boundary.  Small mammals (creeping things) were the survivors of the End Cretaceous extinction. The sparcity of fossils in the Early Paleocene and the fact that they evolved simultaneously probably means that we cannot know the order of early animals of the Paleocene after the End Cretaceous extinction.

On the other hand, the modern order, Carnivora, appeared in 52 Ma. The second classification is cattle. Ungulates generally include the hoofed animals and include deer, hippopotamus, goats, sheep, pigs, musk ox, antelope, cattle, buffalo, rhinoceros, and other grazing animals. One of the primary groups of ungulates are the artiodactyls (even toed ungulates), which include cattle and sheep. Within the artiodactyls, large grazing animals appeared when the grasslands of the earth evolved in the Miocene (23 Ma). The third classification in v. 25 is creeping animals, which probably includes the rodents. The Rodentia appeared in the Late Paleocene (59 – 56 Ma), which was before the others. A common rodent in the Middle East was the house mouse, which only spread into the eastern Mediterranean in 13,000 BC. As with mice, rats evolved in the last few million years. The Lagomorpha include the hares and rabbits, which evolved just after the Paleocene (53 Ma), however common species such as the European rabbit also evolved in the last few million years.

In contrast with tree shrews, primates have depth perception, forward looking eyes, and grasping hands, among other unique characteristics. It is possible that God tweaked the genome in 55 Ma and began the process of primate evolution.  Granted, there are theories for how these characteristics evolved, but maybe these selection pressures operated in combination with God’s intervention. In the following paragraphs, Bing AI describes the selection pressures that led to the evolution of primates in the Cenozoic Era.

“Living in trees has been a significant factor in the evolution of primates. According to a Nature article, primates have developed several traits and habits while living in trees, including adaptations for climbing trees such as rotating shoulder joints, separated big toes and thumbs for grasping, and stereoscopic vision. Another Wikipedia article suggests that primates’ hands and feet evolved to improve locomotion in the trees. For example, the grasping hands and feet of primates are well suited to gripping tree branches of various sizes, and their flexible joints are good for reorienting the extremities in many different ways.

In addition to these physical adaptations, living in trees has also influenced the social behavior of primates. According to a Nature article, living in trees helped reduce predation and provided abundant food in the form of flowers, fruits, nuts, and insects. This abundance of food allowed primates to form larger social groups that could better defend themselves against predators.

In summary, living in trees has played a significant role in the evolution of primates. It has led to the development of physical adaptations such as grasping hands and feet and stereoscopic vision, as well as social adaptations such as larger social groups.”

While God may or may not have intervened in mammal orders, vv. 26-27 indicate that God intervened in human evolution. This intervention was probably within the last few million years, if not more recent

Since the Neolithic Age, animals have been incredibly important to human success. God indicated the important of animals as human companions in the Garden of Eden. Although the eutherian mammals that made it through the End Cretaceous extinction probably had highly advanced social skills, the social behavior and intelligence of placental mammals increased dramatically in the Cenozoic Era. Modern mammals, especially horses, dogs, and cats, are amazingly social and provide human companionship. There are also many mammals and birds that are suitable for domestication as farm animals. Domesticated animals include dogs, cats, chickens, ducks, doves/pigeons, turkeys, geese, horses, cows, sheep, goats, camels, rabbits, hamsters, pigs, and oxen. The animals in the natural ecosystem also benefitted humans. Game animals included buffalo, deer, auroch, and many other large and small animals and birds.

This chapter reviews the evolution of a few mammals. North America is particularly well endowed with Cenozoic fossil sites (Section 5-2). Sections 5-3 and 5-4 describe two of the more interesting mammalian evolutionary sequences, horses and whales, respectively. Three of the oddest groups of mammals are the marsupials in Australia (Section 5-5), the Xenarthra in South America and the Afrotheria in Africa (Section 5-6). One of the interesting questions is why the Xenarthra and Afrotheria became placental mammals even though they diverged from the other eutherian mammals (placental ancestors) over 100 Ma, before any eutherian fossils exhibited placentalism. Finally, this chapter has a relatively long section on the evolution of primates (Section 5-7) during the last 50 million years.

