Update 3: 500 000 Years
The earth shook.
The wave came.
And then, the sun died.
The earth shaking was nothing abnormal to humanity, particularly to the Apa'al and Mnalyaba native to the rocky, broken lands of the far east. The wave that followed it, however, was unlike anything they could have imagined. It rose like a great wall of destruction, rode up onto the shores... and then continued. It struck far inland even in rough, moutainous lands. In the low-lying Gefo valley, it proceeded hundreds of kilometers inland. The same befell the flat jungles of the Abhwal and Oebhwaho, sweeping away everything it reached. At last, after traveling over two thousand kilometers upstream in some particularly flat jungle river basins, the megatsunami peaked, and began to retreat. Behind it was ruination, salted earth, a lunatic landscape of tossed boulders, ruined, matchstick trees, and mud.
It was a bitter blow for humankind. Some people, such as the Abhwal and Oebhwaho, were nearly wiped out. The densely-populated Gefo Valley was reduced to a wasteland. But worse was still to come. In the days to follow, the sky grew dark and grey. Ash fell over green leaves, soon to be followed by snow, landing in places which had never seen such weather before. Weather was disrupted, and soon so were the seasons. Plants died off en masse, as did the herbivores who relied on them. In short time, the famine spread to humans as well. Over the course of little more than a decade, human populations collapsed from a peak of just under a million, to a scant few tens of thousands. The climate would remain abnormally cool for thousands of years thereafter. Particularly hard hit were the populations already living near the limits of human cold tolerance, such as the Tiryats and Amalyafv, and heavily-populated places, such as the Itar Sea, and Ypta Mountains. Doubtlessly the Gefo Valley would have also suffered heavily from the famine, had it not already been exterminated by the callous hand of the great outer sea. Ash clouds were thickest in temperate and arid latitudes, although this reprieve was little-appreciated in the tropics, which had already been scoured by the tsunami.
As food steadily disappeared, the already-strained Fumo cultures began to break down, falling into increasingly vicious conflicts with themselves and the Cao. Famine and warfare defined the Itar Sea, the cradle of humankind, for generations. Some Fumos and Kuku maintained cooperation, but many more found themselves in a bloody struggle for food. Complicating matters was a northward migration of Tiryaps, fleeing the Ypta Mountains, which were becoming progressively drier and less able to support a large population.
In times of utter desperation, a cultural affinity for cannibalism can be very useful. With the Fumos in disarray and unable to properly resist, the invading Tiryaps were able to establish themselves as a major player in the milieu of the Itar Sea.
After all had passed, however, humanity proved resilient, and began to recover what had been lost.
In the far east, the Apa'al people continued their development. The eastern Akgapa'al developed increasingly structured societies, mirroring in some ways those of the distant Fumos and Tiryaps. They followed a semi-nomadic lifestyle, pursuing herds of ungulates across their ranges. The Kogkapal, on the other hand, remained a simple, hunting people, their main advancement being slightly superior stone-chipping techniques. Meanwhile, the Akger Apa'al, who were the worst hit by the disaster, ultimately returned to their home river, as well as playing a major role in repopulating the Gefo (Gero, in their language) basin.
The re-peopling of the Gero Valley has been a rather diverse affair. As a rich land, it was only a matter of time before the survivors returned to claim this prime territory. First to move were the aforementioned Apa'al, followed shortly by the Amalyap, who already had a significant presence in the upper river. Apalo from the great western plains were also swift to return. Much conflict and competition erupted, ultimately leading to the formation of two broad camps: the Gero (derived from Apa'al and Apalo) in the lower river and coastal regions, and the Amalyo (Apalo and Amalyap) in the upper reaches of the river.
Mnayakgu rift-men thrive in the lands between the Akgan and Kogan ranges. Squat and strong, with excellent balance and a great ability to climb, the Mnayakgu thrive where their neighbours cannot. Meanwhile, the Nyamaba are Mnalyaba who have migrated steadily further to the south.
Tharyavs, a mixed population of Taryabs and Ayalyafvs, have steadily migrated northeastwards, at the edge of the northward-creeping boreal zone. Their migration has brought them into contact with the Mnalyaba and Kogkapal, who have managed to put up a resistance to these newcomers, dragging their progress to a halt.
Amalyafvs, while overlapping significantly with the Tharyavs and Taryabs in range, have steadily migrated southwards, adopting the strong, but flexible social structures needed to survive in such harsh conditions.
