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.
Paleocene–Eocene Thermal Maximum
55.8 MYA came the Paleocene–Eocene Thermal Maximum (petm), a period of rapid global warming: 6 ºC in 20,000 years. By the end of the Eocene there were no significant ice sheets on Earth.
A bolide started a chain reaction. Tectonic shifts, including volcanic activity, triggered a spike in what had been a steady pace of warming. The North Atlantic was opening up at the time from seafloor spreading. A feedback loop ensued. In sum, ~1,500 billion tonnes of carbon were released, as well as much methane.
Some marine species went extinct, notably 50% of foraminifera, as did several terrestrial mammals. Primates suffered their first extinction event. Otherwise, climatic changes spurred fast adaptations that resulted in diversification under duress for many land mammals. The reptiles that had survived the dinosaur age enjoyed a brief renaissance.
Plants moved to higher latitudes as quickly as they could. Rain forests and mangrove thickets were found as far north as Belgium and Wyoming, and as far south as Tasmania. The Canadian Arctic sported palm trees.
Temperate forests became wetter and denser. Some mammals adapted by downsizing. Several new mammal groups appeared in the wake of petm: owls, bats, rodents, ungulates, elephants, and whales.
Around 30 mammal orders existed by the end of the Paleocene epoch, twice as many prior to the K–Pg extinction event. Within 10 million years of the passing of the dinosaurs, after petm, all 18 modern mammal orders existed.
Recovery from petm was relatively quick: 120,000 to 170,000 years. A biogeochemical feedback system engendered carbon burial and a rapid reversal of the warming.
~40 MYA mouse-sized primates migrated from Asia into Africa. There they found lush conditions with few carnivores, prompting a burst of speciation; whence arose anthropoids: the clade from which humans descended.
The concentration of greenhouse gases in the atmosphere, predominantly CO2, depends upon the dynamics of generating sources and sinks. Volcanoes are a primary emitter, whereas sequestration is had in sediments, soils, and organisms.
The planet cooled throughout the Eocene epoch owing to tectonic movements. Volcanism subsided as seafloor spreading declined.
The shallow seaway between the Indian and Asian continents disappeared as the plates collided, causing the Himalayan mountain range to rise. The advent of this and other mountain chains removed CO2 from the atmosphere while fewer greenhouse gases were being pumped from volcanoes.
During the early Eocene, the southern tip of South America was joined or adjacent to Antarctica. This configuration engendered warm surface currents from the Atlantic and Pacific oceans to travel south, skimming Antarctica and shedding their heat. These currents moderated Antarctic temperatures, and thus those of the entire planet.
As South America slid north, the seaway around Antarctica broadened and deepened. The emergence of a Southern Ocean created a circular current around the southernmost continent: the Antarctic Circumpolar Current.
This gyre became less susceptible to heat transfer from the warmer south-flowing currents. As Antarctica cooled during the middle Eocene, so did the whole planet.
Continental glaciers formed and sea level dropped. Planetary albedo (reflective power) increased. Global cooling strengthened as deep ocean passages grew deeper: between Antarctica and Australia, and South America and Antarctica.
Antarctic offshore surface waters started to sink as they grew cooler than those below. Horizontal current flow changes were accompanied by vertical seawater reorganization. This had 2 noteworthy effects.
1st, warmer nutrient-rich waters rose offshore. Phytoplankton blooms ensued, which fostered a new ecosystem. For one, early oceanic whales – basilosaurids – went extinct, as they were outcompeted by new whale species that had especially adapted to the new abundance of food.
2nd, coupled with tectonic events in the North Atlantic, an ocean conveyor system developed in the ocean basins that provided aeration, thus opening the ocean floor to colonization.
The upshot to all these changes was a turnover of species. The Eocene–Oligocene (E–O) extinction event ensued. It was relatively modest, occurring over 10 million years.
Species generally became more cold-tolerant or suffered loss of habitat range. The marine food web crashed during the early Oligocene, as temperate zone species faltered from changing deep-sea circulation patterns. By 36 MYA, tropical forests were restricted to the equatorial belt.
From 34 MYA into the Oligocene ushered a gradual change at the higher latitudes to drier climates and cooler temperatures. A feedback loop led to rapid Antarctic glaciation over 300,000 years, with a dramatic 30 °C drop in sea temperatures and concomitant cooling on land.
The cooling culminated in a mass extinction 32 MYA. As tropical forests became open woodlands and grasslands, primates vanished in North America and Europe.
Monkeys originated when Africa and Arabia were joined as an island continent. The animals there evolved in isolation until docking with Eurasia 24–20 MYA. It was only after the continents connected that the mammals of African origin – antelope, pigs, lions, rhinos – entered Eurasia. Monkeys colonized South America via island hopping on vegetative rafts: whence New World monkeys.
More global cooling during the Miocene diminished tropical forest in Africa. It was replaced by savanna and open woodlands.
Waves of adaptive radiation resulted in diverse primate species in Africa, Asia, and southern Europe.
Volcanism entered the evolutionary picture again 29.5 MYA with eruptions in Ethiopia and elsewhere. Lava fields were extensive.
Large bolide strikes 35 MYA – at Chesapeake, Bay Toms Canyon (160 km east of Atlantic City, New Jersey), and Popigai (Siberia) – may have been instrumental in stirring volcanic activity.
The scoured East African landscape recovered, with new ecosystem opportunities. Not coincidentally, apes evolved from Old World monkeys 25 MYA in that region, becoming the dominant primates during the Miocene.
Various lineages of apes descended. Several took hominoid form.
The ancient African-Arabian continental plate collided with the Eurasian plate during the late Mesozoic. These continental collisions created mountains throughout Europe, the Middle East, and west Asia. This episode of mountain-building is known as the Alpine orogeny.
The Mediterranean Sea was formed during this process. As Africa-Arabia drifted northward, it closed over the ancient Tethys Ocean, which had flowed between the supercontinents Laurasia and Gondwana. What remained of the Tethys Sea became the Mediterranean, with a basin by way of stretched tectonic plates subducting from the Sea’s eastern edge.
During the Miocene, when sea levels were low, the Strait of Gibraltar closed. Evaporation ensued. The Mediterranean became a very salty, much smaller sea.
These episodes – of the Strait temporarily closing and reopening – happened multiple times between 6 and 5.3 MYA. It had a wrenching effect on habitats. Eventually, the Mediterranean basin was a desert, dotted by a few salty lakes.
Then, about 5 MYA, at the start of the Pliocene, sea levels again rose. The Atlantic surged past the Strait and refilled the Mediterranean Sea. From the end of the Miocene (5 MYA), climate in the Mediterranean basin changed from subtropical, with summer rainfall supporting laurel forests, to what it is today, with dry summers. Hence, modern life in the Mediterranean reflects an Atlantic heritage rather than the rich fauna inherited from the Tethys and Indian Oceans.
From the ancient Egyptians to Roman rule, the Mediterranean Sea played an essential role in the evolution of Western civilization, both as a natural resource and as a transportation thoroughfare. It is impossible to imagine human societal development in the region without it.