One of the most important factors in primate and mammal evolution in general was the changing climate during the Cenozoic https://youtu.be/yR8cR75iKGU. The first period of the Cenozoic was the Paleogene, which had three epochs: Paleocene, Eocene, and Oligocene. The climate was hot in the Paleocene and early Eocene, leading to jungles over much of the earth, an ideal environment for primates. The climate began to cool (Figure 5I-3) approximately 50 Ma and continued to cool until the end of the Eocene. These climatic eras were incredibly important to the evolution of mammals during the Cenozoic.

At the end of the Eocene, Antarctica became covered with ice. The temperature remained stable during the Oligocene and early Miocene. Mountains that formed in the Miocene started another cooling trend and caused an ecological shift from forests to grasslands. Grasslands favored grazing animals like cows and horses. The climate continued to cool during the Pliocene, and the world entered the Ice Ages, which favored the evolution of large animals such as mammoths. 

The extremely high temperatures at the end of the Paleocene and beginning of the Eocene probably led to the appearance of primates, horses, whales, and other orders. Carbon dioxide and temperature were high at the beginning of the Cenozoic. Temperature rose even higher, 5 to 8 0C in 20,000 years, in an incident at the end of the Paleocene (56 Ma), called the Paleocene-Eocene Thermal Maximum (PETM). Carbon dioxide in the atmosphere reached 2,000 ppm, and it was so warm in the early Eocene that North America and Asia were covered by tropical forests (Figure 5-2). The first definitive ancestors of primates, horses, and whales appear in the Early Eocene fossil record. Giant snakes and turtles also evolved in this hot period. Carbon dioxide and temperature began to drop 49 Ma and dropped during the rest of the Eocene until the beginning of the Oligocene (34 Ma). Scientists think that the temperature decline in the Mid and Late Eocene might have been caused by massive blooms of azola in the ocean that dropped to the sea floor in the Arctic Ocean, thus removing carbon from the carbon cycle and dropping the level of carbon dioxide in the atmosphere. 

The Cenozoic Era went from one of the hottest periods with the least polar ice cover of the Phanerozoic Eon to one of the coldest with the most ice cover (Figure 5-3) at the end of the Phanerozoic Eon.  Glaciers covered much of North American and Eurasia during the coldest periods of the recent Ice Ages. These cold temperatures changes had a major impact on the evolution of humans and animals during the Pleistocene and Holocene epochs of the last 2.6 million years. 

The following play helps to communicate the change that took place between the Cretaceous Period and the Cenozoic Era.

Days (Eons) of Our Lives (Episode 5)

Bob the modern bird
Mamie the modern mammal

Mamie: Hi Bob, I see you survived the End Cretaceous extinction. Do you think we can live in harmony?

Bob: Tweet, tweet, tweet.

Mamie: Things are a little different now. I am a real placental mammal. The definite article. I am no longer a protomammal. You turned into such a sweet and handsome bird. Are you still a carnivore?

Bob: Tweet, tweet.

Mamie: I love your beautiful songs. I am so happy that you made it through the extinction. Are you happy to see me here?

Bob: Tweet

Mamie: It is great that all your carnivorous Ave colleagues, such as Rex, were wiped out. This world would definitely be a worse place with them around.

Bob: Tweet, tweet, tweet.

Mamie: We survived the extinction by eating insects. Did you survive by eating insects?

Bob: Tweet, tweet.

Mamie: Someday, we are going to domesticate you and turn you into pets and farm animals. We will even put chickens in little boxes so they can lay eggs and won't have to move a muscle. Does that sound fun?

Bob: Tweet.

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Hyaenodon horridus, a North American species of hypercarnivore. Credit Daderot. Used here per CCO. Wikipedia.