Like the Amalyafvs, the Tiryats struggled mightily during the coldest years, but have emerged supremely well-adapted for cold conditions. The southernmost Tiryats have become talented sealers, and have become well-accustomed to harvesting the bounties of the southern coast.
The Temekyap have branched into the Mkyaph and Timika. The Timika have spread into and quickly occupied a great, harsh basin, monopolizing the precious rivers that flow into a small, salty sea in the desert's heart. The Mkyaph, on the other hand, grow close to the Myakap people, coming to grow culturally and physically closer due to their overlapping ranges and herd-chasing hunting practices.
At the furthest reaches of Timika lands, are the Mukta. Derived from the long-isolated western Myukyap, the Mukta live in the highlands rising out of the vast desert, rare islands of livability in a harsh, uncompromising land. Skilled climbers and able to subsist off of the limited offerings of their lands, the Mukta have begun to spread back eastwards, towards the Itar Sea.
Here, they have sporadic encounters with the people known as the Ikzil. Understood to be derived from long-isolated mountain populations of Fumos, the Ikzil are among the very first cave painters. While their images are often little more complex than leaf impressions or handprints, it is a major step in human development.
The Fumori suffered greatly through the Tiryap migration, and find themselves an endangered people, clinging on to the edge of an arid band of land south of the Itar Sea.
Fumos have re-established themselves to a degree on the Itar Sea, particularly on the northern coast. Many of the remaining Fumos are those who live in association with the Kukus, although the balance of power in this relationship has shifted distinctly in the favour of the aquatic people, who were able to emerge through the cataclysms relatively unscathed. However, it is the Tyumru who are the new dominant people of the western shores. With a mixture of Tiryap and Fumo ancestry, this hybrid people bear the heritage of two of the more influential cultures in the world.
The Fumme and Apalle, driven from the drying grasslands to the great northern rivers, have merged together, becoming the Vomma. They have spread down the river, to the edge of the Apa'nuk of the great northern rainforest.
Deep inland, the Apfal remain in their band of arid territory, though some of their number have expandeddownstream, finding another basin sea around which they can thrive.
The Apa'nuk, who survived the cataclysm in far greater proportions than the Abhwal and Oebhwaho due to their tendency to live further from the water, have gone on to spread across a vast range, encompassing most of the great jungles of the north. The surviving Abhwal live primarily on the middle and lower reaches of the same river inhabited by the Vomma. Some of them have returned to the coast, though many have chosen to remain on their river, having a faint, almost mythic memory of a terrible thing that awaits downstream.
The Oebhwaho, or at least those who survived, have rebounded. They reside across the northwest, thriving on the bays and rivers that make up the western slope of the great mountains at the continent's spine. The northern population, which emerged from a different group of survivors than the rest, have taken to referring to themselves as 'Wabaho'.
Half a million years have passed from the event, one which future researchers will speculate was a stupendous volcanic explosion and subsequent collapse which generated megatsunami waves hundreds of meters, or perhaps even a kilometer high. Prominent layers of ash found around the world, partcularly in southern temperate latitudes, will make this a valuable landmark for paleolithic archaeologists. It's understood through that this particular eruption disrupted global climate for the better part of fifty thousand years, before the regular cycle of warm and cold periods resumed. The world that emerged afterwards was recognizable in many ways, but still broadly changed. Some peoples, such as those of the far east, and the less dense inland regions, emerged largely the same as they had been before. However, the megatsunami was a devastating blow to the coastal Abhwal and Oebhwaho, who experienced a devastating setback, extirpating them from much of their traditional range. The Gefo, now Gero, Valley was almost completely re-peopled with a new stock of inland survivors, while the Fumo culture was ripped from their position of preeminence in the most densely-populated part of the world.
Finally, after 1.5 million years of reproductive isolation, the amphibious humans of the Itar sea are biologically a distinctive species,
Homo natatus. Hybridization with dryland humans is possible, to a degree, although
H. natatus females giving birth to hybrid young is almost always lethal to the mother.
)Their lungs adapt to the thin air and changing pressures. They also, perhaps, begin to consider longer term projects. First it might be the winding of great yards of rope, to create a bridge between their cave-dwelling and the fish-filled lake below. Learning the ways of the new animals, and the best methods to prevent their nimble escape up the cliff faces, or even frighten them such that they leap to their own doom. Or perhaps, mindfully stockpiling and practice using piles of rock to defend their homes against the raiding Tharyav. Some of the later ones might even be generation-long, or require the cooperation of many tribes for many years. (an extreme example would be cutting stairs into a cliff face to facilitate trade with lowlying tribes, but that might be too much). 
(but seriously great update!)
