Marsupials evolved ~150 MYA, in the late Jurassic. These creatures gave birth to undeveloped live young 2 weeks after conception. Then came a genetic jump, creating eutheria (ευθήριον – Greek for “true beasts”): transforming the uterus from egg production to a nurturing home for a growing embryo. Wide areas of the genome were altered in the sudden saltation that produced placental mammals.
The rather rat-like Protungulatum was the progenitor of all placental mammals. This was within a few hundred thousand years after the K–Pg boundary.
Within 2–3 million years after arrival, placental mammal orders proliferated. Over 6,000 species emerged.
Over 1,500 genes are expressed in the uterus in placental mammals. Uterus gene expression is coordinated by transposons: DNA sequences that can move themselves into new positions within the genome of a single cell, in either a “cut and paste” or “copy and paste” operation. Such transposition can alter traits.
The transposons that gave rise to placental mammals were like prefabricated regulatory units, carrying out entirely new functions, such as facilitating maternal-fetal communication. Their origin remains unknown.
The placenta allows a fetus much more time to develop within the womb. Yet even the most advanced placental infants are no more developed than turtles or crocodiles. The telling difference comes in the post-partum behavior of placental mammal mothers, which nurse with nutritious breast milk. This provides a crucial head start in life.
No one knows how many entangled genes were needed to coordinate the physiological, behavioral, and mental traits which define the placental mammal breeding regime. Without the impulse of maternal care, placental physiology would have been for naught. The idea of DNA concocting knowledge and crafting behavioral complexes is ludicrous. Molecules alone can never explain the miracles of life.
Having few helpless offspring via placental development combined with extended parental care affords prolonged development of sensory and cognitive apparatuses. It also facilitates lives of complex sociality. Mental dexterity also allows animals to be generalists: able to survive in diverse biomes.
Primates adopted this life-history strategy: loosening the reins of instinct in favor of greater ad hoc cognition tied to life experience. This was taken to its biological extreme in humans, who could forsake innate constraints of sanity in favor of cunning and rationalization to satisfy their destructive desires.
Most animal groups that survived the K–Pg extinction underwent adaptive diversifications during the initial epochs of the Paleogene period, filling vacated ecological niches. Besides birds and mammals, radiations included many aquatic groups, including fish, echinoderms, crustaceans, bryozoans, foraminifera, and diatoms.
Primates arose 85 MYA. The first ones were the size of a mouse, eking out a living in the trees.
Mammalian radiation was fostered by the quick comeback of plants following the K–Pg event. Then came a setback.