Dr Ellis Silver offers arguments, based on human physiology, that suggest we may not have evolved alongside other life on Earth - but arrived from elsewhere.
A balloon returned from a high-altitude flight this year covered in microscopic life forms which seemed not to be of this world - and reignited the debate over whether life on Earth actually began here, or somewhere else.
This year, other scientists have argued that life originated on Mars, due to a mineral found in Martian meteorites, thought to be crucial to the genesis of life. Another experiment showed that amino acids could have arrived in impacts with comets - which suggests life might be widespread in the solar system.
But a new book by American ecologist Dr Ellis Silver argues that humans may well not be from Earth - and may have arrived separately. Silver offers arguments, based on human physiology, that suggest we may not have evolved alongside other life on Earth - but arrived from elsewhere, brought here by aliens as recently as a few tens of thousands of years ago.
Silver, an environmentalist who is currently working with the effort to clean plastic debris from the Pacific, says his book aims to provoke debate - and is based on scientific work on the difference between humans and other animals.