Replies

Global temperatures and sea levels dropped during that period,

Glad they solved that whole global warming thing.

which could have made life more difficult for the hunter-gatherer Jomon people.

Never mind.

Since it is easier for a sequence to become common in a small population, this is another indication that the size of the Jomon population decreased during the Late Jomon Period before the arrival of the Yayoi people.

Wouldn't it also be consistent with large numbers of Jomon men being killed off by invading Yayoi?

Let's not leave out the Ainu, Japan's indigenous aboriginal people:

"The Ainu people are historically residents of parts of Hokkaido (the Northern island of Japan) the Kuril Islands, and Sakhalin. According to the government, there are currently 25,000 Ainu living in Japan, but other sources claim there are up to 200,000. The origin of the Ainu people and language is, for the most part, unknown. However, there have been many theories on the subject.
One theory suggests that the Ainu people are remnants of the Jomon-jin, or the hunter-gathers who inhabited Japan during the Jomon Period (14,500 BC – 300 AD) and perhaps even before. Around the year 300 AD, another group of immigrants known as the Yayoi people made their way to the islands of Japan, introducing new agricultural techniques and technology and integrating with the Jomon people. It is believed that the Yayoi group may not have reached as far as the Northern island of Hokkaido, allowing the Jomon hunter-gatherer way of life to survive in that area."

"Physically, the Ainu stand out distinctly from the Japanese as a separate ethnic group. Ainu people tend to have light skin, a stout frame, deep-set eyes with a European shape, and thick, wavy hair. Full-blooded Ainu may have even had blue eyes or brown hair. In the past, the Ainu were proposed to be of Caucasian decent, given their appearance, but recently it has been proved through dental morphology and fingerprinting that the Ainu are in fact Mongoloid, not Caucasoid."

The Ainu

Thanks for the timely post, SunkenCiv.

A couple nights ago I spent several hours reading about the first humans in Japan.

Apparently, the oldest bones they have are in the 14,000 to 15,000 year old range, and, until recently, very little research had been done on sites that might be older than that.

A pre-15,000 year old arrival date for humans would put them right at the center of the coldest part of the last ice age.

Humans could have probably walked, or floated, or island hopped into the Japanese mainland from Korea, Taiwan, Sakhalin, or Kamchatka.