Now Placentos—the youngest and the last
Of Mammos' three great houses—rose to claim
The world entire: for Placentos had surpassed
Both elder siblings in the single game
That mattered most: the womb.
For Placentos
Built inside the mother's body a new
And revolutionary organ—the placenta's
Gift was this: a tissue that grew
Between the mother and the child, a bridge
Of blood that fed the embryo directly
From the mother's circulation—the ridge
Between the two bloods meeting perfectly
But never mixing—oxygen and nutrients
Crossing from the mother's side, the waste
Returning—and the child's long residence
Within the womb allowed a pace
Of development no egg or pouch could match:
The placental mammal was born larger,
More developed, more ready to dispatch
Its first breath—and the larger
Brain that the longer gestation allowed
Was the placental's advantage in a world
Where intelligence would matter—the endowed
And complex brain, its cortex furled
In folds that no marsupial could grow
In its short gestation, was the gift
Of the womb's extended show
Of development—the evolutionary shift
That would produce the elephant, the whale,
The dolphin, the ape, and at last the human mind.
And Placentos divided, and the tale
Of the Cenozoic's mammals is the kind
Of story that geography writes:
Three continents, three lineages, three
Separate and independent flights
Of adaptation—the threefold decree
Of the breaking Gondwana and the northern lands.
Afrotheros was born in Africa—
Isolated by the ocean's bands
From every other continent, a replica
Of the Australian experiment:
A single landmass, alone, where Afrotheros'
Children filled the niches in a different bent
Than their northern cousins—and the heroes
Of the African radiation were strange
And unexpected: the elephant, the aardvark,
The hyrax, the manatee—the range
Of forms that Africa's isolation in the dark
Of the early Cenozoic produced
From a single founding stock.
Xenarthros was born in South America—
The armored wanderer, whose flock
Of children grew in isolation too:
The sloth who hung inverted from the trees,
The armadillo in his banded hue
Of armor-plate, the anteater's expertise
Of tongue that probed the termite mound—
And in the Pleistocene, the giant forms:
The ground sloth tall as a house, the round
And car-sized glyptodont whose armored storms
Of carapace made him a living tank.
And Boreos—born in the north, in Laurasia—
The largest and most successful rank
Of all: for Boreos' intelligentsia
Would produce the carnivores, the ungulates,
The bats, the whales, the primates, the rodents—
The overwhelming majority of the fates
Of mammals were decided in the correspondents
Of Boreos' northern radiation.
Two great houses divided from his line:
Euarchos—whose predestination
Led to the primates and the rodent's design—
And Laurasios—whose children were
The carnivores, the hoofed ones, and the bats—
The northern hunter and the engineer
Of ungulate and predator, the vats
Of diversity that Laurasia's wide
And connected continents allowed.
Honor the threefold division—the pride
Of Placentos' house, the endowed
And scattered sons who took the world's
Three continents and filled them, each alone,
With the mammalian art—the flags unfurled
Of Afrotheros, Xenarthros, and the throne
Of Boreos—three answers to the question
That the empty Paleocene asked:
What will the mammal be? Three directions,
Three continents, three futures unmasked.