Although this world diverged in the 1920s it belongs here.
In 1923
Randolph Carter, while exploring caves in New England, discovered a system of nexal gates leading to far places. A few lead to places on Earth, others to Mars and other worlds around our sun. The gates that fascinated everyone led to extra-solar planets.
Carter's explorations led to the realisation that most of the lifeforms on these worlds came from the Earth. He theorised that whoever created the gates meant to spread Earth's lifeforms far and wide. Perhaps because the conditions for life to start and to become complex and abundant were rare and Earth was especially rich in lifeforms.
Carter's people found human groups from Native American tribal groups (bands of the
Mi'kmaq People) to large civilizations of many different origins. Boston, having a nexal gate under
Beacon Hill built a subway to a large city settled by a mixture of Gaelic speakers and
Narragansetts.
The early enthusiasm for these gates was somewhat dimmed when a plague spread from one of these worlds to the Earth in 1939(some people say the plague was a good thing because Hitler and Stalin both died and thus we were spared a second great war, other say the horrors of 1914-1918 taught Europe to avoid general war for all time). After the Plague was beaten by penicillin the USA focused on exploring the stars through these gates and spreading out to the stars.
During the celebrations of the Tricentennial of Randolph Carter's discovery something happened. No one knows what. But the gates to Earth are closed. But the settlers from Earth settled on the worlds of a thousand stars have stardrives and the engineers to build and crew starships. The journey to Earth shall be made. The Mi-Go and others advise against this or threaten, but humanity is undaunted.