A tsunami wiped out ancient communities in the Atacama Desert 3,800 years ago

It didn't happen all in one week, year, or millennium. But, starting 20,000 years ago, generation after generation found the village its grandparents built washed by waves, and had to move higher and build again. Usually other people already lived higher up, so conflicts arose. Australian oral history still records compacts settling those.

After the rise finally leveled off 8000 years ago, 120 meters upslope and often hundreds of miles inland, the tales came unmoored from personal experience, and shifted gradually, over more millennia, to what we know today.

The whole Persian Gulf had filled in, and the South China Sea, a million square miles below what are now Indonesian islands ("Sundaland") between and around Australia / New Guinea ("Sahul"), off India and Japan, and between Ireland, Britannia and Europe ("Doggerland") were forever gone.

The oldest civilizations we know are uphill from a few of those places, and the oldest rock art we know was painted in the mountains offshore of what was, 40,000 years ago, Sundaland.

Possibly what triggered the beginnings of civilization was the continuous need to adapt to new circumstances, and habit of discovering a new way of life among strangers, generation after generation, over the millennia from 20,000 years ago to 8000 years ago, and then ending, leaving uninterrupted stability.

The Younger Dryas cataclysm 12,000 years ago that wiped out most big animals in North America and darkened the whole world for years after must have had some impact. Gobekli Tepe, the oldest documented temple, was built after.

But Gunung Padang on Java was in use before the water ever started to rise. There were no ice ages there.

At that paper points out, one of the ways that oral traditions might last longer is if "a particular society remains comparatively isolated, not subject to the (repeated) cultural intrusion that might lead to radical alteration or replacement of Indigenous traditions".

Western mythologies are not isolated. In Snorri Sturluson's 13th century telling of the flood after Ymir's death, Bergelmir and his wife escaped by holding on to a hollowed-out tree trunk. Does that reflect pre-Christian Norse traditions, or a syncretization with Christian traditions that in turn drew on older Middle Eastern flood myths?

It's also difficult to determine if the mythology is indeed 7,000 years old, or a more recent mythology. Consider the Cantre'r Gwaelod, "an area of land which, according to legend, was located in an area west of present-day Wales which is now under the waters of Cardigan Bay" (quoting Wikipedia), with oldest records from the 1200s, and similar to the stories of Helig ap Glanawg.

The petrified forest of Ynyslas shows that 4-5,000 years ago the beach was a forest, and that people lived in it.

So, are the stories of Cantre'r Gwaelod the result of oral tradition that passed down for 4,000 years?

Or are they a more modern just-so story that attempts to explain why there are petrified trees on the beach? (Note that the tradition interprets glacial moraines as dikes.)

Or, consider the story of Giant's Causeway. Tradition is that Fionn mac Cumhaill built a path from Ireland to Scotland - both parts of the same lava flow, but with no reason to believe this is a result of an ancient oral tradition from when those land masses were connected.

> some of those themes likely imprinted on us via generations of long forgotten history and religion

On the flip side, https://en.wikipedia.org/wiki/List_of_flood_myths#Japan points out:

] Japan lacks a major flood myth. Japanese scholars in the 19th century such as Hirata Atsutane and Motoori Norinaga used the global flood myths of other cultures to argue for the supremacy of Shinto and promote Japanese nationalism.[22] They claimed that the fact that Japan has no flood myth showed that it was both the centre and highest point on Earth, making it the closest place on Earth to the heavens. As such, this to them demonstrates the veracity of the Japanese creation myth, where Japan comes first and foremost.

Why were Japanese people not similarly imprinted?

> a near extinction event

Our ancestors then may have had no clue that the rest of humans had died out. There may have been no emotional or cultural imprint to pass on.

Consider if everyone died in the year 1600, except for those on Australia. Our history and loss would not have been reflected in their stories.

It comes down, in the end, to plausibility: people everywhere have always been subject to existential catastrophe, including flood, conflagration, desertification, famine, pandemic, invasion, eviction, genocide, cyclone, earthquake, tsunami, volcanic eruption, bolide strike -- need I go on? Yet, only one of these shows up as formative in all places and nearly all peoples.

There are other ways to interpret universality. One is that a progenitor people carried an origin story with them, over tens of millennia, as they spread out to cover the planet. The universal interpretation of the Pleiades as seven sisters (one now lost) can be explained no other way.

The simpler explanation for prominence of flood myths over all others regardless of proximity to recent flooding events is universal experience, in place, continued over millennia.

The end of generation-on-generation flooding that immediately preceded the rise of sedentary agriculture and civilization must have been as profoundly affecting as the flood's previous relentless progress.

There must have been, after 12 millennia, a deeply ingrained expectation that the sea would continue rising indefinitely, the world perhaps ending with the sea finally submerging all land. Building with an expectation not to be soon evicted by water must have seemed audaciously sacrilegious, in many places, for many generations after.

Association of high ground with the gods' favor must have been cemented early.

Peoples accustomed to higher ground would have, instead, continual experience of invasion by people displaced, again finally trailing off.

So, is the null hypothesis that universal flood myths are local memories, with those citing universal older experience having something to prove? Or should the older origin be the baseline, with citations of scattered recent events seeding disconnected, coincidentally matching myths obliged to find support?

The choice is fatuous. Each may be true, in different places. A recent flood event can easily loom larger in memory than legend. Absent a recent flood or tsunami, the universal experience remains.