Now updated to most recent version with the equator marked in black:
Here's a brief description of the core cradle region:
The area surrounds a warm, generally placid sea, sheltered from the currents of the vast ocean to the north. Hurricanes typically evolve from the west traveling southeast, and they smash over the outer islands with regularity, although they trouble the southern part of the region less often.
The lowlands of the north, its islands and coastal regions, are covered by dense jungle, with tropical rainforest the predominant vegetation type. The island chain and the vast inland mountain range it transitions into are highly volcanic, with a number of active volcanoes periodically erupting. Although only one mountain in the island chain is large enough to have a permanent snowpack, the vegetation transitions to light forest cover and finally to alpine tableland on some of the higher peaks.
The equator passes directly over the aforementioned glaciated volcano just south of the largest island in the chain. For the most part, the older islands have a number of sheltered bays both shallow and deep, and the elevation ascends fairly modestly. Between the numerous lagoons, wetlands, and dense forest cover, arable land for traditional agriculture is at a premium, promoting primarily tree-based agriculture among the natives. The region is also metal poor, but rich in outcroppings of granite and obsidian.
The mountainous region the island chain meets is notable for sheer, sharp cliffs, black sand beaches and the intense steepness of the land, which rapidly ascends, in some places, thousands of feet in a few miles, giving the jarring impression of palm trees clinging to life on the shore with perennially snow-capped mountains visible in the background. At least five torrential rivers, fed by a thousand alpine streams, pound down the mountain valleys, creating a series of immensely spectacular waterfalls. Obviously these rivers are only navigable on their lower reaches. An intermediate region of temperate land suitable for terrace farming exists above the sweltering, perpetually humid lowland jungles, but the elevation makes organized labor extremely difficult to marshal on a large scale. Settled people are prone to crowding into the lowland river valleys, with communication between many of them only possible by sea, promoting linguistic diversity and political division. The region is rich in metal deposits.
In the southern reaches of this region, the mountain chains begin to break apart, transitioning to a country of broad plains, rolling hills and dry savannas, broken by the occasional, reddish-brown mountain, standing stark and alone over the hills, and immensely eroded monoliths of piled boulders tossed carelessly about everywhere else. (These mountains are the ancient remnants of a much older chain than their northern neighbors.) This countryside surrounds the great jagged bay carved by the floods that broke through after the end of the last ice age, the land around its southern extremities becoming increasingly desertified.
Along the coastline to the west, a thin stretch of mangrove forest clings to the southern edge of the great salt sea, becoming thickest in the delta islands of the great slow river that empties into the ocean, before giving way to plains as soon as the ocean is out of sight. These open plains and alluvial riverlands are much better suited to traditional field-based agriculture, and the wild grains growing in this region were among the first plants to be domesticated by ancient humans several thousand years ago. The area has a rainy season and a dry season, around which agriculture and social cycles are built. It has reasonably good access to metals and minerals, but is poor in high quality timber. The lack of major terrain barriers makes this region a good candidate for establishing large, unified powers with major population centers.
The far south of the world is covered in a vast, unending dune sea broken by the occasional high, wind-carved mesa island, with the exception of the southwest, where the eroded and scattered mountains and rolling golden plains are believed to continue on, perhaps forever, along the shores of the great freshwater sea.
With the exception of several species of large, mostly-flightless birds with garish plumage and hooked foreclaws, who often hunt in packs and range from an exotic pet to a trivial nuisance to a feared predator (based on size) in some of the deeper jungles, fauna in this region are analogous to what one might normally expect for our planet, with horses, pigs, cows, chickens, goats/goat-like antelopes, and guinea pigs all available as domesticates, although some of these sicken and die in the northern jungle.
