The Dogon stories explain that also. According to their oral traditions, a race people from the Sirius system called the Nommos visited Earth thousands of years ago. The Nommos were ugly, amphibious beings that resembled mermen and mermaids. They also appear in Babylonian, Accadian, and Sumerian myths. The Egyptian Goddess Isis, who is sometimes depicted as a mermaid, is also linked with the star Sirius.
The Nommos, according to the Dogon legend, lived on a planet that orbits another star in the Sirius system. They landed on Earth in an "ark" that made a spinning decent to the ground with great noise and wind. It was the Nommos that gave the Dogon the knowledge about Sirius B