Nasa has released a new book that suggests ancient rock carvings found across the globe were may have been created by extraterrestrials.
As the search for life on planets other than our own continues, the space agency has suggested ways in which we might communicate with aliens from other worlds.
In one section, William Edmondson, from the University of Birmingham, considers the possibility that rock art on Earth is of extraterrestrial origin.
"We can say little, if anything, about what these patterns signify, why they were cut into rocks, or who created them," he writes. "For all intents and purposes, they might have been made by aliens."
Edmondson maintains that suggesting the ancient markings were created by aliens is necessary to reframe the way we go about searching for signals from other worlds - and how we make contact.
"Consider again, therefore, the desirability of establishing symbolic/linguistic communication with ETI [Extra-Terrestrial Intelligence]," it reads. "It is helpful to review some parallels from human existence that pose problems for us today."
"One of these is "rock art," which consists of patterns or shapes cut into rock many thousands of years ago. Such ancient stone carvings can be found in many countries... We can say little, if anything, about what these patterns signify, why they were cut into rocks, or who created them."
The book, Archaeology, Anthropology And Interstellar Communications, is edited by Douglas Vakoch, the director of Interstellar Message Composition at the Seach for Extraterrestrial Intelligence (SETI) Institute.
Vakoch's research studies ways in which different civilisations might "create messages that could be transmitted across interstellar space" - allowing communication between humans and extraterrestrials without face-to-face contact.
The introduction to the book states: "Addressing a field that has been dominated by astronomers, physicists, engineers, and computer scientists, the contributors to this collection raise questions that may have been overlooked by physical scientists about the ease of establishing meaningful communication with an extraterrestrial intelligence."
Elsewhere, authors tackle the question of the possible biology, evolution and physics of an alien race. Vakoch examines methods in which extraterrestrials might communicate, as messaging through sound, as we do on Earth, may not be possible.